This is /home/vagrant/rpmbuild/BUILD/build-eglibc/manual/libc.info,
produced by makeinfo version 4.13 from libc.texinfo.

INFO-DIR-SECTION Software libraries
START-INFO-DIR-ENTRY
* Libc: (libc).                 C library.
END-INFO-DIR-ENTRY

INFO-DIR-SECTION GNU C library functions and macros
START-INFO-DIR-ENTRY
* ALTWERASE: (libc)Local Modes.
* ARGP_ERR_UNKNOWN: (libc)Argp Parser Functions.
* ARG_MAX: (libc)General Limits.
* BC_BASE_MAX: (libc)Utility Limits.
* BC_DIM_MAX: (libc)Utility Limits.
* BC_SCALE_MAX: (libc)Utility Limits.
* BC_STRING_MAX: (libc)Utility Limits.
* BRKINT: (libc)Input Modes.
* BUFSIZ: (libc)Controlling Buffering.
* CCTS_OFLOW: (libc)Control Modes.
* CHILD_MAX: (libc)General Limits.
* CIGNORE: (libc)Control Modes.
* CLK_TCK: (libc)Processor Time.
* CLOCAL: (libc)Control Modes.
* CLOCKS_PER_SEC: (libc)CPU Time.
* COLL_WEIGHTS_MAX: (libc)Utility Limits.
* CPU_CLR: (libc)CPU Affinity.
* CPU_ISSET: (libc)CPU Affinity.
* CPU_SET: (libc)CPU Affinity.
* CPU_SETSIZE: (libc)CPU Affinity.
* CPU_ZERO: (libc)CPU Affinity.
* CREAD: (libc)Control Modes.
* CRTS_IFLOW: (libc)Control Modes.
* CS5: (libc)Control Modes.
* CS6: (libc)Control Modes.
* CS7: (libc)Control Modes.
* CS8: (libc)Control Modes.
* CSIZE: (libc)Control Modes.
* CSTOPB: (libc)Control Modes.
* DES_FAILED: (libc)DES Encryption.
* DTTOIF: (libc)Directory Entries.
* E2BIG: (libc)Error Codes.
* EACCES: (libc)Error Codes.
* EADDRINUSE: (libc)Error Codes.
* EADDRNOTAVAIL: (libc)Error Codes.
* EADV: (libc)Error Codes.
* EAFNOSUPPORT: (libc)Error Codes.
* EAGAIN: (libc)Error Codes.
* EALREADY: (libc)Error Codes.
* EAUTH: (libc)Error Codes.
* EBACKGROUND: (libc)Error Codes.
* EBADE: (libc)Error Codes.
* EBADF: (libc)Error Codes.
* EBADFD: (libc)Error Codes.
* EBADMSG: (libc)Error Codes.
* EBADR: (libc)Error Codes.
* EBADRPC: (libc)Error Codes.
* EBADRQC: (libc)Error Codes.
* EBADSLT: (libc)Error Codes.
* EBFONT: (libc)Error Codes.
* EBUSY: (libc)Error Codes.
* ECANCELED: (libc)Error Codes.
* ECHILD: (libc)Error Codes.
* ECHO: (libc)Local Modes.
* ECHOCTL: (libc)Local Modes.
* ECHOE: (libc)Local Modes.
* ECHOK: (libc)Local Modes.
* ECHOKE: (libc)Local Modes.
* ECHONL: (libc)Local Modes.
* ECHOPRT: (libc)Local Modes.
* ECHRNG: (libc)Error Codes.
* ECOMM: (libc)Error Codes.
* ECONNABORTED: (libc)Error Codes.
* ECONNREFUSED: (libc)Error Codes.
* ECONNRESET: (libc)Error Codes.
* ED: (libc)Error Codes.
* EDEADLK: (libc)Error Codes.
* EDEADLOCK: (libc)Error Codes.
* EDESTADDRREQ: (libc)Error Codes.
* EDIED: (libc)Error Codes.
* EDOM: (libc)Error Codes.
* EDOTDOT: (libc)Error Codes.
* EDQUOT: (libc)Error Codes.
* EEXIST: (libc)Error Codes.
* EFAULT: (libc)Error Codes.
* EFBIG: (libc)Error Codes.
* EFTYPE: (libc)Error Codes.
* EGRATUITOUS: (libc)Error Codes.
* EGREGIOUS: (libc)Error Codes.
* EHOSTDOWN: (libc)Error Codes.
* EHOSTUNREACH: (libc)Error Codes.
* EHWPOISON: (libc)Error Codes.
* EIDRM: (libc)Error Codes.
* EIEIO: (libc)Error Codes.
* EILSEQ: (libc)Error Codes.
* EINPROGRESS: (libc)Error Codes.
* EINTR: (libc)Error Codes.
* EINVAL: (libc)Error Codes.
* EIO: (libc)Error Codes.
* EISCONN: (libc)Error Codes.
* EISDIR: (libc)Error Codes.
* EISNAM: (libc)Error Codes.
* EKEYEXPIRED: (libc)Error Codes.
* EKEYREJECTED: (libc)Error Codes.
* EKEYREVOKED: (libc)Error Codes.
* EL2HLT: (libc)Error Codes.
* EL2NSYNC: (libc)Error Codes.
* EL3HLT: (libc)Error Codes.
* EL3RST: (libc)Error Codes.
* ELIBACC: (libc)Error Codes.
* ELIBBAD: (libc)Error Codes.
* ELIBEXEC: (libc)Error Codes.
* ELIBMAX: (libc)Error Codes.
* ELIBSCN: (libc)Error Codes.
* ELNRNG: (libc)Error Codes.
* ELOOP: (libc)Error Codes.
* EMEDIUMTYPE: (libc)Error Codes.
* EMFILE: (libc)Error Codes.
* EMLINK: (libc)Error Codes.
* EMSGSIZE: (libc)Error Codes.
* EMULTIHOP: (libc)Error Codes.
* ENAMETOOLONG: (libc)Error Codes.
* ENAVAIL: (libc)Error Codes.
* ENEEDAUTH: (libc)Error Codes.
* ENETDOWN: (libc)Error Codes.
* ENETRESET: (libc)Error Codes.
* ENETUNREACH: (libc)Error Codes.
* ENFILE: (libc)Error Codes.
* ENOANO: (libc)Error Codes.
* ENOBUFS: (libc)Error Codes.
* ENOCSI: (libc)Error Codes.
* ENODATA: (libc)Error Codes.
* ENODEV: (libc)Error Codes.
* ENOENT: (libc)Error Codes.
* ENOEXEC: (libc)Error Codes.
* ENOKEY: (libc)Error Codes.
* ENOLCK: (libc)Error Codes.
* ENOLINK: (libc)Error Codes.
* ENOMEDIUM: (libc)Error Codes.
* ENOMEM: (libc)Error Codes.
* ENOMSG: (libc)Error Codes.
* ENONET: (libc)Error Codes.
* ENOPKG: (libc)Error Codes.
* ENOPROTOOPT: (libc)Error Codes.
* ENOSPC: (libc)Error Codes.
* ENOSR: (libc)Error Codes.
* ENOSTR: (libc)Error Codes.
* ENOSYS: (libc)Error Codes.
* ENOTBLK: (libc)Error Codes.
* ENOTCONN: (libc)Error Codes.
* ENOTDIR: (libc)Error Codes.
* ENOTEMPTY: (libc)Error Codes.
* ENOTNAM: (libc)Error Codes.
* ENOTRECOVERABLE: (libc)Error Codes.
* ENOTSOCK: (libc)Error Codes.
* ENOTSUP: (libc)Error Codes.
* ENOTTY: (libc)Error Codes.
* ENOTUNIQ: (libc)Error Codes.
* ENXIO: (libc)Error Codes.
* EOF: (libc)EOF and Errors.
* EOPNOTSUPP: (libc)Error Codes.
* EOVERFLOW: (libc)Error Codes.
* EOWNERDEAD: (libc)Error Codes.
* EPERM: (libc)Error Codes.
* EPFNOSUPPORT: (libc)Error Codes.
* EPIPE: (libc)Error Codes.
* EPROCLIM: (libc)Error Codes.
* EPROCUNAVAIL: (libc)Error Codes.
* EPROGMISMATCH: (libc)Error Codes.
* EPROGUNAVAIL: (libc)Error Codes.
* EPROTO: (libc)Error Codes.
* EPROTONOSUPPORT: (libc)Error Codes.
* EPROTOTYPE: (libc)Error Codes.
* EQUIV_CLASS_MAX: (libc)Utility Limits.
* ERANGE: (libc)Error Codes.
* EREMCHG: (libc)Error Codes.
* EREMOTE: (libc)Error Codes.
* EREMOTEIO: (libc)Error Codes.
* ERESTART: (libc)Error Codes.
* ERFKILL: (libc)Error Codes.
* EROFS: (libc)Error Codes.
* ERPCMISMATCH: (libc)Error Codes.
* ESHUTDOWN: (libc)Error Codes.
* ESOCKTNOSUPPORT: (libc)Error Codes.
* ESPIPE: (libc)Error Codes.
* ESRCH: (libc)Error Codes.
* ESRMNT: (libc)Error Codes.
* ESTALE: (libc)Error Codes.
* ESTRPIPE: (libc)Error Codes.
* ETIME: (libc)Error Codes.
* ETIMEDOUT: (libc)Error Codes.
* ETOOMANYREFS: (libc)Error Codes.
* ETXTBSY: (libc)Error Codes.
* EUCLEAN: (libc)Error Codes.
* EUNATCH: (libc)Error Codes.
* EUSERS: (libc)Error Codes.
* EWOULDBLOCK: (libc)Error Codes.
* EXDEV: (libc)Error Codes.
* EXFULL: (libc)Error Codes.
* EXIT_FAILURE: (libc)Exit Status.
* EXIT_SUCCESS: (libc)Exit Status.
* EXPR_NEST_MAX: (libc)Utility Limits.
* FD_CLOEXEC: (libc)Descriptor Flags.
* FD_CLR: (libc)Waiting for I/O.
* FD_ISSET: (libc)Waiting for I/O.
* FD_SET: (libc)Waiting for I/O.
* FD_SETSIZE: (libc)Waiting for I/O.
* FD_ZERO: (libc)Waiting for I/O.
* FILENAME_MAX: (libc)Limits for Files.
* FLUSHO: (libc)Local Modes.
* FOPEN_MAX: (libc)Opening Streams.
* FP_ILOGB0: (libc)Exponents and Logarithms.
* FP_ILOGBNAN: (libc)Exponents and Logarithms.
* F_DUPFD: (libc)Duplicating Descriptors.
* F_GETFD: (libc)Descriptor Flags.
* F_GETFL: (libc)Getting File Status Flags.
* F_GETLK: (libc)File Locks.
* F_GETOWN: (libc)Interrupt Input.
* F_OK: (libc)Testing File Access.
* F_SETFD: (libc)Descriptor Flags.
* F_SETFL: (libc)Getting File Status Flags.
* F_SETLK: (libc)File Locks.
* F_SETLKW: (libc)File Locks.
* F_SETOWN: (libc)Interrupt Input.
* HUGE_VAL: (libc)Math Error Reporting.
* HUGE_VALF: (libc)Math Error Reporting.
* HUGE_VALL: (libc)Math Error Reporting.
* HUPCL: (libc)Control Modes.
* I: (libc)Complex Numbers.
* ICANON: (libc)Local Modes.
* ICRNL: (libc)Input Modes.
* IEXTEN: (libc)Local Modes.
* IFNAMSIZ: (libc)Interface Naming.
* IFTODT: (libc)Directory Entries.
* IGNBRK: (libc)Input Modes.
* IGNCR: (libc)Input Modes.
* IGNPAR: (libc)Input Modes.
* IMAXBEL: (libc)Input Modes.
* INADDR_ANY: (libc)Host Address Data Type.
* INADDR_BROADCAST: (libc)Host Address Data Type.
* INADDR_LOOPBACK: (libc)Host Address Data Type.
* INADDR_NONE: (libc)Host Address Data Type.
* INFINITY: (libc)Infinity and NaN.
* INLCR: (libc)Input Modes.
* INPCK: (libc)Input Modes.
* IPPORT_RESERVED: (libc)Ports.
* IPPORT_USERRESERVED: (libc)Ports.
* ISIG: (libc)Local Modes.
* ISTRIP: (libc)Input Modes.
* IXANY: (libc)Input Modes.
* IXOFF: (libc)Input Modes.
* IXON: (libc)Input Modes.
* LINE_MAX: (libc)Utility Limits.
* LINK_MAX: (libc)Limits for Files.
* L_ctermid: (libc)Identifying the Terminal.
* L_cuserid: (libc)Who Logged In.
* L_tmpnam: (libc)Temporary Files.
* MAXNAMLEN: (libc)Limits for Files.
* MAXSYMLINKS: (libc)Symbolic Links.
* MAX_CANON: (libc)Limits for Files.
* MAX_INPUT: (libc)Limits for Files.
* MB_CUR_MAX: (libc)Selecting the Conversion.
* MB_LEN_MAX: (libc)Selecting the Conversion.
* MDMBUF: (libc)Control Modes.
* MSG_DONTROUTE: (libc)Socket Data Options.
* MSG_OOB: (libc)Socket Data Options.
* MSG_PEEK: (libc)Socket Data Options.
* NAME_MAX: (libc)Limits for Files.
* NAN: (libc)Infinity and NaN.
* NCCS: (libc)Mode Data Types.
* NGROUPS_MAX: (libc)General Limits.
* NOFLSH: (libc)Local Modes.
* NOKERNINFO: (libc)Local Modes.
* NSIG: (libc)Standard Signals.
* NULL: (libc)Null Pointer Constant.
* ONLCR: (libc)Output Modes.
* ONOEOT: (libc)Output Modes.
* OPEN_MAX: (libc)General Limits.
* OPOST: (libc)Output Modes.
* OXTABS: (libc)Output Modes.
* O_ACCMODE: (libc)Access Modes.
* O_APPEND: (libc)Operating Modes.
* O_ASYNC: (libc)Operating Modes.
* O_CREAT: (libc)Open-time Flags.
* O_EXCL: (libc)Open-time Flags.
* O_EXEC: (libc)Access Modes.
* O_EXLOCK: (libc)Open-time Flags.
* O_FSYNC: (libc)Operating Modes.
* O_IGNORE_CTTY: (libc)Open-time Flags.
* O_NDELAY: (libc)Operating Modes.
* O_NOATIME: (libc)Operating Modes.
* O_NOCTTY: (libc)Open-time Flags.
* O_NOLINK: (libc)Open-time Flags.
* O_NONBLOCK: (libc)Open-time Flags.
* O_NONBLOCK: (libc)Operating Modes.
* O_NOTRANS: (libc)Open-time Flags.
* O_RDONLY: (libc)Access Modes.
* O_RDWR: (libc)Access Modes.
* O_READ: (libc)Access Modes.
* O_SHLOCK: (libc)Open-time Flags.
* O_SYNC: (libc)Operating Modes.
* O_TRUNC: (libc)Open-time Flags.
* O_WRITE: (libc)Access Modes.
* O_WRONLY: (libc)Access Modes.
* PARENB: (libc)Control Modes.
* PARMRK: (libc)Input Modes.
* PARODD: (libc)Control Modes.
* PATH_MAX: (libc)Limits for Files.
* PA_FLAG_MASK: (libc)Parsing a Template String.
* PENDIN: (libc)Local Modes.
* PF_FILE: (libc)Local Namespace Details.
* PF_INET6: (libc)Internet Namespace.
* PF_INET: (libc)Internet Namespace.
* PF_LOCAL: (libc)Local Namespace Details.
* PF_UNIX: (libc)Local Namespace Details.
* PIPE_BUF: (libc)Limits for Files.
* P_tmpdir: (libc)Temporary Files.
* RAND_MAX: (libc)ISO Random.
* RE_DUP_MAX: (libc)General Limits.
* RLIM_INFINITY: (libc)Limits on Resources.
* R_OK: (libc)Testing File Access.
* SA_NOCLDSTOP: (libc)Flags for Sigaction.
* SA_ONSTACK: (libc)Flags for Sigaction.
* SA_RESTART: (libc)Flags for Sigaction.
* SEEK_CUR: (libc)File Positioning.
* SEEK_END: (libc)File Positioning.
* SEEK_SET: (libc)File Positioning.
* SIGABRT: (libc)Program Error Signals.
* SIGALRM: (libc)Alarm Signals.
* SIGBUS: (libc)Program Error Signals.
* SIGCHLD: (libc)Job Control Signals.
* SIGCLD: (libc)Job Control Signals.
* SIGCONT: (libc)Job Control Signals.
* SIGEMT: (libc)Program Error Signals.
* SIGFPE: (libc)Program Error Signals.
* SIGHUP: (libc)Termination Signals.
* SIGILL: (libc)Program Error Signals.
* SIGINFO: (libc)Miscellaneous Signals.
* SIGINT: (libc)Termination Signals.
* SIGIO: (libc)Asynchronous I/O Signals.
* SIGIOT: (libc)Program Error Signals.
* SIGKILL: (libc)Termination Signals.
* SIGLOST: (libc)Operation Error Signals.
* SIGPIPE: (libc)Operation Error Signals.
* SIGPOLL: (libc)Asynchronous I/O Signals.
* SIGPROF: (libc)Alarm Signals.
* SIGQUIT: (libc)Termination Signals.
* SIGSEGV: (libc)Program Error Signals.
* SIGSTOP: (libc)Job Control Signals.
* SIGSYS: (libc)Program Error Signals.
* SIGTERM: (libc)Termination Signals.
* SIGTRAP: (libc)Program Error Signals.
* SIGTSTP: (libc)Job Control Signals.
* SIGTTIN: (libc)Job Control Signals.
* SIGTTOU: (libc)Job Control Signals.
* SIGURG: (libc)Asynchronous I/O Signals.
* SIGUSR1: (libc)Miscellaneous Signals.
* SIGUSR2: (libc)Miscellaneous Signals.
* SIGVTALRM: (libc)Alarm Signals.
* SIGWINCH: (libc)Miscellaneous Signals.
* SIGXCPU: (libc)Operation Error Signals.
* SIGXFSZ: (libc)Operation Error Signals.
* SIG_ERR: (libc)Basic Signal Handling.
* SOCK_DGRAM: (libc)Communication Styles.
* SOCK_RAW: (libc)Communication Styles.
* SOCK_RDM: (libc)Communication Styles.
* SOCK_SEQPACKET: (libc)Communication Styles.
* SOCK_STREAM: (libc)Communication Styles.
* SOL_SOCKET: (libc)Socket-Level Options.
* SSIZE_MAX: (libc)General Limits.
* STREAM_MAX: (libc)General Limits.
* SUN_LEN: (libc)Local Namespace Details.
* SV_INTERRUPT: (libc)BSD Handler.
* SV_ONSTACK: (libc)BSD Handler.
* SV_RESETHAND: (libc)BSD Handler.
* S_IFMT: (libc)Testing File Type.
* S_ISBLK: (libc)Testing File Type.
* S_ISCHR: (libc)Testing File Type.
* S_ISDIR: (libc)Testing File Type.
* S_ISFIFO: (libc)Testing File Type.
* S_ISLNK: (libc)Testing File Type.
* S_ISREG: (libc)Testing File Type.
* S_ISSOCK: (libc)Testing File Type.
* S_TYPEISMQ: (libc)Testing File Type.
* S_TYPEISSEM: (libc)Testing File Type.
* S_TYPEISSHM: (libc)Testing File Type.
* TMP_MAX: (libc)Temporary Files.
* TOSTOP: (libc)Local Modes.
* TZNAME_MAX: (libc)General Limits.
* VDISCARD: (libc)Other Special.
* VDSUSP: (libc)Signal Characters.
* VEOF: (libc)Editing Characters.
* VEOL2: (libc)Editing Characters.
* VEOL: (libc)Editing Characters.
* VERASE: (libc)Editing Characters.
* VINTR: (libc)Signal Characters.
* VKILL: (libc)Editing Characters.
* VLNEXT: (libc)Other Special.
* VMIN: (libc)Noncanonical Input.
* VQUIT: (libc)Signal Characters.
* VREPRINT: (libc)Editing Characters.
* VSTART: (libc)Start/Stop Characters.
* VSTATUS: (libc)Other Special.
* VSTOP: (libc)Start/Stop Characters.
* VSUSP: (libc)Signal Characters.
* VTIME: (libc)Noncanonical Input.
* VWERASE: (libc)Editing Characters.
* WCHAR_MAX: (libc)Extended Char Intro.
* WCHAR_MIN: (libc)Extended Char Intro.
* WCOREDUMP: (libc)Process Completion Status.
* WEOF: (libc)EOF and Errors.
* WEOF: (libc)Extended Char Intro.
* WEXITSTATUS: (libc)Process Completion Status.
* WIFEXITED: (libc)Process Completion Status.
* WIFSIGNALED: (libc)Process Completion Status.
* WIFSTOPPED: (libc)Process Completion Status.
* WSTOPSIG: (libc)Process Completion Status.
* WTERMSIG: (libc)Process Completion Status.
* W_OK: (libc)Testing File Access.
* X_OK: (libc)Testing File Access.
* _Complex_I: (libc)Complex Numbers.
* _Exit: (libc)Termination Internals.
* _IOFBF: (libc)Controlling Buffering.
* _IOLBF: (libc)Controlling Buffering.
* _IONBF: (libc)Controlling Buffering.
* _Imaginary_I: (libc)Complex Numbers.
* _PATH_UTMP: (libc)Manipulating the Database.
* _PATH_WTMP: (libc)Manipulating the Database.
* _POSIX2_C_DEV: (libc)System Options.
* _POSIX2_C_VERSION: (libc)Version Supported.
* _POSIX2_FORT_DEV: (libc)System Options.
* _POSIX2_FORT_RUN: (libc)System Options.
* _POSIX2_LOCALEDEF: (libc)System Options.
* _POSIX2_SW_DEV: (libc)System Options.
* _POSIX_CHOWN_RESTRICTED: (libc)Options for Files.
* _POSIX_JOB_CONTROL: (libc)System Options.
* _POSIX_NO_TRUNC: (libc)Options for Files.
* _POSIX_SAVED_IDS: (libc)System Options.
* _POSIX_VDISABLE: (libc)Options for Files.
* _POSIX_VERSION: (libc)Version Supported.
* __fbufsize: (libc)Controlling Buffering.
* __flbf: (libc)Controlling Buffering.
* __fpending: (libc)Controlling Buffering.
* __fpurge: (libc)Flushing Buffers.
* __freadable: (libc)Opening Streams.
* __freading: (libc)Opening Streams.
* __fsetlocking: (libc)Streams and Threads.
* __fwritable: (libc)Opening Streams.
* __fwriting: (libc)Opening Streams.
* __gconv_end_fct: (libc)glibc iconv Implementation.
* __gconv_fct: (libc)glibc iconv Implementation.
* __gconv_init_fct: (libc)glibc iconv Implementation.
* __ppc_get_timebase: (libc)PowerPC.
* __ppc_get_timebase_freq: (libc)PowerPC.
* __ppc_mdoio: (libc)PowerPC.
* __ppc_mdoom: (libc)PowerPC.
* __ppc_set_ppr_low: (libc)PowerPC.
* __ppc_set_ppr_med: (libc)PowerPC.
* __ppc_set_ppr_med_low: (libc)PowerPC.
* __ppc_yield: (libc)PowerPC.
* __va_copy: (libc)Argument Macros.
* _exit: (libc)Termination Internals.
* _flushlbf: (libc)Flushing Buffers.
* _tolower: (libc)Case Conversion.
* _toupper: (libc)Case Conversion.
* a64l: (libc)Encode Binary Data.
* abort: (libc)Aborting a Program.
* abs: (libc)Absolute Value.
* accept: (libc)Accepting Connections.
* access: (libc)Testing File Access.
* acos: (libc)Inverse Trig Functions.
* acosf: (libc)Inverse Trig Functions.
* acosh: (libc)Hyperbolic Functions.
* acoshf: (libc)Hyperbolic Functions.
* acoshl: (libc)Hyperbolic Functions.
* acosl: (libc)Inverse Trig Functions.
* addmntent: (libc)mtab.
* addseverity: (libc)Adding Severity Classes.
* adjtime: (libc)High-Resolution Calendar.
* adjtimex: (libc)High-Resolution Calendar.
* aio_cancel64: (libc)Cancel AIO Operations.
* aio_cancel: (libc)Cancel AIO Operations.
* aio_error64: (libc)Status of AIO Operations.
* aio_error: (libc)Status of AIO Operations.
* aio_fsync64: (libc)Synchronizing AIO Operations.
* aio_fsync: (libc)Synchronizing AIO Operations.
* aio_init: (libc)Configuration of AIO.
* aio_read64: (libc)Asynchronous Reads/Writes.
* aio_read: (libc)Asynchronous Reads/Writes.
* aio_return64: (libc)Status of AIO Operations.
* aio_return: (libc)Status of AIO Operations.
* aio_suspend64: (libc)Synchronizing AIO Operations.
* aio_suspend: (libc)Synchronizing AIO Operations.
* aio_write64: (libc)Asynchronous Reads/Writes.
* aio_write: (libc)Asynchronous Reads/Writes.
* alarm: (libc)Setting an Alarm.
* alloca: (libc)Variable Size Automatic.
* alphasort64: (libc)Scanning Directory Content.
* alphasort: (libc)Scanning Directory Content.
* argp_error: (libc)Argp Helper Functions.
* argp_failure: (libc)Argp Helper Functions.
* argp_help: (libc)Argp Help.
* argp_parse: (libc)Argp.
* argp_state_help: (libc)Argp Helper Functions.
* argp_usage: (libc)Argp Helper Functions.
* argz_add: (libc)Argz Functions.
* argz_add_sep: (libc)Argz Functions.
* argz_append: (libc)Argz Functions.
* argz_count: (libc)Argz Functions.
* argz_create: (libc)Argz Functions.
* argz_create_sep: (libc)Argz Functions.
* argz_delete: (libc)Argz Functions.
* argz_extract: (libc)Argz Functions.
* argz_insert: (libc)Argz Functions.
* argz_next: (libc)Argz Functions.
* argz_replace: (libc)Argz Functions.
* argz_stringify: (libc)Argz Functions.
* asctime: (libc)Formatting Calendar Time.
* asctime_r: (libc)Formatting Calendar Time.
* asin: (libc)Inverse Trig Functions.
* asinf: (libc)Inverse Trig Functions.
* asinh: (libc)Hyperbolic Functions.
* asinhf: (libc)Hyperbolic Functions.
* asinhl: (libc)Hyperbolic Functions.
* asinl: (libc)Inverse Trig Functions.
* asprintf: (libc)Dynamic Output.
* assert: (libc)Consistency Checking.
* assert_perror: (libc)Consistency Checking.
* atan2: (libc)Inverse Trig Functions.
* atan2f: (libc)Inverse Trig Functions.
* atan2l: (libc)Inverse Trig Functions.
* atan: (libc)Inverse Trig Functions.
* atanf: (libc)Inverse Trig Functions.
* atanh: (libc)Hyperbolic Functions.
* atanhf: (libc)Hyperbolic Functions.
* atanhl: (libc)Hyperbolic Functions.
* atanl: (libc)Inverse Trig Functions.
* atexit: (libc)Cleanups on Exit.
* atof: (libc)Parsing of Floats.
* atoi: (libc)Parsing of Integers.
* atol: (libc)Parsing of Integers.
* atoll: (libc)Parsing of Integers.
* backtrace: (libc)Backtraces.
* backtrace_symbols: (libc)Backtraces.
* backtrace_symbols_fd: (libc)Backtraces.
* basename: (libc)Finding Tokens in a String.
* basename: (libc)Finding Tokens in a String.
* bcmp: (libc)String/Array Comparison.
* bcopy: (libc)Copying and Concatenation.
* bind: (libc)Setting Address.
* bind_textdomain_codeset: (libc)Charset conversion in gettext.
* bindtextdomain: (libc)Locating gettext catalog.
* brk: (libc)Resizing the Data Segment.
* bsearch: (libc)Array Search Function.
* btowc: (libc)Converting a Character.
* bzero: (libc)Copying and Concatenation.
* cabs: (libc)Absolute Value.
* cabsf: (libc)Absolute Value.
* cabsl: (libc)Absolute Value.
* cacos: (libc)Inverse Trig Functions.
* cacosf: (libc)Inverse Trig Functions.
* cacosh: (libc)Hyperbolic Functions.
* cacoshf: (libc)Hyperbolic Functions.
* cacoshl: (libc)Hyperbolic Functions.
* cacosl: (libc)Inverse Trig Functions.
* calloc: (libc)Allocating Cleared Space.
* canonicalize_file_name: (libc)Symbolic Links.
* carg: (libc)Operations on Complex.
* cargf: (libc)Operations on Complex.
* cargl: (libc)Operations on Complex.
* casin: (libc)Inverse Trig Functions.
* casinf: (libc)Inverse Trig Functions.
* casinh: (libc)Hyperbolic Functions.
* casinhf: (libc)Hyperbolic Functions.
* casinhl: (libc)Hyperbolic Functions.
* casinl: (libc)Inverse Trig Functions.
* catan: (libc)Inverse Trig Functions.
* catanf: (libc)Inverse Trig Functions.
* catanh: (libc)Hyperbolic Functions.
* catanhf: (libc)Hyperbolic Functions.
* catanhl: (libc)Hyperbolic Functions.
* catanl: (libc)Inverse Trig Functions.
* catclose: (libc)The catgets Functions.
* catgets: (libc)The catgets Functions.
* catopen: (libc)The catgets Functions.
* cbc_crypt: (libc)DES Encryption.
* cbrt: (libc)Exponents and Logarithms.
* cbrtf: (libc)Exponents and Logarithms.
* cbrtl: (libc)Exponents and Logarithms.
* ccos: (libc)Trig Functions.
* ccosf: (libc)Trig Functions.
* ccosh: (libc)Hyperbolic Functions.
* ccoshf: (libc)Hyperbolic Functions.
* ccoshl: (libc)Hyperbolic Functions.
* ccosl: (libc)Trig Functions.
* ceil: (libc)Rounding Functions.
* ceilf: (libc)Rounding Functions.
* ceill: (libc)Rounding Functions.
* cexp: (libc)Exponents and Logarithms.
* cexpf: (libc)Exponents and Logarithms.
* cexpl: (libc)Exponents and Logarithms.
* cfgetispeed: (libc)Line Speed.
* cfgetospeed: (libc)Line Speed.
* cfmakeraw: (libc)Noncanonical Input.
* cfree: (libc)Freeing after Malloc.
* cfsetispeed: (libc)Line Speed.
* cfsetospeed: (libc)Line Speed.
* cfsetspeed: (libc)Line Speed.
* chdir: (libc)Working Directory.
* chmod: (libc)Setting Permissions.
* chown: (libc)File Owner.
* cimag: (libc)Operations on Complex.
* cimagf: (libc)Operations on Complex.
* cimagl: (libc)Operations on Complex.
* clearenv: (libc)Environment Access.
* clearerr: (libc)Error Recovery.
* clearerr_unlocked: (libc)Error Recovery.
* clock: (libc)CPU Time.
* clog10: (libc)Exponents and Logarithms.
* clog10f: (libc)Exponents and Logarithms.
* clog10l: (libc)Exponents and Logarithms.
* clog: (libc)Exponents and Logarithms.
* clogf: (libc)Exponents and Logarithms.
* clogl: (libc)Exponents and Logarithms.
* close: (libc)Opening and Closing Files.
* closedir: (libc)Reading/Closing Directory.
* closelog: (libc)closelog.
* confstr: (libc)String Parameters.
* conj: (libc)Operations on Complex.
* conjf: (libc)Operations on Complex.
* conjl: (libc)Operations on Complex.
* connect: (libc)Connecting.
* copysign: (libc)FP Bit Twiddling.
* copysignf: (libc)FP Bit Twiddling.
* copysignl: (libc)FP Bit Twiddling.
* cos: (libc)Trig Functions.
* cosf: (libc)Trig Functions.
* cosh: (libc)Hyperbolic Functions.
* coshf: (libc)Hyperbolic Functions.
* coshl: (libc)Hyperbolic Functions.
* cosl: (libc)Trig Functions.
* cpow: (libc)Exponents and Logarithms.
* cpowf: (libc)Exponents and Logarithms.
* cpowl: (libc)Exponents and Logarithms.
* cproj: (libc)Operations on Complex.
* cprojf: (libc)Operations on Complex.
* cprojl: (libc)Operations on Complex.
* creal: (libc)Operations on Complex.
* crealf: (libc)Operations on Complex.
* creall: (libc)Operations on Complex.
* creat64: (libc)Opening and Closing Files.
* creat: (libc)Opening and Closing Files.
* crypt: (libc)crypt.
* crypt_r: (libc)crypt.
* csin: (libc)Trig Functions.
* csinf: (libc)Trig Functions.
* csinh: (libc)Hyperbolic Functions.
* csinhf: (libc)Hyperbolic Functions.
* csinhl: (libc)Hyperbolic Functions.
* csinl: (libc)Trig Functions.
* csqrt: (libc)Exponents and Logarithms.
* csqrtf: (libc)Exponents and Logarithms.
* csqrtl: (libc)Exponents and Logarithms.
* ctan: (libc)Trig Functions.
* ctanf: (libc)Trig Functions.
* ctanh: (libc)Hyperbolic Functions.
* ctanhf: (libc)Hyperbolic Functions.
* ctanhl: (libc)Hyperbolic Functions.
* ctanl: (libc)Trig Functions.
* ctermid: (libc)Identifying the Terminal.
* ctime: (libc)Formatting Calendar Time.
* ctime_r: (libc)Formatting Calendar Time.
* cuserid: (libc)Who Logged In.
* dcgettext: (libc)Translation with gettext.
* dcngettext: (libc)Advanced gettext functions.
* des_setparity: (libc)DES Encryption.
* dgettext: (libc)Translation with gettext.
* difftime: (libc)Elapsed Time.
* dirfd: (libc)Opening a Directory.
* dirname: (libc)Finding Tokens in a String.
* div: (libc)Integer Division.
* dngettext: (libc)Advanced gettext functions.
* drand48: (libc)SVID Random.
* drand48_r: (libc)SVID Random.
* drem: (libc)Remainder Functions.
* dremf: (libc)Remainder Functions.
* dreml: (libc)Remainder Functions.
* dup2: (libc)Duplicating Descriptors.
* dup: (libc)Duplicating Descriptors.
* ecb_crypt: (libc)DES Encryption.
* ecvt: (libc)System V Number Conversion.
* ecvt_r: (libc)System V Number Conversion.
* encrypt: (libc)DES Encryption.
* encrypt_r: (libc)DES Encryption.
* endfsent: (libc)fstab.
* endgrent: (libc)Scanning All Groups.
* endhostent: (libc)Host Names.
* endmntent: (libc)mtab.
* endnetent: (libc)Networks Database.
* endnetgrent: (libc)Lookup Netgroup.
* endprotoent: (libc)Protocols Database.
* endpwent: (libc)Scanning All Users.
* endservent: (libc)Services Database.
* endutent: (libc)Manipulating the Database.
* endutxent: (libc)XPG Functions.
* envz_add: (libc)Envz Functions.
* envz_entry: (libc)Envz Functions.
* envz_get: (libc)Envz Functions.
* envz_merge: (libc)Envz Functions.
* envz_strip: (libc)Envz Functions.
* erand48: (libc)SVID Random.
* erand48_r: (libc)SVID Random.
* erf: (libc)Special Functions.
* erfc: (libc)Special Functions.
* erfcf: (libc)Special Functions.
* erfcl: (libc)Special Functions.
* erff: (libc)Special Functions.
* erfl: (libc)Special Functions.
* err: (libc)Error Messages.
* errno: (libc)Checking for Errors.
* error: (libc)Error Messages.
* error_at_line: (libc)Error Messages.
* errx: (libc)Error Messages.
* execl: (libc)Executing a File.
* execle: (libc)Executing a File.
* execlp: (libc)Executing a File.
* execv: (libc)Executing a File.
* execve: (libc)Executing a File.
* execvp: (libc)Executing a File.
* exit: (libc)Normal Termination.
* exp10: (libc)Exponents and Logarithms.
* exp10f: (libc)Exponents and Logarithms.
* exp10l: (libc)Exponents and Logarithms.
* exp2: (libc)Exponents and Logarithms.
* exp2f: (libc)Exponents and Logarithms.
* exp2l: (libc)Exponents and Logarithms.
* exp: (libc)Exponents and Logarithms.
* expf: (libc)Exponents and Logarithms.
* expl: (libc)Exponents and Logarithms.
* expm1: (libc)Exponents and Logarithms.
* expm1f: (libc)Exponents and Logarithms.
* expm1l: (libc)Exponents and Logarithms.
* fabs: (libc)Absolute Value.
* fabsf: (libc)Absolute Value.
* fabsl: (libc)Absolute Value.
* fchdir: (libc)Working Directory.
* fchmod: (libc)Setting Permissions.
* fchown: (libc)File Owner.
* fclose: (libc)Closing Streams.
* fcloseall: (libc)Closing Streams.
* fcntl: (libc)Control Operations.
* fcvt: (libc)System V Number Conversion.
* fcvt_r: (libc)System V Number Conversion.
* fdatasync: (libc)Synchronizing I/O.
* fdim: (libc)Misc FP Arithmetic.
* fdimf: (libc)Misc FP Arithmetic.
* fdiml: (libc)Misc FP Arithmetic.
* fdopen: (libc)Descriptors and Streams.
* fdopendir: (libc)Opening a Directory.
* feclearexcept: (libc)Status bit operations.
* fedisableexcept: (libc)Control Functions.
* feenableexcept: (libc)Control Functions.
* fegetenv: (libc)Control Functions.
* fegetexcept: (libc)Control Functions.
* fegetexceptflag: (libc)Status bit operations.
* fegetround: (libc)Rounding.
* feholdexcept: (libc)Control Functions.
* feof: (libc)EOF and Errors.
* feof_unlocked: (libc)EOF and Errors.
* feraiseexcept: (libc)Status bit operations.
* ferror: (libc)EOF and Errors.
* ferror_unlocked: (libc)EOF and Errors.
* fesetenv: (libc)Control Functions.
* fesetexceptflag: (libc)Status bit operations.
* fesetround: (libc)Rounding.
* fetestexcept: (libc)Status bit operations.
* feupdateenv: (libc)Control Functions.
* fflush: (libc)Flushing Buffers.
* fflush_unlocked: (libc)Flushing Buffers.
* fgetc: (libc)Character Input.
* fgetc_unlocked: (libc)Character Input.
* fgetgrent: (libc)Scanning All Groups.
* fgetgrent_r: (libc)Scanning All Groups.
* fgetpos64: (libc)Portable Positioning.
* fgetpos: (libc)Portable Positioning.
* fgetpwent: (libc)Scanning All Users.
* fgetpwent_r: (libc)Scanning All Users.
* fgets: (libc)Line Input.
* fgets_unlocked: (libc)Line Input.
* fgetwc: (libc)Character Input.
* fgetwc_unlocked: (libc)Character Input.
* fgetws: (libc)Line Input.
* fgetws_unlocked: (libc)Line Input.
* fileno: (libc)Descriptors and Streams.
* fileno_unlocked: (libc)Descriptors and Streams.
* finite: (libc)Floating Point Classes.
* finitef: (libc)Floating Point Classes.
* finitel: (libc)Floating Point Classes.
* flockfile: (libc)Streams and Threads.
* floor: (libc)Rounding Functions.
* floorf: (libc)Rounding Functions.
* floorl: (libc)Rounding Functions.
* fma: (libc)Misc FP Arithmetic.
* fmaf: (libc)Misc FP Arithmetic.
* fmal: (libc)Misc FP Arithmetic.
* fmax: (libc)Misc FP Arithmetic.
* fmaxf: (libc)Misc FP Arithmetic.
* fmaxl: (libc)Misc FP Arithmetic.
* fmemopen: (libc)String Streams.
* fmin: (libc)Misc FP Arithmetic.
* fminf: (libc)Misc FP Arithmetic.
* fminl: (libc)Misc FP Arithmetic.
* fmod: (libc)Remainder Functions.
* fmodf: (libc)Remainder Functions.
* fmodl: (libc)Remainder Functions.
* fmtmsg: (libc)Printing Formatted Messages.
* fnmatch: (libc)Wildcard Matching.
* fopen64: (libc)Opening Streams.
* fopen: (libc)Opening Streams.
* fopencookie: (libc)Streams and Cookies.
* fork: (libc)Creating a Process.
* forkpty: (libc)Pseudo-Terminal Pairs.
* fpathconf: (libc)Pathconf.
* fpclassify: (libc)Floating Point Classes.
* fprintf: (libc)Formatted Output Functions.
* fputc: (libc)Simple Output.
* fputc_unlocked: (libc)Simple Output.
* fputs: (libc)Simple Output.
* fputs_unlocked: (libc)Simple Output.
* fputwc: (libc)Simple Output.
* fputwc_unlocked: (libc)Simple Output.
* fputws: (libc)Simple Output.
* fputws_unlocked: (libc)Simple Output.
* fread: (libc)Block Input/Output.
* fread_unlocked: (libc)Block Input/Output.
* free: (libc)Freeing after Malloc.
* freopen64: (libc)Opening Streams.
* freopen: (libc)Opening Streams.
* frexp: (libc)Normalization Functions.
* frexpf: (libc)Normalization Functions.
* frexpl: (libc)Normalization Functions.
* fscanf: (libc)Formatted Input Functions.
* fseek: (libc)File Positioning.
* fseeko64: (libc)File Positioning.
* fseeko: (libc)File Positioning.
* fsetpos64: (libc)Portable Positioning.
* fsetpos: (libc)Portable Positioning.
* fstat64: (libc)Reading Attributes.
* fstat: (libc)Reading Attributes.
* fsync: (libc)Synchronizing I/O.
* ftell: (libc)File Positioning.
* ftello64: (libc)File Positioning.
* ftello: (libc)File Positioning.
* ftruncate64: (libc)File Size.
* ftruncate: (libc)File Size.
* ftrylockfile: (libc)Streams and Threads.
* ftw64: (libc)Working with Directory Trees.
* ftw: (libc)Working with Directory Trees.
* funlockfile: (libc)Streams and Threads.
* futimes: (libc)File Times.
* fwide: (libc)Streams and I18N.
* fwprintf: (libc)Formatted Output Functions.
* fwrite: (libc)Block Input/Output.
* fwrite_unlocked: (libc)Block Input/Output.
* fwscanf: (libc)Formatted Input Functions.
* gamma: (libc)Special Functions.
* gammaf: (libc)Special Functions.
* gammal: (libc)Special Functions.
* gcvt: (libc)System V Number Conversion.
* get_avphys_pages: (libc)Query Memory Parameters.
* get_current_dir_name: (libc)Working Directory.
* get_nprocs: (libc)Processor Resources.
* get_nprocs_conf: (libc)Processor Resources.
