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This is gdb.info, produced by makeinfo version 4.8 from ./gdb.texinfo.
INFO-DIR-SECTION Software development
START-INFO-DIR-ENTRY
* Gdb: (gdb). The GNU debugger.
END-INFO-DIR-ENTRY
Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996,
1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009,
2010 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" and "Free Software Needs Free
Documentation", with the Front-Cover Texts being "A GNU Manual," and
with the Back-Cover Texts as in (a) below.
(a) The FSF's Back-Cover Text is: "You are free to copy and modify
this GNU Manual. Buying copies from GNU Press supports the FSF in
developing GNU and promoting software freedom."
This file documents the GNU debugger GDB.
This is the Ninth Edition, of `Debugging with GDB: the GNU
Source-Level Debugger' for GDB (GDB) Version 7.2.
Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996,
1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009,
2010 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" and "Free Software Needs Free
Documentation", with the Front-Cover Texts being "A GNU Manual," and
with the Back-Cover Texts as in (a) below.
(a) The FSF's Back-Cover Text is: "You are free to copy and modify
this GNU Manual. Buying copies from GNU Press supports the FSF in
developing GNU and promoting software freedom."

File: gdb.info, Node: Registers, Next: Floating Point Hardware, Prev: Convenience Vars, Up: Data
10.12 Registers
===============
You can refer to machine register contents, in expressions, as variables
with names starting with `$'. The names of registers are different for
each machine; use `info registers' to see the names used on your
machine.
`info registers'
Print the names and values of all registers except floating-point
and vector registers (in the selected stack frame).
`info all-registers'
Print the names and values of all registers, including
floating-point and vector registers (in the selected stack frame).
`info registers REGNAME ...'
Print the "relativized" value of each specified register REGNAME.
As discussed in detail below, register values are normally
relative to the selected stack frame. REGNAME may be any register
name valid on the machine you are using, with or without the
initial `$'.
GDB has four "standard" register names that are available (in
expressions) on most machines--whenever they do not conflict with an
architecture's canonical mnemonics for registers. The register names
`$pc' and `$sp' are used for the program counter register and the stack
pointer. `$fp' is used for a register that contains a pointer to the
current stack frame, and `$ps' is used for a register that contains the
processor status. For example, you could print the program counter in
hex with
p/x $pc
or print the instruction to be executed next with
x/i $pc
or add four to the stack pointer(1) with
set $sp += 4
Whenever possible, these four standard register names are available
on your machine even though the machine has different canonical
mnemonics, so long as there is no conflict. The `info registers'
command shows the canonical names. For example, on the SPARC, `info
registers' displays the processor status register as `$psr' but you can
also refer to it as `$ps'; and on x86-based machines `$ps' is an alias
for the EFLAGS register.
GDB always considers the contents of an ordinary register as an
integer when the register is examined in this way. Some machines have
special registers which can hold nothing but floating point; these
registers are considered to have floating point values. There is no way
to refer to the contents of an ordinary register as floating point value
(although you can _print_ it as a floating point value with `print/f
$REGNAME').
Some registers have distinct "raw" and "virtual" data formats. This
means that the data format in which the register contents are saved by
the operating system is not the same one that your program normally
sees. For example, the registers of the 68881 floating point
coprocessor are always saved in "extended" (raw) format, but all C
programs expect to work with "double" (virtual) format. In such cases,
GDB normally works with the virtual format only (the format that makes
sense for your program), but the `info registers' command prints the
data in both formats.
Some machines have special registers whose contents can be
interpreted in several different ways. For example, modern x86-based
machines have SSE and MMX registers that can hold several values packed
together in several different formats. GDB refers to such registers in
`struct' notation:
(gdb) print $xmm1
$1 = {
v4_float = {0, 3.43859137e-038, 1.54142831e-044, 1.821688e-044},
v2_double = {9.92129282474342e-303, 2.7585945287983262e-313},
v16_int8 = "\000\000\000\000\3706;\001\v\000\000\000\r\000\000",
v8_int16 = {0, 0, 14072, 315, 11, 0, 13, 0},
v4_int32 = {0, 20657912, 11, 13},
v2_int64 = {88725056443645952, 55834574859},
uint128 = 0x0000000d0000000b013b36f800000000
}
To set values of such registers, you need to tell GDB which view of the
register you wish to change, as if you were assigning value to a
`struct' member:
(gdb) set $xmm1.uint128 = 0x000000000000000000000000FFFFFFFF
Normally, register values are relative to the selected stack frame
(*note Selecting a Frame: Selection.). This means that you get the
value that the register would contain if all stack frames farther in
were exited and their saved registers restored. In order to see the
true contents of hardware registers, you must select the innermost
frame (with `frame 0').
However, GDB must deduce where registers are saved, from the machine
code generated by your compiler. If some registers are not saved, or if
GDB is unable to locate the saved registers, the selected stack frame
makes no difference.
---------- Footnotes ----------
(1) This is a way of removing one word from the stack, on machines
where stacks grow downward in memory (most machines, nowadays). This
assumes that the innermost stack frame is selected; setting `$sp' is
not allowed when other stack frames are selected. To pop entire frames
off the stack, regardless of machine architecture, use `return'; see
*Note Returning from a Function: Returning.