* get_phys_pages: (libc)Query Memory Parameters.
* getauxval: (libc)Auxiliary Vector.
* getc: (libc)Character Input.
* getc_unlocked: (libc)Character Input.
* getchar: (libc)Character Input.
* getchar_unlocked: (libc)Character Input.
* getcontext: (libc)System V contexts.
* getcwd: (libc)Working Directory.
* getdate: (libc)General Time String Parsing.
* getdate_r: (libc)General Time String Parsing.
* getdelim: (libc)Line Input.
* getdomainnname: (libc)Host Identification.
* getegid: (libc)Reading Persona.
* getenv: (libc)Environment Access.
* geteuid: (libc)Reading Persona.
* getfsent: (libc)fstab.
* getfsfile: (libc)fstab.
* getfsspec: (libc)fstab.
* getgid: (libc)Reading Persona.
* getgrent: (libc)Scanning All Groups.
* getgrent_r: (libc)Scanning All Groups.
* getgrgid: (libc)Lookup Group.
* getgrgid_r: (libc)Lookup Group.
* getgrnam: (libc)Lookup Group.
* getgrnam_r: (libc)Lookup Group.
* getgrouplist: (libc)Setting Groups.
* getgroups: (libc)Reading Persona.
* gethostbyaddr: (libc)Host Names.
* gethostbyaddr_r: (libc)Host Names.
* gethostbyname2: (libc)Host Names.
* gethostbyname2_r: (libc)Host Names.
* gethostbyname: (libc)Host Names.
* gethostbyname_r: (libc)Host Names.
* gethostent: (libc)Host Names.
* gethostid: (libc)Host Identification.
* gethostname: (libc)Host Identification.
* getitimer: (libc)Setting an Alarm.
* getline: (libc)Line Input.
* getloadavg: (libc)Processor Resources.
* getlogin: (libc)Who Logged In.
* getmntent: (libc)mtab.
* getmntent_r: (libc)mtab.
* getnetbyaddr: (libc)Networks Database.
* getnetbyname: (libc)Networks Database.
* getnetent: (libc)Networks Database.
* getnetgrent: (libc)Lookup Netgroup.
* getnetgrent_r: (libc)Lookup Netgroup.
* getopt: (libc)Using Getopt.
* getopt_long: (libc)Getopt Long Options.
* getopt_long_only: (libc)Getopt Long Options.
* getpagesize: (libc)Query Memory Parameters.
* getpass: (libc)getpass.
* getpeername: (libc)Who is Connected.
* getpgid: (libc)Process Group Functions.
* getpgrp: (libc)Process Group Functions.
* getpgrp: (libc)Process Group Functions.
* getpid: (libc)Process Identification.
* getppid: (libc)Process Identification.
* getpriority: (libc)Traditional Scheduling Functions.
* getprotobyname: (libc)Protocols Database.
* getprotobynumber: (libc)Protocols Database.
* getprotoent: (libc)Protocols Database.
* getpt: (libc)Allocation.
* getpwent: (libc)Scanning All Users.
* getpwent_r: (libc)Scanning All Users.
* getpwnam: (libc)Lookup User.
* getpwnam_r: (libc)Lookup User.
* getpwuid: (libc)Lookup User.
* getpwuid_r: (libc)Lookup User.
* getrlimit64: (libc)Limits on Resources.
* getrlimit: (libc)Limits on Resources.
* getrusage: (libc)Resource Usage.
* gets: (libc)Line Input.
* getservbyname: (libc)Services Database.
* getservbyport: (libc)Services Database.
* getservent: (libc)Services Database.
* getsid: (libc)Process Group Functions.
* getsockname: (libc)Reading Address.
* getsockopt: (libc)Socket Option Functions.
* getsubopt: (libc)Suboptions.
* gettext: (libc)Translation with gettext.
* gettimeofday: (libc)High-Resolution Calendar.
* getuid: (libc)Reading Persona.
* getumask: (libc)Setting Permissions.
* getutent: (libc)Manipulating the Database.
* getutent_r: (libc)Manipulating the Database.
* getutid: (libc)Manipulating the Database.
* getutid_r: (libc)Manipulating the Database.
* getutline: (libc)Manipulating the Database.
* getutline_r: (libc)Manipulating the Database.
* getutmp: (libc)XPG Functions.
* getutmpx: (libc)XPG Functions.
* getutxent: (libc)XPG Functions.
* getutxid: (libc)XPG Functions.
* getutxline: (libc)XPG Functions.
* getw: (libc)Character Input.
* getwc: (libc)Character Input.
* getwc_unlocked: (libc)Character Input.
* getwchar: (libc)Character Input.
* getwchar_unlocked: (libc)Character Input.
* getwd: (libc)Working Directory.
* glob64: (libc)Calling Glob.
* glob: (libc)Calling Glob.
* globfree64: (libc)More Flags for Globbing.
* globfree: (libc)More Flags for Globbing.
* gmtime: (libc)Broken-down Time.
* gmtime_r: (libc)Broken-down Time.
* grantpt: (libc)Allocation.
* gsignal: (libc)Signaling Yourself.
* gtty: (libc)BSD Terminal Modes.
* hasmntopt: (libc)mtab.
* hcreate: (libc)Hash Search Function.
* hcreate_r: (libc)Hash Search Function.
* hdestroy: (libc)Hash Search Function.
* hdestroy_r: (libc)Hash Search Function.
* hsearch: (libc)Hash Search Function.
* hsearch_r: (libc)Hash Search Function.
* htonl: (libc)Byte Order.
* htons: (libc)Byte Order.
* hypot: (libc)Exponents and Logarithms.
* hypotf: (libc)Exponents and Logarithms.
* hypotl: (libc)Exponents and Logarithms.
* iconv: (libc)Generic Conversion Interface.
* iconv_close: (libc)Generic Conversion Interface.
* iconv_open: (libc)Generic Conversion Interface.
* if_freenameindex: (libc)Interface Naming.
* if_indextoname: (libc)Interface Naming.
* if_nameindex: (libc)Interface Naming.
* if_nametoindex: (libc)Interface Naming.
* ilogb: (libc)Exponents and Logarithms.
* ilogbf: (libc)Exponents and Logarithms.
* ilogbl: (libc)Exponents and Logarithms.
* imaxabs: (libc)Absolute Value.
* imaxdiv: (libc)Integer Division.
* in6addr_any: (libc)Host Address Data Type.
* in6addr_loopback: (libc)Host Address Data Type.
* index: (libc)Search Functions.
* inet_addr: (libc)Host Address Functions.
* inet_aton: (libc)Host Address Functions.
* inet_lnaof: (libc)Host Address Functions.
* inet_makeaddr: (libc)Host Address Functions.
* inet_netof: (libc)Host Address Functions.
* inet_network: (libc)Host Address Functions.
* inet_ntoa: (libc)Host Address Functions.
* inet_ntop: (libc)Host Address Functions.
* inet_pton: (libc)Host Address Functions.
* initgroups: (libc)Setting Groups.
* initstate: (libc)BSD Random.
* initstate_r: (libc)BSD Random.
* innetgr: (libc)Netgroup Membership.
* ioctl: (libc)IOCTLs.
* isalnum: (libc)Classification of Characters.
* isalpha: (libc)Classification of Characters.
* isascii: (libc)Classification of Characters.
* isatty: (libc)Is It a Terminal.
* isblank: (libc)Classification of Characters.
* iscntrl: (libc)Classification of Characters.
* isdigit: (libc)Classification of Characters.
* isfinite: (libc)Floating Point Classes.
* isgraph: (libc)Classification of Characters.
* isgreater: (libc)FP Comparison Functions.
* isgreaterequal: (libc)FP Comparison Functions.
* isinf: (libc)Floating Point Classes.
* isinff: (libc)Floating Point Classes.
* isinfl: (libc)Floating Point Classes.
* isless: (libc)FP Comparison Functions.
* islessequal: (libc)FP Comparison Functions.
* islessgreater: (libc)FP Comparison Functions.
* islower: (libc)Classification of Characters.
* isnan: (libc)Floating Point Classes.
* isnan: (libc)Floating Point Classes.
* isnanf: (libc)Floating Point Classes.
* isnanl: (libc)Floating Point Classes.
* isnormal: (libc)Floating Point Classes.
* isprint: (libc)Classification of Characters.
* ispunct: (libc)Classification of Characters.
* issignaling: (libc)Floating Point Classes.
* isspace: (libc)Classification of Characters.
* isunordered: (libc)FP Comparison Functions.
* isupper: (libc)Classification of Characters.
* iswalnum: (libc)Classification of Wide Characters.
* iswalpha: (libc)Classification of Wide Characters.
* iswblank: (libc)Classification of Wide Characters.
* iswcntrl: (libc)Classification of Wide Characters.
* iswctype: (libc)Classification of Wide Characters.
* iswdigit: (libc)Classification of Wide Characters.
* iswgraph: (libc)Classification of Wide Characters.
* iswlower: (libc)Classification of Wide Characters.
* iswprint: (libc)Classification of Wide Characters.
* iswpunct: (libc)Classification of Wide Characters.
* iswspace: (libc)Classification of Wide Characters.
* iswupper: (libc)Classification of Wide Characters.
* iswxdigit: (libc)Classification of Wide Characters.
* isxdigit: (libc)Classification of Characters.
* j0: (libc)Special Functions.
* j0f: (libc)Special Functions.
* j0l: (libc)Special Functions.
* j1: (libc)Special Functions.
* j1f: (libc)Special Functions.
* j1l: (libc)Special Functions.
* jn: (libc)Special Functions.
* jnf: (libc)Special Functions.
* jnl: (libc)Special Functions.
* jrand48: (libc)SVID Random.
* jrand48_r: (libc)SVID Random.
* kill: (libc)Signaling Another Process.
* killpg: (libc)Signaling Another Process.
* l64a: (libc)Encode Binary Data.
* labs: (libc)Absolute Value.
* lcong48: (libc)SVID Random.
* lcong48_r: (libc)SVID Random.
* ldexp: (libc)Normalization Functions.
* ldexpf: (libc)Normalization Functions.
* ldexpl: (libc)Normalization Functions.
* ldiv: (libc)Integer Division.
* lfind: (libc)Array Search Function.
* lgamma: (libc)Special Functions.
* lgamma_r: (libc)Special Functions.
* lgammaf: (libc)Special Functions.
* lgammaf_r: (libc)Special Functions.
* lgammal: (libc)Special Functions.
* lgammal_r: (libc)Special Functions.
* link: (libc)Hard Links.
* lio_listio64: (libc)Asynchronous Reads/Writes.
* lio_listio: (libc)Asynchronous Reads/Writes.
* listen: (libc)Listening.
* llabs: (libc)Absolute Value.
* lldiv: (libc)Integer Division.
* llrint: (libc)Rounding Functions.
* llrintf: (libc)Rounding Functions.
* llrintl: (libc)Rounding Functions.
* llround: (libc)Rounding Functions.
* llroundf: (libc)Rounding Functions.
* llroundl: (libc)Rounding Functions.
* localeconv: (libc)The Lame Way to Locale Data.
* localtime: (libc)Broken-down Time.
* localtime_r: (libc)Broken-down Time.
* log10: (libc)Exponents and Logarithms.
* log10f: (libc)Exponents and Logarithms.
* log10l: (libc)Exponents and Logarithms.
* log1p: (libc)Exponents and Logarithms.
* log1pf: (libc)Exponents and Logarithms.
* log1pl: (libc)Exponents and Logarithms.
* log2: (libc)Exponents and Logarithms.
* log2f: (libc)Exponents and Logarithms.
* log2l: (libc)Exponents and Logarithms.
* log: (libc)Exponents and Logarithms.
* logb: (libc)Exponents and Logarithms.
* logbf: (libc)Exponents and Logarithms.
* logbl: (libc)Exponents and Logarithms.
* logf: (libc)Exponents and Logarithms.
* login: (libc)Logging In and Out.
* login_tty: (libc)Logging In and Out.
* logl: (libc)Exponents and Logarithms.
* logout: (libc)Logging In and Out.
* logwtmp: (libc)Logging In and Out.
* longjmp: (libc)Non-Local Details.
* lrand48: (libc)SVID Random.
* lrand48_r: (libc)SVID Random.
* lrint: (libc)Rounding Functions.
* lrintf: (libc)Rounding Functions.
* lrintl: (libc)Rounding Functions.
* lround: (libc)Rounding Functions.
* lroundf: (libc)Rounding Functions.
* lroundl: (libc)Rounding Functions.
* lsearch: (libc)Array Search Function.
* lseek64: (libc)File Position Primitive.
* lseek: (libc)File Position Primitive.
* lstat64: (libc)Reading Attributes.
* lstat: (libc)Reading Attributes.
* lutimes: (libc)File Times.
* madvise: (libc)Memory-mapped I/O.
* makecontext: (libc)System V contexts.
* mallinfo: (libc)Statistics of Malloc.
* malloc: (libc)Basic Allocation.
* mallopt: (libc)Malloc Tunable Parameters.
* mblen: (libc)Non-reentrant Character Conversion.
* mbrlen: (libc)Converting a Character.
* mbrtowc: (libc)Converting a Character.
* mbsinit: (libc)Keeping the state.
* mbsnrtowcs: (libc)Converting Strings.
* mbsrtowcs: (libc)Converting Strings.
* mbstowcs: (libc)Non-reentrant String Conversion.
* mbtowc: (libc)Non-reentrant Character Conversion.
* mcheck: (libc)Heap Consistency Checking.
* memalign: (libc)Aligned Memory Blocks.
* memccpy: (libc)Copying and Concatenation.
* memchr: (libc)Search Functions.
* memcmp: (libc)String/Array Comparison.
* memcpy: (libc)Copying and Concatenation.
* memfrob: (libc)Trivial Encryption.
* memmem: (libc)Search Functions.
* memmove: (libc)Copying and Concatenation.
* mempcpy: (libc)Copying and Concatenation.
* memrchr: (libc)Search Functions.
* memset: (libc)Copying and Concatenation.
* mkdir: (libc)Creating Directories.
* mkdtemp: (libc)Temporary Files.
* mkfifo: (libc)FIFO Special Files.
* mknod: (libc)Making Special Files.
* mkstemp: (libc)Temporary Files.
* mktemp: (libc)Temporary Files.
* mktime: (libc)Broken-down Time.
* mlock: (libc)Page Lock Functions.
* mlockall: (libc)Page Lock Functions.
* mmap64: (libc)Memory-mapped I/O.
* mmap: (libc)Memory-mapped I/O.
* modf: (libc)Rounding Functions.
* modff: (libc)Rounding Functions.
* modfl: (libc)Rounding Functions.
* mount: (libc)Mount-Unmount-Remount.
* mprobe: (libc)Heap Consistency Checking.
* mrand48: (libc)SVID Random.
* mrand48_r: (libc)SVID Random.
* mremap: (libc)Memory-mapped I/O.
* msync: (libc)Memory-mapped I/O.
* mtrace: (libc)Tracing malloc.
* munlock: (libc)Page Lock Functions.
* munlockall: (libc)Page Lock Functions.
* munmap: (libc)Memory-mapped I/O.
* muntrace: (libc)Tracing malloc.
* nan: (libc)FP Bit Twiddling.
* nanf: (libc)FP Bit Twiddling.
* nanl: (libc)FP Bit Twiddling.
* nanosleep: (libc)Sleeping.
* nearbyint: (libc)Rounding Functions.
* nearbyintf: (libc)Rounding Functions.
* nearbyintl: (libc)Rounding Functions.
* nextafter: (libc)FP Bit Twiddling.
* nextafterf: (libc)FP Bit Twiddling.
* nextafterl: (libc)FP Bit Twiddling.
* nexttoward: (libc)FP Bit Twiddling.
* nexttowardf: (libc)FP Bit Twiddling.
* nexttowardl: (libc)FP Bit Twiddling.
* nftw64: (libc)Working with Directory Trees.
* nftw: (libc)Working with Directory Trees.
* ngettext: (libc)Advanced gettext functions.
* nice: (libc)Traditional Scheduling Functions.
* nl_langinfo: (libc)The Elegant and Fast Way.
* nrand48: (libc)SVID Random.
* nrand48_r: (libc)SVID Random.
* ntohl: (libc)Byte Order.
* ntohs: (libc)Byte Order.
* ntp_adjtime: (libc)High Accuracy Clock.
* ntp_gettime: (libc)High Accuracy Clock.
* obstack_1grow: (libc)Growing Objects.
* obstack_1grow_fast: (libc)Extra Fast Growing.
* obstack_alignment_mask: (libc)Obstacks Data Alignment.
* obstack_alloc: (libc)Allocation in an Obstack.
* obstack_base: (libc)Status of an Obstack.
* obstack_blank: (libc)Growing Objects.
* obstack_blank_fast: (libc)Extra Fast Growing.
* obstack_chunk_size: (libc)Obstack Chunks.
* obstack_copy0: (libc)Allocation in an Obstack.
* obstack_copy: (libc)Allocation in an Obstack.
* obstack_finish: (libc)Growing Objects.
* obstack_free: (libc)Freeing Obstack Objects.
* obstack_grow0: (libc)Growing Objects.
* obstack_grow: (libc)Growing Objects.
* obstack_init: (libc)Preparing for Obstacks.
* obstack_int_grow: (libc)Growing Objects.
* obstack_int_grow_fast: (libc)Extra Fast Growing.
* obstack_next_free: (libc)Status of an Obstack.
* obstack_object_size: (libc)Growing Objects.
* obstack_object_size: (libc)Status of an Obstack.
* obstack_printf: (libc)Dynamic Output.
* obstack_ptr_grow: (libc)Growing Objects.
* obstack_ptr_grow_fast: (libc)Extra Fast Growing.
* obstack_room: (libc)Extra Fast Growing.
* obstack_vprintf: (libc)Variable Arguments Output.
* offsetof: (libc)Structure Measurement.
* on_exit: (libc)Cleanups on Exit.
* open64: (libc)Opening and Closing Files.
* open: (libc)Opening and Closing Files.
* open_memstream: (libc)String Streams.
* opendir: (libc)Opening a Directory.
* openlog: (libc)openlog.
* openpty: (libc)Pseudo-Terminal Pairs.
* parse_printf_format: (libc)Parsing a Template String.
* pathconf: (libc)Pathconf.
* pause: (libc)Using Pause.
* pclose: (libc)Pipe to a Subprocess.
* perror: (libc)Error Messages.
* pipe: (libc)Creating a Pipe.
* popen: (libc)Pipe to a Subprocess.
* posix_memalign: (libc)Aligned Memory Blocks.
* pow10: (libc)Exponents and Logarithms.
* pow10f: (libc)Exponents and Logarithms.
* pow10l: (libc)Exponents and Logarithms.
* pow: (libc)Exponents and Logarithms.
* powf: (libc)Exponents and Logarithms.
* powl: (libc)Exponents and Logarithms.
* pread64: (libc)I/O Primitives.
* pread: (libc)I/O Primitives.
* printf: (libc)Formatted Output Functions.
* printf_size: (libc)Predefined Printf Handlers.
* printf_size_info: (libc)Predefined Printf Handlers.
* psignal: (libc)Signal Messages.
* pthread_getattr_default_np: (libc)Default Thread Attributes.
* pthread_getattr_default_np: (libc)Default Thread Attributes.
* ptsname: (libc)Allocation.
* ptsname_r: (libc)Allocation.
* putc: (libc)Simple Output.
* putc_unlocked: (libc)Simple Output.
* putchar: (libc)Simple Output.
* putchar_unlocked: (libc)Simple Output.
* putenv: (libc)Environment Access.
* putpwent: (libc)Writing a User Entry.
* puts: (libc)Simple Output.
* pututline: (libc)Manipulating the Database.
* pututxline: (libc)XPG Functions.
* putw: (libc)Simple Output.
* putwc: (libc)Simple Output.
* putwc_unlocked: (libc)Simple Output.
* putwchar: (libc)Simple Output.
* putwchar_unlocked: (libc)Simple Output.
* pwrite64: (libc)I/O Primitives.
* pwrite: (libc)I/O Primitives.
* qecvt: (libc)System V Number Conversion.
* qecvt_r: (libc)System V Number Conversion.
* qfcvt: (libc)System V Number Conversion.
* qfcvt_r: (libc)System V Number Conversion.
* qgcvt: (libc)System V Number Conversion.
* qsort: (libc)Array Sort Function.
* raise: (libc)Signaling Yourself.
* rand: (libc)ISO Random.
* rand_r: (libc)ISO Random.
* random: (libc)BSD Random.
* random_r: (libc)BSD Random.
* rawmemchr: (libc)Search Functions.
* read: (libc)I/O Primitives.
* readdir64: (libc)Reading/Closing Directory.
* readdir64_r: (libc)Reading/Closing Directory.
* readdir: (libc)Reading/Closing Directory.
* readdir_r: (libc)Reading/Closing Directory.
* readlink: (libc)Symbolic Links.
* readv: (libc)Scatter-Gather.
* realloc: (libc)Changing Block Size.
* realpath: (libc)Symbolic Links.
* recv: (libc)Receiving Data.
* recvfrom: (libc)Receiving Datagrams.
* recvmsg: (libc)Receiving Datagrams.
* regcomp: (libc)POSIX Regexp Compilation.
* regerror: (libc)Regexp Cleanup.
* regexec: (libc)Matching POSIX Regexps.
* regfree: (libc)Regexp Cleanup.
* register_printf_function: (libc)Registering New Conversions.
* remainder: (libc)Remainder Functions.
* remainderf: (libc)Remainder Functions.
* remainderl: (libc)Remainder Functions.
* remove: (libc)Deleting Files.
* rename: (libc)Renaming Files.
* rewind: (libc)File Positioning.
* rewinddir: (libc)Random Access Directory.
* rindex: (libc)Search Functions.
* rint: (libc)Rounding Functions.
* rintf: (libc)Rounding Functions.
* rintl: (libc)Rounding Functions.
* rmdir: (libc)Deleting Files.
* round: (libc)Rounding Functions.
* roundf: (libc)Rounding Functions.
* roundl: (libc)Rounding Functions.
* rpmatch: (libc)Yes-or-No Questions.
* sbrk: (libc)Resizing the Data Segment.
* scalb: (libc)Normalization Functions.
* scalbf: (libc)Normalization Functions.
* scalbl: (libc)Normalization Functions.
* scalbln: (libc)Normalization Functions.
* scalblnf: (libc)Normalization Functions.
* scalblnl: (libc)Normalization Functions.
* scalbn: (libc)Normalization Functions.
* scalbnf: (libc)Normalization Functions.
* scalbnl: (libc)Normalization Functions.
* scandir64: (libc)Scanning Directory Content.
* scandir: (libc)Scanning Directory Content.
* scanf: (libc)Formatted Input Functions.
* sched_get_priority_max: (libc)Basic Scheduling Functions.
* sched_get_priority_min: (libc)Basic Scheduling Functions.
* sched_getaffinity: (libc)CPU Affinity.
* sched_getparam: (libc)Basic Scheduling Functions.
* sched_getscheduler: (libc)Basic Scheduling Functions.
* sched_rr_get_interval: (libc)Basic Scheduling Functions.
* sched_setaffinity: (libc)CPU Affinity.
* sched_setparam: (libc)Basic Scheduling Functions.
* sched_setscheduler: (libc)Basic Scheduling Functions.
* sched_yield: (libc)Basic Scheduling Functions.
* secure_getenv: (libc)Environment Access.
* seed48: (libc)SVID Random.
* seed48_r: (libc)SVID Random.
* seekdir: (libc)Random Access Directory.
* select: (libc)Waiting for I/O.
* send: (libc)Sending Data.
* sendmsg: (libc)Receiving Datagrams.
* sendto: (libc)Sending Datagrams.
* setbuf: (libc)Controlling Buffering.
* setbuffer: (libc)Controlling Buffering.
* setcontext: (libc)System V contexts.
* setdomainname: (libc)Host Identification.
* setegid: (libc)Setting Groups.
* setenv: (libc)Environment Access.
* seteuid: (libc)Setting User ID.
* setfsent: (libc)fstab.
* setgid: (libc)Setting Groups.
* setgrent: (libc)Scanning All Groups.
* setgroups: (libc)Setting Groups.
* sethostent: (libc)Host Names.
* sethostid: (libc)Host Identification.
* sethostname: (libc)Host Identification.
* setitimer: (libc)Setting an Alarm.
* setjmp: (libc)Non-Local Details.
* setkey: (libc)DES Encryption.
* setkey_r: (libc)DES Encryption.
* setlinebuf: (libc)Controlling Buffering.
* setlocale: (libc)Setting the Locale.
* setlogmask: (libc)setlogmask.
* setmntent: (libc)mtab.
* setnetent: (libc)Networks Database.
* setnetgrent: (libc)Lookup Netgroup.
* setpgid: (libc)Process Group Functions.
* setpgrp: (libc)Process Group Functions.
* setpriority: (libc)Traditional Scheduling Functions.
* setprotoent: (libc)Protocols Database.
* setpwent: (libc)Scanning All Users.
* setregid: (libc)Setting Groups.
* setreuid: (libc)Setting User ID.
* setrlimit64: (libc)Limits on Resources.
* setrlimit: (libc)Limits on Resources.
* setservent: (libc)Services Database.
* setsid: (libc)Process Group Functions.
* setsockopt: (libc)Socket Option Functions.
* setstate: (libc)BSD Random.
* setstate_r: (libc)BSD Random.
* settimeofday: (libc)High-Resolution Calendar.
* setuid: (libc)Setting User ID.
* setutent: (libc)Manipulating the Database.
* setutxent: (libc)XPG Functions.
* setvbuf: (libc)Controlling Buffering.
* shutdown: (libc)Closing a Socket.
* sigaction: (libc)Advanced Signal Handling.
* sigaddset: (libc)Signal Sets.
* sigaltstack: (libc)Signal Stack.
* sigblock: (libc)Blocking in BSD.
* sigdelset: (libc)Signal Sets.
* sigemptyset: (libc)Signal Sets.
* sigfillset: (libc)Signal Sets.
* siginterrupt: (libc)BSD Handler.
* sigismember: (libc)Signal Sets.
* siglongjmp: (libc)Non-Local Exits and Signals.
* sigmask: (libc)Blocking in BSD.
* signal: (libc)Basic Signal Handling.
* signbit: (libc)FP Bit Twiddling.
* significand: (libc)Normalization Functions.
* significandf: (libc)Normalization Functions.
* significandl: (libc)Normalization Functions.
* sigpause: (libc)Blocking in BSD.
* sigpending: (libc)Checking for Pending Signals.
* sigprocmask: (libc)Process Signal Mask.
* sigsetjmp: (libc)Non-Local Exits and Signals.
* sigsetmask: (libc)Blocking in BSD.
* sigstack: (libc)Signal Stack.
* sigsuspend: (libc)Sigsuspend.
* sigvec: (libc)BSD Handler.
* sin: (libc)Trig Functions.
* sincos: (libc)Trig Functions.
* sincosf: (libc)Trig Functions.
* sincosl: (libc)Trig Functions.
* sinf: (libc)Trig Functions.
* sinh: (libc)Hyperbolic Functions.
* sinhf: (libc)Hyperbolic Functions.
* sinhl: (libc)Hyperbolic Functions.
* sinl: (libc)Trig Functions.
* sleep: (libc)Sleeping.
* snprintf: (libc)Formatted Output Functions.
* socket: (libc)Creating a Socket.
* socketpair: (libc)Socket Pairs.
* sprintf: (libc)Formatted Output Functions.
* sqrt: (libc)Exponents and Logarithms.
* sqrtf: (libc)Exponents and Logarithms.
* sqrtl: (libc)Exponents and Logarithms.
* srand48: (libc)SVID Random.
* srand48_r: (libc)SVID Random.
* srand: (libc)ISO Random.
* srandom: (libc)BSD Random.
* srandom_r: (libc)BSD Random.
* sscanf: (libc)Formatted Input Functions.
* ssignal: (libc)Basic Signal Handling.
* stat64: (libc)Reading Attributes.
* stat: (libc)Reading Attributes.
* stime: (libc)Simple Calendar Time.
* stpcpy: (libc)Copying and Concatenation.
* stpncpy: (libc)Copying and Concatenation.
* strcasecmp: (libc)String/Array Comparison.
* strcasestr: (libc)Search Functions.
* strcat: (libc)Copying and Concatenation.
* strchr: (libc)Search Functions.
* strchrnul: (libc)Search Functions.
* strcmp: (libc)String/Array Comparison.
* strcoll: (libc)Collation Functions.
* strcpy: (libc)Copying and Concatenation.
* strcspn: (libc)Search Functions.
* strdup: (libc)Copying and Concatenation.
* strdupa: (libc)Copying and Concatenation.
* strerror: (libc)Error Messages.
* strerror_r: (libc)Error Messages.
* strfmon: (libc)Formatting Numbers.
* strfry: (libc)strfry.
* strftime: (libc)Formatting Calendar Time.
* strlen: (libc)String Length.
* strncasecmp: (libc)String/Array Comparison.
* strncat: (libc)Copying and Concatenation.
* strncmp: (libc)String/Array Comparison.
* strncpy: (libc)Copying and Concatenation.
* strndup: (libc)Copying and Concatenation.
* strndupa: (libc)Copying and Concatenation.
* strnlen: (libc)String Length.
* strpbrk: (libc)Search Functions.
* strptime: (libc)Low-Level Time String Parsing.
* strrchr: (libc)Search Functions.
* strsep: (libc)Finding Tokens in a String.
* strsignal: (libc)Signal Messages.
* strspn: (libc)Search Functions.
* strstr: (libc)Search Functions.
* strtod: (libc)Parsing of Floats.
* strtof: (libc)Parsing of Floats.
* strtoimax: (libc)Parsing of Integers.
* strtok: (libc)Finding Tokens in a String.
* strtok_r: (libc)Finding Tokens in a String.
* strtol: (libc)Parsing of Integers.
* strtold: (libc)Parsing of Floats.
* strtoll: (libc)Parsing of Integers.
* strtoq: (libc)Parsing of Integers.
* strtoul: (libc)Parsing of Integers.
* strtoull: (libc)Parsing of Integers.
* strtoumax: (libc)Parsing of Integers.
* strtouq: (libc)Parsing of Integers.
* strverscmp: (libc)String/Array Comparison.
* strxfrm: (libc)Collation Functions.
* stty: (libc)BSD Terminal Modes.
* swapcontext: (libc)System V contexts.
* swprintf: (libc)Formatted Output Functions.
* swscanf: (libc)Formatted Input Functions.
* symlink: (libc)Symbolic Links.
* sync: (libc)Synchronizing I/O.
* syscall: (libc)System Calls.
* sysconf: (libc)Sysconf Definition.
* sysctl: (libc)System Parameters.
* syslog: (libc)syslog; vsyslog.
* system: (libc)Running a Command.
* sysv_signal: (libc)Basic Signal Handling.
* tan: (libc)Trig Functions.
* tanf: (libc)Trig Functions.
* tanh: (libc)Hyperbolic Functions.
* tanhf: (libc)Hyperbolic Functions.
* tanhl: (libc)Hyperbolic Functions.
* tanl: (libc)Trig Functions.
* tcdrain: (libc)Line Control.
* tcflow: (libc)Line Control.
* tcflush: (libc)Line Control.
* tcgetattr: (libc)Mode Functions.
* tcgetpgrp: (libc)Terminal Access Functions.
* tcgetsid: (libc)Terminal Access Functions.
* tcsendbreak: (libc)Line Control.
* tcsetattr: (libc)Mode Functions.
* tcsetpgrp: (libc)Terminal Access Functions.
* tdelete: (libc)Tree Search Function.
* tdestroy: (libc)Tree Search Function.
* telldir: (libc)Random Access Directory.
* tempnam: (libc)Temporary Files.
* textdomain: (libc)Locating gettext catalog.
* tfind: (libc)Tree Search Function.
* tgamma: (libc)Special Functions.
* tgammaf: (libc)Special Functions.
* tgammal: (libc)Special Functions.
* time: (libc)Simple Calendar Time.
* timegm: (libc)Broken-down Time.
* timelocal: (libc)Broken-down Time.
* times: (libc)Processor Time.
* tmpfile64: (libc)Temporary Files.
* tmpfile: (libc)Temporary Files.
* tmpnam: (libc)Temporary Files.
* tmpnam_r: (libc)Temporary Files.
* toascii: (libc)Case Conversion.
* tolower: (libc)Case Conversion.
* toupper: (libc)Case Conversion.
* towctrans: (libc)Wide Character Case Conversion.
* towlower: (libc)Wide Character Case Conversion.
* towupper: (libc)Wide Character Case Conversion.
* trunc: (libc)Rounding Functions.
* truncate64: (libc)File Size.
* truncate: (libc)File Size.
* truncf: (libc)Rounding Functions.
* truncl: (libc)Rounding Functions.
* tsearch: (libc)Tree Search Function.
* ttyname: (libc)Is It a Terminal.
* ttyname_r: (libc)Is It a Terminal.
* twalk: (libc)Tree Search Function.
* tzset: (libc)Time Zone Functions.
* ulimit: (libc)Limits on Resources.
* umask: (libc)Setting Permissions.
* umount2: (libc)Mount-Unmount-Remount.
* umount: (libc)Mount-Unmount-Remount.
* uname: (libc)Platform Type.
* ungetc: (libc)How Unread.
* ungetwc: (libc)How Unread.
* unlink: (libc)Deleting Files.
* unlockpt: (libc)Allocation.
* unsetenv: (libc)Environment Access.
* updwtmp: (libc)Manipulating the Database.
* utime: (libc)File Times.
* utimes: (libc)File Times.
* utmpname: (libc)Manipulating the Database.
* utmpxname: (libc)XPG Functions.
* va_arg: (libc)Argument Macros.
* va_copy: (libc)Argument Macros.
* va_end: (libc)Argument Macros.
* va_start: (libc)Argument Macros.
* valloc: (libc)Aligned Memory Blocks.
* vasprintf: (libc)Variable Arguments Output.
* verr: (libc)Error Messages.
* verrx: (libc)Error Messages.
* versionsort64: (libc)Scanning Directory Content.
* versionsort: (libc)Scanning Directory Content.
* vfork: (libc)Creating a Process.
* vfprintf: (libc)Variable Arguments Output.
* vfscanf: (libc)Variable Arguments Input.
* vfwprintf: (libc)Variable Arguments Output.
* vfwscanf: (libc)Variable Arguments Input.
* vlimit: (libc)Limits on Resources.
* vprintf: (libc)Variable Arguments Output.
* vscanf: (libc)Variable Arguments Input.
* vsnprintf: (libc)Variable Arguments Output.
* vsprintf: (libc)Variable Arguments Output.
* vsscanf: (libc)Variable Arguments Input.
* vswprintf: (libc)Variable Arguments Output.
* vswscanf: (libc)Variable Arguments Input.
* vsyslog: (libc)syslog; vsyslog.
* vtimes: (libc)Resource Usage.
* vwarn: (libc)Error Messages.
* vwarnx: (libc)Error Messages.
* vwprintf: (libc)Variable Arguments Output.
* vwscanf: (libc)Variable Arguments Input.
* wait3: (libc)BSD Wait Functions.
* wait4: (libc)Process Completion.
* wait: (libc)Process Completion.
* waitpid: (libc)Process Completion.
* warn: (libc)Error Messages.
* warnx: (libc)Error Messages.
* wcpcpy: (libc)Copying and Concatenation.
* wcpncpy: (libc)Copying and Concatenation.
* wcrtomb: (libc)Converting a Character.
* wcscasecmp: (libc)String/Array Comparison.
* wcscat: (libc)Copying and Concatenation.
* wcschr: (libc)Search Functions.
* wcschrnul: (libc)Search Functions.
* wcscmp: (libc)String/Array Comparison.
* wcscoll: (libc)Collation Functions.
* wcscpy: (libc)Copying and Concatenation.
* wcscspn: (libc)Search Functions.
* wcsdup: (libc)Copying and Concatenation.
* wcsftime: (libc)Formatting Calendar Time.
* wcslen: (libc)String Length.
* wcsncasecmp: (libc)String/Array Comparison.
* wcsncat: (libc)Copying and Concatenation.
* wcsncmp: (libc)String/Array Comparison.
* wcsncpy: (libc)Copying and Concatenation.
* wcsnlen: (libc)String Length.
* wcsnrtombs: (libc)Converting Strings.
* wcspbrk: (libc)Search Functions.
* wcsrchr: (libc)Search Functions.
* wcsrtombs: (libc)Converting Strings.
* wcsspn: (libc)Search Functions.
* wcsstr: (libc)Search Functions.
* wcstod: (libc)Parsing of Floats.
* wcstof: (libc)Parsing of Floats.
* wcstoimax: (libc)Parsing of Integers.
* wcstok: (libc)Finding Tokens in a String.
* wcstol: (libc)Parsing of Integers.
* wcstold: (libc)Parsing of Floats.
* wcstoll: (libc)Parsing of Integers.
* wcstombs: (libc)Non-reentrant String Conversion.
* wcstoq: (libc)Parsing of Integers.
* wcstoul: (libc)Parsing of Integers.
* wcstoull: (libc)Parsing of Integers.
* wcstoumax: (libc)Parsing of Integers.
* wcstouq: (libc)Parsing of Integers.
* wcswcs: (libc)Search Functions.
* wcsxfrm: (libc)Collation Functions.
* wctob: (libc)Converting a Character.
* wctomb: (libc)Non-reentrant Character Conversion.
* wctrans: (libc)Wide Character Case Conversion.
* wctype: (libc)Classification of Wide Characters.
* wmemchr: (libc)Search Functions.
* wmemcmp: (libc)String/Array Comparison.
* wmemcpy: (libc)Copying and Concatenation.
* wmemmove: (libc)Copying and Concatenation.
* wmempcpy: (libc)Copying and Concatenation.
* wmemset: (libc)Copying and Concatenation.
* wordexp: (libc)Calling Wordexp.
* wordfree: (libc)Calling Wordexp.
* wprintf: (libc)Formatted Output Functions.
* write: (libc)I/O Primitives.
* writev: (libc)Scatter-Gather.
* wscanf: (libc)Formatted Input Functions.
* y0: (libc)Special Functions.
* y0f: (libc)Special Functions.
* y0l: (libc)Special Functions.
* y1: (libc)Special Functions.
* y1f: (libc)Special Functions.
* y1l: (libc)Special Functions.
* yn: (libc)Special Functions.
* ynf: (libc)Special Functions.
* ynl: (libc)Special Functions.
END-INFO-DIR-ENTRY

   This file documents the GNU C Library.

   This is `The GNU C Library Reference Manual', for version
2.18-2013.10 (EGLIBC).

   Copyright (C) 1993-2013 Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version
1.3 or any later version published by the Free Software Foundation;
with the Invariant Sections being "Free Software Needs Free
Documentation" and "GNU Lesser General Public License", the Front-Cover
texts being "A GNU Manual", and with the Back-Cover Texts as in (a)
below.  A copy of the license is included in the section entitled "GNU
Free Documentation License".

   (a) The FSF's Back-Cover Text is: "You have the freedom to copy and
modify this GNU manual.  Buying copies from the FSF supports it in
developing GNU and promoting software freedom."


File: libc.info,  Node: Other Kinds of Streams,  Next: Formatted Messages,  Prev: Stream Buffering,  Up: I/O on Streams

12.21 Other Kinds of Streams
============================

The GNU C Library provides ways for you to define additional kinds of
streams that do not necessarily correspond to an open file.

   One such type of stream takes input from or writes output to a
string.  These kinds of streams are used internally to implement the
`sprintf' and `sscanf' functions.  You can also create such a stream
explicitly, using the functions described in *note String Streams::.

   More generally, you can define streams that do input/output to
arbitrary objects using functions supplied by your program.  This
protocol is discussed in *note Custom Streams::.

   *Portability Note:* The facilities described in this section are
specific to GNU.  Other systems or C implementations might or might not
provide equivalent functionality.

* Menu:

* String Streams::              Streams that get data from or put data in
				 a string or memory buffer.
* Custom Streams::              Defining your own streams with an arbitrary
				 input data source and/or output data sink.


File: libc.info,  Node: String Streams,  Next: Custom Streams,  Up: Other Kinds of Streams

12.21.1 String Streams
----------------------

The `fmemopen' and `open_memstream' functions allow you to do I/O to a
string or memory buffer.  These facilities are declared in `stdio.h'.  

 -- Function: FILE * fmemopen (void *BUF, size_t SIZE, const char
          *OPENTYPE)
     This function opens a stream that allows the access specified by
     the OPENTYPE argument, that reads from or writes to the buffer
     specified by the argument BUF.  This array must be at least SIZE
     bytes long.

     If you specify a null pointer as the BUF argument, `fmemopen'
     dynamically allocates an array SIZE bytes long (as with `malloc';
     *note Unconstrained Allocation::).  This is really only useful if
     you are going to write things to the buffer and then read them back
     in again, because you have no way of actually getting a pointer to
     the buffer (for this, try `open_memstream', below).  The buffer is
     freed when the stream is closed.

     The argument OPENTYPE is the same as in `fopen' (*note Opening
     Streams::).  If the OPENTYPE specifies append mode, then the
     initial file position is set to the first null character in the
     buffer.  Otherwise the initial file position is at the beginning
     of the buffer.

     When a stream open for writing is flushed or closed, a null
     character (zero byte) is written at the end of the buffer if it
     fits.  You should add an extra byte to the SIZE argument to
     account for this.  Attempts to write more than SIZE bytes to the
     buffer result in an error.

     For a stream open for reading, null characters (zero bytes) in the
     buffer do not count as "end of file".  Read operations indicate
     end of file only when the file position advances past SIZE bytes.
     So, if you want to read characters from a null-terminated string,
     you should supply the length of the string as the SIZE argument.

   Here is an example of using `fmemopen' to create a stream for
reading from a string:


     #include <stdio.h>

     static char buffer[] = "foobar";

     int
     main (void)
     {
       int ch;
       FILE *stream;

       stream = fmemopen (buffer, strlen (buffer), "r");
       while ((ch = fgetc (stream)) != EOF)
         printf ("Got %c\n", ch);
       fclose (stream);

       return 0;
     }

   This program produces the following output:

     Got f
     Got o
     Got o
     Got b
     Got a
     Got r

 -- Function: FILE * open_memstream (char **PTR, size_t *SIZELOC)
     This function opens a stream for writing to a buffer.  The buffer
     is allocated dynamically and grown as necessary, using `malloc'.
     After you've closed the stream, this buffer is your responsibility
     to clean up using `free' or `realloc'.  *Note Unconstrained
     Allocation::.

     When the stream is closed with `fclose' or flushed with `fflush',
     the locations PTR and SIZELOC are updated to contain the pointer
     to the buffer and its size.  The values thus stored remain valid
     only as long as no further output on the stream takes place.  If
     you do more output, you must flush the stream again to store new
     values before you use them again.

     A null character is written at the end of the buffer.  This null
     character is _not_ included in the size value stored at SIZELOC.

     You can move the stream's file position with `fseek' or `fseeko'
     (*note File Positioning::).  Moving the file position past the end
     of the data already written fills the intervening space with
     zeroes.

   Here is an example of using `open_memstream':


     #include <stdio.h>

     int
     main (void)
     {
       char *bp;
       size_t size;
       FILE *stream;

       stream = open_memstream (&bp, &size);
       fprintf (stream, "hello");
       fflush (stream);
       printf ("buf = `%s', size = %d\n", bp, size);
       fprintf (stream, ", world");
       fclose (stream);
       printf ("buf = `%s', size = %d\n", bp, size);

       return 0;
     }

   This program produces the following output:

     buf = `hello', size = 5
     buf = `hello, world', size = 12


File: libc.info,  Node: Custom Streams,  Prev: String Streams,  Up: Other Kinds of Streams

12.21.2 Programming Your Own Custom Streams
-------------------------------------------

This section describes how you can make a stream that gets input from an
arbitrary data source or writes output to an arbitrary data sink
programmed by you.  We call these "custom streams".  The functions and
types described here are all GNU extensions.

* Menu:

* Streams and Cookies::         The "cookie" records where to fetch or
				 store data that is read or written.
* Hook Functions::              How you should define the four "hook
				 functions" that a custom stream needs.


File: libc.info,  Node: Streams and Cookies,  Next: Hook Functions,  Up: Custom Streams

12.21.2.1 Custom Streams and Cookies
....................................

Inside every custom stream is a special object called the "cookie".
This is an object supplied by you which records where to fetch or store
the data read or written.  It is up to you to define a data type to use
for the cookie.  The stream functions in the library never refer
directly to its contents, and they don't even know what the type is;
they record its address with type `void *'.

   To implement a custom stream, you must specify _how_ to fetch or
store the data in the specified place.  You do this by defining "hook
functions" to read, write, change "file position", and close the
stream.  All four of these functions will be passed the stream's cookie
so they can tell where to fetch or store the data.  The library
functions don't know what's inside the cookie, but your functions will
know.

   When you create a custom stream, you must specify the cookie pointer,
and also the four hook functions stored in a structure of type
`cookie_io_functions_t'.

   These facilities are declared in `stdio.h'.  

 -- Data Type: cookie_io_functions_t
     This is a structure type that holds the functions that define the
     communications protocol between the stream and its cookie.  It has
     the following members:

    `cookie_read_function_t *read'
          This is the function that reads data from the cookie.  If the
          value is a null pointer instead of a function, then read
          operations on this stream always return `EOF'.

    `cookie_write_function_t *write'
          This is the function that writes data to the cookie.  If the
          value is a null pointer instead of a function, then data
          written to the stream is discarded.

    `cookie_seek_function_t *seek'
          This is the function that performs the equivalent of file
          positioning on the cookie.  If the value is a null pointer
          instead of a function, calls to `fseek' or `fseeko' on this
          stream can only seek to locations within the buffer; any
          attempt to seek outside the buffer will return an `ESPIPE'
          error.

    `cookie_close_function_t *close'
          This function performs any appropriate cleanup on the cookie
          when closing the stream.  If the value is a null pointer
          instead of a function, nothing special is done to close the
          cookie when the stream is closed.

 -- Function: FILE * fopencookie (void *COOKIE, const char *OPENTYPE,
          cookie_io_functions_t IO-FUNCTIONS)
     This function actually creates the stream for communicating with
     the COOKIE using the functions in the IO-FUNCTIONS argument.  The
     OPENTYPE argument is interpreted as for `fopen'; see *note Opening
     Streams::.  (But note that the "truncate on open" option is
     ignored.)  The new stream is fully buffered.

     The `fopencookie' function returns the newly created stream, or a
     null pointer in case of an error.


File: libc.info,  Node: Hook Functions,  Prev: Streams and Cookies,  Up: Custom Streams

12.21.2.2 Custom Stream Hook Functions
......................................

Here are more details on how you should define the four hook functions
that a custom stream needs.

   You should define the function to read data from the cookie as:

     ssize_t READER (void *COOKIE, char *BUFFER, size_t SIZE)

   This is very similar to the `read' function; see *note I/O
Primitives::.  Your function should transfer up to SIZE bytes into the
BUFFER, and return the number of bytes read, or zero to indicate
end-of-file.  You can return a value of `-1' to indicate an error.

   You should define the function to write data to the cookie as:

     ssize_t WRITER (void *COOKIE, const char *BUFFER, size_t SIZE)

   This is very similar to the `write' function; see *note I/O
Primitives::.  Your function should transfer up to SIZE bytes from the
buffer, and return the number of bytes written.  You can return a value
of `0' to indicate an error.  You must not return any negative value.

   You should define the function to perform seek operations on the
cookie as:

     int SEEKER (void *COOKIE, off64_t *POSITION, int WHENCE)

   For this function, the POSITION and WHENCE arguments are interpreted
as for `fgetpos'; see *note Portable Positioning::.

   After doing the seek operation, your function should store the
resulting file position relative to the beginning of the file in
POSITION.  Your function should return a value of `0' on success and
`-1' to indicate an error.

   You should define the function to do cleanup operations on the cookie
appropriate for closing the stream as:

     int CLEANER (void *COOKIE)

   Your function should return `-1' to indicate an error, and `0'
otherwise.

 -- Data Type: cookie_read_function
     This is the data type that the read function for a custom stream
     should have.  If you declare the function as shown above, this is
     the type it will have.

 -- Data Type: cookie_write_function
     The data type of the write function for a custom stream.

 -- Data Type: cookie_seek_function
     The data type of the seek function for a custom stream.

 -- Data Type: cookie_close_function
     The data type of the close function for a custom stream.


File: libc.info,  Node: Formatted Messages,  Prev: Other Kinds of Streams,  Up: I/O on Streams

12.22 Formatted Messages
========================

On systems which are based on System V messages of programs (especially
the system tools) are printed in a strict form using the `fmtmsg'
function.  The uniformity sometimes helps the user to interpret messages
and the strictness tests of the `fmtmsg' function ensure that the
programmer follows some minimal requirements.

* Menu:

* Printing Formatted Messages::   The `fmtmsg' function.
* Adding Severity Classes::       Add more severity classes.
* Example::                       How to use `fmtmsg' and `addseverity'.


File: libc.info,  Node: Printing Formatted Messages,  Next: Adding Severity Classes,  Up: Formatted Messages

12.22.1 Printing Formatted Messages
-----------------------------------

Messages can be printed to standard error and/or to the console.  To
select the destination the programmer can use the following two values,
bitwise OR combined if wanted, for the CLASSIFICATION parameter of
`fmtmsg':

`MM_PRINT'
     Display the message in standard error.

`MM_CONSOLE'
     Display the message on the system console.

   The erroneous piece of the system can be signalled by exactly one of
the following values which also is bitwise ORed with the CLASSIFICATION
parameter to `fmtmsg':

`MM_HARD'
     The source of the condition is some hardware.

`MM_SOFT'
     The source of the condition is some software.

`MM_FIRM'
     The source of the condition is some firmware.

   A third component of the CLASSIFICATION parameter to `fmtmsg' can
describe the part of the system which detects the problem.  This is
done by using exactly one of the following values:

`MM_APPL'
     The erroneous condition is detected by the application.

`MM_UTIL'
     The erroneous condition is detected by a utility.

`MM_OPSYS'
     The erroneous condition is detected by the operating system.

   A last component of CLASSIFICATION can signal the results of this
message.  Exactly one of the following values can be used:

`MM_RECOVER'
     It is a recoverable error.

`MM_NRECOV'
     It is a non-recoverable error.

 -- Function: int fmtmsg (long int CLASSIFICATION, const char *LABEL,
          int SEVERITY, const char *TEXT, const char *ACTION, const
          char *TAG)
     Display a message described by its parameters on the device(s)
     specified in the CLASSIFICATION parameter.  The LABEL parameter
     identifies the source of the message.  The string should consist
     of two colon separated parts where the first part has not more
     than 10 and the second part not more than 14 characters.  The TEXT
     parameter describes the condition of the error, the ACTION
     parameter possible steps to recover from the error and the TAG
     parameter is a reference to the online documentation where more
     information can be found.  It should contain the LABEL value and a
     unique identification number.

     Each of the parameters can be a special value which means this
     value is to be omitted.  The symbolic names for these values are:

    `MM_NULLLBL'
          Ignore LABEL parameter.

    `MM_NULLSEV'
          Ignore SEVERITY parameter.

    `MM_NULLMC'
          Ignore CLASSIFICATION parameter.  This implies that nothing is
          actually printed.

    `MM_NULLTXT'
          Ignore TEXT parameter.

    `MM_NULLACT'
          Ignore ACTION parameter.

    `MM_NULLTAG'
          Ignore TAG parameter.

     There is another way certain fields can be omitted from the output
     to standard error.  This is described below in the description of
     environment variables influencing the behavior.

     The SEVERITY parameter can have one of the values in the following
     table: 

    `MM_NOSEV'
          Nothing is printed, this value is the same as `MM_NULLSEV'.

    `MM_HALT'
          This value is printed as `HALT'.

    `MM_ERROR'
          This value is printed as `ERROR'.

    `MM_WARNING'
          This value is printed as `WARNING'.

    `MM_INFO'
          This value is printed as `INFO'.

     The numeric value of these five macros are between `0' and `4'.
     Using the environment variable `SEV_LEVEL' or using the
     `addseverity' function one can add more severity levels with their
     corresponding string to print.  This is described below (*note
     Adding Severity Classes::).

     If no parameter is ignored the output looks like this:

          LABEL: SEVERITY-STRING: TEXT
          TO FIX: ACTION TAG

     The colons, new line characters and the `TO FIX' string are
     inserted if necessary, i.e., if the corresponding parameter is not
     ignored.

     This function is specified in the X/Open Portability Guide.  It is
     also available on all systems derived from System V.

     The function returns the value `MM_OK' if no error occurred.  If
     only the printing to standard error failed, it returns `MM_NOMSG'.
     If printing to the console fails, it returns `MM_NOCON'.  If
     nothing is printed `MM_NOTOK' is returned.  Among situations where
     all outputs fail this last value is also returned if a parameter
     value is incorrect.

   There are two environment variables which influence the behavior of
`fmtmsg'.  The first is `MSGVERB'.  It is used to control the output
actually happening on standard error (_not_ the console output).  Each
of the five fields can explicitly be enabled.  To do this the user has
to put the `MSGVERB' variable with a format like the following in the
environment before calling the `fmtmsg' function the first time:

     MSGVERB=KEYWORD[:KEYWORD[:...]]

   Valid KEYWORDs are `label', `severity', `text', `action', and `tag'.
If the environment variable is not given or is the empty string, a not
supported keyword is given or the value is somehow else invalid, no
part of the message is masked out.

   The second environment variable which influences the behavior of
`fmtmsg' is `SEV_LEVEL'.  This variable and the change in the behavior
of `fmtmsg' is not specified in the X/Open Portability Guide.  It is
available in System V systems, though.  It can be used to introduce new
severity levels.  By default, only the five severity levels described
above are available.  Any other numeric value would make `fmtmsg' print
nothing.

   If the user puts `SEV_LEVEL' with a format like

     SEV_LEVEL=[DESCRIPTION[:DESCRIPTION[:...]]]

in the environment of the process before the first call to `fmtmsg',
where DESCRIPTION has a value of the form

     SEVERITY-KEYWORD,LEVEL,PRINTSTRING

   The SEVERITY-KEYWORD part is not used by `fmtmsg' but it has to be
present.  The LEVEL part is a string representation of a number.  The
numeric value must be a number greater than 4.  This value must be used
in the SEVERITY parameter of `fmtmsg' to select this class.  It is not
possible to overwrite any of the predefined classes.  The PRINTSTRING
is the string printed when a message of this class is processed by
`fmtmsg' (see above, `fmtsmg' does not print the numeric value but
instead the string representation).


File: libc.info,  Node: Adding Severity Classes,  Next: Example,  Prev: Printing Formatted Messages,  Up: Formatted Messages

12.22.2 Adding Severity Classes
-------------------------------

There is another possibility to introduce severity classes besides using
the environment variable `SEV_LEVEL'.  This simplifies the task of
introducing new classes in a running program.  One could use the
`setenv' or `putenv' function to set the environment variable, but this
is toilsome.

 -- Function: int addseverity (int SEVERITY, const char *STRING)
     This function allows the introduction of new severity classes
     which can be addressed by the SEVERITY parameter of the `fmtmsg'
     function.  The SEVERITY parameter of `addseverity' must match the
     value for the parameter with the same name of `fmtmsg', and STRING
     is the string printed in the actual messages instead of the numeric
     value.

     If STRING is `NULL' the severity class with the numeric value
     according to SEVERITY is removed.

     It is not possible to overwrite or remove one of the default
     severity classes.  All calls to `addseverity' with SEVERITY set to
     one of the values for the default classes will fail.

     The return value is `MM_OK' if the task was successfully performed.
     If the return value is `MM_NOTOK' something went wrong.  This could
     mean that no more memory is available or a class is not available
     when it has to be removed.

     This function is not specified in the X/Open Portability Guide
     although the `fmtsmg' function is.  It is available on System V
     systems.


File: libc.info,  Node: Example,  Prev: Adding Severity Classes,  Up: Formatted Messages

12.22.3 How to use `fmtmsg' and `addseverity'
---------------------------------------------

Here is a simple example program to illustrate the use of the both
functions described in this section.


     #include <fmtmsg.h>

     int
     main (void)
     {
       addseverity (5, "NOTE2");
       fmtmsg (MM_PRINT, "only1field", MM_INFO, "text2", "action2", "tag2");
       fmtmsg (MM_PRINT, "UX:cat", 5, "invalid syntax", "refer to manual",
               "UX:cat:001");
       fmtmsg (MM_PRINT, "label:foo", 6, "text", "action", "tag");
       return 0;
     }

   The second call to `fmtmsg' illustrates a use of this function as it
usually occurs on System V systems, which heavily use this function.
It seems worthwhile to give a short explanation here of how this system
works on System V.  The value of the LABEL field (`UX:cat') says that
the error occurred in the Unix program `cat'.  The explanation of the
error follows and the value for the ACTION parameter is `"refer to
manual"'.  One could be more specific here, if necessary.  The TAG
field contains, as proposed above, the value of the string given for
the LABEL parameter, and additionally a unique ID (`001' in this case).
For a GNU environment this string could contain a reference to the
corresponding node in the Info page for the program.

Running this program without specifying the `MSGVERB' and `SEV_LEVEL'
function produces the following output:

     UX:cat: NOTE2: invalid syntax
     TO FIX: refer to manual UX:cat:001

   We see the different fields of the message and how the extra glue
(the colons and the `TO FIX' string) are printed.  But only one of the
three calls to `fmtmsg' produced output.  The first call does not print
anything because the LABEL parameter is not in the correct form.  The
string must contain two fields, separated by a colon (*note Printing
Formatted Messages::).  The third `fmtmsg' call produced no output
since the class with the numeric value `6' is not defined.  Although a
class with numeric value `5' is also not defined by default, the call
to `addseverity' introduces it and the second call to `fmtmsg' produces
the above output.

   When we change the environment of the program to contain
`SEV_LEVEL=XXX,6,NOTE' when running it we get a different result:

     UX:cat: NOTE2: invalid syntax
     TO FIX: refer to manual UX:cat:001
     label:foo: NOTE: text
     TO FIX: action tag

   Now the third call to `fmtmsg' produced some output and we see how
the string `NOTE' from the environment variable appears in the message.

   Now we can reduce the output by specifying which fields we are
interested in.  If we additionally set the environment variable
`MSGVERB' to the value `severity:label:action' we get the following
output:

     UX:cat: NOTE2
     TO FIX: refer to manual
     label:foo: NOTE
     TO FIX: action

I.e., the output produced by the TEXT and the TAG parameters to
`fmtmsg' vanished.  Please also note that now there is no colon after
the `NOTE' and `NOTE2' strings in the output.  This is not necessary
since there is no more output on this line because the text is missing.


File: libc.info,  Node: Low-Level I/O,  Next: File System Interface,  Prev: I/O on Streams,  Up: Top

13 Low-Level Input/Output
*************************

This chapter describes functions for performing low-level input/output
operations on file descriptors.  These functions include the primitives
for the higher-level I/O functions described in *note I/O on Streams::,
as well as functions for performing low-level control operations for
which there are no equivalents on streams.

   Stream-level I/O is more flexible and usually more convenient;
therefore, programmers generally use the descriptor-level functions only
when necessary.  These are some of the usual reasons:

   * For reading binary files in large chunks.

   * For reading an entire file into core before parsing it.

   * To perform operations other than data transfer, which can only be
     done with a descriptor.  (You can use `fileno' to get the
     descriptor corresponding to a stream.)

   * To pass descriptors to a child process.  (The child can create its
     own stream to use a descriptor that it inherits, but cannot
     inherit a stream directly.)

* Menu:

* Opening and Closing Files::           How to open and close file
                                         descriptors.
* I/O Primitives::                      Reading and writing data.
* File Position Primitive::             Setting a descriptor's file
                                         position.
* Descriptors and Streams::             Converting descriptor to stream
                                         or vice-versa.
* Stream/Descriptor Precautions::       Precautions needed if you use both
                                         descriptors and streams.
* Scatter-Gather::                      Fast I/O to discontinuous buffers.
* Memory-mapped I/O::                   Using files like memory.
* Waiting for I/O::                     How to check for input or output
					 on multiple file descriptors.
* Synchronizing I/O::                   Making sure all I/O actions completed.
* Asynchronous I/O::                    Perform I/O in parallel.
* Control Operations::                  Various other operations on file
					 descriptors.
* Duplicating Descriptors::             Fcntl commands for duplicating
                                         file descriptors.
* Descriptor Flags::                    Fcntl commands for manipulating
                                         flags associated with file
                                         descriptors.
* File Status Flags::                   Fcntl commands for manipulating
                                         flags associated with open files.
* File Locks::                          Fcntl commands for implementing
                                         file locking.
* Interrupt Input::                     Getting an asynchronous signal when
                                         input arrives.
* IOCTLs::                              Generic I/O Control operations.


File: libc.info,  Node: Opening and Closing Files,  Next: I/O Primitives,  Up: Low-Level I/O

13.1 Opening and Closing Files
==============================

This section describes the primitives for opening and closing files
using file descriptors.  The `open' and `creat' functions are declared
in the header file `fcntl.h', while `close' is declared in `unistd.h'.  

 -- Function: int open (const char *FILENAME, int FLAGS[, mode_t MODE])
     The `open' function creates and returns a new file descriptor for
     the file named by FILENAME.  Initially, the file position
     indicator for the file is at the beginning of the file.  The
     argument MODE (*note Permission Bits::) is used only when a file is
     created, but it doesn't hurt to supply the argument in any case.

     The FLAGS argument controls how the file is to be opened.  This is
     a bit mask; you create the value by the bitwise OR of the
     appropriate parameters (using the `|' operator in C).  *Note File
     Status Flags::, for the parameters available.

     The normal return value from `open' is a non-negative integer file
     descriptor.  In the case of an error, a value of -1 is returned
     instead.  In addition to the usual file name errors (*note File
     Name Errors::), the following `errno' error conditions are defined
     for this function:

    `EACCES'
          The file exists but is not readable/writable as requested by
          the FLAGS argument, the file does not exist and the directory
          is unwritable so it cannot be created.

    `EEXIST'
          Both `O_CREAT' and `O_EXCL' are set, and the named file
          already exists.

    `EINTR'
          The `open' operation was interrupted by a signal.  *Note
          Interrupted Primitives::.

    `EISDIR'
          The FLAGS argument specified write access, and the file is a
          directory.

    `EMFILE'
          The process has too many files open.  The maximum number of
          file descriptors is controlled by the `RLIMIT_NOFILE'
          resource limit; *note Limits on Resources::.

    `ENFILE'
          The entire system, or perhaps the file system which contains
          the directory, cannot support any additional open files at
          the moment.  (This problem cannot happen on GNU/Hurd systems.)

    `ENOENT'
          The named file does not exist, and `O_CREAT' is not specified.

    `ENOSPC'
          The directory or file system that would contain the new file
          cannot be extended, because there is no disk space left.

    `ENXIO'
          `O_NONBLOCK' and `O_WRONLY' are both set in the FLAGS
          argument, the file named by FILENAME is a FIFO (*note Pipes
          and FIFOs::), and no process has the file open for reading.

    `EROFS'
          The file resides on a read-only file system and any of
          `O_WRONLY', `O_RDWR', and `O_TRUNC' are set in the FLAGS
          argument, or `O_CREAT' is set and the file does not already
          exist.

     If on a 32 bit machine the sources are translated with
     `_FILE_OFFSET_BITS == 64' the function `open' returns a file
     descriptor opened in the large file mode which enables the file
     handling functions to use files up to 2^63 bytes in size and
     offset from -2^63 to 2^63.  This happens transparently for the user
     since all of the lowlevel file handling functions are equally
     replaced.

     This function is a cancellation point in multi-threaded programs.
     This is a problem if the thread allocates some resources (like
     memory, file descriptors, semaphores or whatever) at the time
     `open' is called.  If the thread gets canceled these resources
     stay allocated until the program ends.  To avoid this calls to
     `open' should be protected using cancellation handlers.

     The `open' function is the underlying primitive for the `fopen'
     and `freopen' functions, that create streams.

 -- Function: int open64 (const char *FILENAME, int FLAGS[, mode_t
          MODE])
     This function is similar to `open'.  It returns a file descriptor
     which can be used to access the file named by FILENAME.  The only
     difference is that on 32 bit systems the file is opened in the
     large file mode.  I.e., file length and file offsets can exceed 31
     bits.

     When the sources are translated with `_FILE_OFFSET_BITS == 64' this
     function is actually available under the name `open'.  I.e., the
     new, extended API using 64 bit file sizes and offsets transparently
     replaces the old API.

 -- Obsolete function: int creat (const char *FILENAME, mode_t MODE)
     This function is obsolete.  The call:

          creat (FILENAME, MODE)

     is equivalent to:

          open (FILENAME, O_WRONLY | O_CREAT | O_TRUNC, MODE)

     If on a 32 bit machine the sources are translated with
     `_FILE_OFFSET_BITS == 64' the function `creat' returns a file
     descriptor opened in the large file mode which enables the file
     handling functions to use files up to 2^63 in size and offset from
     -2^63 to 2^63.  This happens transparently for the user since all
     of the lowlevel file handling functions are equally replaced.

 -- Obsolete function: int creat64 (const char *FILENAME, mode_t MODE)
     This function is similar to `creat'.  It returns a file descriptor
     which can be used to access the file named by FILENAME.  The only
     the difference is that on 32 bit systems the file is opened in the
     large file mode.  I.e., file length and file offsets can exceed 31
     bits.

     To use this file descriptor one must not use the normal operations
     but instead the counterparts named `*64', e.g., `read64'.

     When the sources are translated with `_FILE_OFFSET_BITS == 64' this
     function is actually available under the name `open'.  I.e., the
     new, extended API using 64 bit file sizes and offsets transparently
     replaces the old API.

 -- Function: int close (int FILEDES)
     The function `close' closes the file descriptor FILEDES.  Closing
     a file has the following consequences:

        * The file descriptor is deallocated.

        * Any record locks owned by the process on the file are
          unlocked.

        * When all file descriptors associated with a pipe or FIFO have
          been closed, any unread data is discarded.

     This function is a cancellation point in multi-threaded programs.
     This is a problem if the thread allocates some resources (like
     memory, file descriptors, semaphores or whatever) at the time
     `close' is called.  If the thread gets canceled these resources
     stay allocated until the program ends.  To avoid this, calls to
     `close' should be protected using cancellation handlers.

     The normal return value from `close' is 0; a value of -1 is
     returned in case of failure.  The following `errno' error
     conditions are defined for this function:

    `EBADF'
          The FILEDES argument is not a valid file descriptor.

    `EINTR'
          The `close' call was interrupted by a signal.  *Note
          Interrupted Primitives::.  Here is an example of how to
          handle `EINTR' properly:

               TEMP_FAILURE_RETRY (close (desc));

    `ENOSPC'
    `EIO'
    `EDQUOT'
          When the file is accessed by NFS, these errors from `write'
          can sometimes not be detected until `close'.  *Note I/O
          Primitives::, for details on their meaning.

     Please note that there is _no_ separate `close64' function.  This
     is not necessary since this function does not determine nor depend
     on the mode of the file.  The kernel which performs the `close'
     operation knows which mode the descriptor is used for and can
     handle this situation.

   To close a stream, call `fclose' (*note Closing Streams::) instead
of trying to close its underlying file descriptor with `close'.  This
flushes any buffered output and updates the stream object to indicate
that it is closed.


File: libc.info,  Node: I/O Primitives,  Next: File Position Primitive,  Prev: Opening and Closing Files,  Up: Low-Level I/O

13.2 Input and Output Primitives
================================

This section describes the functions for performing primitive input and
output operations on file descriptors: `read', `write', and `lseek'.
These functions are declared in the header file `unistd.h'.  

 -- Data Type: ssize_t
     This data type is used to represent the sizes of blocks that can be
     read or written in a single operation.  It is similar to `size_t',
     but must be a signed type.

 -- Function: ssize_t read (int FILEDES, void *BUFFER, size_t SIZE)
     The `read' function reads up to SIZE bytes from the file with
     descriptor FILEDES, storing the results in the BUFFER.  (This is
     not necessarily a character string, and no terminating null
     character is added.)

     The return value is the number of bytes actually read.  This might
     be less than SIZE; for example, if there aren't that many bytes
     left in the file or if there aren't that many bytes immediately
     available.  The exact behavior depends on what kind of file it is.
     Note that reading less than SIZE bytes is not an error.

     A value of zero indicates end-of-file (except if the value of the
     SIZE argument is also zero).  This is not considered an error.  If
     you keep calling `read' while at end-of-file, it will keep
     returning zero and doing nothing else.

     If `read' returns at least one character, there is no way you can
     tell whether end-of-file was reached.  But if you did reach the
     end, the next read will return zero.

     In case of an error, `read' returns -1.  The following `errno'
     error conditions are defined for this function:

    `EAGAIN'
          Normally, when no input is immediately available, `read'
          waits for some input.  But if the `O_NONBLOCK' flag is set
          for the file (*note File Status Flags::), `read' returns
          immediately without reading any data, and reports this error.

          *Compatibility Note:* Most versions of BSD Unix use a
          different error code for this: `EWOULDBLOCK'.  In the GNU C
          Library, `EWOULDBLOCK' is an alias for `EAGAIN', so it
          doesn't matter which name you use.

          On some systems, reading a large amount of data from a
          character special file can also fail with `EAGAIN' if the
          kernel cannot find enough physical memory to lock down the
          user's pages.  This is limited to devices that transfer with
          direct memory access into the user's memory, which means it
          does not include terminals, since they always use separate
          buffers inside the kernel.  This problem never happens on
          GNU/Hurd systems.

          Any condition that could result in `EAGAIN' can instead
          result in a successful `read' which returns fewer bytes than
          requested.  Calling `read' again immediately would result in
          `EAGAIN'.

    `EBADF'
          The FILEDES argument is not a valid file descriptor, or is
          not open for reading.

    `EINTR'
          `read' was interrupted by a signal while it was waiting for
          input.  *Note Interrupted Primitives::.  A signal will not
          necessary cause `read' to return `EINTR'; it may instead
          result in a successful `read' which returns fewer bytes than
          requested.

    `EIO'
          For many devices, and for disk files, this error code
          indicates a hardware error.

          `EIO' also occurs when a background process tries to read
          from the controlling terminal, and the normal action of
          stopping the process by sending it a `SIGTTIN' signal isn't
          working.  This might happen if the signal is being blocked or
          ignored, or because the process group is orphaned.  *Note Job
          Control::, for more information about job control, and *note
          Signal Handling::, for information about signals.

    `EINVAL'
          In some systems, when reading from a character or block
          device, position and size offsets must be aligned to a
          particular block size.  This error indicates that the offsets
          were not properly aligned.

     Please note that there is no function named `read64'.  This is not
     necessary since this function does not directly modify or handle
     the possibly wide file offset.  Since the kernel handles this state
     internally, the `read' function can be used for all cases.

     This function is a cancellation point in multi-threaded programs.
     This is a problem if the thread allocates some resources (like
     memory, file descriptors, semaphores or whatever) at the time
     `read' is called.  If the thread gets canceled these resources
     stay allocated until the program ends.  To avoid this, calls to
     `read' should be protected using cancellation handlers.

     The `read' function is the underlying primitive for all of the
     functions that read from streams, such as `fgetc'.

 -- Function: ssize_t pread (int FILEDES, void *BUFFER, size_t SIZE,
          off_t OFFSET)
     The `pread' function is similar to the `read' function.  The first
     three arguments are identical, and the return values and error
     codes also correspond.

     The difference is the fourth argument and its handling.  The data
     block is not read from the current position of the file descriptor
     `filedes'.  Instead the data is read from the file starting at
     position OFFSET.  The position of the file descriptor itself is
     not affected by the operation.  The value is the same as before
     the call.

     When the source file is compiled with `_FILE_OFFSET_BITS == 64' the
     `pread' function is in fact `pread64' and the type `off_t' has 64
     bits, which makes it possible to handle files up to 2^63 bytes in
     length.

     The return value of `pread' describes the number of bytes read.
     In the error case it returns -1 like `read' does and the error
     codes are also the same, with these additions:

    `EINVAL'
          The value given for OFFSET is negative and therefore illegal.

    `ESPIPE'
          The file descriptor FILEDES is associate with a pipe or a
          FIFO and this device does not allow positioning of the file
          pointer.

     The function is an extension defined in the Unix Single
     Specification version 2.

 -- Function: ssize_t pread64 (int FILEDES, void *BUFFER, size_t SIZE,
          off64_t OFFSET)
     This function is similar to the `pread' function.  The difference
     is that the OFFSET parameter is of type `off64_t' instead of
     `off_t' which makes it possible on 32 bit machines to address
     files larger than 2^31 bytes and up to 2^63 bytes.  The file
     descriptor `filedes' must be opened using `open64' since otherwise
     the large offsets possible with `off64_t' will lead to errors with
     a descriptor in small file mode.

     When the source file is compiled with `_FILE_OFFSET_BITS == 64' on
     a 32 bit machine this function is actually available under the name
     `pread' and so transparently replaces the 32 bit interface.

 -- Function: ssize_t write (int FILEDES, const void *BUFFER, size_t
          SIZE)
     The `write' function writes up to SIZE bytes from BUFFER to the
     file with descriptor FILEDES.  The data in BUFFER is not
     necessarily a character string and a null character is output like
     any other character.

     The return value is the number of bytes actually written.  This
     may be SIZE, but can always be smaller.  Your program should
     always call `write' in a loop, iterating until all the data is
     written.

     Once `write' returns, the data is enqueued to be written and can be
     read back right away, but it is not necessarily written out to
     permanent storage immediately.  You can use `fsync' when you need
     to be sure your data has been permanently stored before
     continuing.  (It is more efficient for the system to batch up
     consecutive writes and do them all at once when convenient.
     Normally they will always be written to disk within a minute or
     less.)  Modern systems provide another function `fdatasync' which
     guarantees integrity only for the file data and is therefore
     faster.  You can use the `O_FSYNC' open mode to make `write' always
     store the data to disk before returning; *note Operating Modes::.

     In the case of an error, `write' returns -1.  The following
     `errno' error conditions are defined for this function:

    `EAGAIN'
          Normally, `write' blocks until the write operation is
          complete.  But if the `O_NONBLOCK' flag is set for the file
          (*note Control Operations::), it returns immediately without
          writing any data and reports this error.  An example of a
          situation that might cause the process to block on output is
          writing to a terminal device that supports flow control,
          where output has been suspended by receipt of a STOP
          character.

          *Compatibility Note:* Most versions of BSD Unix use a
          different error code for this: `EWOULDBLOCK'.  In the GNU C
          Library, `EWOULDBLOCK' is an alias for `EAGAIN', so it
          doesn't matter which name you use.

          On some systems, writing a large amount of data from a
          character special file can also fail with `EAGAIN' if the
          kernel cannot find enough physical memory to lock down the
          user's pages.  This is limited to devices that transfer with
          direct memory access into the user's memory, which means it
          does not include terminals, since they always use separate
          buffers inside the kernel.  This problem does not arise on
          GNU/Hurd systems.

    `EBADF'
          The FILEDES argument is not a valid file descriptor, or is
          not open for writing.

    `EFBIG'
          The size of the file would become larger than the
          implementation can support.

    `EINTR'
          The `write' operation was interrupted by a signal while it was
          blocked waiting for completion.  A signal will not
          necessarily cause `write' to return `EINTR'; it may instead
          result in a successful `write' which writes fewer bytes than
          requested.  *Note Interrupted Primitives::.

    `EIO'
          For many devices, and for disk files, this error code
          indicates a hardware error.

    `ENOSPC'
          The device containing the file is full.

    `EPIPE'
          This error is returned when you try to write to a pipe or
          FIFO that isn't open for reading by any process.  When this
          happens, a `SIGPIPE' signal is also sent to the process; see
          *note Signal Handling::.

    `EINVAL'
          In some systems, when writing to a character or block device,
          position and size offsets must be aligned to a particular
          block size.  This error indicates that the offsets were not
          properly aligned.

     Unless you have arranged to prevent `EINTR' failures, you should
     check `errno' after each failing call to `write', and if the error
     was `EINTR', you should simply repeat the call.  *Note Interrupted
     Primitives::.  The easy way to do this is with the macro
     `TEMP_FAILURE_RETRY', as follows:

          nbytes = TEMP_FAILURE_RETRY (write (desc, buffer, count));

     Please note that there is no function named `write64'.  This is not
     necessary since this function does not directly modify or handle
     the possibly wide file offset.  Since the kernel handles this state
     internally the `write' function can be used for all cases.

     This function is a cancellation point in multi-threaded programs.
     This is a problem if the thread allocates some resources (like
     memory, file descriptors, semaphores or whatever) at the time
     `write' is called.  If the thread gets canceled these resources
     stay allocated until the program ends.  To avoid this, calls to
     `write' should be protected using cancellation handlers.

     The `write' function is the underlying primitive for all of the
     functions that write to streams, such as `fputc'.

 -- Function: ssize_t pwrite (int FILEDES, const void *BUFFER, size_t
          SIZE, off_t OFFSET)
     The `pwrite' function is similar to the `write' function.  The
     first three arguments are identical, and the return values and
     error codes also correspond.

     The difference is the fourth argument and its handling.  The data
     block is not written to the current position of the file descriptor
     `filedes'.  Instead the data is written to the file starting at
     position OFFSET.  The position of the file descriptor itself is
     not affected by the operation.  The value is the same as before
     the call.

     When the source file is compiled with `_FILE_OFFSET_BITS == 64' the
     `pwrite' function is in fact `pwrite64' and the type `off_t' has
     64 bits, which makes it possible to handle files up to 2^63 bytes
     in length.

     The return value of `pwrite' describes the number of written bytes.
     In the error case it returns -1 like `write' does and the error
     codes are also the same, with these additions:

    `EINVAL'
          The value given for OFFSET is negative and therefore illegal.

    `ESPIPE'
          The file descriptor FILEDES is associated with a pipe or a
          FIFO and this device does not allow positioning of the file
          pointer.

     The function is an extension defined in the Unix Single
     Specification version 2.