File: gdb.info, Node: Floating Point Hardware, Next: Vector Unit, Prev: Registers, Up: Data
10.13 Floating Point Hardware
=============================
Depending on the configuration, GDB may be able to give you more
information about the status of the floating point hardware.
`info float'
Display hardware-dependent information about the floating point
unit. The exact contents and layout vary depending on the
floating point chip. Currently, `info float' is supported on the
ARM and x86 machines.

File: gdb.info, Node: Vector Unit, Next: OS Information, Prev: Floating Point Hardware, Up: Data
10.14 Vector Unit
=================
Depending on the configuration, GDB may be able to give you more
information about the status of the vector unit.
`info vector'
Display information about the vector unit. The exact contents and
layout vary depending on the hardware.

File: gdb.info, Node: OS Information, Next: Memory Region Attributes, Prev: Vector Unit, Up: Data
10.15 Operating System Auxiliary Information
============================================
GDB provides interfaces to useful OS facilities that can help you debug
your program.
When GDB runs on a "Posix system" (such as GNU or Unix machines), it
interfaces with the inferior via the `ptrace' system call. The
operating system creates a special sata structure, called `struct
user', for this interface. You can use the command `info udot' to
display the contents of this data structure.
`info udot'
Display the contents of the `struct user' maintained by the OS
kernel for the program being debugged. GDB displays the contents
of `struct user' as a list of hex numbers, similar to the
`examine' command.
Some operating systems supply an "auxiliary vector" to programs at
startup. This is akin to the arguments and environment that you
specify for a program, but contains a system-dependent variety of
binary values that tell system libraries important details about the
hardware, operating system, and process. Each value's purpose is
identified by an integer tag; the meanings are well-known but
system-specific. Depending on the configuration and operating system
facilities, GDB may be able to show you this information. For remote
targets, this functionality may further depend on the remote stub's
support of the `qXfer:auxv:read' packet, see *Note qXfer auxiliary
vector read::.
`info auxv'
Display the auxiliary vector of the inferior, which can be either a
live process or a core dump file. GDB prints each tag value
numerically, and also shows names and text descriptions for
recognized tags. Some values in the vector are numbers, some bit
masks, and some pointers to strings or other data. GDB displays
each value in the most appropriate form for a recognized tag, and
in hexadecimal for an unrecognized tag.
On some targets, GDB can access operating-system-specific information
and display it to user, without interpretation. For remote targets,
this functionality depends on the remote stub's support of the
`qXfer:osdata:read' packet, see *Note qXfer osdata read::.
`info os'
List the types of OS information available for the target. If the
target does not return a list of possible types, this command will
report an error.
`info os processes'
Display the list of processes on the target. For each process,
GDB prints the process identifier, the name of the user, and the
command corresponding to the process.