 -- Function: ssize_t pwrite64 (int FILEDES, const void *BUFFER, size_t
          SIZE, off64_t OFFSET)
     This function is similar to the `pwrite' function.  The difference
     is that the OFFSET parameter is of type `off64_t' instead of
     `off_t' which makes it possible on 32 bit machines to address
     files larger than 2^31 bytes and up to 2^63 bytes.  The file
     descriptor `filedes' must be opened using `open64' since otherwise
     the large offsets possible with `off64_t' will lead to errors with
     a descriptor in small file mode.

     When the source file is compiled using `_FILE_OFFSET_BITS == 64'
     on a 32 bit machine this function is actually available under the
     name `pwrite' and so transparently replaces the 32 bit interface.


File: libc.info,  Node: File Position Primitive,  Next: Descriptors and Streams,  Prev: I/O Primitives,  Up: Low-Level I/O

13.3 Setting the File Position of a Descriptor
==============================================

Just as you can set the file position of a stream with `fseek', you can
set the file position of a descriptor with `lseek'.  This specifies the
position in the file for the next `read' or `write' operation.  *Note
File Positioning::, for more information on the file position and what
it means.

   To read the current file position value from a descriptor, use
`lseek (DESC, 0, SEEK_CUR)'.

 -- Function: off_t lseek (int FILEDES, off_t OFFSET, int WHENCE)
     The `lseek' function is used to change the file position of the
     file with descriptor FILEDES.

     The WHENCE argument specifies how the OFFSET should be
     interpreted, in the same way as for the `fseek' function, and it
     must be one of the symbolic constants `SEEK_SET', `SEEK_CUR', or
     `SEEK_END'.

    `SEEK_SET'
          Specifies that OFFSET is a count of characters from the
          beginning of the file.

    `SEEK_CUR'
          Specifies that OFFSET is a count of characters from the
          current file position.  This count may be positive or
          negative.

    `SEEK_END'
          Specifies that OFFSET is a count of characters from the end of
          the file.  A negative count specifies a position within the
          current extent of the file; a positive count specifies a
          position past the current end.  If you set the position past
          the current end, and actually write data, you will extend the
          file with zeros up to that position.

     The return value from `lseek' is normally the resulting file
     position, measured in bytes from the beginning of the file.  You
     can use this feature together with `SEEK_CUR' to read the current
     file position.

     If you want to append to the file, setting the file position to the
     current end of file with `SEEK_END' is not sufficient.  Another
     process may write more data after you seek but before you write,
     extending the file so the position you write onto clobbers their
     data.  Instead, use the `O_APPEND' operating mode; *note Operating
     Modes::.

     You can set the file position past the current end of the file.
     This does not by itself make the file longer; `lseek' never
     changes the file.  But subsequent output at that position will
     extend the file.  Characters between the previous end of file and
     the new position are filled with zeros.  Extending the file in
     this way can create a "hole": the blocks of zeros are not actually
     allocated on disk, so the file takes up less space than it appears
     to; it is then called a "sparse file".  

     If the file position cannot be changed, or the operation is in
     some way invalid, `lseek' returns a value of -1.  The following
     `errno' error conditions are defined for this function:

    `EBADF'
          The FILEDES is not a valid file descriptor.

    `EINVAL'
          The WHENCE argument value is not valid, or the resulting file
          offset is not valid.  A file offset is invalid.

    `ESPIPE'
          The FILEDES corresponds to an object that cannot be
          positioned, such as a pipe, FIFO or terminal device.
          (POSIX.1 specifies this error only for pipes and FIFOs, but
          on GNU systems, you always get `ESPIPE' if the object is not
          seekable.)

     When the source file is compiled with `_FILE_OFFSET_BITS == 64' the
     `lseek' function is in fact `lseek64' and the type `off_t' has 64
     bits which makes it possible to handle files up to 2^63 bytes in
     length.

     This function is a cancellation point in multi-threaded programs.
     This is a problem if the thread allocates some resources (like
     memory, file descriptors, semaphores or whatever) at the time
     `lseek' is called.  If the thread gets canceled these resources
     stay allocated until the program ends.  To avoid this calls to
     `lseek' should be protected using cancellation handlers.

     The `lseek' function is the underlying primitive for the `fseek',
     `fseeko', `ftell', `ftello' and `rewind' functions, which operate
     on streams instead of file descriptors.

 -- Function: off64_t lseek64 (int FILEDES, off64_t OFFSET, int WHENCE)
     This function is similar to the `lseek' function.  The difference
     is that the OFFSET parameter is of type `off64_t' instead of
     `off_t' which makes it possible on 32 bit machines to address
     files larger than 2^31 bytes and up to 2^63 bytes.  The file
     descriptor `filedes' must be opened using `open64' since otherwise
     the large offsets possible with `off64_t' will lead to errors with
     a descriptor in small file mode.

     When the source file is compiled with `_FILE_OFFSET_BITS == 64' on
     a 32 bits machine this function is actually available under the
     name `lseek' and so transparently replaces the 32 bit interface.

   You can have multiple descriptors for the same file if you open the
file more than once, or if you duplicate a descriptor with `dup'.
Descriptors that come from separate calls to `open' have independent
file positions; using `lseek' on one descriptor has no effect on the
other.  For example,

     {
       int d1, d2;
       char buf[4];
       d1 = open ("foo", O_RDONLY);
       d2 = open ("foo", O_RDONLY);
       lseek (d1, 1024, SEEK_SET);
       read (d2, buf, 4);
     }

will read the first four characters of the file `foo'.  (The
error-checking code necessary for a real program has been omitted here
for brevity.)

   By contrast, descriptors made by duplication share a common file
position with the original descriptor that was duplicated.  Anything
which alters the file position of one of the duplicates, including
reading or writing data, affects all of them alike.  Thus, for example,

     {
       int d1, d2, d3;
       char buf1[4], buf2[4];
       d1 = open ("foo", O_RDONLY);
       d2 = dup (d1);
       d3 = dup (d2);
       lseek (d3, 1024, SEEK_SET);
       read (d1, buf1, 4);
       read (d2, buf2, 4);
     }

will read four characters starting with the 1024'th character of `foo',
and then four more characters starting with the 1028'th character.

 -- Data Type: off_t
     This is a signed integer type used to represent file sizes.  In
     the GNU C Library, this type is no narrower than `int'.

     If the source is compiled with `_FILE_OFFSET_BITS == 64' this type
     is transparently replaced by `off64_t'.

 -- Data Type: off64_t
     This type is used similar to `off_t'.  The difference is that even
     on 32 bit machines, where the `off_t' type would have 32 bits,
     `off64_t' has 64 bits and so is able to address files up to 2^63
     bytes in length.

     When compiling with `_FILE_OFFSET_BITS == 64' this type is
     available under the name `off_t'.

   These aliases for the `SEEK_...' constants exist for the sake of
compatibility with older BSD systems.  They are defined in two
different header files: `fcntl.h' and `sys/file.h'.

`L_SET'
     An alias for `SEEK_SET'.

`L_INCR'
     An alias for `SEEK_CUR'.

`L_XTND'
     An alias for `SEEK_END'.


File: libc.info,  Node: Descriptors and Streams,  Next: Stream/Descriptor Precautions,  Prev: File Position Primitive,  Up: Low-Level I/O

13.4 Descriptors and Streams
============================

Given an open file descriptor, you can create a stream for it with the
`fdopen' function.  You can get the underlying file descriptor for an
existing stream with the `fileno' function.  These functions are
declared in the header file `stdio.h'.  

 -- Function: FILE * fdopen (int FILEDES, const char *OPENTYPE)
     The `fdopen' function returns a new stream for the file descriptor
     FILEDES.

     The OPENTYPE argument is interpreted in the same way as for the
     `fopen' function (*note Opening Streams::), except that the `b'
     option is not permitted; this is because GNU systems make no
     distinction between text and binary files.  Also, `"w"' and `"w+"'
     do not cause truncation of the file; these have an effect only
     when opening a file, and in this case the file has already been
     opened.  You must make sure that the OPENTYPE argument matches the
     actual mode of the open file descriptor.

     The return value is the new stream.  If the stream cannot be
     created (for example, if the modes for the file indicated by the
     file descriptor do not permit the access specified by the OPENTYPE
     argument), a null pointer is returned instead.

     In some other systems, `fdopen' may fail to detect that the modes
     for file descriptor do not permit the access specified by
     `opentype'.  The GNU C Library always checks for this.

   For an example showing the use of the `fdopen' function, see *note
Creating a Pipe::.

 -- Function: int fileno (FILE *STREAM)
     This function returns the file descriptor associated with the
     stream STREAM.  If an error is detected (for example, if the STREAM
     is not valid) or if STREAM does not do I/O to a file, `fileno'
     returns -1.

 -- Function: int fileno_unlocked (FILE *STREAM)
     The `fileno_unlocked' function is equivalent to the `fileno'
     function except that it does not implicitly lock the stream if the
     state is `FSETLOCKING_INTERNAL'.

     This function is a GNU extension.

   There are also symbolic constants defined in `unistd.h' for the file
descriptors belonging to the standard streams `stdin', `stdout', and
`stderr'; see *note Standard Streams::.  

`STDIN_FILENO'
     This macro has value `0', which is the file descriptor for
     standard input.  

`STDOUT_FILENO'
     This macro has value `1', which is the file descriptor for
     standard output.  

`STDERR_FILENO'
     This macro has value `2', which is the file descriptor for
     standard error output.
   

File: libc.info,  Node: Stream/Descriptor Precautions,  Next: Scatter-Gather,  Prev: Descriptors and Streams,  Up: Low-Level I/O

13.5 Dangers of Mixing Streams and Descriptors
==============================================

You can have multiple file descriptors and streams (let's call both
streams and descriptors "channels" for short) connected to the same
file, but you must take care to avoid confusion between channels.  There
are two cases to consider: "linked" channels that share a single file
position value, and "independent" channels that have their own file
positions.

   It's best to use just one channel in your program for actual data
transfer to any given file, except when all the access is for input.
For example, if you open a pipe (something you can only do at the file
descriptor level), either do all I/O with the descriptor, or construct a
stream from the descriptor with `fdopen' and then do all I/O with the
stream.

* Menu:

* Linked Channels::	   Dealing with channels sharing a file position.
* Independent Channels::   Dealing with separately opened, unlinked channels.
* Cleaning Streams::	   Cleaning a stream makes it safe to use
                            another channel.


File: libc.info,  Node: Linked Channels,  Next: Independent Channels,  Up: Stream/Descriptor Precautions

13.5.1 Linked Channels
----------------------

Channels that come from a single opening share the same file position;
we call them "linked" channels.  Linked channels result when you make a
stream from a descriptor using `fdopen', when you get a descriptor from
a stream with `fileno', when you copy a descriptor with `dup' or
`dup2', and when descriptors are inherited during `fork'.  For files
that don't support random access, such as terminals and pipes, _all_
channels are effectively linked.  On random-access files, all
append-type output streams are effectively linked to each other.

   If you have been using a stream for I/O (or have just opened the
stream), and you want to do I/O using another channel (either a stream
or a descriptor) that is linked to it, you must first "clean up" the
stream that you have been using.  *Note Cleaning Streams::.

   Terminating a process, or executing a new program in the process,
destroys all the streams in the process.  If descriptors linked to these
streams persist in other processes, their file positions become
undefined as a result.  To prevent this, you must clean up the streams
before destroying them.


File: libc.info,  Node: Independent Channels,  Next: Cleaning Streams,  Prev: Linked Channels,  Up: Stream/Descriptor Precautions

13.5.2 Independent Channels
---------------------------

When you open channels (streams or descriptors) separately on a seekable
file, each channel has its own file position.  These are called
"independent channels".

   The system handles each channel independently.  Most of the time,
this is quite predictable and natural (especially for input): each
channel can read or write sequentially at its own place in the file.
However, if some of the channels are streams, you must take these
precautions:

   * You should clean an output stream after use, before doing anything
     else that might read or write from the same part of the file.

   * You should clean an input stream before reading data that may have
     been modified using an independent channel.  Otherwise, you might
     read obsolete data that had been in the stream's buffer.

   If you do output to one channel at the end of the file, this will
certainly leave the other independent channels positioned somewhere
before the new end.  You cannot reliably set their file positions to the
new end of file before writing, because the file can always be extended
by another process between when you set the file position and when you
write the data.  Instead, use an append-type descriptor or stream; they
always output at the current end of the file.  In order to make the
end-of-file position accurate, you must clean the output channel you
were using, if it is a stream.

   It's impossible for two channels to have separate file pointers for a
file that doesn't support random access.  Thus, channels for reading or
writing such files are always linked, never independent.  Append-type
channels are also always linked.  For these channels, follow the rules
for linked channels; see *note Linked Channels::.


File: libc.info,  Node: Cleaning Streams,  Prev: Independent Channels,  Up: Stream/Descriptor Precautions

13.5.3 Cleaning Streams
-----------------------

You can use `fflush' to clean a stream in most cases.

   You can skip the `fflush' if you know the stream is already clean.
A stream is clean whenever its buffer is empty.  For example, an
unbuffered stream is always clean.  An input stream that is at
end-of-file is clean.  A line-buffered stream is clean when the last
character output was a newline.  However, a just-opened input stream
might not be clean, as its input buffer might not be empty.

   There is one case in which cleaning a stream is impossible on most
systems.  This is when the stream is doing input from a file that is not
random-access.  Such streams typically read ahead, and when the file is
not random access, there is no way to give back the excess data already
read.  When an input stream reads from a random-access file, `fflush'
does clean the stream, but leaves the file pointer at an unpredictable
place; you must set the file pointer before doing any further I/O.

   Closing an output-only stream also does `fflush', so this is a valid
way of cleaning an output stream.

   You need not clean a stream before using its descriptor for control
operations such as setting terminal modes; these operations don't affect
the file position and are not affected by it.  You can use any
descriptor for these operations, and all channels are affected
simultaneously.  However, text already "output" to a stream but still
buffered by the stream will be subject to the new terminal modes when
subsequently flushed.  To make sure "past" output is covered by the
terminal settings that were in effect at the time, flush the output
streams for that terminal before setting the modes.  *Note Terminal
Modes::.


File: libc.info,  Node: Scatter-Gather,  Next: Memory-mapped I/O,  Prev: Stream/Descriptor Precautions,  Up: Low-Level I/O

13.6 Fast Scatter-Gather I/O
============================

Some applications may need to read or write data to multiple buffers,
which are separated in memory.  Although this can be done easily enough
with multiple calls to `read' and `write', it is inefficient because
there is overhead associated with each kernel call.

   Instead, many platforms provide special high-speed primitives to
perform these "scatter-gather" operations in a single kernel call.  The
GNU C Library will provide an emulation on any system that lacks these
primitives, so they are not a portability threat.  They are defined in
`sys/uio.h'.

   These functions are controlled with arrays of `iovec' structures,
which describe the location and size of each buffer.

 -- Data Type: struct iovec
     The `iovec' structure describes a buffer. It contains two fields:

    `void *iov_base'
          Contains the address of a buffer.

    `size_t iov_len'
          Contains the length of the buffer.


 -- Function: ssize_t readv (int FILEDES, const struct iovec *VECTOR,
          int COUNT)
     The `readv' function reads data from FILEDES and scatters it into
     the buffers described in VECTOR, which is taken to be COUNT
     structures long.  As each buffer is filled, data is sent to the
     next.

     Note that `readv' is not guaranteed to fill all the buffers.  It
     may stop at any point, for the same reasons `read' would.

     The return value is a count of bytes (_not_ buffers) read, 0
     indicating end-of-file, or -1 indicating an error.  The possible
     errors are the same as in `read'.


 -- Function: ssize_t writev (int FILEDES, const struct iovec *VECTOR,
          int COUNT)
     The `writev' function gathers data from the buffers described in
     VECTOR, which is taken to be COUNT structures long, and writes
     them to `filedes'.  As each buffer is written, it moves on to the
     next.

     Like `readv', `writev' may stop midstream under the same
     conditions `write' would.

     The return value is a count of bytes written, or -1 indicating an
     error.  The possible errors are the same as in `write'.


   Note that if the buffers are small (under about 1kB), high-level
streams may be easier to use than these functions.  However, `readv' and
`writev' are more efficient when the individual buffers themselves (as
opposed to the total output), are large.  In that case, a high-level
stream would not be able to cache the data effectively.


File: libc.info,  Node: Memory-mapped I/O,  Next: Waiting for I/O,  Prev: Scatter-Gather,  Up: Low-Level I/O

13.7 Memory-mapped I/O
======================

On modern operating systems, it is possible to "mmap" (pronounced
"em-map") a file to a region of memory.  When this is done, the file can
be accessed just like an array in the program.

   This is more efficient than `read' or `write', as only the regions
of the file that a program actually accesses are loaded.  Accesses to
not-yet-loaded parts of the mmapped region are handled in the same way
as swapped out pages.

   Since mmapped pages can be stored back to their file when physical
memory is low, it is possible to mmap files orders of magnitude larger
than both the physical memory _and_ swap space.  The only limit is
address space.  The theoretical limit is 4GB on a 32-bit machine -
however, the actual limit will be smaller since some areas will be
reserved for other purposes.  If the LFS interface is used the file size
on 32-bit systems is not limited to 2GB (offsets are signed which
reduces the addressable area of 4GB by half); the full 64-bit are
available.

   Memory mapping only works on entire pages of memory.  Thus, addresses
for mapping must be page-aligned, and length values will be rounded up.
To determine the size of a page the machine uses one should use

     size_t page_size = (size_t) sysconf (_SC_PAGESIZE);

These functions are declared in `sys/mman.h'.

 -- Function: void * mmap (void *ADDRESS, size_t LENGTH, int PROTECT,
          int FLAGS, int FILEDES, off_t OFFSET)
     The `mmap' function creates a new mapping, connected to bytes
     (OFFSET) to (OFFSET + LENGTH - 1) in the file open on FILEDES.  A
     new reference for the file specified by FILEDES is created, which
     is not removed by closing the file.

     ADDRESS gives a preferred starting address for the mapping.
     `NULL' expresses no preference. Any previous mapping at that
     address is automatically removed. The address you give may still be
     changed, unless you use the `MAP_FIXED' flag.

     PROTECT contains flags that control what kind of access is
     permitted.  They include `PROT_READ', `PROT_WRITE', and
     `PROT_EXEC', which permit reading, writing, and execution,
     respectively.  Inappropriate access will cause a segfault (*note
     Program Error Signals::).

     Note that most hardware designs cannot support write permission
     without read permission, and many do not distinguish read and
     execute permission.  Thus, you may receive wider permissions than
     you ask for, and mappings of write-only files may be denied even
     if you do not use `PROT_READ'.

     FLAGS contains flags that control the nature of the map.  One of
     `MAP_SHARED' or `MAP_PRIVATE' must be specified.

     They include:

    `MAP_PRIVATE'
          This specifies that writes to the region should never be
          written back to the attached file.  Instead, a copy is made
          for the process, and the region will be swapped normally if
          memory runs low.  No other process will see the changes.

          Since private mappings effectively revert to ordinary memory
          when written to, you must have enough virtual memory for a
          copy of the entire mmapped region if you use this mode with
          `PROT_WRITE'.

    `MAP_SHARED'
          This specifies that writes to the region will be written back
          to the file.  Changes made will be shared immediately with
          other processes mmaping the same file.

          Note that actual writing may take place at any time.  You
          need to use `msync', described below, if it is important that
          other processes using conventional I/O get a consistent view
          of the file.

    `MAP_FIXED'
          This forces the system to use the exact mapping address
          specified in ADDRESS and fail if it can't.

    `MAP_ANONYMOUS'
    `MAP_ANON'
          This flag tells the system to create an anonymous mapping,
          not connected to a file.  FILEDES and OFF are ignored, and
          the region is initialized with zeros.

          Anonymous maps are used as the basic primitive to extend the
          heap on some systems.  They are also useful to share data
          between multiple tasks without creating a file.

          On some systems using private anonymous mmaps is more
          efficient than using `malloc' for large blocks.  This is not
          an issue with the GNU C Library, as the included `malloc'
          automatically uses `mmap' where appropriate.


     `mmap' returns the address of the new mapping, or -1 for an error.

     Possible errors include:

    `EINVAL'
          Either ADDRESS was unusable, or inconsistent FLAGS were given.

    `EACCES'
          FILEDES was not open for the type of access specified in
          PROTECT.

    `ENOMEM'
          Either there is not enough memory for the operation, or the
          process is out of address space.

    `ENODEV'
          This file is of a type that doesn't support mapping.

    `ENOEXEC'
          The file is on a filesystem that doesn't support mapping.


 -- Function: void * mmap64 (void *ADDRESS, size_t LENGTH, int PROTECT,
          int FLAGS, int FILEDES, off64_t OFFSET)
     The `mmap64' function is equivalent to the `mmap' function but the
     OFFSET parameter is of type `off64_t'.  On 32-bit systems this
     allows the file associated with the FILEDES descriptor to be
     larger than 2GB.  FILEDES must be a descriptor returned from a
     call to `open64' or `fopen64' and `freopen64' where the descriptor
     is retrieved with `fileno'.

     When the sources are translated with `_FILE_OFFSET_BITS == 64' this
     function is actually available under the name `mmap'.  I.e., the
     new, extended API using 64 bit file sizes and offsets transparently
     replaces the old API.

 -- Function: int munmap (void *ADDR, size_t LENGTH)
     `munmap' removes any memory maps from (ADDR) to (ADDR + LENGTH).
     LENGTH should be the length of the mapping.

     It is safe to unmap multiple mappings in one command, or include
     unmapped space in the range.  It is also possible to unmap only
     part of an existing mapping.  However, only entire pages can be
     removed.  If LENGTH is not an even number of pages, it will be
     rounded up.

     It returns 0 for success and -1 for an error.

     One error is possible:

    `EINVAL'
          The memory range given was outside the user mmap range or
          wasn't page aligned.


 -- Function: int msync (void *ADDRESS, size_t LENGTH, int FLAGS)
     When using shared mappings, the kernel can write the file at any
     time before the mapping is removed.  To be certain data has
     actually been written to the file and will be accessible to
     non-memory-mapped I/O, it is necessary to use this function.

     It operates on the region ADDRESS to (ADDRESS + LENGTH).  It may
     be used on part of a mapping or multiple mappings, however the
     region given should not contain any unmapped space.

     FLAGS can contain some options:

    `MS_SYNC'
          This flag makes sure the data is actually written _to disk_.
          Normally `msync' only makes sure that accesses to a file with
          conventional I/O reflect the recent changes.

    `MS_ASYNC'
          This tells `msync' to begin the synchronization, but not to
          wait for it to complete.


     `msync' returns 0 for success and -1 for error.  Errors include:

    `EINVAL'
          An invalid region was given, or the FLAGS were invalid.

    `EFAULT'
          There is no existing mapping in at least part of the given
          region.


 -- Function: void * mremap (void *ADDRESS, size_t LENGTH, size_t
          NEW_LENGTH, int FLAG)
     This function can be used to change the size of an existing memory
     area. ADDRESS and LENGTH must cover a region entirely mapped in
     the same `mmap' statement. A new mapping with the same
     characteristics will be returned with the length NEW_LENGTH.

     One option is possible, `MREMAP_MAYMOVE'. If it is given in FLAGS,
     the system may remove the existing mapping and create a new one of
     the desired length in another location.

     The address of the resulting mapping is returned, or -1. Possible
     error codes include:

    `EFAULT'
          There is no existing mapping in at least part of the original
          region, or the region covers two or more distinct mappings.

    `EINVAL'
          The address given is misaligned or inappropriate.

    `EAGAIN'
          The region has pages locked, and if extended it would exceed
          the process's resource limit for locked pages.  *Note Limits
          on Resources::.

    `ENOMEM'
          The region is private writable, and insufficient virtual
          memory is available to extend it.  Also, this error will
          occur if `MREMAP_MAYMOVE' is not given and the extension
          would collide with another mapped region.


   This function is only available on a few systems.  Except for
performing optional optimizations one should not rely on this function.

   Not all file descriptors may be mapped.  Sockets, pipes, and most
devices only allow sequential access and do not fit into the mapping
abstraction.  In addition, some regular files may not be mmapable, and
older kernels may not support mapping at all.  Thus, programs using
`mmap' should have a fallback method to use should it fail. *Note Mmap:
(standards)Mmap.

 -- Function: int madvise (void *ADDR, size_t LENGTH, int ADVICE)
     This function can be used to provide the system with ADVICE about
     the intended usage patterns of the memory region starting at ADDR
     and extending LENGTH bytes.

     The valid BSD values for ADVICE are:

    `MADV_NORMAL'
          The region should receive no further special treatment.

    `MADV_RANDOM'
          The region will be accessed via random page references. The
          kernel should page-in the minimal number of pages for each
          page fault.

    `MADV_SEQUENTIAL'
          The region will be accessed via sequential page references.
          This may cause the kernel to aggressively read-ahead,
          expecting further sequential references after any page fault
          within this region.

    `MADV_WILLNEED'
          The region will be needed.  The pages within this region may
          be pre-faulted in by the kernel.

    `MADV_DONTNEED'
          The region is no longer needed.  The kernel may free these
          pages, causing any changes to the pages to be lost, as well
          as swapped out pages to be discarded.


     The POSIX names are slightly different, but with the same meanings:

    `POSIX_MADV_NORMAL'
          This corresponds with BSD's `MADV_NORMAL'.

    `POSIX_MADV_RANDOM'
          This corresponds with BSD's `MADV_RANDOM'.

    `POSIX_MADV_SEQUENTIAL'
          This corresponds with BSD's `MADV_SEQUENTIAL'.

    `POSIX_MADV_WILLNEED'
          This corresponds with BSD's `MADV_WILLNEED'.

    `POSIX_MADV_DONTNEED'
          This corresponds with BSD's `MADV_DONTNEED'.


     `madvise' returns 0 for success and -1 for error.  Errors include:
    `EINVAL'
          An invalid region was given, or the ADVICE was invalid.

    `EFAULT'
          There is no existing mapping in at least part of the given
          region.



File: libc.info,  Node: Waiting for I/O,  Next: Synchronizing I/O,  Prev: Memory-mapped I/O,  Up: Low-Level I/O

13.8 Waiting for Input or Output
================================

Sometimes a program needs to accept input on multiple input channels
whenever input arrives.  For example, some workstations may have devices
such as a digitizing tablet, function button box, or dial box that are
connected via normal asynchronous serial interfaces; good user interface
style requires responding immediately to input on any device.  Another
example is a program that acts as a server to several other processes
via pipes or sockets.

   You cannot normally use `read' for this purpose, because this blocks
the program until input is available on one particular file descriptor;
input on other channels won't wake it up.  You could set nonblocking
mode and poll each file descriptor in turn, but this is very
inefficient.

   A better solution is to use the `select' function.  This blocks the
program until input or output is ready on a specified set of file
descriptors, or until a timer expires, whichever comes first.  This
facility is declared in the header file `sys/types.h'.  

   In the case of a server socket (*note Listening::), we say that
"input" is available when there are pending connections that could be
accepted (*note Accepting Connections::).  `accept' for server sockets
blocks and interacts with `select' just as `read' does for normal input.

   The file descriptor sets for the `select' function are specified as
`fd_set' objects.  Here is the description of the data type and some
macros for manipulating these objects.

 -- Data Type: fd_set
     The `fd_set' data type represents file descriptor sets for the
     `select' function.  It is actually a bit array.

 -- Macro: int FD_SETSIZE
     The value of this macro is the maximum number of file descriptors
     that a `fd_set' object can hold information about.  On systems
     with a fixed maximum number, `FD_SETSIZE' is at least that number.
     On some systems, including GNU, there is no absolute limit on the
     number of descriptors open, but this macro still has a constant
     value which controls the number of bits in an `fd_set'; if you get
     a file descriptor with a value as high as `FD_SETSIZE', you cannot
     put that descriptor into an `fd_set'.

 -- Macro: void FD_ZERO (fd_set *SET)
     This macro initializes the file descriptor set SET to be the empty
     set.

 -- Macro: void FD_SET (int FILEDES, fd_set *SET)
     This macro adds FILEDES to the file descriptor set SET.

     The FILEDES parameter must not have side effects since it is
     evaluated more than once.

 -- Macro: void FD_CLR (int FILEDES, fd_set *SET)
     This macro removes FILEDES from the file descriptor set SET.

     The FILEDES parameter must not have side effects since it is
     evaluated more than once.

 -- Macro: int FD_ISSET (int FILEDES, const fd_set *SET)
     This macro returns a nonzero value (true) if FILEDES is a member
     of the file descriptor set SET, and zero (false) otherwise.

     The FILEDES parameter must not have side effects since it is
     evaluated more than once.

   Next, here is the description of the `select' function itself.

 -- Function: int select (int NFDS, fd_set *READ-FDS, fd_set
          *WRITE-FDS, fd_set *EXCEPT-FDS, struct timeval *TIMEOUT)
     The `select' function blocks the calling process until there is
     activity on any of the specified sets of file descriptors, or
     until the timeout period has expired.

     The file descriptors specified by the READ-FDS argument are
     checked to see if they are ready for reading; the WRITE-FDS file
     descriptors are checked to see if they are ready for writing; and
     the EXCEPT-FDS file descriptors are checked for exceptional
     conditions.  You can pass a null pointer for any of these
     arguments if you are not interested in checking for that kind of
     condition.

     A file descriptor is considered ready for reading if a `read' call
     will not block.  This usually includes the read offset being at
     the end of the file or there is an error to report.  A server
     socket is considered ready for reading if there is a pending
     connection which can be accepted with `accept'; *note Accepting
     Connections::.  A client socket is ready for writing when its
     connection is fully established; *note Connecting::.

     "Exceptional conditions" does not mean errors--errors are reported
     immediately when an erroneous system call is executed, and do not
     constitute a state of the descriptor.  Rather, they include
     conditions such as the presence of an urgent message on a socket.
     (*Note Sockets::, for information on urgent messages.)

     The `select' function checks only the first NFDS file descriptors.
     The usual thing is to pass `FD_SETSIZE' as the value of this
     argument.

     The TIMEOUT specifies the maximum time to wait.  If you pass a
     null pointer for this argument, it means to block indefinitely
     until one of the file descriptors is ready.  Otherwise, you should
     provide the time in `struct timeval' format; see *note
     High-Resolution Calendar::.  Specify zero as the time (a `struct
     timeval' containing all zeros) if you want to find out which
     descriptors are ready without waiting if none are ready.

     The normal return value from `select' is the total number of ready
     file descriptors in all of the sets.  Each of the argument sets is
     overwritten with information about the descriptors that are ready
     for the corresponding operation.  Thus, to see if a particular
     descriptor DESC has input, use `FD_ISSET (DESC, READ-FDS)' after
     `select' returns.

     If `select' returns because the timeout period expires, it returns
     a value of zero.

     Any signal will cause `select' to return immediately.  So if your
     program uses signals, you can't rely on `select' to keep waiting
     for the full time specified.  If you want to be sure of waiting
     for a particular amount of time, you must check for `EINTR' and
     repeat the `select' with a newly calculated timeout based on the
     current time.  See the example below.  See also *note Interrupted
     Primitives::.

     If an error occurs, `select' returns `-1' and does not modify the
     argument file descriptor sets.  The following `errno' error
     conditions are defined for this function:

    `EBADF'
          One of the file descriptor sets specified an invalid file
          descriptor.

    `EINTR'
          The operation was interrupted by a signal.  *Note Interrupted
          Primitives::.

    `EINVAL'
          The TIMEOUT argument is invalid; one of the components is
          negative or too large.

   *Portability Note:*  The `select' function is a BSD Unix feature.

   Here is an example showing how you can use `select' to establish a
timeout period for reading from a file descriptor.  The `input_timeout'
function blocks the calling process until input is available on the
file descriptor, or until the timeout period expires.


     #include <errno.h>
     #include <stdio.h>
     #include <unistd.h>
     #include <sys/types.h>
     #include <sys/time.h>

     int
     input_timeout (int filedes, unsigned int seconds)
     {
       fd_set set;
       struct timeval timeout;

       /* Initialize the file descriptor set. */
       FD_ZERO (&set);
       FD_SET (filedes, &set);

       /* Initialize the timeout data structure. */
       timeout.tv_sec = seconds;
       timeout.tv_usec = 0;

       /* `select' returns 0 if timeout, 1 if input available, -1 if error. */
       return TEMP_FAILURE_RETRY (select (FD_SETSIZE,
                                          &set, NULL, NULL,
                                          &timeout));
     }

     int
     main (void)
     {
       fprintf (stderr, "select returned %d.\n",
                input_timeout (STDIN_FILENO, 5));
       return 0;
     }

   There is another example showing the use of `select' to multiplex
input from multiple sockets in *note Server Example::.


File: libc.info,  Node: Synchronizing I/O,  Next: Asynchronous I/O,  Prev: Waiting for I/O,  Up: Low-Level I/O

13.9 Synchronizing I/O operations
=================================

In most modern operating systems, the normal I/O operations are not
executed synchronously.  I.e., even if a `write' system call returns,
this does not mean the data is actually written to the media, e.g., the
disk.

   In situations where synchronization points are necessary, you can use
special functions which ensure that all operations finish before they
return.

 -- Function: void sync (void)
     A call to this function will not return as long as there is data
     which has not been written to the device.  All dirty buffers in
     the kernel will be written and so an overall consistent system can
     be achieved (if no other process in parallel writes data).

     A prototype for `sync' can be found in `unistd.h'.

   Programs more often want to ensure that data written to a given file
is committed, rather than all data in the system.  For this, `sync' is
overkill.

 -- Function: int fsync (int FILDES)
     The `fsync' function can be used to make sure all data associated
     with the open file FILDES is written to the device associated with
     the descriptor.  The function call does not return unless all
     actions have finished.

     A prototype for `fsync' can be found in `unistd.h'.

     This function is a cancellation point in multi-threaded programs.
     This is a problem if the thread allocates some resources (like
     memory, file descriptors, semaphores or whatever) at the time
     `fsync' is called.  If the thread gets canceled these resources
     stay allocated until the program ends.  To avoid this, calls to
     `fsync' should be protected using cancellation handlers.

     The return value of the function is zero if no error occurred.
     Otherwise it is -1 and the global variable ERRNO is set to the
     following values:
    `EBADF'
          The descriptor FILDES is not valid.

    `EINVAL'
          No synchronization is possible since the system does not
          implement this.

   Sometimes it is not even necessary to write all data associated with
a file descriptor.  E.g., in database files which do not change in size
it is enough to write all the file content data to the device.
Meta-information, like the modification time etc., are not that
important and leaving such information uncommitted does not prevent a
successful recovering of the file in case of a problem.

 -- Function: int fdatasync (int FILDES)
     When a call to the `fdatasync' function returns, it is ensured
     that all of the file data is written to the device.  For all
     pending I/O operations, the parts guaranteeing data integrity
     finished.

     Not all systems implement the `fdatasync' operation.  On systems
     missing this functionality `fdatasync' is emulated by a call to
     `fsync' since the performed actions are a superset of those
     required by `fdatasync'.

     The prototype for `fdatasync' is in `unistd.h'.

     The return value of the function is zero if no error occurred.
     Otherwise it is -1 and the global variable ERRNO is set to the
     following values:
    `EBADF'
          The descriptor FILDES is not valid.

    `EINVAL'
          No synchronization is possible since the system does not
          implement this.


File: libc.info,  Node: Asynchronous I/O,  Next: Control Operations,  Prev: Synchronizing I/O,  Up: Low-Level I/O

13.10 Perform I/O Operations in Parallel
========================================

The POSIX.1b standard defines a new set of I/O operations which can
significantly reduce the time an application spends waiting at I/O.  The
new functions allow a program to initiate one or more I/O operations and
then immediately resume normal work while the I/O operations are
executed in parallel.  This functionality is available if the
`unistd.h' file defines the symbol `_POSIX_ASYNCHRONOUS_IO'.

   These functions are part of the library with realtime functions named
`librt'.  They are not actually part of the `libc' binary.  The
implementation of these functions can be done using support in the
kernel (if available) or using an implementation based on threads at
userlevel.  In the latter case it might be necessary to link
applications with the thread library `libpthread' in addition to
`librt'.

   All AIO operations operate on files which were opened previously.
There might be arbitrarily many operations running for one file.  The
asynchronous I/O operations are controlled using a data structure named
`struct aiocb' ("AIO control block").  It is defined in `aio.h' as
follows.

 -- Data Type: struct aiocb
     The POSIX.1b standard mandates that the `struct aiocb' structure
     contains at least the members described in the following table.
     There might be more elements which are used by the implementation,
     but depending upon these elements is not portable and is highly
     deprecated.

    `int aio_fildes'
          This element specifies the file descriptor to be used for the
          operation.  It must be a legal descriptor, otherwise the
          operation will fail.

          The device on which the file is opened must allow the seek
          operation.  I.e., it is not possible to use any of the AIO
          operations on devices like terminals where an `lseek' call
          would lead to an error.

    `off_t aio_offset'
          This element specifies the offset in the file at which the
          operation (input or output) is performed.  Since the
          operations are carried out in arbitrary order and more than
          one operation for one file descriptor can be started, one
          cannot expect a current read/write position of the file
          descriptor.

    `volatile void *aio_buf'
          This is a pointer to the buffer with the data to be written
          or the place where the read data is stored.

    `size_t aio_nbytes'
          This element specifies the length of the buffer pointed to by
          `aio_buf'.

    `int aio_reqprio'
          If the platform has defined `_POSIX_PRIORITIZED_IO' and
          `_POSIX_PRIORITY_SCHEDULING', the AIO requests are processed
          based on the current scheduling priority.  The `aio_reqprio'
          element can then be used to lower the priority of the AIO
          operation.

    `struct sigevent aio_sigevent'
          This element specifies how the calling process is notified
          once the operation terminates.  If the `sigev_notify' element
          is `SIGEV_NONE', no notification is sent.  If it is
          `SIGEV_SIGNAL', the signal determined by `sigev_signo' is
          sent.  Otherwise, `sigev_notify' must be `SIGEV_THREAD'.  In
          this case, a thread is created which starts executing the
          function pointed to by `sigev_notify_function'.

    `int aio_lio_opcode'
          This element is only used by the `lio_listio' and
          `lio_listio64' functions.  Since these functions allow an
          arbitrary number of operations to start at once, and each
          operation can be input or output (or nothing), the
          information must be stored in the control block.  The
          possible values are:

         `LIO_READ'
               Start a read operation.  Read from the file at position
               `aio_offset' and store the next `aio_nbytes' bytes in the
               buffer pointed to by `aio_buf'.

         `LIO_WRITE'
               Start a write operation.  Write `aio_nbytes' bytes
               starting at `aio_buf' into the file starting at position
               `aio_offset'.

         `LIO_NOP'
               Do nothing for this control block.  This value is useful
               sometimes when an array of `struct aiocb' values
               contains holes, i.e., some of the values must not be
               handled although the whole array is presented to the
               `lio_listio' function.

     When the sources are compiled using `_FILE_OFFSET_BITS == 64' on a
     32 bit machine, this type is in fact `struct aiocb64', since the
     LFS interface transparently replaces the `struct aiocb' definition.

   For use with the AIO functions defined in the LFS, there is a
similar type defined which replaces the types of the appropriate
members with larger types but otherwise is equivalent to `struct
aiocb'.  Particularly, all member names are the same.

 -- Data Type: struct aiocb64
    `int aio_fildes'
          This element specifies the file descriptor which is used for
          the operation.  It must be a legal descriptor since otherwise
          the operation fails for obvious reasons.

          The device on which the file is opened must allow the seek
          operation.  I.e., it is not possible to use any of the AIO
          operations on devices like terminals where an `lseek' call
          would lead to an error.

    `off64_t aio_offset'
          This element specifies at which offset in the file the
          operation (input or output) is performed.  Since the
          operation are carried in arbitrary order and more than one
          operation for one file descriptor can be started, one cannot
          expect a current read/write position of the file descriptor.

    `volatile void *aio_buf'
          This is a pointer to the buffer with the data to be written
          or the place where the read data is stored.

    `size_t aio_nbytes'
          This element specifies the length of the buffer pointed to by
          `aio_buf'.

    `int aio_reqprio'
          If for the platform `_POSIX_PRIORITIZED_IO' and
          `_POSIX_PRIORITY_SCHEDULING' are defined the AIO requests are
          processed based on the current scheduling priority.  The
          `aio_reqprio' element can then be used to lower the priority
          of the AIO operation.

    `struct sigevent aio_sigevent'
          This element specifies how the calling process is notified
          once the operation terminates.  If the `sigev_notify',
          element is `SIGEV_NONE' no notification is sent.  If it is
          `SIGEV_SIGNAL', the signal determined by `sigev_signo' is
          sent.  Otherwise, `sigev_notify' must be `SIGEV_THREAD' in
          which case a thread which starts executing the function
          pointed to by `sigev_notify_function'.

    `int aio_lio_opcode'
          This element is only used by the `lio_listio' and
          `[lio_listio64' functions.  Since these functions allow an
          arbitrary number of operations to start at once, and since
          each operation can be input or output (or nothing), the
          information must be stored in the control block.  See the
          description of `struct aiocb' for a description of the
          possible values.

     When the sources are compiled using `_FILE_OFFSET_BITS == 64' on a
     32 bit machine, this type is available under the name `struct
     aiocb64', since the LFS transparently replaces the old interface.

* Menu:

* Asynchronous Reads/Writes::    Asynchronous Read and Write Operations.
* Status of AIO Operations::     Getting the Status of AIO Operations.
* Synchronizing AIO Operations:: Getting into a consistent state.
* Cancel AIO Operations::        Cancellation of AIO Operations.
* Configuration of AIO::         How to optimize the AIO implementation.


File: libc.info,  Node: Asynchronous Reads/Writes,  Next: Status of AIO Operations,  Up: Asynchronous I/O

13.10.1 Asynchronous Read and Write Operations
----------------------------------------------

 -- Function: int aio_read (struct aiocb *AIOCBP)
     This function initiates an asynchronous read operation.  It
     immediately returns after the operation was enqueued or when an
     error was encountered.

     The first `aiocbp->aio_nbytes' bytes of the file for which
     `aiocbp->aio_fildes' is a descriptor are written to the buffer
     starting at `aiocbp->aio_buf'.  Reading starts at the absolute
     position `aiocbp->aio_offset' in the file.

     If prioritized I/O is supported by the platform the
     `aiocbp->aio_reqprio' value is used to adjust the priority before
     the request is actually enqueued.

     The calling process is notified about the termination of the read
     request according to the `aiocbp->aio_sigevent' value.

     When `aio_read' returns, the return value is zero if no error
     occurred that can be found before the process is enqueued.  If
     such an early error is found, the function returns -1 and sets
     `errno' to one of the following values:

    `EAGAIN'
          The request was not enqueued due to (temporarily) exceeded
          resource limitations.

    `ENOSYS'
          The `aio_read' function is not implemented.

    `EBADF'
          The `aiocbp->aio_fildes' descriptor is not valid.  This
          condition need not be recognized before enqueueing the
          request and so this error might also be signaled
          asynchronously.

    `EINVAL'
          The `aiocbp->aio_offset' or `aiocbp->aio_reqpiro' value is
          invalid.  This condition need not be recognized before
          enqueueing the request and so this error might also be
          signaled asynchronously.

     If `aio_read' returns zero, the current status of the request can
     be queried using `aio_error' and `aio_return' functions.  As long
     as the value returned by `aio_error' is `EINPROGRESS' the
     operation has not yet completed.  If `aio_error' returns zero, the
     operation successfully terminated, otherwise the value is to be
     interpreted as an error code.  If the function terminated, the
     result of the operation can be obtained using a call to
     `aio_return'.  The returned value is the same as an equivalent
     call to `read' would have returned.  Possible error codes returned
     by `aio_error' are:

    `EBADF'
          The `aiocbp->aio_fildes' descriptor is not valid.

    `ECANCELED'
          The operation was canceled before the operation was finished
          (*note Cancel AIO Operations::)

    `EINVAL'
          The `aiocbp->aio_offset' value is invalid.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is in fact `aio_read64' since the LFS interface
     transparently replaces the normal implementation.

 -- Function: int aio_read64 (struct aiocb64 *AIOCBP)
     This function is similar to the `aio_read' function.  The only
     difference is that on 32 bit machines, the file descriptor should
     be opened in the large file mode.  Internally, `aio_read64' uses
     functionality equivalent to `lseek64' (*note File Position
     Primitive::) to position the file descriptor correctly for the
     reading, as opposed to `lseek' functionality used in `aio_read'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64', this
     function is available under the name `aio_read' and so
     transparently replaces the interface for small files on 32 bit
     machines.

   To write data asynchronously to a file, there exists an equivalent
pair of functions with a very similar interface.

 -- Function: int aio_write (struct aiocb *AIOCBP)
     This function initiates an asynchronous write operation.  The
     function call immediately returns after the operation was enqueued
     or if before this happens an error was encountered.

     The first `aiocbp->aio_nbytes' bytes from the buffer starting at
     `aiocbp->aio_buf' are written to the file for which
     `aiocbp->aio_fildes' is an descriptor, starting at the absolute
     position `aiocbp->aio_offset' in the file.

     If prioritized I/O is supported by the platform, the
     `aiocbp->aio_reqprio' value is used to adjust the priority before
     the request is actually enqueued.

     The calling process is notified about the termination of the read
     request according to the `aiocbp->aio_sigevent' value.

     When `aio_write' returns, the return value is zero if no error
     occurred that can be found before the process is enqueued.  If
     such an early error is found the function returns -1 and sets
     `errno' to one of the following values.

    `EAGAIN'
          The request was not enqueued due to (temporarily) exceeded
          resource limitations.

    `ENOSYS'
          The `aio_write' function is not implemented.

    `EBADF'
          The `aiocbp->aio_fildes' descriptor is not valid.  This
          condition may not be recognized before enqueueing the
          request, and so this error might also be signaled
          asynchronously.

    `EINVAL'
          The `aiocbp->aio_offset' or `aiocbp->aio_reqprio' value is
          invalid.  This condition may not be recognized before
          enqueueing the request and so this error might also be
          signaled asynchronously.

     In the case `aio_write' returns zero, the current status of the
     request can be queried using `aio_error' and `aio_return'
     functions.  As long as the value returned by `aio_error' is
     `EINPROGRESS' the operation has not yet completed.  If `aio_error'
     returns zero, the operation successfully terminated, otherwise the
     value is to be interpreted as an error code.  If the function
     terminated, the result of the operation can be get using a call to
     `aio_return'.  The returned value is the same as an equivalent
     call to `read' would have returned.  Possible error codes returned
     by `aio_error' are:

    `EBADF'
          The `aiocbp->aio_fildes' descriptor is not valid.

    `ECANCELED'
          The operation was canceled before the operation was finished.
          (*note Cancel AIO Operations::)

    `EINVAL'
          The `aiocbp->aio_offset' value is invalid.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64', this
     function is in fact `aio_write64' since the LFS interface
     transparently replaces the normal implementation.

 -- Function: int aio_write64 (struct aiocb64 *AIOCBP)
     This function is similar to the `aio_write' function.  The only
     difference is that on 32 bit machines the file descriptor should
     be opened in the large file mode.  Internally `aio_write64' uses
     functionality equivalent to `lseek64' (*note File Position
     Primitive::) to position the file descriptor correctly for the
     writing, as opposed to `lseek' functionality used in `aio_write'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64', this
     function is available under the name `aio_write' and so
     transparently replaces the interface for small files on 32 bit
     machines.

   Besides these functions with the more or less traditional interface,
POSIX.1b also defines a function which can initiate more than one
operation at a time, and which can handle freely mixed read and write
operations.  It is therefore similar to a combination of `readv' and
`writev'.

 -- Function: int lio_listio (int MODE, struct aiocb *const LIST[], int
          NENT, struct sigevent *SIG)
     The `lio_listio' function can be used to enqueue an arbitrary
     number of read and write requests at one time.  The requests can
     all be meant for the same file, all for different files or every
     solution in between.

     `lio_listio' gets the NENT requests from the array pointed to by
     LIST.  The operation to be performed is determined by the
     `aio_lio_opcode' member in each element of LIST.  If this field is
     `LIO_READ' a read operation is enqueued, similar to a call of
     `aio_read' for this element of the array (except that the way the
     termination is signalled is different, as we will see below).  If
     the `aio_lio_opcode' member is `LIO_WRITE' a write operation is
     enqueued.  Otherwise the `aio_lio_opcode' must be `LIO_NOP' in
     which case this element of LIST is simply ignored.  This
     "operation" is useful in situations where one has a fixed array of
     `struct aiocb' elements from which only a few need to be handled at
     a time.  Another situation is where the `lio_listio' call was
     canceled before all requests are processed (*note Cancel AIO
     Operations::) and the remaining requests have to be reissued.

     The other members of each element of the array pointed to by
     `list' must have values suitable for the operation as described in
     the documentation for `aio_read' and `aio_write' above.

     The MODE argument determines how `lio_listio' behaves after having
     enqueued all the requests.  If MODE is `LIO_WAIT' it waits until
     all requests terminated.  Otherwise MODE must be `LIO_NOWAIT' and
     in this case the function returns immediately after having
     enqueued all the requests.  In this case the caller gets a
     notification of the termination of all requests according to the
     SIG parameter.  If SIG is `NULL' no notification is send.
     Otherwise a signal is sent or a thread is started, just as
     described in the description for `aio_read' or `aio_write'.

     If MODE is `LIO_WAIT', the return value of `lio_listio' is 0 when
     all requests completed successfully.  Otherwise the function
     return -1 and `errno' is set accordingly.  To find out which
     request or requests failed one has to use the `aio_error' function
     on all the elements of the array LIST.

     In case MODE is `LIO_NOWAIT', the function returns 0 if all
     requests were enqueued correctly.  The current state of the
     requests can be found using `aio_error' and `aio_return' as
     described above.  If `lio_listio' returns -1 in this mode, the
     global variable `errno' is set accordingly.  If a request did not
     yet terminate, a call to `aio_error' returns `EINPROGRESS'.  If
     the value is different, the request is finished and the error
     value (or 0) is returned and the result of the operation can be
     retrieved using `aio_return'.

     Possible values for `errno' are:

    `EAGAIN'
          The resources necessary to queue all the requests are not
          available at the moment.  The error status for each element
          of LIST must be checked to determine which request failed.

          Another reason could be that the system wide limit of AIO
          requests is exceeded.  This cannot be the case for the
          implementation on GNU systems since no arbitrary limits exist.

    `EINVAL'
          The MODE parameter is invalid or NENT is larger than
          `AIO_LISTIO_MAX'.

    `EIO'
          One or more of the request's I/O operations failed.  The
          error status of each request should be checked to determine
          which one failed.

    `ENOSYS'
          The `lio_listio' function is not supported.

     If the MODE parameter is `LIO_NOWAIT' and the caller cancels a
     request, the error status for this request returned by `aio_error'
     is `ECANCELED'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64', this
     function is in fact `lio_listio64' since the LFS interface
     transparently replaces the normal implementation.

 -- Function: int lio_listio64 (int MODE, struct aiocb64 *const LIST[],
          int NENT, struct sigevent *SIG)
     This function is similar to the `lio_listio' function.  The only
     difference is that on 32 bit machines, the file descriptor should
     be opened in the large file mode.  Internally, `lio_listio64' uses
     functionality equivalent to `lseek64' (*note File Position
     Primitive::) to position the file descriptor correctly for the
     reading or writing, as opposed to `lseek' functionality used in
     `lio_listio'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64', this
     function is available under the name `lio_listio' and so
     transparently replaces the interface for small files on 32 bit
     machines.


File: libc.info,  Node: Status of AIO Operations,  Next: Synchronizing AIO Operations,  Prev: Asynchronous Reads/Writes,  Up: Asynchronous I/O

13.10.2 Getting the Status of AIO Operations
--------------------------------------------

As already described in the documentation of the functions in the last
section, it must be possible to get information about the status of an
I/O request.  When the operation is performed truly asynchronously (as
with `aio_read' and `aio_write' and with `lio_listio' when the mode is
`LIO_NOWAIT'), one sometimes needs to know whether a specific request
already terminated and if so, what the result was.  The following two
functions allow you to get this kind of information.

 -- Function: int aio_error (const struct aiocb *AIOCBP)
     This function determines the error state of the request described
     by the `struct aiocb' variable pointed to by AIOCBP.  If the
     request has not yet terminated the value returned is always
     `EINPROGRESS'.  Once the request has terminated the value
     `aio_error' returns is either 0 if the request completed
     successfully or it returns the value which would be stored in the
     `errno' variable if the request would have been done using `read',
     `write', or `fsync'.

     The function can return `ENOSYS' if it is not implemented.  It
     could also return `EINVAL' if the AIOCBP parameter does not refer
     to an asynchronous operation whose return status is not yet known.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is in fact `aio_error64' since the LFS interface
     transparently replaces the normal implementation.

 -- Function: int aio_error64 (const struct aiocb64 *AIOCBP)
     This function is similar to `aio_error' with the only difference
     that the argument is a reference to a variable of type `struct
     aiocb64'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is available under the name `aio_error' and so
     transparently replaces the interface for small files on 32 bit
     machines.

 -- Function: ssize_t aio_return (struct aiocb *AIOCBP)
     This function can be used to retrieve the return status of the
     operation carried out by the request described in the variable
     pointed to by AIOCBP.  As long as the error status of this request
     as returned by `aio_error' is `EINPROGRESS' the return of this
     function is undefined.

     Once the request is finished this function can be used exactly
     once to retrieve the return value.  Following calls might lead to
     undefined behavior.  The return value itself is the value which
     would have been returned by the `read', `write', or `fsync' call.

     The function can return `ENOSYS' if it is not implemented.  It
     could also return `EINVAL' if the AIOCBP parameter does not refer
     to an asynchronous operation whose return status is not yet known.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is in fact `aio_return64' since the LFS interface
     transparently replaces the normal implementation.

 -- Function: ssize_t aio_return64 (struct aiocb64 *AIOCBP)
     This function is similar to `aio_return' with the only difference
     that the argument is a reference to a variable of type `struct
     aiocb64'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is available under the name `aio_return' and so
     transparently replaces the interface for small files on 32 bit
     machines.


File: libc.info,  Node: Synchronizing AIO Operations,  Next: Cancel AIO Operations,  Prev: Status of AIO Operations,  Up: Asynchronous I/O

13.10.3 Getting into a Consistent State
---------------------------------------

When dealing with asynchronous operations it is sometimes necessary to
get into a consistent state.  This would mean for AIO that one wants to
know whether a certain request or a group of request were processed.
This could be done by waiting for the notification sent by the system
after the operation terminated, but this sometimes would mean wasting
resources (mainly computation time).  Instead POSIX.1b defines two
functions which will help with most kinds of consistency.

   The `aio_fsync' and `aio_fsync64' functions are only available if
the symbol `_POSIX_SYNCHRONIZED_IO' is defined in `unistd.h'.

 -- Function: int aio_fsync (int OP, struct aiocb *AIOCBP)
     Calling this function forces all I/O operations operating queued
     at the time of the function call operating on the file descriptor
     `aiocbp->aio_fildes' into the synchronized I/O completion state
     (*note Synchronizing I/O::).  The `aio_fsync' function returns
     immediately but the notification through the method described in
     `aiocbp->aio_sigevent' will happen only after all requests for this
     file descriptor have terminated and the file is synchronized.
     This also means that requests for this very same file descriptor
     which are queued after the synchronization request are not
     affected.

     If OP is `O_DSYNC' the synchronization happens as with a call to
     `fdatasync'.  Otherwise OP should be `O_SYNC' and the
     synchronization happens as with `fsync'.

     As long as the synchronization has not happened, a call to
     `aio_error' with the reference to the object pointed to by AIOCBP
     returns `EINPROGRESS'.  Once the synchronization is done
     `aio_error' return 0 if the synchronization was not successful.
     Otherwise the value returned is the value to which the `fsync' or
     `fdatasync' function would have set the `errno' variable.  In this
     case nothing can be assumed about the consistency for the data
     written to this file descriptor.

     The return value of this function is 0 if the request was
     successfully enqueued.  Otherwise the return value is -1 and
     `errno' is set to one of the following values:

    `EAGAIN'
          The request could not be enqueued due to temporary lack of
          resources.

    `EBADF'
          The file descriptor `AIOCBP->aio_fildes' is not valid.

    `EINVAL'
          The implementation does not support I/O synchronization or
          the OP parameter is other than `O_DSYNC' and `O_SYNC'.

    `ENOSYS'
          This function is not implemented.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is in fact `aio_fsync64' since the LFS interface
     transparently replaces the normal implementation.

 -- Function: int aio_fsync64 (int OP, struct aiocb64 *AIOCBP)
     This function is similar to `aio_fsync' with the only difference
     that the argument is a reference to a variable of type `struct
     aiocb64'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is available under the name `aio_fsync' and so
     transparently replaces the interface for small files on 32 bit
     machines.

   Another method of synchronization is to wait until one or more
requests of a specific set terminated.  This could be achieved by the
`aio_*' functions to notify the initiating process about the
termination but in some situations this is not the ideal solution.  In
a program which constantly updates clients somehow connected to the
server it is not always the best solution to go round robin since some
connections might be slow.  On the other hand letting the `aio_*'
function notify the caller might also be not the best solution since
whenever the process works on preparing data for on client it makes no
sense to be interrupted by a notification since the new client will not
be handled before the current client is served.  For situations like
this `aio_suspend' should be used.

 -- Function: int aio_suspend (const struct aiocb *const LIST[], int
          NENT, const struct timespec *TIMEOUT)
     When calling this function, the calling thread is suspended until
     at least one of the requests pointed to by the NENT elements of the
     array LIST has completed.  If any of the requests has already
     completed at the time `aio_suspend' is called, the function returns
     immediately.  Whether a request has terminated or not is
     determined by comparing the error status of the request with
     `EINPROGRESS'.  If an element of LIST is `NULL', the entry is
     simply ignored.

     If no request has finished, the calling process is suspended.  If
     TIMEOUT is `NULL', the process is not woken until a request has
     finished.  If TIMEOUT is not `NULL', the process remains suspended
     at least as long as specified in TIMEOUT.  In this case,
     `aio_suspend' returns with an error.

     The return value of the function is 0 if one or more requests from
     the LIST have terminated.  Otherwise the function returns -1 and
     `errno' is set to one of the following values:

    `EAGAIN'
          None of the requests from the LIST completed in the time
          specified by TIMEOUT.

    `EINTR'
          A signal interrupted the `aio_suspend' function.  This signal
          might also be sent by the AIO implementation while signalling
          the termination of one of the requests.

    `ENOSYS'
          The `aio_suspend' function is not implemented.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is in fact `aio_suspend64' since the LFS interface
     transparently replaces the normal implementation.

 -- Function: int aio_suspend64 (const struct aiocb64 *const LIST[],
          int NENT, const struct timespec *TIMEOUT)
     This function is similar to `aio_suspend' with the only difference
     that the argument is a reference to a variable of type `struct
     aiocb64'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is available under the name `aio_suspend' and so
     transparently replaces the interface for small files on 32 bit
     machines.


File: libc.info,  Node: Cancel AIO Operations,  Next: Configuration of AIO,  Prev: Synchronizing AIO Operations,  Up: Asynchronous I/O

13.10.4 Cancellation of AIO Operations
--------------------------------------

When one or more requests are asynchronously processed, it might be
useful in some situations to cancel a selected operation, e.g., if it
becomes obvious that the written data is no longer accurate and would
have to be overwritten soon.  As an example, assume an application,
which writes data in files in a situation where new incoming data would
have to be written in a file which will be updated by an enqueued
request.  The POSIX AIO implementation provides such a function, but
this function is not capable of forcing the cancellation of the
request.  It is up to the implementation to decide whether it is
possible to cancel the operation or not.  Therefore using this function
is merely a hint.

 -- Function: int aio_cancel (int FILDES, struct aiocb *AIOCBP)
     The `aio_cancel' function can be used to cancel one or more
     outstanding requests.  If the AIOCBP parameter is `NULL', the
     function tries to cancel all of the outstanding requests which
     would process the file descriptor FILDES (i.e., whose `aio_fildes'
     member is FILDES).  If AIOCBP is not `NULL', `aio_cancel' attempts
     to cancel the specific request pointed to by AIOCBP.

     For requests which were successfully canceled, the normal
     notification about the termination of the request should take
     place.  I.e., depending on the `struct sigevent' object which
     controls this, nothing happens, a signal is sent or a thread is
     started.  If the request cannot be canceled, it terminates the
     usual way after performing the operation.

     After a request is successfully canceled, a call to `aio_error'
     with a reference to this request as the parameter will return
     `ECANCELED' and a call to `aio_return' will return -1.  If the
     request wasn't canceled and is still running the error status is
     still `EINPROGRESS'.

     The return value of the function is `AIO_CANCELED' if there were
     requests which haven't terminated and which were successfully
     canceled.  If there is one or more requests left which couldn't be
     canceled, the return value is `AIO_NOTCANCELED'.  In this case
     `aio_error' must be used to find out which of the, perhaps
     multiple, requests (in AIOCBP is `NULL') weren't successfully
     canceled.  If all requests already terminated at the time
     `aio_cancel' is called the return value is `AIO_ALLDONE'.

     If an error occurred during the execution of `aio_cancel' the
     function returns -1 and sets `errno' to one of the following
     values.

    `EBADF'
          The file descriptor FILDES is not valid.

    `ENOSYS'
          `aio_cancel' is not implemented.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64', this
     function is in fact `aio_cancel64' since the LFS interface
     transparently replaces the normal implementation.

 -- Function: int aio_cancel64 (int FILDES, struct aiocb64 *AIOCBP)
     This function is similar to `aio_cancel' with the only difference
     that the argument is a reference to a variable of type `struct
     aiocb64'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64', this
     function is available under the name `aio_cancel' and so
     transparently replaces the interface for small files on 32 bit
     machines.


File: libc.info,  Node: Configuration of AIO,  Prev: Cancel AIO Operations,  Up: Asynchronous I/O

13.10.5 How to optimize the AIO implementation
----------------------------------------------

The POSIX standard does not specify how the AIO functions are
implemented.  They could be system calls, but it is also possible to
emulate them at userlevel.

   At the point of this writing, the available implementation is a
userlevel implementation which uses threads for handling the enqueued
requests.  While this implementation requires making some decisions
about limitations, hard limitations are something which is best avoided
in the GNU C Library.  Therefore, the GNU C Library provides a means
for tuning the AIO implementation according to the individual use.

 -- Data Type: struct aioinit
     This data type is used to pass the configuration or tunable
     parameters to the implementation.  The program has to initialize
     the members of this struct and pass it to the implementation using
     the `aio_init' function.

    `int aio_threads'
          This member specifies the maximal number of threads which may
          be used at any one time.

    `int aio_num'
          This number provides an estimate on the maximal number of
          simultaneously enqueued requests.

    `int aio_locks'
          Unused.

    `int aio_usedba'
          Unused.

    `int aio_debug'
          Unused.

    `int aio_numusers'
          Unused.

    `int aio_reserved[2]'
          Unused.

 -- Function: void aio_init (const struct aioinit *INIT)
     This function must be called before any other AIO function.
     Calling it is completely voluntary, as it is only meant to help
     the AIO implementation perform better.

     Before calling the `aio_init', function the members of a variable
     of type `struct aioinit' must be initialized.  Then a reference to
     this variable is passed as the parameter to `aio_init' which itself
     may or may not pay attention to the hints.

     The function has no return value and no error cases are defined.
     It is a extension which follows a proposal from the SGI
     implementation in Irix 6.  It is not covered by POSIX.1b or Unix98.


File: libc.info,  Node: Control Operations,  Next: Duplicating Descriptors,  Prev: Asynchronous I/O,  Up: Low-Level I/O

13.11 Control Operations on Files
=================================

This section describes how you can perform various other operations on
file descriptors, such as inquiring about or setting flags describing
the status of the file descriptor, manipulating record locks, and the
like.  All of these operations are performed by the function `fcntl'.

   The second argument to the `fcntl' function is a command that
specifies which operation to perform.  The function and macros that name
various flags that are used with it are declared in the header file
`fcntl.h'.  Many of these flags are also used by the `open' function;
see *note Opening and Closing Files::.  

 -- Function: int fcntl (int FILEDES, int COMMAND, ...)
     The `fcntl' function performs the operation specified by COMMAND
     on the file descriptor FILEDES.  Some commands require additional
     arguments to be supplied.  These additional arguments and the
     return value and error conditions are given in the detailed
     descriptions of the individual commands.

     Briefly, here is a list of what the various commands are.

    `F_DUPFD'
          Duplicate the file descriptor (return another file descriptor
          pointing to the same open file).  *Note Duplicating
          Descriptors::.

    `F_GETFD'
          Get flags associated with the file descriptor.  *Note
          Descriptor Flags::.

    `F_SETFD'
          Set flags associated with the file descriptor.  *Note
          Descriptor Flags::.

    `F_GETFL'
          Get flags associated with the open file.  *Note File Status
          Flags::.

    `F_SETFL'
          Set flags associated with the open file.  *Note File Status
          Flags::.

    `F_GETLK'
          Get a file lock.  *Note File Locks::.

    `F_SETLK'
          Set or clear a file lock.  *Note File Locks::.

    `F_SETLKW'
          Like `F_SETLK', but wait for completion.  *Note File Locks::.

    `F_GETOWN'
          Get process or process group ID to receive `SIGIO' signals.
          *Note Interrupt Input::.

    `F_SETOWN'
          Set process or process group ID to receive `SIGIO' signals.
          *Note Interrupt Input::.

     This function is a cancellation point in multi-threaded programs.
     This is a problem if the thread allocates some resources (like
     memory, file descriptors, semaphores or whatever) at the time
     `fcntl' is called.  If the thread gets canceled these resources
     stay allocated until the program ends.  To avoid this calls to
     `fcntl' should be protected using cancellation handlers.


File: libc.info,  Node: Duplicating Descriptors,  Next: Descriptor Flags,  Prev: Control Operations,  Up: Low-Level I/O

13.12 Duplicating Descriptors
=============================

You can "duplicate" a file descriptor, or allocate another file
descriptor that refers to the same open file as the original.  Duplicate
descriptors share one file position and one set of file status flags
(*note File Status Flags::), but each has its own set of file descriptor
flags (*note Descriptor Flags::).

   The major use of duplicating a file descriptor is to implement
"redirection" of input or output:  that is, to change the file or pipe
that a particular file descriptor corresponds to.

   You can perform this operation using the `fcntl' function with the
`F_DUPFD' command, but there are also convenient functions `dup' and
`dup2' for duplicating descriptors.

   The `fcntl' function and flags are declared in `fcntl.h', while
prototypes for `dup' and `dup2' are in the header file `unistd.h'.

 -- Function: int dup (int OLD)
     This function copies descriptor OLD to the first available
     descriptor number (the first number not currently open).  It is
     equivalent to `fcntl (OLD, F_DUPFD, 0)'.

 -- Function: int dup2 (int OLD, int NEW)
     This function copies the descriptor OLD to descriptor number NEW.

     If OLD is an invalid descriptor, then `dup2' does nothing; it does
     not close NEW.  Otherwise, the new duplicate of OLD replaces any
     previous meaning of descriptor NEW, as if NEW were closed first.

     If OLD and NEW are different numbers, and OLD is a valid
     descriptor number, then `dup2' is equivalent to:

          close (NEW);
          fcntl (OLD, F_DUPFD, NEW)

     However, `dup2' does this atomically; there is no instant in the
     middle of calling `dup2' at which NEW is closed and not yet a
     duplicate of OLD.

 -- Macro: int F_DUPFD
     This macro is used as the COMMAND argument to `fcntl', to copy the
     file descriptor given as the first argument.

     The form of the call in this case is:

          fcntl (OLD, F_DUPFD, NEXT-FILEDES)

     The NEXT-FILEDES argument is of type `int' and specifies that the
     file descriptor returned should be the next available one greater
     than or equal to this value.

     The return value from `fcntl' with this command is normally the
     value of the new file descriptor.  A return value of -1 indicates
     an error.  The following `errno' error conditions are defined for
     this command:

    `EBADF'
          The OLD argument is invalid.

    `EINVAL'
          The NEXT-FILEDES argument is invalid.

    `EMFILE'
          There are no more file descriptors available--your program is
          already using the maximum.  In BSD and GNU, the maximum is
          controlled by a resource limit that can be changed; *note
          Limits on Resources::, for more information about the
          `RLIMIT_NOFILE' limit.

     `ENFILE' is not a possible error code for `dup2' because `dup2'
     does not create a new opening of a file; duplicate descriptors do
     not count toward the limit which `ENFILE' indicates.  `EMFILE' is
     possible because it refers to the limit on distinct descriptor
     numbers in use in one process.

   Here is an example showing how to use `dup2' to do redirection.
Typically, redirection of the standard streams (like `stdin') is done
by a shell or shell-like program before calling one of the `exec'
functions (*note Executing a File::) to execute a new program in a
child process.  When the new program is executed, it creates and
initializes the standard streams to point to the corresponding file
descriptors, before its `main' function is invoked.

   So, to redirect standard input to a file, the shell could do
something like:

     pid = fork ();
     if (pid == 0)
       {
         char *filename;
         char *program;
         int file;
         ...
         file = TEMP_FAILURE_RETRY (open (filename, O_RDONLY));
         dup2 (file, STDIN_FILENO);
         TEMP_FAILURE_RETRY (close (file));
         execv (program, NULL);
       }

   There is also a more detailed example showing how to implement
redirection in the context of a pipeline of processes in *note
Launching Jobs::.


File: libc.info,  Node: Descriptor Flags,  Next: File Status Flags,  Prev: Duplicating Descriptors,  Up: Low-Level I/O

13.13 File Descriptor Flags
===========================

"File descriptor flags" are miscellaneous attributes of a file
descriptor.  These flags are associated with particular file
descriptors, so that if you have created duplicate file descriptors
from a single opening of a file, each descriptor has its own set of
flags.

   Currently there is just one file descriptor flag: `FD_CLOEXEC',
which causes the descriptor to be closed if you use any of the
`exec...' functions (*note Executing a File::).

   The symbols in this section are defined in the header file `fcntl.h'.  

 -- Macro: int F_GETFD
     This macro is used as the COMMAND argument to `fcntl', to specify
     that it should return the file descriptor flags associated with
     the FILEDES argument.

     The normal return value from `fcntl' with this command is a
     nonnegative number which can be interpreted as the bitwise OR of
     the individual flags (except that currently there is only one flag
     to use).

     In case of an error, `fcntl' returns -1.  The following `errno'
     error conditions are defined for this command:

    `EBADF'
          The FILEDES argument is invalid.

 -- Macro: int F_SETFD
     This macro is used as the COMMAND argument to `fcntl', to specify
     that it should set the file descriptor flags associated with the
     FILEDES argument.  This requires a third `int' argument to specify
     the new flags, so the form of the call is:

          fcntl (FILEDES, F_SETFD, NEW-FLAGS)

     The normal return value from `fcntl' with this command is an
     unspecified value other than -1, which indicates an error.  The
     flags and error conditions are the same as for the `F_GETFD'
     command.

   The following macro is defined for use as a file descriptor flag with
the `fcntl' function.  The value is an integer constant usable as a bit
mask value.

 -- Macro: int FD_CLOEXEC
     This flag specifies that the file descriptor should be closed when
     an `exec' function is invoked; see *note Executing a File::.  When
     a file descriptor is allocated (as with `open' or `dup'), this bit
     is initially cleared on the new file descriptor, meaning that
     descriptor will survive into the new program after `exec'.

   If you want to modify the file descriptor flags, you should get the
current flags with `F_GETFD' and modify the value.  Don't assume that
the flags listed here are the only ones that are implemented; your
program may be run years from now and more flags may exist then.  For
example, here is a function to set or clear the flag `FD_CLOEXEC'
without altering any other flags:

     /* Set the `FD_CLOEXEC' flag of DESC if VALUE is nonzero,
        or clear the flag if VALUE is 0.
        Return 0 on success, or -1 on error with `errno' set. */

     int
     set_cloexec_flag (int desc, int value)
     {
       int oldflags = fcntl (desc, F_GETFD, 0);
       /* If reading the flags failed, return error indication now. */
       if (oldflags < 0)
         return oldflags;
       /* Set just the flag we want to set. */
       if (value != 0)
         oldflags |= FD_CLOEXEC;
       else
         oldflags &= ~FD_CLOEXEC;
       /* Store modified flag word in the descriptor. */
       return fcntl (desc, F_SETFD, oldflags);
     }


File: libc.info,  Node: File Status Flags,  Next: File Locks,  Prev: Descriptor Flags,  Up: Low-Level I/O

13.14 File Status Flags
=======================

"File status flags" are used to specify attributes of the opening of a
file.  Unlike the file descriptor flags discussed in *note Descriptor
Flags::, the file status flags are shared by duplicated file descriptors
resulting from a single opening of the file.  The file status flags are
specified with the FLAGS argument to `open'; *note Opening and Closing
Files::.

   File status flags fall into three categories, which are described in
the following sections.

   * *note Access Modes::, specify what type of access is allowed to the
     file: reading, writing, or both.  They are set by `open' and are
     returned by `fcntl', but cannot be changed.

   * *note Open-time Flags::, control details of what `open' will do.
     These flags are not preserved after the `open' call.

   * *note Operating Modes::, affect how operations such as `read' and
     `write' are done.  They are set by `open', and can be fetched or
     changed with `fcntl'.

   The symbols in this section are defined in the header file `fcntl.h'.  

* Menu:

* Access Modes::                Whether the descriptor can read or write.
* Open-time Flags::             Details of `open'.
* Operating Modes::             Special modes to control I/O operations.
* Getting File Status Flags::   Fetching and changing these flags.


File: libc.info,  Node: Access Modes,  Next: Open-time Flags,  Up: File Status Flags

13.14.1 File Access Modes
-------------------------

The file access modes allow a file descriptor to be used for reading,
writing, or both.  (On GNU/Hurd systems, they can also allow none of
these, and allow execution of the file as a program.)  The access modes
are chosen when the file is opened, and never change.

 -- Macro: int O_RDONLY
     Open the file for read access.

 -- Macro: int O_WRONLY
     Open the file for write access.

 -- Macro: int O_RDWR
     Open the file for both reading and writing.

   On GNU/Hurd systems (and not on other systems), `O_RDONLY' and
`O_WRONLY' are independent bits that can be bitwise-ORed together, and
it is valid for either bit to be set or clear.  This means that
`O_RDWR' is the same as `O_RDONLY|O_WRONLY'.  A file access mode of
zero is permissible; it allows no operations that do input or output to
the file, but does allow other operations such as `fchmod'.  On
GNU/Hurd systems, since "read-only" or "write-only" is a misnomer,
`fcntl.h' defines additional names for the file access modes.  These
names are preferred when writing GNU-specific code.  But most programs
will want to be portable to other POSIX.1 systems and should use the
POSIX.1 names above instead.

 -- Macro: int O_READ
     Open the file for reading.  Same as `O_RDONLY'; only defined on
     GNU.

 -- Macro: int O_WRITE
     Open the file for writing.  Same as `O_WRONLY'; only defined on
     GNU.

 -- Macro: int O_EXEC
     Open the file for executing.  Only defined on GNU.

   To determine the file access mode with `fcntl', you must extract the
access mode bits from the retrieved file status flags.  On GNU/Hurd
systems, you can just test the `O_READ' and `O_WRITE' bits in the flags
word.  But in other POSIX.1 systems, reading and writing access modes
are not stored as distinct bit flags.  The portable way to extract the
file access mode bits is with `O_ACCMODE'.

 -- Macro: int O_ACCMODE
     This macro stands for a mask that can be bitwise-ANDed with the
     file status flag value to produce a value representing the file
     access mode.  The mode will be `O_RDONLY', `O_WRONLY', or `O_RDWR'.
     (On GNU/Hurd systems it could also be zero, and it never includes
     the `O_EXEC' bit.)


File: libc.info,  Node: Open-time Flags,  Next: Operating Modes,  Prev: Access Modes,  Up: File Status Flags

13.14.2 Open-time Flags
-----------------------

The open-time flags specify options affecting how `open' will behave.
These options are not preserved once the file is open.  The exception to
this is `O_NONBLOCK', which is also an I/O operating mode and so it
_is_ saved.  *Note Opening and Closing Files::, for how to call `open'.

   There are two sorts of options specified by open-time flags.

   * "File name translation flags" affect how `open' looks up the file
     name to locate the file, and whether the file can be created.  

   * "Open-time action flags" specify extra operations that `open' will
     perform on the file once it is open.  

   Here are the file name translation flags.

 -- Macro: int O_CREAT
     If set, the file will be created if it doesn't already exist.  

 -- Macro: int O_EXCL
     If both `O_CREAT' and `O_EXCL' are set, then `open' fails if the
     specified file already exists.  This is guaranteed to never
     clobber an existing file.

 -- Macro: int O_NONBLOCK
     This prevents `open' from blocking for a "long time" to open the
     file.  This is only meaningful for some kinds of files, usually
     devices such as serial ports; when it is not meaningful, it is
     harmless and ignored.  Often opening a port to a modem blocks
     until the modem reports carrier detection; if `O_NONBLOCK' is
     specified, `open' will return immediately without a carrier.

     Note that the `O_NONBLOCK' flag is overloaded as both an I/O
     operating mode and a file name translation flag.  This means that
     specifying `O_NONBLOCK' in `open' also sets nonblocking I/O mode;
     *note Operating Modes::.  To open the file without blocking but do
     normal I/O that blocks, you must call `open' with `O_NONBLOCK' set
     and then call `fcntl' to turn the bit off.

 -- Macro: int O_NOCTTY
     If the named file is a terminal device, don't make it the
     controlling terminal for the process.  *Note Job Control::, for
     information about what it means to be the controlling terminal.

     On GNU/Hurd systems and 4.4 BSD, opening a file never makes it the
     controlling terminal and `O_NOCTTY' is zero.  However, GNU/Linux
     systems and some other systems use a nonzero value for `O_NOCTTY'
     and set the controlling terminal when you open a file that is a
     terminal device; so to be portable, use `O_NOCTTY' when it is
     important to avoid this.  

   The following three file name translation flags exist only on
GNU/Hurd systems.

 -- Macro: int O_IGNORE_CTTY
     Do not recognize the named file as the controlling terminal, even
     if it refers to the process's existing controlling terminal
     device.  Operations on the new file descriptor will never induce
     job control signals.  *Note Job Control::.

 -- Macro: int O_NOLINK
     If the named file is a symbolic link, open the link itself instead
     of the file it refers to.  (`fstat' on the new file descriptor will
     return the information returned by `lstat' on the link's name.)  

 -- Macro: int O_NOTRANS
     If the named file is specially translated, do not invoke the
     translator.  Open the bare file the translator itself sees.

   The open-time action flags tell `open' to do additional operations
which are not really related to opening the file.  The reason to do them
as part of `open' instead of in separate calls is that `open' can do
them atomically.

 -- Macro: int O_TRUNC
     Truncate the file to zero length.  This option is only useful for
     regular files, not special files such as directories or FIFOs.
     POSIX.1 requires that you open the file for writing to use
     `O_TRUNC'.  In BSD and GNU you must have permission to write the
     file to truncate it, but you need not open for write access.

     This is the only open-time action flag specified by POSIX.1.
     There is no good reason for truncation to be done by `open',
     instead of by calling `ftruncate' afterwards.  The `O_TRUNC' flag
     existed in Unix before `ftruncate' was invented, and is retained
     for backward compatibility.

   The remaining operating modes are BSD extensions.  They exist only
on some systems.  On other systems, these macros are not defined.

 -- Macro: int O_SHLOCK
     Acquire a shared lock on the file, as with `flock'.  *Note File
     Locks::.

     If `O_CREAT' is specified, the locking is done atomically when
     creating the file.  You are guaranteed that no other process will
     get the lock on the new file first.

 -- Macro: int O_EXLOCK
     Acquire an exclusive lock on the file, as with `flock'.  *Note
     File Locks::.  This is atomic like `O_SHLOCK'.


File: libc.info,  Node: Operating Modes,  Next: Getting File Status Flags,  Prev: Open-time Flags,  Up: File Status Flags

13.14.3 I/O Operating Modes
---------------------------

The operating modes affect how input and output operations using a file
descriptor work.  These flags are set by `open' and can be fetched and
changed with `fcntl'.

 -- Macro: int O_APPEND
     The bit that enables append mode for the file.  If set, then all
     `write' operations write the data at the end of the file, extending
     it, regardless of the current file position.  This is the only
     reliable way to append to a file.  In append mode, you are
     guaranteed that the data you write will always go to the current
     end of the file, regardless of other processes writing to the
     file.  Conversely, if you simply set the file position to the end
     of file and write, then another process can extend the file after
     you set the file position but before you write, resulting in your
     data appearing someplace before the real end of file.

 -- Macro: int O_NONBLOCK
     The bit that enables nonblocking mode for the file.  If this bit
     is set, `read' requests on the file can return immediately with a
     failure status if there is no input immediately available, instead
     of blocking.  Likewise, `write' requests can also return
     immediately with a failure status if the output can't be written
     immediately.

     Note that the `O_NONBLOCK' flag is overloaded as both an I/O
     operating mode and a file name translation flag; *note Open-time
     Flags::.

 -- Macro: int O_NDELAY
     This is an obsolete name for `O_NONBLOCK', provided for
     compatibility with BSD.  It is not defined by the POSIX.1 standard.

   The remaining operating modes are BSD and GNU extensions.  They
exist only on some systems.  On other systems, these macros are not
defined.

 -- Macro: int O_ASYNC
     The bit that enables asynchronous input mode.  If set, then `SIGIO'
     signals will be generated when input is available.  *Note
     Interrupt Input::.

     Asynchronous input mode is a BSD feature.

 -- Macro: int O_FSYNC
     The bit that enables synchronous writing for the file.  If set,
     each `write' call will make sure the data is reliably stored on
     disk before returning.  Synchronous writing is a BSD feature.

 -- Macro: int O_SYNC
     This is another name for `O_FSYNC'.  They have the same value.

 -- Macro: int O_NOATIME
     If this bit is set, `read' will not update the access time of the
     file.  *Note File Times::.  This is used by programs that do
     backups, so that backing a file up does not count as reading it.
     Only the owner of the file or the superuser may use this bit.

     This is a GNU extension.


File: libc.info,  Node: Getting File Status Flags,  Prev: Operating Modes,  Up: File Status Flags

13.14.4 Getting and Setting File Status Flags
---------------------------------------------

The `fcntl' function can fetch or change file status flags.

 -- Macro: int F_GETFL
     This macro is used as the COMMAND argument to `fcntl', to read the
     file status flags for the open file with descriptor FILEDES.

     The normal return value from `fcntl' with this command is a
     nonnegative number which can be interpreted as the bitwise OR of
     the individual flags.  Since the file access modes are not
     single-bit values, you can mask off other bits in the returned
     flags with `O_ACCMODE' to compare them.

     In case of an error, `fcntl' returns -1.  The following `errno'
     error conditions are defined for this command:

    `EBADF'
          The FILEDES argument is invalid.

 -- Macro: int F_SETFL
     This macro is used as the COMMAND argument to `fcntl', to set the
     file status flags for the open file corresponding to the FILEDES
     argument.  This command requires a third `int' argument to specify
     the new flags, so the call looks like this:

          fcntl (FILEDES, F_SETFL, NEW-FLAGS)

     You can't change the access mode for the file in this way; that is,
     whether the file descriptor was opened for reading or writing.

     The normal return value from `fcntl' with this command is an
     unspecified value other than -1, which indicates an error.  The
     error conditions are the same as for the `F_GETFL' command.

   If you want to modify the file status flags, you should get the
current flags with `F_GETFL' and modify the value.  Don't assume that
the flags listed here are the only ones that are implemented; your
program may be run years from now and more flags may exist then.  For
example, here is a function to set or clear the flag `O_NONBLOCK'
without altering any other flags:

     /* Set the `O_NONBLOCK' flag of DESC if VALUE is nonzero,
        or clear the flag if VALUE is 0.
        Return 0 on success, or -1 on error with `errno' set. */

     int
     set_nonblock_flag (int desc, int value)
     {
       int oldflags = fcntl (desc, F_GETFL, 0);
       /* If reading the flags failed, return error indication now. */
       if (oldflags == -1)
         return -1;
       /* Set just the flag we want to set. */
       if (value != 0)
         oldflags |= O_NONBLOCK;
       else
         oldflags &= ~O_NONBLOCK;
       /* Store modified flag word in the descriptor. */
       return fcntl (desc, F_SETFL, oldflags);
     }


File: libc.info,  Node: File Locks,  Next: Interrupt Input,  Prev: File Status Flags,  Up: Low-Level I/O

13.15 File Locks
================

The remaining `fcntl' commands are used to support "record locking",
which permits multiple cooperating programs to prevent each other from
simultaneously accessing parts of a file in error-prone ways.

   An "exclusive" or "write" lock gives a process exclusive access for
writing to the specified part of the file.  While a write lock is in
place, no other process can lock that part of the file.

   A "shared" or "read" lock prohibits any other process from
requesting a write lock on the specified part of the file.  However,
other processes can request read locks.

   The `read' and `write' functions do not actually check to see
whether there are any locks in place.  If you want to implement a
locking protocol for a file shared by multiple processes, your
application must do explicit `fcntl' calls to request and clear locks
at the appropriate points.

   Locks are associated with processes.  A process can only have one
kind of lock set for each byte of a given file.  When any file
descriptor for that file is closed by the process, all of the locks
that process holds on that file are released, even if the locks were
made using other descriptors that remain open.  Likewise, locks are
released when a process exits, and are not inherited by child processes
created using `fork' (*note Creating a Process::).

   When making a lock, use a `struct flock' to specify what kind of
lock and where.  This data type and the associated macros for the
`fcntl' function are declared in the header file `fcntl.h'.  

 -- Data Type: struct flock
     This structure is used with the `fcntl' function to describe a file
     lock.  It has these members:

    `short int l_type'
          Specifies the type of the lock; one of `F_RDLCK', `F_WRLCK',
          or `F_UNLCK'.

    `short int l_whence'
          This corresponds to the WHENCE argument to `fseek' or
          `lseek', and specifies what the offset is relative to.  Its
          value can be one of `SEEK_SET', `SEEK_CUR', or `SEEK_END'.

    `off_t l_start'
          This specifies the offset of the start of the region to which
          the lock applies, and is given in bytes relative to the point
          specified by `l_whence' member.

    `off_t l_len'
          This specifies the length of the region to be locked.  A
          value of `0' is treated specially; it means the region
          extends to the end of the file.

    `pid_t l_pid'
          This field is the process ID (*note Process Creation
          Concepts::) of the process holding the lock.  It is filled in
          by calling `fcntl' with the `F_GETLK' command, but is ignored
          when making a lock.

 -- Macro: int F_GETLK
     This macro is used as the COMMAND argument to `fcntl', to specify
     that it should get information about a lock.  This command
     requires a third argument of type `struct flock *' to be passed to
     `fcntl', so that the form of the call is:

          fcntl (FILEDES, F_GETLK, LOCKP)

     If there is a lock already in place that would block the lock
     described by the LOCKP argument, information about that lock
     overwrites `*LOCKP'.  Existing locks are not reported if they are
     compatible with making a new lock as specified.  Thus, you should
     specify a lock type of `F_WRLCK' if you want to find out about both
     read and write locks, or `F_RDLCK' if you want to find out about
     write locks only.

     There might be more than one lock affecting the region specified
     by the LOCKP argument, but `fcntl' only returns information about
     one of them.  The `l_whence' member of the LOCKP structure is set
     to `SEEK_SET' and the `l_start' and `l_len' fields set to identify
     the locked region.

     If no lock applies, the only change to the LOCKP structure is to
     update the `l_type' to a value of `F_UNLCK'.

     The normal return value from `fcntl' with this command is an
     unspecified value other than -1, which is reserved to indicate an
     error.  The following `errno' error conditions are defined for
     this command:

    `EBADF'
          The FILEDES argument is invalid.

    `EINVAL'
          Either the LOCKP argument doesn't specify valid lock
          information, or the file associated with FILEDES doesn't
          support locks.

 -- Macro: int F_SETLK
     This macro is used as the COMMAND argument to `fcntl', to specify
     that it should set or clear a lock.  This command requires a third
     argument of type `struct flock *' to be passed to `fcntl', so that
     the form of the call is:

          fcntl (FILEDES, F_SETLK, LOCKP)

     If the process already has a lock on any part of the region, the
     old lock on that part is replaced with the new lock.  You can
     remove a lock by specifying a lock type of `F_UNLCK'.

     If the lock cannot be set, `fcntl' returns immediately with a value
     of -1.  This function does not block waiting for other processes
     to release locks.  If `fcntl' succeeds, it return a value other
     than -1.

     The following `errno' error conditions are defined for this
     function:

    `EAGAIN'
    `EACCES'
          The lock cannot be set because it is blocked by an existing
          lock on the file.  Some systems use `EAGAIN' in this case,
          and other systems use `EACCES'; your program should treat
          them alike, after `F_SETLK'.  (GNU/Linux and GNU/Hurd systems
          always use `EAGAIN'.)

    `EBADF'
          Either: the FILEDES argument is invalid; you requested a read
          lock but the FILEDES is not open for read access; or, you
          requested a write lock but the FILEDES is not open for write
          access.

    `EINVAL'
          Either the LOCKP argument doesn't specify valid lock
          information, or the file associated with FILEDES doesn't
          support locks.

    `ENOLCK'
          The system has run out of file lock resources; there are
          already too many file locks in place.

          Well-designed file systems never report this error, because
          they have no limitation on the number of locks.  However, you
          must still take account of the possibility of this error, as
          it could result from network access to a file system on
          another machine.

 -- Macro: int F_SETLKW
     This macro is used as the COMMAND argument to `fcntl', to specify
     that it should set or clear a lock.  It is just like the `F_SETLK'
     command, but causes the process to block (or wait) until the
     request can be specified.

     This command requires a third argument of type `struct flock *', as
     for the `F_SETLK' command.

     The `fcntl' return values and errors are the same as for the
     `F_SETLK' command, but these additional `errno' error conditions
     are defined for this command:

    `EINTR'
          The function was interrupted by a signal while it was waiting.
          *Note Interrupted Primitives::.

    `EDEADLK'
          The specified region is being locked by another process.  But
          that process is waiting to lock a region which the current
          process has locked, so waiting for the lock would result in
          deadlock.  The system does not guarantee that it will detect
          all such conditions, but it lets you know if it notices one.

   The following macros are defined for use as values for the `l_type'
member of the `flock' structure.  The values are integer constants.

`F_RDLCK'
     This macro is used to specify a read (or shared) lock.

`F_WRLCK'
     This macro is used to specify a write (or exclusive) lock.

`F_UNLCK'
     This macro is used to specify that the region is unlocked.

   As an example of a situation where file locking is useful, consider a
program that can be run simultaneously by several different users, that
logs status information to a common file.  One example of such a program
might be a game that uses a file to keep track of high scores.  Another
example might be a program that records usage or accounting information
for billing purposes.

   Having multiple copies of the program simultaneously writing to the
file could cause the contents of the file to become mixed up.  But you
can prevent this kind of problem by setting a write lock on the file
before actually writing to the file.

   If the program also needs to read the file and wants to make sure
that the contents of the file are in a consistent state, then it can
also use a read lock.  While the read lock is set, no other process can
lock that part of the file for writing.

   Remember that file locks are only a _voluntary_ protocol for
controlling access to a file.  There is still potential for access to
the file by programs that don't use the lock protocol.


File: libc.info,  Node: Interrupt Input,  Next: IOCTLs,  Prev: File Locks,  Up: Low-Level I/O

13.16 Interrupt-Driven Input
============================

If you set the `O_ASYNC' status flag on a file descriptor (*note File
Status Flags::), a `SIGIO' signal is sent whenever input or output
becomes possible on that file descriptor.  The process or process group
to receive the signal can be selected by using the `F_SETOWN' command
to the `fcntl' function.  If the file descriptor is a socket, this also
selects the recipient of `SIGURG' signals that are delivered when
out-of-band data arrives on that socket; see *note Out-of-Band Data::.
(`SIGURG' is sent in any situation where `select' would report the
socket as having an "exceptional condition".  *Note Waiting for I/O::.)

   If the file descriptor corresponds to a terminal device, then `SIGIO'
signals are sent to the foreground process group of the terminal.
*Note Job Control::.

   The symbols in this section are defined in the header file `fcntl.h'.

 -- Macro: int F_GETOWN
     This macro is used as the COMMAND argument to `fcntl', to specify
     that it should get information about the process or process group
     to which `SIGIO' signals are sent.  (For a terminal, this is
     actually the foreground process group ID, which you can get using
     `tcgetpgrp'; see *note Terminal Access Functions::.)

     The return value is interpreted as a process ID; if negative, its
     absolute value is the process group ID.

     The following `errno' error condition is defined for this command:

    `EBADF'
          The FILEDES argument is invalid.

 -- Macro: int F_SETOWN
     This macro is used as the COMMAND argument to `fcntl', to specify
     that it should set the process or process group to which `SIGIO'
     signals are sent.  This command requires a third argument of type
     `pid_t' to be passed to `fcntl', so that the form of the call is:

          fcntl (FILEDES, F_SETOWN, PID)

     The PID argument should be a process ID.  You can also pass a
     negative number whose absolute value is a process group ID.

     The return value from `fcntl' with this command is -1 in case of
     error and some other value if successful.  The following `errno'
     error conditions are defined for this command:

    `EBADF'
          The FILEDES argument is invalid.

    `ESRCH'
          There is no process or process group corresponding to PID.


File: libc.info,  Node: IOCTLs,  Prev: Interrupt Input,  Up: Low-Level I/O

13.17 Generic I/O Control operations
====================================

GNU systems can handle most input/output operations on many different
devices and objects in terms of a few file primitives - `read', `write'
and `lseek'.  However, most devices also have a few peculiar operations
which do not fit into this model. Such as:

   * Changing the character font used on a terminal.

   * Telling a magnetic tape system to rewind or fast forward.  (Since
     they cannot move in byte increments, `lseek' is inapplicable).

   * Ejecting a disk from a drive.

   * Playing an audio track from a CD-ROM drive.

   * Maintaining routing tables for a network.


   Although some such objects such as sockets and terminals (1) have
special functions of their own, it would not be practical to create
functions for all these cases.

   Instead these minor operations, known as "IOCTL"s, are assigned code
numbers and multiplexed through the `ioctl' function, defined in
`sys/ioctl.h'.  The code numbers themselves are defined in many
different headers.

 -- Function: int ioctl (int FILEDES, int COMMAND, ...)
     The `ioctl' function performs the generic I/O operation COMMAND on
     FILEDES.

     A third argument is usually present, either a single number or a
     pointer to a structure.  The meaning of this argument, the
     returned value, and any error codes depends upon the command used.
     Often -1 is returned for a failure.


   On some systems, IOCTLs used by different devices share the same
numbers.  Thus, although use of an inappropriate IOCTL _usually_ only
produces an error, you should not attempt to use device-specific IOCTLs
on an unknown device.

   Most IOCTLs are OS-specific and/or only used in special system
utilities, and are thus beyond the scope of this document.  For an
example of the use of an IOCTL, see *note Out-of-Band Data::.

   ---------- Footnotes ----------

   (1) Actually, the terminal-specific functions are implemented with
IOCTLs on many platforms.


File: libc.info,  Node: File System Interface,  Next: Pipes and FIFOs,  Prev: Low-Level I/O,  Up: Top

14 File System Interface
************************

This chapter describes the GNU C Library's functions for manipulating
files.  Unlike the input and output functions (*note I/O on Streams::;
*note Low-Level I/O::), these functions are concerned with operating on
the files themselves rather than on their contents.

   Among the facilities described in this chapter are functions for
examining or modifying directories, functions for renaming and deleting
files, and functions for examining and setting file attributes such as
access permissions and modification times.

* Menu:

* Working Directory::           This is used to resolve relative
				 file names.
* Accessing Directories::       Finding out what files a directory
				 contains.
* Working with Directory Trees:: Apply actions to all files or a selectable
                                 subset of a directory hierarchy.
* Hard Links::                  Adding alternate names to a file.
* Symbolic Links::              A file that ``points to'' a file name.
* Deleting Files::              How to delete a file, and what that means.
* Renaming Files::              Changing a file's name.
* Creating Directories::        A system call just for creating a directory.
* File Attributes::             Attributes of individual files.
* Making Special Files::        How to create special files.
* Temporary Files::             Naming and creating temporary files.


File: libc.info,  Node: Working Directory,  Next: Accessing Directories,  Up: File System Interface

14.1 Working Directory
======================

Each process has associated with it a directory, called its "current
working directory" or simply "working directory", that is used in the
resolution of relative file names (*note File Name Resolution::).

   When you log in and begin a new session, your working directory is
initially set to the home directory associated with your login account
in the system user database.  You can find any user's home directory
using the `getpwuid' or `getpwnam' functions; see *note User Database::.

   Users can change the working directory using shell commands like
`cd'.  The functions described in this section are the primitives used
by those commands and by other programs for examining and changing the
working directory.  

   Prototypes for these functions are declared in the header file
`unistd.h'.  

 -- Function: char * getcwd (char *BUFFER, size_t SIZE)
     The `getcwd' function returns an absolute file name representing
     the current working directory, storing it in the character array
     BUFFER that you provide.  The SIZE argument is how you tell the
     system the allocation size of BUFFER.

     The GNU C Library version of this function also permits you to
     specify a null pointer for the BUFFER argument.  Then `getcwd'
     allocates a buffer automatically, as with `malloc' (*note
     Unconstrained Allocation::).  If the SIZE is greater than zero,
     then the buffer is that large; otherwise, the buffer is as large
     as necessary to hold the result.

     The return value is BUFFER on success and a null pointer on
     failure.  The following `errno' error conditions are defined for
     this function:

    `EINVAL'
          The SIZE argument is zero and BUFFER is not a null pointer.

    `ERANGE'
          The SIZE argument is less than the length of the working
          directory name.  You need to allocate a bigger array and try
          again.

    `EACCES'
          Permission to read or search a component of the file name was
          denied.

   You could implement the behavior of GNU's `getcwd (NULL, 0)' using
only the standard behavior of `getcwd':

     char *
     gnu_getcwd ()
     {
       size_t size = 100;

       while (1)
         {
           char *buffer = (char *) xmalloc (size);
           if (getcwd (buffer, size) == buffer)
             return buffer;
           free (buffer);
           if (errno != ERANGE)
             return 0;
           size *= 2;
         }
     }

*Note Malloc Examples::, for information about `xmalloc', which is not
a library function but is a customary name used in most GNU software.

 -- Deprecated Function: char * getwd (char *BUFFER)
     This is similar to `getcwd', but has no way to specify the size of
     the buffer.  The GNU C Library provides `getwd' only for backwards
     compatibility with BSD.

     The BUFFER argument should be a pointer to an array at least
     `PATH_MAX' bytes long (*note Limits for Files::).  On GNU/Hurd
     systems there is no limit to the size of a file name, so this is
     not necessarily enough space to contain the directory name.  That
     is why this function is deprecated.

 -- Function: char * get_current_dir_name (void)
     This `get_current_dir_name' function is basically equivalent to
     `getcwd (NULL, 0)'.  The only difference is that the value of the
     `PWD' variable is returned if this value is correct.  This is a
     subtle difference which is visible if the path described by the
     `PWD' value is using one or more symbol links in which case the
     value returned by `getcwd' can resolve the symbol links and
     therefore yield a different result.

     This function is a GNU extension.

 -- Function: int chdir (const char *FILENAME)
     This function is used to set the process's working directory to
     FILENAME.

     The normal, successful return value from `chdir' is `0'.  A value
     of `-1' is returned to indicate an error.  The `errno' error
     conditions defined for this function are the usual file name
     syntax errors (*note File Name Errors::), plus `ENOTDIR' if the
     file FILENAME is not a directory.

 -- Function: int fchdir (int FILEDES)
     This function is used to set the process's working directory to
     directory associated with the file descriptor FILEDES.

     The normal, successful return value from `fchdir' is `0'.  A value
     of `-1' is returned to indicate an error.  The following `errno'
     error conditions are defined for this function:

    `EACCES'
          Read permission is denied for the directory named by
          `dirname'.

    `EBADF'
          The FILEDES argument is not a valid file descriptor.

    `ENOTDIR'
          The file descriptor FILEDES is not associated with a
          directory.

    `EINTR'
          The function call was interrupt by a signal.

    `EIO'
          An I/O error occurred.


File: libc.info,  Node: Accessing Directories,  Next: Working with Directory Trees,  Prev: Working Directory,  Up: File System Interface

14.2 Accessing Directories
==========================

The facilities described in this section let you read the contents of a
directory file.  This is useful if you want your program to list all the
files in a directory, perhaps as part of a menu.

   The `opendir' function opens a "directory stream" whose elements are
directory entries.  Alternatively `fdopendir' can be used which can
have advantages if the program needs to have more control over the way
the directory is opened for reading.  This allows, for instance, to
pass the `O_NOATIME' flag to `open'.

   You use the `readdir' function on the directory stream to retrieve
these entries, represented as `struct dirent' objects.  The name of the
file for each entry is stored in the `d_name' member of this structure.
There are obvious parallels here to the stream facilities for ordinary
files, described in *note I/O on Streams::.

* Menu:

* Directory Entries::           Format of one directory entry.
* Opening a Directory::         How to open a directory stream.
* Reading/Closing Directory::   How to read directory entries from the stream.
* Simple Directory Lister::     A very simple directory listing program.
* Random Access Directory::     Rereading part of the directory
                                 already read with the same stream.
* Scanning Directory Content::  Get entries for user selected subset of
                                 contents in given directory.
* Simple Directory Lister Mark II::  Revised version of the program.


File: libc.info,  Node: Directory Entries,  Next: Opening a Directory,  Up: Accessing Directories

14.2.1 Format of a Directory Entry
----------------------------------

This section describes what you find in a single directory entry, as you
might obtain it from a directory stream.  All the symbols are declared
in the header file `dirent.h'.

 -- Data Type: struct dirent
     This is a structure type used to return information about directory
     entries.  It contains the following fields:

    `char d_name[]'
          This is the null-terminated file name component.  This is the
          only field you can count on in all POSIX systems.

    `ino_t d_fileno'
          This is the file serial number.  For BSD compatibility, you
          can also refer to this member as `d_ino'.  On GNU/Linux and
          GNU/Hurd systems and most POSIX systems, for most files this
          the same as the `st_ino' member that `stat' will return for
          the file.  *Note File Attributes::.

    `unsigned char d_namlen'
          This is the length of the file name, not including the
          terminating null character.  Its type is `unsigned char'
          because that is the integer type of the appropriate size.
          This member is a BSD extension.  The symbol
          `_DIRENT_HAVE_D_NAMLEN' is defined if this member is
          available.

    `unsigned char d_type'
          This is the type of the file, possibly unknown.  The
          following constants are defined for its value:

         `DT_UNKNOWN'
               The type is unknown.  Only some filesystems have full
               support to return the type of the file, others might
               always return this value.

         `DT_REG'
               A regular file.

         `DT_DIR'
               A directory.

         `DT_FIFO'
               A named pipe, or FIFO.  *Note FIFO Special Files::.

         `DT_SOCK'
               A local-domain socket.

         `DT_CHR'
               A character device.

         `DT_BLK'
               A block device.

         `DT_LNK'
               A symbolic link.

          This member is a BSD extension.  The symbol
          `_DIRENT_HAVE_D_TYPE' is defined if this member is available.
          On systems where it is used, it corresponds to the file type
          bits in the `st_mode' member of `struct stat'.  If the value
          cannot be determine the member value is DT_UNKNOWN.  These
          two macros convert between `d_type' values and `st_mode'
          values:

           -- Function: int IFTODT (mode_t MODE)
               This returns the `d_type' value corresponding to MODE.

           -- Function: mode_t DTTOIF (int DTYPE)
               This returns the `st_mode' value corresponding to DTYPE.

     This structure may contain additional members in the future.  Their
     availability is always announced in the compilation environment by
     a macro names `_DIRENT_HAVE_D_XXX' where XXX is replaced by the
     name of the new member.  For instance, the member `d_reclen'
     available on some systems is announced through the macro
     `_DIRENT_HAVE_D_RECLEN'.

     When a file has multiple names, each name has its own directory
     entry.  The only way you can tell that the directory entries
     belong to a single file is that they have the same value for the
     `d_fileno' field.

     File attributes such as size, modification times etc., are part of
     the file itself, not of any particular directory entry.  *Note
     File Attributes::.


File: libc.info,  Node: Opening a Directory,  Next: Reading/Closing Directory,  Prev: Directory Entries,  Up: Accessing Directories

14.2.2 Opening a Directory Stream
---------------------------------

This section describes how to open a directory stream.  All the symbols
are declared in the header file `dirent.h'.

 -- Data Type: DIR
     The `DIR' data type represents a directory stream.

   You shouldn't ever allocate objects of the `struct dirent' or `DIR'
data types, since the directory access functions do that for you.
Instead, you refer to these objects using the pointers returned by the
following functions.

 -- Function: DIR * opendir (const char *DIRNAME)
     The `opendir' function opens and returns a directory stream for
     reading the directory whose file name is DIRNAME.  The stream has
     type `DIR *'.

     If unsuccessful, `opendir' returns a null pointer.  In addition to
     the usual file name errors (*note File Name Errors::), the
     following `errno' error conditions are defined for this function:

    `EACCES'
          Read permission is denied for the directory named by
          `dirname'.

    `EMFILE'
          The process has too many files open.

    `ENFILE'
          The entire system, or perhaps the file system which contains
          the directory, cannot support any additional open files at
          the moment.  (This problem cannot happen on GNU/Hurd systems.)

    `ENOMEM'
          Not enough memory available.

     The `DIR' type is typically implemented using a file descriptor,
     and the `opendir' function in terms of the `open' function.  *Note
     Low-Level I/O::.  Directory streams and the underlying file
     descriptors are closed on `exec' (*note Executing a File::).

   The directory which is opened for reading by `opendir' is identified
by the name.  In some situations this is not sufficient.  Or the way
`opendir' implicitly creates a file descriptor for the directory is not
the way a program might want it.  In these cases an alternative
interface can be used.

 -- Function: DIR * fdopendir (int FD)
     The `fdopendir' function works just like `opendir' but instead of
     taking a file name and opening a file descriptor for the directory
     the caller is required to provide a file descriptor.  This file
     descriptor is then used in subsequent uses of the returned
     directory stream object.

     The caller must make sure the file descriptor is associated with a
     directory and it allows reading.

     If the `fdopendir' call returns successfully the file descriptor
     is now under the control of the system.  It can be used in the same
     way the descriptor implicitly created by `opendir' can be used but
     the program must not close the descriptor.

     In case the function is unsuccessful it returns a null pointer and
     the file descriptor remains to be usable by the program.  The
     following `errno' error conditions are defined for this function:

    `EBADF'
          The file descriptor is not valid.

    `ENOTDIR'
          The file descriptor is not associated with a directory.

    `EINVAL'
          The descriptor does not allow reading the directory content.

    `ENOMEM'
          Not enough memory available.

   In some situations it can be desirable to get hold of the file
descriptor which is created by the `opendir' call.  For instance, to
switch the current working directory to the directory just read the
`fchdir' function could be used.  Historically the `DIR' type was
exposed and programs could access the fields.  This does not happen in
the GNU C Library.  Instead a separate function is provided to allow
access.

 -- Function: int dirfd (DIR *DIRSTREAM)
     The function `dirfd' returns the file descriptor associated with
     the directory stream DIRSTREAM.  This descriptor can be used until
     the directory is closed with `closedir'.  If the directory stream
     implementation is not using file descriptors the return value is
     `-1'.


File: libc.info,  Node: Reading/Closing Directory,  Next: Simple Directory Lister,  Prev: Opening a Directory,  Up: Accessing Directories

14.2.3 Reading and Closing a Directory Stream
---------------------------------------------

This section describes how to read directory entries from a directory
stream, and how to close the stream when you are done with it.  All the
symbols are declared in the header file `dirent.h'.

 -- Function: struct dirent * readdir (DIR *DIRSTREAM)
     This function reads the next entry from the directory.  It normally
     returns a pointer to a structure containing information about the
     file.  This structure is associated with the DIRSTREAM handle and
     can be rewritten by a subsequent call.

     *Portability Note:* On some systems `readdir' may not return
     entries for `.' and `..', even though these are always valid file
     names in any directory.  *Note File Name Resolution::.

     If there are no more entries in the directory or an error is
     detected, `readdir' returns a null pointer.  The following `errno'
     error conditions are defined for this function:

    `EBADF'
          The DIRSTREAM argument is not valid.

     To distinguish between an end-of-directory condition or an error,
     you must set `errno' to zero before calling `readdir'.  To avoid
     entering an infinite loop, you should stop reading from the
     directory after the first error.

     In POSIX.1-2008, `readdir' is not thread-safe.  In the GNU C
     Library implementation, it is safe to call `readdir' concurrently
     on different DIRSTREAMs, but multiple threads accessing the same
     DIRSTREAM result in undefined behavior.  `readdir_r' is a fully
     thread-safe alternative, but suffers from poor portability (see
     below).  It is recommended that you use `readdir', with external
     locking if multiple threads access the same DIRSTREAM.

 -- Function: int readdir_r (DIR *DIRSTREAM, struct dirent *ENTRY,
          struct dirent **RESULT)
     This function is a version of `readdir' which performs internal
     locking.  Like `readdir' it returns the next entry from the
     directory.  To prevent conflicts between simultaneously running
     threads the result is stored inside the ENTRY object.

     *Portability Note:* It is recommended to use `readdir' instead of
     `readdir_r' for the following reasons:

        * On systems which do not define `NAME_MAX', it may not be
          possible to use `readdir_r' safely because the caller does
          not specify the length of the buffer for the directory entry.

        * On some systems, `readdir_r' cannot read directory entries
          with very long names.  If such a name is encountered, the GNU
          C Library implementation of `readdir_r' returns with an error
          code of `ENAMETOOLONG' after the final directory entry has
          been read.  On other systems, `readdir_r' may return
          successfully, but the `d_name' member may not be
          NUL-terminated or may be truncated.

        * POSIX-1.2008 does not guarantee that `readdir' is thread-safe,
          even when access to the same DIRSTREAM is serialized.  But in
          current implementations (including the GNU C Library), it is
          safe to call `readdir' concurrently on different DIRSTREAMs,
          so there is no need to use `readdir_r' in most multi-threaded
          programs.  In the rare case that multiple threads need to
          read from the same DIRSTREAM, it is still better to use
          `readdir' and external synchronization.

        * It is expected that future versions of POSIX will obsolete
          `readdir_r' and mandate the level of thread safety for
          `readdir' which is provided by the GNU C Library and other
          implementations today.

     Normally `readdir_r' returns zero and sets `*RESULT' to ENTRY.  If
     there are no more entries in the directory or an error is
     detected, `readdir_r' sets `*RESULT' to a null pointer and returns
     a nonzero error code, also stored in `errno', as described for
     `readdir'.

     It is also important to look at the definition of the `struct
     dirent' type.  Simply passing a pointer to an object of this type
     for the second parameter of `readdir_r' might not be enough.  Some
     systems don't define the `d_name' element sufficiently long.  In
     this case the user has to provide additional space.  There must be
     room for at least `NAME_MAX + 1' characters in the `d_name' array.
     Code to call `readdir_r' could look like this:

            union
            {
              struct dirent d;
              char b[offsetof (struct dirent, d_name) + NAME_MAX + 1];
            } u;

            if (readdir_r (dir, &u.d, &res) == 0)
              ...

   To support large filesystems on 32-bit machines there are LFS
variants of the last two functions.

 -- Function: struct dirent64 * readdir64 (DIR *DIRSTREAM)
     The `readdir64' function is just like the `readdir' function
     except that it returns a pointer to a record of type `struct
     dirent64'.  Some of the members of this data type (notably `d_ino')
     might have a different size to allow large filesystems.

     In all other aspects this function is equivalent to `readdir'.

 -- Function: int readdir64_r (DIR *DIRSTREAM, struct dirent64 *ENTRY,
          struct dirent64 **RESULT)
     The `readdir64_r' function is equivalent to the `readdir_r'
     function except that it takes parameters of base type `struct
     dirent64' instead of `struct dirent' in the second and third
     position.  The same precautions mentioned in the documentation of
     `readdir_r' also apply here.

 -- Function: int closedir (DIR *DIRSTREAM)
     This function closes the directory stream DIRSTREAM.  It returns
     `0' on success and `-1' on failure.

     The following `errno' error conditions are defined for this
     function:

    `EBADF'
          The DIRSTREAM argument is not valid.


File: libc.info,  Node: Simple Directory Lister,  Next: Random Access Directory,  Prev: Reading/Closing Directory,  Up: Accessing Directories

14.2.4 Simple Program to List a Directory
-----------------------------------------

Here's a simple program that prints the names of the files in the
current working directory:


     #include <stdio.h>
     #include <sys/types.h>
     #include <dirent.h>

     int
     main (void)
     {
       DIR *dp;
       struct dirent *ep;

       dp = opendir ("./");
       if (dp != NULL)
         {
           while (ep = readdir (dp))
             puts (ep->d_name);
           (void) closedir (dp);
         }
       else
         perror ("Couldn't open the directory");

       return 0;
     }

   The order in which files appear in a directory tends to be fairly
random.  A more useful program would sort the entries (perhaps by
alphabetizing them) before printing them; see *note Scanning Directory
Content::, and *note Array Sort Function::.


File: libc.info,  Node: Random Access Directory,  Next: Scanning Directory Content,  Prev: Simple Directory Lister,  Up: Accessing Directories

14.2.5 Random Access in a Directory Stream
------------------------------------------

This section describes how to reread parts of a directory that you have
already read from an open directory stream.  All the symbols are
declared in the header file `dirent.h'.

 -- Function: void rewinddir (DIR *DIRSTREAM)
     The `rewinddir' function is used to reinitialize the directory
     stream DIRSTREAM, so that if you call `readdir' it returns
     information about the first entry in the directory again.  This
     function also notices if files have been added or removed to the
     directory since it was opened with `opendir'.  (Entries for these
     files might or might not be returned by `readdir' if they were
     added or removed since you last called `opendir' or `rewinddir'.)

 -- Function: long int telldir (DIR *DIRSTREAM)
     The `telldir' function returns the file position of the directory
     stream DIRSTREAM.  You can use this value with `seekdir' to
     restore the directory stream to that position.

 -- Function: void seekdir (DIR *DIRSTREAM, long int POS)
     The `seekdir' function sets the file position of the directory
     stream DIRSTREAM to POS.  The value POS must be the result of a
     previous call to `telldir' on this particular stream; closing and
     reopening the directory can invalidate values returned by
     `telldir'.


File: libc.info,  Node: Scanning Directory Content,  Next: Simple Directory Lister Mark II,  Prev: Random Access Directory,  Up: Accessing Directories

14.2.6 Scanning the Content of a Directory
------------------------------------------

A higher-level interface to the directory handling functions is the
`scandir' function.  With its help one can select a subset of the
entries in a directory, possibly sort them and get a list of names as
the result.

 -- Function: int scandir (const char *DIR, struct dirent ***NAMELIST,
          int (*SELECTOR) (const struct dirent *), int (*CMP) (const
          struct dirent **, const struct dirent **))
     The `scandir' function scans the contents of the directory selected
     by DIR.  The result in *NAMELIST is an array of pointers to
     structure of type `struct dirent' which describe all selected
     directory entries and which is allocated using `malloc'.  Instead
     of always getting all directory entries returned, the user supplied
     function SELECTOR can be used to decide which entries are in the
     result.  Only the entries for which SELECTOR returns a non-zero
     value are selected.

     Finally the entries in *NAMELIST are sorted using the
     user-supplied function CMP.  The arguments passed to the CMP
     function are of type `struct dirent **', therefore one cannot
     directly use the `strcmp' or `strcoll' functions; instead see the
     functions `alphasort' and `versionsort' below.

     The return value of the function is the number of entries placed in
     *NAMELIST.  If it is `-1' an error occurred (either the directory
     could not be opened for reading or the malloc call failed) and the
     global variable `errno' contains more information on the error.

   As described above the fourth argument to the `scandir' function
must be a pointer to a sorting function.  For the convenience of the
programmer the GNU C Library contains implementations of functions which
are very helpful for this purpose.

 -- Function: int alphasort (const void *A, const void *B)
     The `alphasort' function behaves like the `strcoll' function
     (*note String/Array Comparison::).  The difference is that the
     arguments are not string pointers but instead they are of type
     `struct dirent **'.

     The return value of `alphasort' is less than, equal to, or greater
     than zero depending on the order of the two entries A and B.

 -- Function: int versionsort (const void *A, const void *B)
     The `versionsort' function is like `alphasort' except that it uses
     the `strverscmp' function internally.

   If the filesystem supports large files we cannot use the `scandir'
anymore since the `dirent' structure might not able to contain all the
information.  The LFS provides the new type `struct dirent64'.  To use
this we need a new function.

 -- Function: int scandir64 (const char *DIR, struct dirent64
          ***NAMELIST, int (*SELECTOR) (const struct dirent64 *), int
          (*CMP) (const struct dirent64 **, const struct dirent64 **))
     The `scandir64' function works like the `scandir' function except
     that the directory entries it returns are described by elements of
     type `struct dirent64'.  The function pointed to by SELECTOR is
     again used to select the desired entries, except that SELECTOR now
     must point to a function which takes a `struct dirent64 *'
     parameter.

     Similarly the CMP function should expect its two arguments to be
     of type `struct dirent64 **'.

   As CMP is now a function of a different type, the functions
`alphasort' and `versionsort' cannot be supplied for that argument.
Instead we provide the two replacement functions below.

 -- Function: int alphasort64 (const void *A, const void *B)
     The `alphasort64' function behaves like the `strcoll' function
     (*note String/Array Comparison::).  The difference is that the
     arguments are not string pointers but instead they are of type
     `struct dirent64 **'.

     Return value of `alphasort64' is less than, equal to, or greater
     than zero depending on the order of the two entries A and B.

 -- Function: int versionsort64 (const void *A, const void *B)
     The `versionsort64' function is like `alphasort64', excepted that
     it uses the `strverscmp' function internally.

   It is important not to mix the use of `scandir' and the 64-bit
comparison functions or vice versa.  There are systems on which this
works but on others it will fail miserably.


File: libc.info,  Node: Simple Directory Lister Mark II,  Prev: Scanning Directory Content,  Up: Accessing Directories

14.2.7 Simple Program to List a Directory, Mark II
--------------------------------------------------

Here is a revised version of the directory lister found above (*note
Simple Directory Lister::).  Using the `scandir' function we can avoid
the functions which work directly with the directory contents.  After
the call the returned entries are available for direct use.


     #include <stdio.h>
     #include <dirent.h>

     static int
     one (const struct dirent *unused)
     {
       return 1;
     }

     int
     main (void)
     {
       struct dirent **eps;
       int n;

       n = scandir ("./", &eps, one, alphasort);
       if (n >= 0)
         {
           int cnt;
           for (cnt = 0; cnt < n; ++cnt)
             puts (eps[cnt]->d_name);
         }
       else
         perror ("Couldn't open the directory");

       return 0;
     }

   Note the simple selector function in this example.  Since we want to
see all directory entries we always return `1'.


File: libc.info,  Node: Working with Directory Trees,  Next: Hard Links,  Prev: Accessing Directories,  Up: File System Interface

14.3 Working with Directory Trees
=================================

The functions described so far for handling the files in a directory
have allowed you to either retrieve the information bit by bit, or to
process all the files as a group (see `scandir').  Sometimes it is
useful to process whole hierarchies of directories and their contained
files.  The X/Open specification defines two functions to do this.  The
simpler form is derived from an early definition in System V systems
and therefore this function is available on SVID-derived systems.  The
prototypes and required definitions can be found in the `ftw.h' header.

   There are four functions in this family: `ftw', `nftw' and their
64-bit counterparts `ftw64' and `nftw64'.  These functions take as one
of their arguments a pointer to a callback function of the appropriate
type.

 -- Data Type: __ftw_func_t
          int (*) (const char *, const struct stat *, int)

     The type of callback functions given to the `ftw' function.  The
     first parameter points to the file name, the second parameter to an
     object of type `struct stat' which is filled in for the file named
     in the first parameter.

     The last parameter is a flag giving more information about the
     current file.  It can have the following values:

    `FTW_F'
          The item is either a normal file or a file which does not fit
          into one of the following categories.  This could be special
          files, sockets etc.

    `FTW_D'
          The item is a directory.

    `FTW_NS'
          The `stat' call failed and so the information pointed to by
          the second paramater is invalid.

    `FTW_DNR'
          The item is a directory which cannot be read.

    `FTW_SL'
          The item is a symbolic link.  Since symbolic links are
          normally followed seeing this value in a `ftw' callback
          function means the referenced file does not exist.  The
          situation for `nftw' is different.

          This value is only available if the program is compiled with
          `_BSD_SOURCE' or `_XOPEN_EXTENDED' defined before including
          the first header.  The original SVID systems do not have
          symbolic links.

     If the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     type is in fact `__ftw64_func_t' since this mode changes `struct
     stat' to be `struct stat64'.

   For the LFS interface and for use in the function `ftw64', the
header `ftw.h' defines another function type.

 -- Data Type: __ftw64_func_t
          int (*) (const char *, const struct stat64 *, int)

     This type is used just like `__ftw_func_t' for the callback
     function, but this time is called from `ftw64'.  The second
     parameter to the function is a pointer to a variable of type
     `struct stat64' which is able to represent the larger values.

 -- Data Type: __nftw_func_t
          int (*) (const char *, const struct stat *, int, struct FTW *)

     The first three arguments are the same as for the `__ftw_func_t'
     type.  However for the third argument some additional values are
     defined to allow finer differentiation:
    `FTW_DP'
          The current item is a directory and all subdirectories have
          already been visited and reported.  This flag is returned
          instead of `FTW_D' if the `FTW_DEPTH' flag is passed to
          `nftw' (see below).

    `FTW_SLN'
          The current item is a stale symbolic link.  The file it
          points to does not exist.

     The last parameter of the callback function is a pointer to a
     structure with some extra information as described below.

     If the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     type is in fact `__nftw64_func_t' since this mode changes `struct
     stat' to be `struct stat64'.

   For the LFS interface there is also a variant of this data type
available which has to be used with the `nftw64' function.

 -- Data Type: __nftw64_func_t
          int (*) (const char *, const struct stat64 *, int, struct FTW *)

     This type is used just like `__nftw_func_t' for the callback
     function, but this time is called from `nftw64'.  The second
     parameter to the function is this time a pointer to a variable of
     type `struct stat64' which is able to represent the larger values.

 -- Data Type: struct FTW
     The information contained in this structure helps in interpreting
     the name parameter and gives some information about the current
     state of the traversal of the directory hierarchy.

    `int base'
          The value is the offset into the string passed in the first
          parameter to the callback function of the beginning of the
          file name.  The rest of the string is the path of the file.
          This information is especially important if the `FTW_CHDIR'
          flag was set in calling `nftw' since then the current
          directory is the one the current item is found in.

    `int level'
          Whilst processing, the code tracks how many directories down
          it has gone to find the current file.  This nesting level
          starts at 0 for files in the initial directory (or is zero
          for the initial file if a file was passed).

 -- Function: int ftw (const char *FILENAME, __ftw_func_t FUNC, int
          DESCRIPTORS)
     The `ftw' function calls the callback function given in the
     parameter FUNC for every item which is found in the directory
     specified by FILENAME and all directories below.  The function
     follows symbolic links if necessary but does not process an item
     twice.  If FILENAME is not a directory then it itself is the only
     object returned to the callback function.

     The file name passed to the callback function is constructed by
     taking the FILENAME parameter and appending the names of all passed
     directories and then the local file name.  So the callback
     function can use this parameter to access the file.  `ftw' also
     calls `stat' for the file and passes that information on to the
     callback function.  If this `stat' call was not successful the
     failure is indicated by setting the third argument of the callback
     function to `FTW_NS'.  Otherwise it is set according to the
     description given in the account of `__ftw_func_t' above.

     The callback function is expected to return 0 to indicate that no
     error occurred and that processing should continue.  If an error
     occurred in the callback function or it wants `ftw' to return
     immediately, the callback function can return a value other than
     0.  This is the only correct way to stop the function.  The
     program must not use `setjmp' or similar techniques to continue
     from another place.  This would leave resources allocated by the
     `ftw' function unfreed.

     The DESCRIPTORS parameter to `ftw' specifies how many file
     descriptors it is allowed to consume.  The function runs faster
     the more descriptors it can use.  For each level in the directory
     hierarchy at most one descriptor is used, but for very deep ones
     any limit on open file descriptors for the process or the system
     may be exceeded.  Moreover, file descriptor limits in a
     multi-threaded program apply to all the threads as a group, and
     therefore it is a good idea to supply a reasonable limit to the
     number of open descriptors.

     The return value of the `ftw' function is 0 if all callback
     function calls returned 0 and all actions performed by the `ftw'
     succeeded.  If a function call failed (other than calling `stat'
     on an item) the function returns -1.  If a callback function
     returns a value other than 0 this value is returned as the return
     value of `ftw'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a
     32-bit system this function is in fact `ftw64', i.e., the LFS
     interface transparently replaces the old interface.

 -- Function: int ftw64 (const char *FILENAME, __ftw64_func_t FUNC, int
          DESCRIPTORS)
     This function is similar to `ftw' but it can work on filesystems
     with large files.  File information is reported using a variable
     of type `struct stat64' which is passed by reference to the
     callback function.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a
     32-bit system this function is available under the name `ftw' and
     transparently replaces the old implementation.

 -- Function: int nftw (const char *FILENAME, __nftw_func_t FUNC, int
          DESCRIPTORS, int FLAG)
     The `nftw' function works like the `ftw' functions.  They call the
     callback function FUNC for all items found in the directory
     FILENAME and below.  At most DESCRIPTORS file descriptors are
     consumed during the `nftw' call.

     One difference is that the callback function is of a different
     type.  It is of type `struct FTW *' and provides the callback
     function with the extra information described above.

     A second difference is that `nftw' takes a fourth argument, which
     is 0 or a bitwise-OR combination of any of the following values.

    `FTW_PHYS'
          While traversing the directory symbolic links are not
          followed.  Instead symbolic links are reported using the
          `FTW_SL' value for the type parameter to the callback
          function.  If the file referenced by a symbolic link does not
          exist `FTW_SLN' is returned instead.

    `FTW_MOUNT'
          The callback function is only called for items which are on
          the same mounted filesystem as the directory given by the
          FILENAME parameter to `nftw'.

    `FTW_CHDIR'
          If this flag is given the current working directory is
          changed to the directory of the reported object before the
          callback function is called.  When `ntfw' finally returns the
          current directory is restored to its original value.

    `FTW_DEPTH'
          If this option is specified then all subdirectories and files
          within them are processed before processing the top directory
          itself (depth-first processing).  This also means the type
          flag given to the callback function is `FTW_DP' and not
          `FTW_D'.

    `FTW_ACTIONRETVAL'
          If this option is specified then return values from callbacks
          are handled differently.  If the callback returns
          `FTW_CONTINUE', walking continues normally.  `FTW_STOP' means
          walking stops and `FTW_STOP' is returned to the caller.  If
          `FTW_SKIP_SUBTREE' is returned by the callback with `FTW_D'
          argument, the subtree is skipped and walking continues with
          next sibling of the directory.  If `FTW_SKIP_SIBLINGS' is
          returned by the callback, all siblings of the current entry
          are skipped and walking continues in its parent.  No other
          return values should be returned from the callbacks if this
          option is set.  This option is a GNU extension.

     The return value is computed in the same way as for `ftw'.  `nftw'
     returns 0 if no failures occurred and all callback functions
     returned 0.  In case of internal errors, such as memory problems,
     the return value is -1 and ERRNO is set accordingly.  If the
     return value of a callback invocation was non-zero then that value
     is returned.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a
     32-bit system this function is in fact `nftw64', i.e., the LFS
     interface transparently replaces the old interface.

 -- Function: int nftw64 (const char *FILENAME, __nftw64_func_t FUNC,
          int DESCRIPTORS, int FLAG)
     This function is similar to `nftw' but it can work on filesystems
     with large files.  File information is reported using a variable
     of type `struct stat64' which is passed by reference to the
     callback function.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a
     32-bit system this function is available under the name `nftw' and
     transparently replaces the old implementation.


File: libc.info,  Node: Hard Links,  Next: Symbolic Links,  Prev: Working with Directory Trees,  Up: File System Interface

14.4 Hard Links
===============

In POSIX systems, one file can have many names at the same time.  All of
the names are equally real, and no one of them is preferred to the
others.

   To add a name to a file, use the `link' function.  (The new name is
also called a "hard link" to the file.)  Creating a new link to a file
does not copy the contents of the file; it simply makes a new name by
which the file can be known, in addition to the file's existing name or
names.

   One file can have names in several directories, so the organization
of the file system is not a strict hierarchy or tree.

   In most implementations, it is not possible to have hard links to the
same file in multiple file systems.  `link' reports an error if you try
to make a hard link to the file from another file system when this
cannot be done.

   The prototype for the `link' function is declared in the header file
`unistd.h'.  

 -- Function: int link (const char *OLDNAME, const char *NEWNAME)
     The `link' function makes a new link to the existing file named by
     OLDNAME, under the new name NEWNAME.

     This function returns a value of `0' if it is successful and `-1'
     on failure.  In addition to the usual file name errors (*note File
     Name Errors::) for both OLDNAME and NEWNAME, the following `errno'
     error conditions are defined for this function:

    `EACCES'
          You are not allowed to write to the directory in which the
          new link is to be written.

    `EEXIST'
          There is already a file named NEWNAME.  If you want to replace
          this link with a new link, you must remove the old link
          explicitly first.

    `EMLINK'
          There are already too many links to the file named by OLDNAME.
          (The maximum number of links to a file is `LINK_MAX'; see
          *note Limits for Files::.)

    `ENOENT'
          The file named by OLDNAME doesn't exist.  You can't make a
          link to a file that doesn't exist.

    `ENOSPC'
          The directory or file system that would contain the new link
          is full and cannot be extended.

    `EPERM'
          On GNU/Linux and GNU/Hurd systems and some others, you cannot
          make links to directories.  Many systems allow only
          privileged users to do so.  This error is used to report the
          problem.

    `EROFS'
          The directory containing the new link can't be modified
          because it's on a read-only file system.

    `EXDEV'
          The directory specified in NEWNAME is on a different file
          system than the existing file.

    `EIO'
          A hardware error occurred while trying to read or write the
          to filesystem.


File: libc.info,  Node: Symbolic Links,  Next: Deleting Files,  Prev: Hard Links,  Up: File System Interface

14.5 Symbolic Links
===================

GNU systems support "soft links" or "symbolic links".  This is a kind
of "file" that is essentially a pointer to another file name.  Unlike
hard links, symbolic links can be made to directories or across file
systems with no restrictions.  You can also make a symbolic link to a
name which is not the name of any file.  (Opening this link will fail
until a file by that name is created.)  Likewise, if the symbolic link
points to an existing file which is later deleted, the symbolic link
continues to point to the same file name even though the name no longer
names any file.

   The reason symbolic links work the way they do is that special things
happen when you try to open the link.  The `open' function realizes you
have specified the name of a link, reads the file name contained in the
link, and opens that file name instead.  The `stat' function likewise
operates on the file that the symbolic link points to, instead of on
the link itself.

   By contrast, other operations such as deleting or renaming the file
operate on the link itself.  The functions `readlink' and `lstat' also
refrain from following symbolic links, because their purpose is to
obtain information about the link.  `link', the function that makes a
hard link, does too.  It makes a hard link to the symbolic link, which
one rarely wants.

   Some systems have for some functions operating on files have a limit
on how many symbolic links are followed when resolving a path name.  The
limit if it exists is published in the `sys/param.h' header file.

 -- Macro: int MAXSYMLINKS
     The macro `MAXSYMLINKS' specifies how many symlinks some function
     will follow before returning `ELOOP'.  Not all functions behave the
     same and this value is not the same a that returned for
     `_SC_SYMLOOP' by `sysconf'.  In fact, the `sysconf' result can
     indicate that there is no fixed limit although `MAXSYMLINKS'
     exists and has a finite value.

   Prototypes for most of the functions listed in this section are in
`unistd.h'.  

 -- Function: int symlink (const char *OLDNAME, const char *NEWNAME)
     The `symlink' function makes a symbolic link to OLDNAME named
     NEWNAME.

     The normal return value from `symlink' is `0'.  A return value of
     `-1' indicates an error.  In addition to the usual file name
     syntax errors (*note File Name Errors::), the following `errno'
     error conditions are defined for this function:

    `EEXIST'
          There is already an existing file named NEWNAME.

    `EROFS'
          The file NEWNAME would exist on a read-only file system.

    `ENOSPC'
          The directory or file system cannot be extended to make the
          new link.

    `EIO'
          A hardware error occurred while reading or writing data on
          the disk.


 -- Function: ssize_t readlink (const char *FILENAME, char *BUFFER,
          size_t SIZE)
     The `readlink' function gets the value of the symbolic link
     FILENAME.  The file name that the link points to is copied into
     BUFFER.  This file name string is _not_ null-terminated;
     `readlink' normally returns the number of characters copied.  The
     SIZE argument specifies the maximum number of characters to copy,
     usually the allocation size of BUFFER.

     If the return value equals SIZE, you cannot tell whether or not
     there was room to return the entire name.  So make a bigger buffer
     and call `readlink' again.  Here is an example:

          char *
          readlink_malloc (const char *filename)
          {
            int size = 100;
            char *buffer = NULL;

            while (1)
              {
                buffer = (char *) xrealloc (buffer, size);
                int nchars = readlink (filename, buffer, size);
                if (nchars < 0)
                  {
                    free (buffer);
                    return NULL;
                  }
                if (nchars < size)
                  return buffer;
                size *= 2;
              }
          }

     A value of `-1' is returned in case of error.  In addition to the
     usual file name errors (*note File Name Errors::), the following
     `errno' error conditions are defined for this function:

    `EINVAL'
          The named file is not a symbolic link.

    `EIO'
          A hardware error occurred while reading or writing data on
          the disk.

   In some situations it is desirable to resolve all the symbolic links
to get the real name of a file where no prefix names a symbolic link
which is followed and no filename in the path is `.' or `..'.  This is
for instance desirable if files have to be compare in which case
different names can refer to the same inode.

 -- Function: char * canonicalize_file_name (const char *NAME)
     The `canonicalize_file_name' function returns the absolute name of
     the file named by NAME which contains no `.', `..' components nor
     any repeated path separators (`/') or symlinks.  The result is
     passed back as the return value of the function in a block of
     memory allocated with `malloc'.  If the result is not used anymore
     the memory should be freed with a call to `free'.

     If any of the path components is missing the function returns a
     NULL pointer.  This is also what is returned if the length of the
     path reaches or exceeds `PATH_MAX' characters.  In any case
     `errno' is set accordingly.

    `ENAMETOOLONG'
          The resulting path is too long.  This error only occurs on
          systems which have a limit on the file name length.

    `EACCES'
          At least one of the path components is not readable.

    `ENOENT'
          The input file name is empty.

    `ENOENT'
          At least one of the path components does not exist.

    `ELOOP'
          More than `MAXSYMLINKS' many symlinks have been followed.

     This function is a GNU extension and is declared in `stdlib.h'.

   The Unix standard includes a similar function which differs from
`canonicalize_file_name' in that the user has to provide the buffer
where the result is placed in.

 -- Function: char * realpath (const char *restrict NAME, char
          *restrict RESOLVED)
     A call to `realpath' where the RESOLVED parameter is `NULL'
     behaves exactly like `canonicalize_file_name'.  The function
     allocates a buffer for the file name and returns a pointer to it.
     If RESOLVED is not `NULL' it points to a buffer into which the
     result is copied.  It is the callers responsibility to allocate a
     buffer which is large enough.  On systems which define `PATH_MAX'
     this means the buffer must be large enough for a pathname of this
     size.  For systems without limitations on the pathname length the
     requirement cannot be met and programs should not call `realpath'
     with anything but `NULL' for the second parameter.

     One other difference is that the buffer RESOLVED (if nonzero) will
     contain the part of the path component which does not exist or is
     not readable if the function returns `NULL' and `errno' is set to
     `EACCES' or `ENOENT'.

     This function is declared in `stdlib.h'.

   The advantage of using this function is that it is more widely
available.  The drawback is that it reports failures for long path on
systems which have no limits on the file name length.


File: libc.info,  Node: Deleting Files,  Next: Renaming Files,  Prev: Symbolic Links,  Up: File System Interface

14.6 Deleting Files
===================

You can delete a file with `unlink' or `remove'.

   Deletion actually deletes a file name.  If this is the file's only
name, then the file is deleted as well.  If the file has other
remaining names (*note Hard Links::), it remains accessible under those
names.

 -- Function: int unlink (const char *FILENAME)
     The `unlink' function deletes the file name FILENAME.  If this is
     a file's sole name, the file itself is also deleted.  (Actually,
     if any process has the file open when this happens, deletion is
     postponed until all processes have closed the file.)

     The function `unlink' is declared in the header file `unistd.h'.

     This function returns `0' on successful completion, and `-1' on
     error.  In addition to the usual file name errors (*note File Name
     Errors::), the following `errno' error conditions are defined for
     this function:

    `EACCES'
          Write permission is denied for the directory from which the
          file is to be removed, or the directory has the sticky bit
          set and you do not own the file.

    `EBUSY'
          This error indicates that the file is being used by the
          system in such a way that it can't be unlinked.  For example,
          you might see this error if the file name specifies the root
          directory or a mount point for a file system.

    `ENOENT'
          The file name to be deleted doesn't exist.

    `EPERM'
          On some systems `unlink' cannot be used to delete the name of
          a directory, or at least can only be used this way by a
          privileged user.  To avoid such problems, use `rmdir' to
          delete directories.  (On GNU/Linux and GNU/Hurd systems
          `unlink' can never delete the name of a directory.)

    `EROFS'
          The directory containing the file name to be deleted is on a
          read-only file system and can't be modified.

 -- Function: int rmdir (const char *FILENAME)
     The `rmdir' function deletes a directory.  The directory must be
     empty before it can be removed; in other words, it can only contain
     entries for `.' and `..'.

     In most other respects, `rmdir' behaves like `unlink'.  There are
     two additional `errno' error conditions defined for `rmdir':

    `ENOTEMPTY'
    `EEXIST'
          The directory to be deleted is not empty.

     These two error codes are synonymous; some systems use one, and
     some use the other.  GNU/Linux and GNU/Hurd systems always use
     `ENOTEMPTY'.

     The prototype for this function is declared in the header file
     `unistd.h'.  

 -- Function: int remove (const char *FILENAME)
     This is the ISO C function to remove a file.  It works like
     `unlink' for files and like `rmdir' for directories.  `remove' is
     declared in `stdio.h'.  


File: libc.info,  Node: Renaming Files,  Next: Creating Directories,  Prev: Deleting Files,  Up: File System Interface

14.7 Renaming Files
===================

The `rename' function is used to change a file's name.

 -- Function: int rename (const char *OLDNAME, const char *NEWNAME)
     The `rename' function renames the file OLDNAME to NEWNAME.  The
     file formerly accessible under the name OLDNAME is afterwards
     accessible as NEWNAME instead.  (If the file had any other names
     aside from OLDNAME, it continues to have those names.)

     The directory containing the name NEWNAME must be on the same file
     system as the directory containing the name OLDNAME.

     One special case for `rename' is when OLDNAME and NEWNAME are two
     names for the same file.  The consistent way to handle this case
     is to delete OLDNAME.  However, in this case POSIX requires that
     `rename' do nothing and report success--which is inconsistent.  We
     don't know what your operating system will do.

     If OLDNAME is not a directory, then any existing file named
     NEWNAME is removed during the renaming operation.  However, if
     NEWNAME is the name of a directory, `rename' fails in this case.

     If OLDNAME is a directory, then either NEWNAME must not exist or
     it must name a directory that is empty.  In the latter case, the
     existing directory named NEWNAME is deleted first.  The name
     NEWNAME must not specify a subdirectory of the directory `oldname'
     which is being renamed.

     One useful feature of `rename' is that the meaning of NEWNAME
     changes "atomically" from any previously existing file by that
     name to its new meaning (i.e., the file that was called OLDNAME).
     There is no instant at which NEWNAME is non-existent "in between"
     the old meaning and the new meaning.  If there is a system crash
     during the operation, it is possible for both names to still
     exist; but NEWNAME will always be intact if it exists at all.

     If `rename' fails, it returns `-1'.  In addition to the usual file
     name errors (*note File Name Errors::), the following `errno'
     error conditions are defined for this function:

    `EACCES'
          One of the directories containing NEWNAME or OLDNAME refuses
          write permission; or NEWNAME and OLDNAME are directories and
          write permission is refused for one of them.

    `EBUSY'
          A directory named by OLDNAME or NEWNAME is being used by the
          system in a way that prevents the renaming from working.
          This includes directories that are mount points for
          filesystems, and directories that are the current working
          directories of processes.

    `ENOTEMPTY'
    `EEXIST'
          The directory NEWNAME isn't empty.  GNU/Linux and GNU/Hurd
          systems always return `ENOTEMPTY' for this, but some other
          systems return `EEXIST'.

    `EINVAL'
          OLDNAME is a directory that contains NEWNAME.

    `EISDIR'
          NEWNAME is a directory but the OLDNAME isn't.

    `EMLINK'
          The parent directory of NEWNAME would have too many links
          (entries).

    `ENOENT'
          The file OLDNAME doesn't exist.

    `ENOSPC'
          The directory that would contain NEWNAME has no room for
          another entry, and there is no space left in the file system
          to expand it.

    `EROFS'
          The operation would involve writing to a directory on a
          read-only file system.

    `EXDEV'
          The two file names NEWNAME and OLDNAME are on different file
          systems.


File: libc.info,  Node: Creating Directories,  Next: File Attributes,  Prev: Renaming Files,  Up: File System Interface

14.8 Creating Directories
=========================

Directories are created with the `mkdir' function.  (There is also a
shell command `mkdir' which does the same thing.)

 -- Function: int mkdir (const char *FILENAME, mode_t MODE)
     The `mkdir' function creates a new, empty directory with name
     FILENAME.

     The argument MODE specifies the file permissions for the new
     directory file.  *Note Permission Bits::, for more information
     about this.

     A return value of `0' indicates successful completion, and `-1'
     indicates failure.  In addition to the usual file name syntax
     errors (*note File Name Errors::), the following `errno' error
     conditions are defined for this function:

    `EACCES'
          Write permission is denied for the parent directory in which
          the new directory is to be added.

    `EEXIST'
          A file named FILENAME already exists.

    `EMLINK'
          The parent directory has too many links (entries).

          Well-designed file systems never report this error, because
          they permit more links than your disk could possibly hold.
          However, you must still take account of the possibility of
          this error, as it could result from network access to a file
          system on another machine.

    `ENOSPC'
          The file system doesn't have enough room to create the new
          directory.

    `EROFS'
          The parent directory of the directory being created is on a
          read-only file system and cannot be modified.

     To use this function, your program should include the header file
     `sys/stat.h'.  


File: libc.info,  Node: File Attributes,  Next: Making Special Files,  Prev: Creating Directories,  Up: File System Interface

14.9 File Attributes
====================

When you issue an `ls -l' shell command on a file, it gives you
information about the size of the file, who owns it, when it was last
modified, etc.  These are called the "file attributes", and are
associated with the file itself and not a particular one of its names.

   This section contains information about how you can inquire about and
modify the attributes of a file.

* Menu:

* Attribute Meanings::          The names of the file attributes,
                                 and what their values mean.
* Reading Attributes::          How to read the attributes of a file.
* Testing File Type::           Distinguishing ordinary files,
                                 directories, links...
* File Owner::                  How ownership for new files is determined,
			         and how to change it.
* Permission Bits::             How information about a file's access
                                 mode is stored.
* Access Permission::           How the system decides who can access a file.
* Setting Permissions::         How permissions for new files are assigned,
			         and how to change them.
* Testing File Access::         How to find out if your process can
                                 access a file.
* File Times::                  About the time attributes of a file.
* File Size::			Manually changing the size of a file.


File: libc.info,  Node: Attribute Meanings,  Next: Reading Attributes,  Up: File Attributes

14.9.1 The meaning of the File Attributes
-----------------------------------------

When you read the attributes of a file, they come back in a structure
called `struct stat'.  This section describes the names of the
attributes, their data types, and what they mean.  For the functions to
read the attributes of a file, see *note Reading Attributes::.

   The header file `sys/stat.h' declares all the symbols defined in
this section.  

 -- Data Type: struct stat
     The `stat' structure type is used to return information about the
     attributes of a file.  It contains at least the following members:

    `mode_t st_mode'
          Specifies the mode of the file.  This includes file type
          information (*note Testing File Type::) and the file
          permission bits (*note Permission Bits::).

    `ino_t st_ino'
          The file serial number, which distinguishes this file from
          all other files on the same device.

    `dev_t st_dev'
          Identifies the device containing the file.  The `st_ino' and
          `st_dev', taken together, uniquely identify the file.  The
          `st_dev' value is not necessarily consistent across reboots or
          system crashes, however.

    `nlink_t st_nlink'
          The number of hard links to the file.  This count keeps track
          of how many directories have entries for this file.  If the
          count is ever decremented to zero, then the file itself is
          discarded as soon as no process still holds it open.
          Symbolic links are not counted in the total.

    `uid_t st_uid'
          The user ID of the file's owner.  *Note File Owner::.

    `gid_t st_gid'
          The group ID of the file.  *Note File Owner::.

    `off_t st_size'
          This specifies the size of a regular file in bytes.  For
          files that are really devices this field isn't usually
          meaningful.  For symbolic links this specifies the length of
          the file name the link refers to.

    `time_t st_atime'
          This is the last access time for the file.  *Note File
          Times::.

    `unsigned long int st_atime_usec'
          This is the fractional part of the last access time for the
          file.  *Note File Times::.

    `time_t st_mtime'
          This is the time of the last modification to the contents of
          the file.  *Note File Times::.

    `unsigned long int st_mtime_usec'
          This is the fractional part of the time of the last
          modification to the contents of the file.  *Note File Times::.

    `time_t st_ctime'
          This is the time of the last modification to the attributes
          of the file.  *Note File Times::.

    `unsigned long int st_ctime_usec'
          This is the fractional part of the time of the last
          modification to the attributes of the file.  *Note File
          Times::.

    `blkcnt_t st_blocks'
          This is the amount of disk space that the file occupies,
          measured in units of 512-byte blocks.

          The number of disk blocks is not strictly proportional to the
          size of the file, for two reasons: the file system may use
          some blocks for internal record keeping; and the file may be
          sparse--it may have "holes" which contain zeros but do not
          actually take up space on the disk.

          You can tell (approximately) whether a file is sparse by
          comparing this value with `st_size', like this:

               (st.st_blocks * 512 < st.st_size)

          This test is not perfect because a file that is just slightly
          sparse might not be detected as sparse at all.  For practical
          applications, this is not a problem.

    `unsigned int st_blksize'
          The optimal block size for reading of writing this file, in
          bytes.  You might use this size for allocating the buffer
          space for reading of writing the file.  (This is unrelated to
          `st_blocks'.)

   The extensions for the Large File Support (LFS) require, even on
32-bit machines, types which can handle file sizes up to 2^63.
Therefore a new definition of `struct stat' is necessary.

 -- Data Type: struct stat64
     The members of this type are the same and have the same names as
     those in `struct stat'.  The only difference is that the members
     `st_ino', `st_size', and `st_blocks' have a different type to
     support larger values.

    `mode_t st_mode'
          Specifies the mode of the file.  This includes file type
          information (*note Testing File Type::) and the file
          permission bits (*note Permission Bits::).

    `ino64_t st_ino'
          The file serial number, which distinguishes this file from
          all other files on the same device.

    `dev_t st_dev'
          Identifies the device containing the file.  The `st_ino' and
          `st_dev', taken together, uniquely identify the file.  The
          `st_dev' value is not necessarily consistent across reboots or
          system crashes, however.

    `nlink_t st_nlink'
          The number of hard links to the file.  This count keeps track
          of how many directories have entries for this file.  If the
          count is ever decremented to zero, then the file itself is
          discarded as soon as no process still holds it open.
          Symbolic links are not counted in the total.

    `uid_t st_uid'
          The user ID of the file's owner.  *Note File Owner::.

    `gid_t st_gid'
          The group ID of the file.  *Note File Owner::.

    `off64_t st_size'
          This specifies the size of a regular file in bytes.  For
          files that are really devices this field isn't usually
          meaningful.  For symbolic links this specifies the length of
          the file name the link refers to.

    `time_t st_atime'
          This is the last access time for the file.  *Note File
          Times::.

    `unsigned long int st_atime_usec'
          This is the fractional part of the last access time for the
          file.  *Note File Times::.

    `time_t st_mtime'
          This is the time of the last modification to the contents of
          the file.  *Note File Times::.

    `unsigned long int st_mtime_usec'
          This is the fractional part of the time of the last
          modification to the contents of the file.  *Note File Times::.

    `time_t st_ctime'
          This is the time of the last modification to the attributes
          of the file.  *Note File Times::.

    `unsigned long int st_ctime_usec'
          This is the fractional part of the time of the last
          modification to the attributes of the file.  *Note File
          Times::.

    `blkcnt64_t st_blocks'
          This is the amount of disk space that the file occupies,
          measured in units of 512-byte blocks.

    `unsigned int st_blksize'
          The optimal block size for reading of writing this file, in
          bytes.  You might use this size for allocating the buffer
          space for reading of writing the file.  (This is unrelated to
          `st_blocks'.)

   Some of the file attributes have special data type names which exist
specifically for those attributes.  (They are all aliases for well-known
integer types that you know and love.)  These typedef names are defined
in the header file `sys/types.h' as well as in `sys/stat.h'.  Here is a
list of them.

 -- Data Type: mode_t
     This is an integer data type used to represent file modes.  In the
     GNU C Library, this is an unsigned type no narrower than `unsigned
     int'.

 -- Data Type: ino_t
     This is an unsigned integer type used to represent file serial
     numbers.  (In Unix jargon, these are sometimes called "inode
     numbers".)  In the GNU C Library, this type is no narrower than
     `unsigned int'.

     If the source is compiled with `_FILE_OFFSET_BITS == 64' this type
     is transparently replaced by `ino64_t'.

 -- Data Type: ino64_t
     This is an unsigned integer type used to represent file serial
     numbers for the use in LFS.  In the GNU C Library, this type is no
     narrower than `unsigned int'.

     When compiling with `_FILE_OFFSET_BITS == 64' this type is
     available under the name `ino_t'.

 -- Data Type: dev_t
     This is an arithmetic data type used to represent file device
     numbers.  In the GNU C Library, this is an integer type no
     narrower than `int'.

 -- Data Type: nlink_t
     This is an integer type used to represent file link counts.

 -- Data Type: blkcnt_t
     This is a signed integer type used to represent block counts.  In
     the GNU C Library, this type is no narrower than `int'.

     If the source is compiled with `_FILE_OFFSET_BITS == 64' this type
     is transparently replaced by `blkcnt64_t'.

 -- Data Type: blkcnt64_t
     This is a signed integer type used to represent block counts for
     the use in LFS.  In the GNU C Library, this type is no narrower
     than `int'.

     When compiling with `_FILE_OFFSET_BITS == 64' this type is
     available under the name `blkcnt_t'.


File: libc.info,  Node: Reading Attributes,  Next: Testing File Type,  Prev: Attribute Meanings,  Up: File Attributes

14.9.2 Reading the Attributes of a File
---------------------------------------

To examine the attributes of files, use the functions `stat', `fstat'
and `lstat'.  They return the attribute information in a `struct stat'
object.  All three functions are declared in the header file
`sys/stat.h'.

 -- Function: int stat (const char *FILENAME, struct stat *BUF)
     The `stat' function returns information about the attributes of the
     file named by FILENAME in the structure pointed to by BUF.

     If FILENAME is the name of a symbolic link, the attributes you get
     describe the file that the link points to.  If the link points to a
     nonexistent file name, then `stat' fails reporting a nonexistent
     file.

     The return value is `0' if the operation is successful, or `-1' on
     failure.  In addition to the usual file name errors (*note File
     Name Errors::, the following `errno' error conditions are defined
     for this function:

    `ENOENT'
          The file named by FILENAME doesn't exist.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is in fact `stat64' since the LFS interface transparently
     replaces the normal implementation.

 -- Function: int stat64 (const char *FILENAME, struct stat64 *BUF)
     This function is similar to `stat' but it is also able to work on
     files larger then 2^31 bytes on 32-bit systems.  To be able to do
     this the result is stored in a variable of type `struct stat64' to
     which BUF must point.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is available under the name `stat' and so transparently
     replaces the interface for small files on 32-bit machines.

 -- Function: int fstat (int FILEDES, struct stat *BUF)
     The `fstat' function is like `stat', except that it takes an open
     file descriptor as an argument instead of a file name.  *Note
     Low-Level I/O::.

     Like `stat', `fstat' returns `0' on success and `-1' on failure.
     The following `errno' error conditions are defined for `fstat':

    `EBADF'
          The FILEDES argument is not a valid file descriptor.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is in fact `fstat64' since the LFS interface transparently
     replaces the normal implementation.

 -- Function: int fstat64 (int FILEDES, struct stat64 *BUF)
     This function is similar to `fstat' but is able to work on large
     files on 32-bit platforms.  For large files the file descriptor
     FILEDES should be obtained by `open64' or `creat64'.  The BUF
     pointer points to a variable of type `struct stat64' which is able
     to represent the larger values.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is available under the name `fstat' and so transparently
     replaces the interface for small files on 32-bit machines.

 -- Function: int lstat (const char *FILENAME, struct stat *BUF)
     The `lstat' function is like `stat', except that it does not
     follow symbolic links.  If FILENAME is the name of a symbolic
     link, `lstat' returns information about the link itself; otherwise
     `lstat' works like `stat'.  *Note Symbolic Links::.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is in fact `lstat64' since the LFS interface transparently
     replaces the normal implementation.

 -- Function: int lstat64 (const char *FILENAME, struct stat64 *BUF)
     This function is similar to `lstat' but it is also able to work on
     files larger then 2^31 bytes on 32-bit systems.  To be able to do
     this the result is stored in a variable of type `struct stat64' to
     which BUF must point.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is available under the name `lstat' and so transparently
     replaces the interface for small files on 32-bit machines.


File: libc.info,  Node: Testing File Type,  Next: File Owner,  Prev: Reading Attributes,  Up: File Attributes

14.9.3 Testing the Type of a File
---------------------------------

The "file mode", stored in the `st_mode' field of the file attributes,
contains two kinds of information: the file type code, and the access
permission bits.  This section discusses only the type code, which you
can use to tell whether the file is a directory, socket, symbolic link,
and so on.  For details about access permissions see *note Permission
Bits::.

   There are two ways you can access the file type information in a file
mode.  Firstly, for each file type there is a "predicate macro" which
examines a given file mode and returns whether it is of that type or
not.  Secondly, you can mask out the rest of the file mode to leave
just the file type code, and compare this against constants for each of
the supported file types.

   All of the symbols listed in this section are defined in the header
file `sys/stat.h'.  

   The following predicate macros test the type of a file, given the
value M which is the `st_mode' field returned by `stat' on that file:

 -- Macro: int S_ISDIR (mode_t M)
     This macro returns non-zero if the file is a directory.

 -- Macro: int S_ISCHR (mode_t M)
     This macro returns non-zero if the file is a character special
     file (a device like a terminal).

 -- Macro: int S_ISBLK (mode_t M)
     This macro returns non-zero if the file is a block special file (a
     device like a disk).

 -- Macro: int S_ISREG (mode_t M)
     This macro returns non-zero if the file is a regular file.

 -- Macro: int S_ISFIFO (mode_t M)
     This macro returns non-zero if the file is a FIFO special file, or
     a pipe.  *Note Pipes and FIFOs::.

 -- Macro: int S_ISLNK (mode_t M)
     This macro returns non-zero if the file is a symbolic link.  *Note
     Symbolic Links::.

 -- Macro: int S_ISSOCK (mode_t M)
     This macro returns non-zero if the file is a socket.  *Note
     Sockets::.

   An alternate non-POSIX method of testing the file type is supported
for compatibility with BSD.  The mode can be bitwise AND-ed with
`S_IFMT' to extract the file type code, and compared to the appropriate
constant.  For example,

     S_ISCHR (MODE)

is equivalent to:

     ((MODE & S_IFMT) == S_IFCHR)

 -- Macro: int S_IFMT
     This is a bit mask used to extract the file type code from a mode
     value.

   These are the symbolic names for the different file type codes:

`S_IFDIR'
     This is the file type constant of a directory file.

`S_IFCHR'
     This is the file type constant of a character-oriented device file.

`S_IFBLK'
     This is the file type constant of a block-oriented device file.

`S_IFREG'
     This is the file type constant of a regular file.

`S_IFLNK'
     This is the file type constant of a symbolic link.

`S_IFSOCK'
     This is the file type constant of a socket.

`S_IFIFO'
     This is the file type constant of a FIFO or pipe.

   The POSIX.1b standard introduced a few more objects which possibly
can be implemented as object in the filesystem.  These are message
queues, semaphores, and shared memory objects.  To allow
differentiating these objects from other files the POSIX standard
introduces three new test macros.  But unlike the other macros it does
not take the value of the `st_mode' field as the parameter.  Instead
they expect a pointer to the whole `struct stat' structure.

 -- Macro: int S_TYPEISMQ (struct stat *S)
     If the system implement POSIX message queues as distinct objects
     and the file is a message queue object, this macro returns a
     non-zero value.  In all other cases the result is zero.

 -- Macro: int S_TYPEISSEM (struct stat *S)
     If the system implement POSIX semaphores as distinct objects and
     the file is a semaphore object, this macro returns a non-zero
     value.  In all other cases the result is zero.

 -- Macro: int S_TYPEISSHM (struct stat *S)
     If the system implement POSIX shared memory objects as distinct
     objects and the file is an shared memory object, this macro
     returns a non-zero value.  In all other cases the result is zero.


File: libc.info,  Node: File Owner,  Next: Permission Bits,  Prev: Testing File Type,  Up: File Attributes

14.9.4 File Owner
-----------------

Every file has an "owner" which is one of the registered user names
defined on the system.  Each file also has a "group" which is one of
the defined groups.  The file owner can often be useful for showing you
who edited the file (especially when you edit with GNU Emacs), but its
main purpose is for access control.

   The file owner and group play a role in determining access because
the file has one set of access permission bits for the owner, another
set that applies to users who belong to the file's group, and a third
set of bits that applies to everyone else.  *Note Access Permission::,
for the details of how access is decided based on this data.

   When a file is created, its owner is set to the effective user ID of
the process that creates it (*note Process Persona::).  The file's
group ID may be set to either the effective group ID of the process, or
the group ID of the directory that contains the file, depending on the
system where the file is stored.  When you access a remote file system,
it behaves according to its own rules, not according to the system your
program is running on.  Thus, your program must be prepared to encounter
either kind of behavior no matter what kind of system you run it on.

   You can change the owner and/or group owner of an existing file using
the `chown' function.  This is the primitive for the `chown' and
`chgrp' shell commands.

   The prototype for this function is declared in `unistd.h'.

 -- Function: int chown (const char *FILENAME, uid_t OWNER, gid_t GROUP)
     The `chown' function changes the owner of the file FILENAME to
     OWNER, and its group owner to GROUP.

     Changing the owner of the file on certain systems clears the
     set-user-ID and set-group-ID permission bits.  (This is because
     those bits may not be appropriate for the new owner.)  Other file
     permission bits are not changed.

     The return value is `0' on success and `-1' on failure.  In
     addition to the usual file name errors (*note File Name Errors::),
     the following `errno' error conditions are defined for this
     function:

    `EPERM'
          This process lacks permission to make the requested change.

          Only privileged users or the file's owner can change the
          file's group.  On most file systems, only privileged users
          can change the file owner; some file systems allow you to
          change the owner if you are currently the owner.  When you
          access a remote file system, the behavior you encounter is
          determined by the system that actually holds the file, not by
          the system your program is running on.

          *Note Options for Files::, for information about the
          `_POSIX_CHOWN_RESTRICTED' macro.

    `EROFS'
          The file is on a read-only file system.

 -- Function: int fchown (int FILEDES, uid_t OWNER, gid_t GROUP)
     This is like `chown', except that it changes the owner of the open
     file with descriptor FILEDES.

     The return value from `fchown' is `0' on success and `-1' on
     failure.  The following `errno' error codes are defined for this
     function:

    `EBADF'
          The FILEDES argument is not a valid file descriptor.

    `EINVAL'
          The FILEDES argument corresponds to a pipe or socket, not an
          ordinary file.

    `EPERM'
          This process lacks permission to make the requested change.
          For details see `chmod' above.

    `EROFS'
          The file resides on a read-only file system.


File: libc.info,  Node: Permission Bits,  Next: Access Permission,  Prev: File Owner,  Up: File Attributes

14.9.5 The Mode Bits for Access Permission
------------------------------------------

The "file mode", stored in the `st_mode' field of the file attributes,
contains two kinds of information: the file type code, and the access
permission bits.  This section discusses only the access permission
bits, which control who can read or write the file.  *Note Testing File
Type::, for information about the file type code.

   All of the symbols listed in this section are defined in the header
file `sys/stat.h'.  

   These symbolic constants are defined for the file mode bits that
control access permission for the file:

`S_IRUSR'
`S_IREAD'
     Read permission bit for the owner of the file.  On many systems
     this bit is 0400.  `S_IREAD' is an obsolete synonym provided for
     BSD compatibility.

`S_IWUSR'
`S_IWRITE'
     Write permission bit for the owner of the file.  Usually 0200.
     `S_IWRITE' is an obsolete synonym provided for BSD compatibility.

`S_IXUSR'
`S_IEXEC'
     Execute (for ordinary files) or search (for directories)
     permission bit for the owner of the file.  Usually 0100.
     `S_IEXEC' is an obsolete synonym provided for BSD compatibility.

`S_IRWXU'
     This is equivalent to `(S_IRUSR | S_IWUSR | S_IXUSR)'.

`S_IRGRP'
     Read permission bit for the group owner of the file.  Usually 040.

`S_IWGRP'
     Write permission bit for the group owner of the file.  Usually 020.

`S_IXGRP'
     Execute or search permission bit for the group owner of the file.
     Usually 010.

`S_IRWXG'
     This is equivalent to `(S_IRGRP | S_IWGRP | S_IXGRP)'.

`S_IROTH'
     Read permission bit for other users.  Usually 04.

`S_IWOTH'
     Write permission bit for other users.  Usually 02.

`S_IXOTH'
     Execute or search permission bit for other users.  Usually 01.

`S_IRWXO'
     This is equivalent to `(S_IROTH | S_IWOTH | S_IXOTH)'.

`S_ISUID'
     This is the set-user-ID on execute bit, usually 04000.  *Note How
     Change Persona::.

`S_ISGID'
     This is the set-group-ID on execute bit, usually 02000.  *Note How
     Change Persona::.

`S_ISVTX'
     This is the "sticky" bit, usually 01000.

     For a directory it gives permission to delete a file in that
     directory only if you own that file.  Ordinarily, a user can
     either delete all the files in a directory or cannot delete any of
     them (based on whether the user has write permission for the
     directory).  The same restriction applies--you must have both
     write permission for the directory and own the file you want to
     delete.  The one exception is that the owner of the directory can
     delete any file in the directory, no matter who owns it (provided
     the owner has given himself write permission for the directory).
     This is commonly used for the `/tmp' directory, where anyone may
     create files but not delete files created by other users.

     Originally the sticky bit on an executable file modified the
     swapping policies of the system.  Normally, when a program
     terminated, its pages in core were immediately freed and reused.
     If the sticky bit was set on the executable file, the system kept
     the pages in core for a while as if the program were still
     running.  This was advantageous for a program likely to be run
     many times in succession.  This usage is obsolete in modern
     systems.  When a program terminates, its pages always remain in
     core as long as there is no shortage of memory in the system.
     When the program is next run, its pages will still be in core if
     no shortage arose since the last run.

     On some modern systems where the sticky bit has no useful meaning
     for an executable file, you cannot set the bit at all for a
     non-directory.  If you try, `chmod' fails with `EFTYPE'; *note
     Setting Permissions::.

     Some systems (particularly SunOS) have yet another use for the
     sticky bit.  If the sticky bit is set on a file that is _not_
     executable, it means the opposite: never cache the pages of this
     file at all.  The main use of this is for the files on an NFS
     server machine which are used as the swap area of diskless client
     machines.  The idea is that the pages of the file will be cached
     in the client's memory, so it is a waste of the server's memory to
     cache them a second time.  With this usage the sticky bit also
     implies that the filesystem may fail to record the file's
     modification time onto disk reliably (the idea being that no-one
     cares for a swap file).

     This bit is only available on BSD systems (and those derived from
     them).  Therefore one has to use the `_BSD_SOURCE' feature select
     macro to get the definition (*note Feature Test Macros::).

   The actual bit values of the symbols are listed in the table above
so you can decode file mode values when debugging your programs.  These
bit values are correct for most systems, but they are not guaranteed.

   *Warning:* Writing explicit numbers for file permissions is bad
practice.  Not only is it not portable, it also requires everyone who
reads your program to remember what the bits mean.  To make your program
clean use the symbolic names.


File: libc.info,  Node: Access Permission,  Next: Setting Permissions,  Prev: Permission Bits,  Up: File Attributes

14.9.6 How Your Access to a File is Decided
-------------------------------------------

Recall that the operating system normally decides access permission for
a file based on the effective user and group IDs of the process and its
supplementary group IDs, together with the file's owner, group and
permission bits.  These concepts are discussed in detail in *note
Process Persona::.

   If the effective user ID of the process matches the owner user ID of
the file, then permissions for read, write, and execute/search are
controlled by the corresponding "user" (or "owner") bits.  Likewise, if
any of the effective group ID or supplementary group IDs of the process
matches the group owner ID of the file, then permissions are controlled
by the "group" bits.  Otherwise, permissions are controlled by the
"other" bits.

   Privileged users, like `root', can access any file regardless of its
permission bits.  As a special case, for a file to be executable even
by a privileged user, at least one of its execute bits must be set.


File: libc.info,  Node: Setting Permissions,  Next: Testing File Access,  Prev: Access Permission,  Up: File Attributes

14.9.7 Assigning File Permissions
---------------------------------

The primitive functions for creating files (for example, `open' or
`mkdir') take a MODE argument, which specifies the file permissions to
give the newly created file.  This mode is modified by the process's
"file creation mask", or "umask", before it is used.

   The bits that are set in the file creation mask identify permissions
that are always to be disabled for newly created files.  For example, if
you set all the "other" access bits in the mask, then newly created
files are not accessible at all to processes in the "other" category,
even if the MODE argument passed to the create function would permit
such access.  In other words, the file creation mask is the complement
of the ordinary access permissions you want to grant.

   Programs that create files typically specify a MODE argument that
includes all the permissions that make sense for the particular file.
For an ordinary file, this is typically read and write permission for
all classes of users.  These permissions are then restricted as
specified by the individual user's own file creation mask.

   To change the permission of an existing file given its name, call
`chmod'.  This function uses the specified permission bits and ignores
the file creation mask.

   In normal use, the file creation mask is initialized by the user's
login shell (using the `umask' shell command), and inherited by all
subprocesses.  Application programs normally don't need to worry about
the file creation mask.  It will automatically do what it is supposed to
do.

   When your program needs to create a file and bypass the umask for its
access permissions, the easiest way to do this is to use `fchmod' after
opening the file, rather than changing the umask.  In fact, changing
the umask is usually done only by shells.  They use the `umask'
function.

   The functions in this section are declared in `sys/stat.h'.  

 -- Function: mode_t umask (mode_t MASK)
     The `umask' function sets the file creation mask of the current
     process to MASK, and returns the previous value of the file
     creation mask.

     Here is an example showing how to read the mask with `umask'
     without changing it permanently:

          mode_t
          read_umask (void)
          {
            mode_t mask = umask (0);
            umask (mask);
            return mask;
          }

     However, on GNU/Hurd systems it is better to use `getumask' if you
     just want to read the mask value, because it is reentrant.

 -- Function: mode_t getumask (void)
     Return the current value of the file creation mask for the current
     process.  This function is a GNU extension and is only available on
     GNU/Hurd systems.

 -- Function: int chmod (const char *FILENAME, mode_t MODE)
     The `chmod' function sets the access permission bits for the file
     named by FILENAME to MODE.

     If FILENAME is a symbolic link, `chmod' changes the permissions of
     the file pointed to by the link, not those of the link itself.

     This function returns `0' if successful and `-1' if not.  In
     addition to the usual file name errors (*note File Name Errors::),
     the following `errno' error conditions are defined for this
     function:

    `ENOENT'
          The named file doesn't exist.

    `EPERM'
          This process does not have permission to change the access
          permissions of this file.  Only the file's owner (as judged
          by the effective user ID of the process) or a privileged user
          can change them.

    `EROFS'
          The file resides on a read-only file system.

    `EFTYPE'
          MODE has the `S_ISVTX' bit (the "sticky bit") set, and the
          named file is not a directory.  Some systems do not allow
          setting the sticky bit on non-directory files, and some do
          (and only some of those assign a useful meaning to the bit
          for non-directory files).

          You only get `EFTYPE' on systems where the sticky bit has no
          useful meaning for non-directory files, so it is always safe
          to just clear the bit in MODE and call `chmod' again.  *Note
          Permission Bits::, for full details on the sticky bit.

 -- Function: int fchmod (int FILEDES, mode_t MODE)
     This is like `chmod', except that it changes the permissions of the
     currently open file given by FILEDES.

     The return value from `fchmod' is `0' on success and `-1' on
     failure.  The following `errno' error codes are defined for this
     function:

    `EBADF'
          The FILEDES argument is not a valid file descriptor.

    `EINVAL'
          The FILEDES argument corresponds to a pipe or socket, or
          something else that doesn't really have access permissions.

    `EPERM'
          This process does not have permission to change the access
          permissions of this file.  Only the file's owner (as judged
          by the effective user ID of the process) or a privileged user
          can change them.

    `EROFS'
          The file resides on a read-only file system.


File: libc.info,  Node: Testing File Access,  Next: File Times,  Prev: Setting Permissions,  Up: File Attributes

14.9.8 Testing Permission to Access a File
------------------------------------------

In some situations it is desirable to allow programs to access files or
devices even if this is not possible with the permissions granted to the
user.  One possible solution is to set the setuid-bit of the program
file.  If such a program is started the _effective_ user ID of the
process is changed to that of the owner of the program file.  So to
allow write access to files like `/etc/passwd', which normally can be
written only by the super-user, the modifying program will have to be
owned by `root' and the setuid-bit must be set.

   But beside the files the program is intended to change the user
should not be allowed to access any file to which s/he would not have
access anyway.  The program therefore must explicitly check whether _the
user_ would have the necessary access to a file, before it reads or
writes the file.

   To do this, use the function `access', which checks for access
permission based on the process's _real_ user ID rather than the
effective user ID.  (The setuid feature does not alter the real user ID,
so it reflects the user who actually ran the program.)

   There is another way you could check this access, which is easy to
describe, but very hard to use.  This is to examine the file mode bits
and mimic the system's own access computation.  This method is
undesirable because many systems have additional access control
features; your program cannot portably mimic them, and you would not
want to try to keep track of the diverse features that different systems
have.  Using `access' is simple and automatically does whatever is
appropriate for the system you are using.

   `access' is _only_ only appropriate to use in setuid programs.  A
non-setuid program will always use the effective ID rather than the
real ID.

   The symbols in this section are declared in `unistd.h'.

 -- Function: int access (const char *FILENAME, int HOW)
     The `access' function checks to see whether the file named by
     FILENAME can be accessed in the way specified by the HOW argument.
     The HOW argument either can be the bitwise OR of the flags `R_OK',
     `W_OK', `X_OK', or the existence test `F_OK'.

     This function uses the _real_ user and group IDs of the calling
     process, rather than the _effective_ IDs, to check for access
     permission.  As a result, if you use the function from a `setuid'
     or `setgid' program (*note How Change Persona::), it gives
     information relative to the user who actually ran the program.

     The return value is `0' if the access is permitted, and `-1'
     otherwise.  (In other words, treated as a predicate function,
     `access' returns true if the requested access is _denied_.)

     In addition to the usual file name errors (*note File Name
     Errors::), the following `errno' error conditions are defined for
     this function:

    `EACCES'
          The access specified by HOW is denied.

    `ENOENT'
          The file doesn't exist.

    `EROFS'
          Write permission was requested for a file on a read-only file
          system.

   These macros are defined in the header file `unistd.h' for use as
the HOW argument to the `access' function.  The values are integer
constants.  

 -- Macro: int R_OK
     Flag meaning test for read permission.

 -- Macro: int W_OK
     Flag meaning test for write permission.

 -- Macro: int X_OK
     Flag meaning test for execute/search permission.

 -- Macro: int F_OK
     Flag meaning test for existence of the file.


File: libc.info,  Node: File Times,  Next: File Size,  Prev: Testing File Access,  Up: File Attributes

14.9.9 File Times
-----------------

Each file has three time stamps associated with it:  its access time,
its modification time, and its attribute modification time.  These
correspond to the `st_atime', `st_mtime', and `st_ctime' members of the
`stat' structure; see *note File Attributes::.

   All of these times are represented in calendar time format, as
`time_t' objects.  This data type is defined in `time.h'.  For more
information about representation and manipulation of time values, see
*note Calendar Time::.  

   Reading from a file updates its access time attribute, and writing
updates its modification time.  When a file is created, all three time
stamps for that file are set to the current time.  In addition, the
attribute change time and modification time fields of the directory that
contains the new entry are updated.

   Adding a new name for a file with the `link' function updates the
attribute change time field of the file being linked, and both the
attribute change time and modification time fields of the directory
containing the new name.  These same fields are affected if a file name
is deleted with `unlink', `remove' or `rmdir'.  Renaming a file with
`rename' affects only the attribute change time and modification time
fields of the two parent directories involved, and not the times for
the file being renamed.

   Changing the attributes of a file (for example, with `chmod')
updates its attribute change time field.

   You can also change some of the time stamps of a file explicitly
using the `utime' function--all except the attribute change time.  You
need to include the header file `utime.h' to use this facility.  

 -- Data Type: struct utimbuf
     The `utimbuf' structure is used with the `utime' function to
     specify new access and modification times for a file.  It contains
     the following members:

    `time_t actime'
          This is the access time for the file.

    `time_t modtime'
          This is the modification time for the file.

 -- Function: int utime (const char *FILENAME, const struct utimbuf
          *TIMES)
     This function is used to modify the file times associated with the
     file named FILENAME.

     If TIMES is a null pointer, then the access and modification times
     of the file are set to the current time.  Otherwise, they are set
     to the values from the `actime' and `modtime' members
     (respectively) of the `utimbuf' structure pointed to by TIMES.

     The attribute modification time for the file is set to the current
     time in either case (since changing the time stamps is itself a
     modification of the file attributes).

     The `utime' function returns `0' if successful and `-1' on
     failure.  In addition to the usual file name errors (*note File
     Name Errors::), the following `errno' error conditions are defined
     for this function:

    `EACCES'
          There is a permission problem in the case where a null
          pointer was passed as the TIMES argument.  In order to update
          the time stamp on the file, you must either be the owner of
          the file, have write permission for the file, or be a
          privileged user.

    `ENOENT'
          The file doesn't exist.

    `EPERM'
          If the TIMES argument is not a null pointer, you must either
          be the owner of the file or be a privileged user.

    `EROFS'
          The file lives on a read-only file system.

   Each of the three time stamps has a corresponding microsecond part,
which extends its resolution.  These fields are called `st_atime_usec',
`st_mtime_usec', and `st_ctime_usec'; each has a value between 0 and
999,999, which indicates the time in microseconds.  They correspond to
the `tv_usec' field of a `timeval' structure; see *note High-Resolution
Calendar::.

   The `utimes' function is like `utime', but also lets you specify the
fractional part of the file times.  The prototype for this function is
in the header file `sys/time.h'.  

 -- Function: int utimes (const char *FILENAME, const struct timeval
          TVP[2])
     This function sets the file access and modification times of the
     file FILENAME.  The new file access time is specified by `TVP[0]',
     and the new modification time by `TVP[1]'.  Similar to `utime', if
     TVP is a null pointer then the access and modification times of
     the file are set to the current time.  This function comes from
     BSD.

     The return values and error conditions are the same as for the
     `utime' function.

 -- Function: int lutimes (const char *FILENAME, const struct timeval
          TVP[2])
     This function is like `utimes', except that it does not follow
     symbolic links.  If FILENAME is the name of a symbolic link,
     `lutimes' sets the file access and modification times of the
     symbolic link special file itself (as seen by `lstat'; *note
     Symbolic Links::) while `utimes' sets the file access and
     modification times of the file the symbolic link refers to.  This
     function comes from FreeBSD, and is not available on all platforms
     (if not available, it will fail with `ENOSYS').

     The return values and error conditions are the same as for the
     `utime' function.

 -- Function: int futimes (int FD, const struct timeval TVP[2])
     This function is like `utimes', except that it takes an open file
     descriptor as an argument instead of a file name.  *Note Low-Level
     I/O::.  This function comes from FreeBSD, and is not available on
     all platforms (if not available, it will fail with `ENOSYS').

     Like `utimes', `futimes' returns `0' on success and `-1' on
     failure.  The following `errno' error conditions are defined for
     `futimes':

    `EACCES'
          There is a permission problem in the case where a null
          pointer was passed as the TIMES argument.  In order to update
          the time stamp on the file, you must either be the owner of
          the file, have write permission for the file, or be a
          privileged user.

    `EBADF'
          The FILEDES argument is not a valid file descriptor.

    `EPERM'
          If the TIMES argument is not a null pointer, you must either
          be the owner of the file or be a privileged user.

    `EROFS'
          The file lives on a read-only file system.


File: libc.info,  Node: File Size,  Prev: File Times,  Up: File Attributes

14.9.10 File Size
-----------------

Normally file sizes are maintained automatically.  A file begins with a
size of 0 and is automatically extended when data is written past its
end.  It is also possible to empty a file completely by an `open' or
`fopen' call.

   However, sometimes it is necessary to _reduce_ the size of a file.
This can be done with the `truncate' and `ftruncate' functions.  They
were introduced in BSD Unix.  `ftruncate' was later added to POSIX.1.

   Some systems allow you to extend a file (creating holes) with these
functions.  This is useful when using memory-mapped I/O (*note
Memory-mapped I/O::), where files are not automatically extended.
However, it is not portable but must be implemented if `mmap' allows
mapping of files (i.e., `_POSIX_MAPPED_FILES' is defined).

   Using these functions on anything other than a regular file gives
_undefined_ results.  On many systems, such a call will appear to
succeed, without actually accomplishing anything.

 -- Function: int truncate (const char *FILENAME, off_t LENGTH)
     The `truncate' function changes the size of FILENAME to LENGTH.
     If LENGTH is shorter than the previous length, data at the end
     will be lost.  The file must be writable by the user to perform
     this operation.

     If LENGTH is longer, holes will be added to the end.  However, some
     systems do not support this feature and will leave the file
     unchanged.

     When the source file is compiled with `_FILE_OFFSET_BITS == 64' the
     `truncate' function is in fact `truncate64' and the type `off_t'
     has 64 bits which makes it possible to handle files up to 2^63
     bytes in length.

     The return value is 0 for success, or -1 for an error.  In
     addition to the usual file name errors, the following errors may
     occur:

    `EACCES'
          The file is a directory or not writable.

    `EINVAL'
          LENGTH is negative.

    `EFBIG'
          The operation would extend the file beyond the limits of the
          operating system.

    `EIO'
          A hardware I/O error occurred.

    `EPERM'
          The file is "append-only" or "immutable".

    `EINTR'
          The operation was interrupted by a signal.


 -- Function: int truncate64 (const char *NAME, off64_t LENGTH)
     This function is similar to the `truncate' function.  The
     difference is that the LENGTH argument is 64 bits wide even on 32
     bits machines, which allows the handling of files with sizes up to
     2^63 bytes.

     When the source file is compiled with `_FILE_OFFSET_BITS == 64' on
     a 32 bits machine this function is actually available under the
     name `truncate' and so transparently replaces the 32 bits
     interface.

 -- Function: int ftruncate (int FD, off_t LENGTH)
     This is like `truncate', but it works on a file descriptor FD for
     an opened file instead of a file name to identify the object.  The
     file must be opened for writing to successfully carry out the
     operation.

     The POSIX standard leaves it implementation defined what happens
     if the specified new LENGTH of the file is bigger than the
     original size.  The `ftruncate' function might simply leave the
     file alone and do nothing or it can increase the size to the
     desired size.  In this later case the extended area should be
     zero-filled.  So using `ftruncate' is no reliable way to increase
     the file size but if it is possible it is probably the fastest
     way.  The function also operates on POSIX shared memory segments
     if these are implemented by the system.

     `ftruncate' is especially useful in combination with `mmap'.
     Since the mapped region must have a fixed size one cannot enlarge
     the file by writing something beyond the last mapped page.
     Instead one has to enlarge the file itself and then remap the file
     with the new size.  The example below shows how this works.

     When the source file is compiled with `_FILE_OFFSET_BITS == 64' the
     `ftruncate' function is in fact `ftruncate64' and the type `off_t'
     has 64 bits which makes it possible to handle files up to 2^63
     bytes in length.

     The return value is 0 for success, or -1 for an error.  The
     following errors may occur:

    `EBADF'
          FD does not correspond to an open file.

    `EACCES'
          FD is a directory or not open for writing.

    `EINVAL'
          LENGTH is negative.

    `EFBIG'
          The operation would extend the file beyond the limits of the
          operating system.

    `EIO'
          A hardware I/O error occurred.

    `EPERM'
          The file is "append-only" or "immutable".

    `EINTR'
          The operation was interrupted by a signal.


 -- Function: int ftruncate64 (int ID, off64_t LENGTH)
     This function is similar to the `ftruncate' function.  The
     difference is that the LENGTH argument is 64 bits wide even on 32
     bits machines which allows the handling of files with sizes up to
     2^63 bytes.

     When the source file is compiled with `_FILE_OFFSET_BITS == 64' on
     a 32 bits machine this function is actually available under the
     name `ftruncate' and so transparently replaces the 32 bits
     interface.

   As announced here is a little example of how to use `ftruncate' in
combination with `mmap':

     int fd;
     void *start;
     size_t len;

     int
     add (off_t at, void *block, size_t size)
     {
       if (at + size > len)
         {
           /* Resize the file and remap.  */
           size_t ps = sysconf (_SC_PAGESIZE);
           size_t ns = (at + size + ps - 1) & ~(ps - 1);
           void *np;
           if (ftruncate (fd, ns) < 0)
             return -1;
           np = mmap (NULL, ns, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
           if (np == MAP_FAILED)
             return -1;
           start = np;
           len = ns;
         }
       memcpy ((char *) start + at, block, size);
       return 0;
     }

   The function `add' writes a block of memory at an arbitrary position
in the file.  If the current size of the file is too small it is
extended.  Note the it is extended by a round number of pages.  This is
a requirement of `mmap'.  The program has to keep track of the real
size, and when it has finished a final `ftruncate' call should set the
real size of the file.


File: libc.info,  Node: Making Special Files,  Next: Temporary Files,  Prev: File Attributes,  Up: File System Interface

14.10 Making Special Files
==========================

The `mknod' function is the primitive for making special files, such as
files that correspond to devices.  The GNU C Library includes this
function for compatibility with BSD.

   The prototype for `mknod' is declared in `sys/stat.h'.  

 -- Function: int mknod (const char *FILENAME, mode_t MODE, dev_t DEV)
     The `mknod' function makes a special file with name FILENAME.  The
     MODE specifies the mode of the file, and may include the various
     special file bits, such as `S_IFCHR' (for a character special file)
     or `S_IFBLK' (for a block special file).  *Note Testing File
     Type::.

     The DEV argument specifies which device the special file refers to.
     Its exact interpretation depends on the kind of special file being
     created.

     The return value is `0' on success and `-1' on error.  In addition
     to the usual file name errors (*note File Name Errors::), the
     following `errno' error conditions are defined for this function:

    `EPERM'
          The calling process is not privileged.  Only the superuser
          can create special files.

    `ENOSPC'
          The directory or file system that would contain the new file
          is full and cannot be extended.

    `EROFS'
          The directory containing the new file can't be modified
          because it's on a read-only file system.

    `EEXIST'
          There is already a file named FILENAME.  If you want to
          replace this file, you must remove the old file explicitly
          first.


File: libc.info,  Node: Temporary Files,  Prev: Making Special Files,  Up: File System Interface

14.11 Temporary Files
=====================

If you need to use a temporary file in your program, you can use the
`tmpfile' function to open it.  Or you can use the `tmpnam' (better:
`tmpnam_r') function to provide a name for a temporary file and then
you can open it in the usual way with `fopen'.

   The `tempnam' function is like `tmpnam' but lets you choose what
directory temporary files will go in, and something about what their
file names will look like.  Important for multi-threaded programs is
that `tempnam' is reentrant, while `tmpnam' is not since it returns a
pointer to a static buffer.

   These facilities are declared in the header file `stdio.h'.  

 -- Function: FILE * tmpfile (void)
     This function creates a temporary binary file for update mode, as
     if by calling `fopen' with mode `"wb+"'.  The file is deleted
     automatically when it is closed or when the program terminates.
     (On some other ISO C systems the file may fail to be deleted if
     the program terminates abnormally).

     This function is reentrant.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a
     32-bit system this function is in fact `tmpfile64', i.e., the LFS
     interface transparently replaces the old interface.

 -- Function: FILE * tmpfile64 (void)
     This function is similar to `tmpfile', but the stream it returns a
     pointer to was opened using `tmpfile64'.  Therefore this stream can
     be used for files larger then 2^31 bytes on 32-bit machines.

     Please note that the return type is still `FILE *'.  There is no
     special `FILE' type for the LFS interface.

     If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32
     bits machine this function is available under the name `tmpfile'
     and so transparently replaces the old interface.

 -- Function: char * tmpnam (char *RESULT)
     This function constructs and returns a valid file name that does
     not refer to any existing file.  If the RESULT argument is a null
     pointer, the return value is a pointer to an internal static
     string, which might be modified by subsequent calls and therefore
     makes this function non-reentrant.  Otherwise, the RESULT argument
     should be a pointer to an array of at least `L_tmpnam' characters,
     and the result is written into that array.

     It is possible for `tmpnam' to fail if you call it too many times
     without removing previously-created files.  This is because the
     limited length of the temporary file names gives room for only a
     finite number of different names.  If `tmpnam' fails it returns a
     null pointer.

     *Warning:* Between the time the pathname is constructed and the
     file is created another process might have created a file with the
     same name using `tmpnam', leading to a possible security hole.  The
     implementation generates names which can hardly be predicted, but
     when opening the file you should use the `O_EXCL' flag.  Using
     `tmpfile' or `mkstemp' is a safe way to avoid this problem.

 -- Function: char * tmpnam_r (char *RESULT)
     This function is nearly identical to the `tmpnam' function, except
     that if RESULT is a null pointer it returns a null pointer.

     This guarantees reentrancy because the non-reentrant situation of
     `tmpnam' cannot happen here.

     *Warning*: This function has the same security problems as
     `tmpnam'.

 -- Macro: int L_tmpnam
     The value of this macro is an integer constant expression that
     represents the minimum size of a string large enough to hold a
     file name generated by the `tmpnam' function.

 -- Macro: int TMP_MAX
     The macro `TMP_MAX' is a lower bound for how many temporary names
     you can create with `tmpnam'.  You can rely on being able to call
     `tmpnam' at least this many times before it might fail saying you
     have made too many temporary file names.

     With the GNU C Library, you can create a very large number of
     temporary file names.  If you actually created the files, you
     would probably run out of disk space before you ran out of names.
     Some other systems have a fixed, small limit on the number of
     temporary files.  The limit is never less than `25'.

 -- Function: char * tempnam (const char *DIR, const char *PREFIX)
     This function generates a unique temporary file name.  If PREFIX
     is not a null pointer, up to five characters of this string are
     used as a prefix for the file name.  The return value is a string
     newly allocated with `malloc', so you should release its storage
     with `free' when it is no longer needed.

     Because the string is dynamically allocated this function is
     reentrant.

     The directory prefix for the temporary file name is determined by
     testing each of the following in sequence.  The directory must
     exist and be writable.

        * The environment variable `TMPDIR', if it is defined.  For
          security reasons this only happens if the program is not SUID
          or SGID enabled.

        * The DIR argument, if it is not a null pointer.

        * The value of the `P_tmpdir' macro.

        * The directory `/tmp'.

     This function is defined for SVID compatibility.

     *Warning:* Between the time the pathname is constructed and the
     file is created another process might have created a file with the
     same name using `tempnam', leading to a possible security hole.
     The implementation generates names which can hardly be predicted,
     but when opening the file you should use the `O_EXCL' flag.  Using
     `tmpfile' or `mkstemp' is a safe way to avoid this problem.
   
 -- SVID Macro: char * P_tmpdir
     This macro is the name of the default directory for temporary
     files.

   Older Unix systems did not have the functions just described.
Instead they used `mktemp' and `mkstemp'.  Both of these functions work
by modifying a file name template string you pass.  The last six
characters of this string must be `XXXXXX'.  These six `X's are
replaced with six characters which make the whole string a unique file
name.  Usually the template string is something like
`/tmp/PREFIXXXXXXX', and each program uses a unique PREFIX.

   *NB:* Because `mktemp' and `mkstemp' modify the template string, you
_must not_ pass string constants to them.  String constants are
normally in read-only storage, so your program would crash when
`mktemp' or `mkstemp' tried to modify the string.  These functions are
declared in the header file `stdlib.h'.  

 -- Function: char * mktemp (char *TEMPLATE)
     The `mktemp' function generates a unique file name by modifying
     TEMPLATE as described above.  If successful, it returns TEMPLATE
     as modified.  If `mktemp' cannot find a unique file name, it makes
     TEMPLATE an empty string and returns that.  If TEMPLATE does not
     end with `XXXXXX', `mktemp' returns a null pointer.

     *Warning:* Between the time the pathname is constructed and the
     file is created another process might have created a file with the
     same name using `mktemp', leading to a possible security hole.  The
     implementation generates names which can hardly be predicted, but
     when opening the file you should use the `O_EXCL' flag.  Using
     `mkstemp' is a safe way to avoid this problem.

 -- Function: int mkstemp (char *TEMPLATE)
     The `mkstemp' function generates a unique file name just as
     `mktemp' does, but it also opens the file for you with `open'
     (*note Opening and Closing Files::).  If successful, it modifies
     TEMPLATE in place and returns a file descriptor for that file open
     for reading and writing.  If `mkstemp' cannot create a
     uniquely-named file, it returns `-1'.  If TEMPLATE does not end
     with `XXXXXX', `mkstemp' returns `-1' and does not modify TEMPLATE.

     The file is opened using mode `0600'.  If the file is meant to be
     used by other users this mode must be changed explicitly.

   Unlike `mktemp', `mkstemp' is actually guaranteed to create a unique
file that cannot possibly clash with any other program trying to create
a temporary file.  This is because it works by calling `open' with the
`O_EXCL' flag, which says you want to create a new file and get an
error if the file already exists.

 -- Function: char * mkdtemp (char *TEMPLATE)
     The `mkdtemp' function creates a directory with a unique name.  If
     it succeeds, it overwrites TEMPLATE with the name of the
     directory, and returns TEMPLATE.  As with `mktemp' and `mkstemp',
     TEMPLATE should be a string ending with `XXXXXX'.

     If `mkdtemp' cannot create an uniquely named directory, it returns
     `NULL' and sets ERRNO appropriately.  If TEMPLATE does not end
     with `XXXXXX', `mkdtemp' returns `NULL' and does not modify
     TEMPLATE.  ERRNO will be set to `EINVAL' in this case.

     The directory is created using mode `0700'.

   The directory created by `mkdtemp' cannot clash with temporary files
or directories created by other users.  This is because directory
creation always works like `open' with `O_EXCL'.  *Note Creating
Directories::.

   The `mkdtemp' function comes from OpenBSD.


File: libc.info,  Node: Pipes and FIFOs,  Next: Sockets,  Prev: File System Interface,  Up: Top

15 Pipes and FIFOs
******************

A "pipe" is a mechanism for interprocess communication; data written to
the pipe by one process can be read by another process.  The data is
handled in a first-in, first-out (FIFO) order.  The pipe has no name; it
is created for one use and both ends must be inherited from the single
process which created the pipe.

   A "FIFO special file" is similar to a pipe, but instead of being an
anonymous, temporary connection, a FIFO has a name or names like any
other file.  Processes open the FIFO by name in order to communicate
through it.

   A pipe or FIFO has to be open at both ends simultaneously.  If you
read from a pipe or FIFO file that doesn't have any processes writing
to it (perhaps because they have all closed the file, or exited), the
read returns end-of-file.  Writing to a pipe or FIFO that doesn't have a
reading process is treated as an error condition; it generates a
`SIGPIPE' signal, and fails with error code `EPIPE' if the signal is
handled or blocked.

   Neither pipes nor FIFO special files allow file positioning.  Both
reading and writing operations happen sequentially; reading from the
beginning of the file and writing at the end.

* Menu:

* Creating a Pipe::             Making a pipe with the `pipe' function.
* Pipe to a Subprocess::        Using a pipe to communicate with a
				 child process.
* FIFO Special Files::          Making a FIFO special file.
* Pipe Atomicity::		When pipe (or FIFO) I/O is atomic.


File: libc.info,  Node: Creating a Pipe,  Next: Pipe to a Subprocess,  Up: Pipes and FIFOs

15.1 Creating a Pipe
====================

The primitive for creating a pipe is the `pipe' function.  This creates
both the reading and writing ends of the pipe.  It is not very useful
for a single process to use a pipe to talk to itself.  In typical use,
a process creates a pipe just before it forks one or more child
processes (*note Creating a Process::).  The pipe is then used for
communication either between the parent or child processes, or between
two sibling processes.

   The `pipe' function is declared in the header file `unistd.h'.  

 -- Function: int pipe (int FILEDES[2])
     The `pipe' function creates a pipe and puts the file descriptors
     for the reading and writing ends of the pipe (respectively) into
     `FILEDES[0]' and `FILEDES[1]'.

     An easy way to remember that the input end comes first is that file
     descriptor `0' is standard input, and file descriptor `1' is
     standard output.

     If successful, `pipe' returns a value of `0'.  On failure, `-1' is
     returned.  The following `errno' error conditions are defined for
     this function:

    `EMFILE'
          The process has too many files open.

    `ENFILE'
          There are too many open files in the entire system.  *Note
          Error Codes::, for more information about `ENFILE'.  This
          error never occurs on GNU/Hurd systems.

   Here is an example of a simple program that creates a pipe.  This
program uses the `fork' function (*note Creating a Process::) to create
a child process.  The parent process writes data to the pipe, which is
read by the child process.


     #include <sys/types.h>
     #include <unistd.h>
     #include <stdio.h>
     #include <stdlib.h>

     /* Read characters from the pipe and echo them to `stdout'. */

     void
     read_from_pipe (int file)
     {
       FILE *stream;
       int c;
       stream = fdopen (file, "r");
       while ((c = fgetc (stream)) != EOF)
         putchar (c);
       fclose (stream);
     }

     /* Write some random text to the pipe. */

     void
     write_to_pipe (int file)
     {
       FILE *stream;
       stream = fdopen (file, "w");
       fprintf (stream, "hello, world!\n");
       fprintf (stream, "goodbye, world!\n");
       fclose (stream);
     }

     int
     main (void)
     {
       pid_t pid;
       int mypipe[2];

       /* Create the pipe. */
       if (pipe (mypipe))
         {
           fprintf (stderr, "Pipe failed.\n");
           return EXIT_FAILURE;
         }

       /* Create the child process. */
       pid = fork ();
       if (pid == (pid_t) 0)
         {
           /* This is the child process.
              Close other end first. */
           close (mypipe[1]);
           read_from_pipe (mypipe[0]);
           return EXIT_SUCCESS;
         }
       else if (pid < (pid_t) 0)
         {
           /* The fork failed. */
           fprintf (stderr, "Fork failed.\n");
           return EXIT_FAILURE;
         }
       else
         {
           /* This is the parent process.
              Close other end first. */
           close (mypipe[0]);
           write_to_pipe (mypipe[1]);
           return EXIT_SUCCESS;
         }
     }


File: libc.info,  Node: Pipe to a Subprocess,  Next: FIFO Special Files,  Prev: Creating a Pipe,  Up: Pipes and FIFOs

15.2 Pipe to a Subprocess
=========================

A common use of pipes is to send data to or receive data from a program
being run as a subprocess.  One way of doing this is by using a
combination of `pipe' (to create the pipe), `fork' (to create the
subprocess), `dup2' (to force the subprocess to use the pipe as its
standard input or output channel), and `exec' (to execute the new
program).  Or, you can use `popen' and `pclose'.

   The advantage of using `popen' and `pclose' is that the interface is
much simpler and easier to use.  But it doesn't offer as much
flexibility as using the low-level functions directly.

 -- Function: FILE * popen (const char *COMMAND, const char *MODE)
     The `popen' function is closely related to the `system' function;
     see *note Running a Command::.  It executes the shell command
     COMMAND as a subprocess.  However, instead of waiting for the
     command to complete, it creates a pipe to the subprocess and
     returns a stream that corresponds to that pipe.

     If you specify a MODE argument of `"r"', you can read from the
     stream to retrieve data from the standard output channel of the
     subprocess.  The subprocess inherits its standard input channel
     from the parent process.

     Similarly, if you specify a MODE argument of `"w"', you can write
     to the stream to send data to the standard input channel of the
     subprocess.  The subprocess inherits its standard output channel
     from the parent process.

     In the event of an error `popen' returns a null pointer.  This
     might happen if the pipe or stream cannot be created, if the
     subprocess cannot be forked, or if the program cannot be executed.

 -- Function: int pclose (FILE *STREAM)
     The `pclose' function is used to close a stream created by `popen'.
     It waits for the child process to terminate and returns its status
     value, as for the `system' function.

   Here is an example showing how to use `popen' and `pclose' to filter
output through another program, in this case the paging program `more'.


     #include <stdio.h>
     #include <stdlib.h>

     void
     write_data (FILE * stream)
     {
       int i;
       for (i = 0; i < 100; i++)
         fprintf (stream, "%d\n", i);
       if (ferror (stream))
         {
           fprintf (stderr, "Output to stream failed.\n");
           exit (EXIT_FAILURE);
         }
     }

     int
     main (void)
     {
       FILE *output;

       output = popen ("more", "w");
       if (!output)
         {
           fprintf (stderr,
                    "incorrect parameters or too many files.\n");
           return EXIT_FAILURE;
         }
       write_data (output);
       if (pclose (output) != 0)
         {
           fprintf (stderr,
                    "Could not run more or other error.\n");
         }
       return EXIT_SUCCESS;
     }


File: libc.info,  Node: FIFO Special Files,  Next: Pipe Atomicity,  Prev: Pipe to a Subprocess,  Up: Pipes and FIFOs

15.3 FIFO Special Files
=======================

A FIFO special file is similar to a pipe, except that it is created in a
different way.  Instead of being an anonymous communications channel, a
FIFO special file is entered into the file system by calling `mkfifo'.

   Once you have created a FIFO special file in this way, any process
can open it for reading or writing, in the same way as an ordinary file.
However, it has to be open at both ends simultaneously before you can
proceed to do any input or output operations on it.  Opening a FIFO for
reading normally blocks until some other process opens the same FIFO for
writing, and vice versa.

   The `mkfifo' function is declared in the header file `sys/stat.h'.  

 -- Function: int mkfifo (const char *FILENAME, mode_t MODE)
     The `mkfifo' function makes a FIFO special file with name
     FILENAME.  The MODE argument is used to set the file's
     permissions; see *note Setting Permissions::.

     The normal, successful return value from `mkfifo' is `0'.  In the
     case of an error, `-1' is returned.  In addition to the usual file
     name errors (*note File Name Errors::), the following `errno'
     error conditions are defined for this function:

    `EEXIST'
          The named file already exists.

    `ENOSPC'
          The directory or file system cannot be extended.

    `EROFS'
          The directory that would contain the file resides on a
          read-only file system.


File: libc.info,  Node: Pipe Atomicity,  Prev: FIFO Special Files,  Up: Pipes and FIFOs

15.4 Atomicity of Pipe I/O
==========================

Reading or writing pipe data is "atomic" if the size of data written is
not greater than `PIPE_BUF'.  This means that the data transfer seems
to be an instantaneous unit, in that nothing else in the system can
observe a state in which it is partially complete.  Atomic I/O may not
begin right away (it may need to wait for buffer space or for data),
but once it does begin it finishes immediately.

   Reading or writing a larger amount of data may not be atomic; for
example, output data from other processes sharing the descriptor may be
interspersed.  Also, once `PIPE_BUF' characters have been written,
further writes will block until some characters are read.

   *Note Limits for Files::, for information about the `PIPE_BUF'
parameter.