File: gdb.info, Node: Memory Region Attributes, Next: Dump/Restore Files, Prev: OS Information, Up: Data
10.16 Memory Region Attributes
==============================
"Memory region attributes" allow you to describe special handling
required by regions of your target's memory. GDB uses attributes to
determine whether to allow certain types of memory accesses; whether to
use specific width accesses; and whether to cache target memory. By
default the description of memory regions is fetched from the target
(if the current target supports this), but the user can override the
fetched regions.
Defined memory regions can be individually enabled and disabled.
When a memory region is disabled, GDB uses the default attributes when
accessing memory in that region. Similarly, if no memory regions have
been defined, GDB uses the default attributes when accessing all memory.
When a memory region is defined, it is given a number to identify it;
to enable, disable, or remove a memory region, you specify that number.
`mem LOWER UPPER ATTRIBUTES...'
Define a memory region bounded by LOWER and UPPER with attributes
ATTRIBUTES..., and add it to the list of regions monitored by GDB.
Note that UPPER == 0 is a special case: it is treated as the
target's maximum memory address. (0xffff on 16 bit targets,
0xffffffff on 32 bit targets, etc.)
`mem auto'
Discard any user changes to the memory regions and use
target-supplied regions, if available, or no regions if the target
does not support.
`delete mem NUMS...'
Remove memory regions NUMS... from the list of regions monitored
by GDB.
`disable mem NUMS...'
Disable monitoring of memory regions NUMS.... A disabled memory
region is not forgotten. It may be enabled again later.
`enable mem NUMS...'
Enable monitoring of memory regions NUMS....
`info mem'
Print a table of all defined memory regions, with the following
columns for each region:
_Memory Region Number_
_Enabled or Disabled._
Enabled memory regions are marked with `y'. Disabled memory
regions are marked with `n'.
_Lo Address_
The address defining the inclusive lower bound of the memory
region.
_Hi Address_
The address defining the exclusive upper bound of the memory
region.
_Attributes_
The list of attributes set for this memory region.
10.16.1 Attributes
------------------
10.16.1.1 Memory Access Mode
............................
The access mode attributes set whether GDB may make read or write
accesses to a memory region.
While these attributes prevent GDB from performing invalid memory
accesses, they do nothing to prevent the target system, I/O DMA, etc.
from accessing memory.
`ro'
Memory is read only.
`wo'
Memory is write only.
`rw'
Memory is read/write. This is the default.
10.16.1.2 Memory Access Size
............................
The access size attribute tells GDB to use specific sized accesses in
the memory region. Often memory mapped device registers require
specific sized accesses. If no access size attribute is specified, GDB
may use accesses of any size.
`8'
Use 8 bit memory accesses.
`16'
Use 16 bit memory accesses.
`32'
Use 32 bit memory accesses.
`64'
Use 64 bit memory accesses.
10.16.1.3 Data Cache
....................
The data cache attributes set whether GDB will cache target memory.
While this generally improves performance by reducing debug protocol
overhead, it can lead to incorrect results because GDB does not know
about volatile variables or memory mapped device registers.
`cache'
Enable GDB to cache target memory.
`nocache'
Disable GDB from caching target memory. This is the default.
10.16.2 Memory Access Checking
------------------------------
GDB can be instructed to refuse accesses to memory that is not
explicitly described. This can be useful if accessing such regions has
undesired effects for a specific target, or to provide better error
checking. The following commands control this behaviour.
`set mem inaccessible-by-default [on|off]'
If `on' is specified, make GDB treat memory not explicitly
described by the memory ranges as non-existent and refuse accesses
to such memory. The checks are only performed if there's at least
one memory range defined. If `off' is specified, make GDB treat
the memory not explicitly described by the memory ranges as RAM.
The default value is `on'.
`show mem inaccessible-by-default'
Show the current handling of accesses to unknown memory.

File: gdb.info, Node: Dump/Restore Files, Next: Core File Generation, Prev: Memory Region Attributes, Up: Data
10.17 Copy Between Memory and a File
====================================
You can use the commands `dump', `append', and `restore' to copy data
between target memory and a file. The `dump' and `append' commands
write data to a file, and the `restore' command reads data from a file
back into the inferior's memory. Files may be in binary, Motorola
S-record, Intel hex, or Tektronix Hex format; however, GDB can only
append to binary files.
`dump [FORMAT] memory FILENAME START_ADDR END_ADDR'
`dump [FORMAT] value FILENAME EXPR'
Dump the contents of memory from START_ADDR to END_ADDR, or the
value of EXPR, to FILENAME in the given format.
The FORMAT parameter may be any one of:
`binary'
Raw binary form.
`ihex'
Intel hex format.
`srec'
Motorola S-record format.
`tekhex'
Tektronix Hex format.
GDB uses the same definitions of these formats as the GNU binary
utilities, like `objdump' and `objcopy'. If FORMAT is omitted,
GDB dumps the data in raw binary form.
`append [binary] memory FILENAME START_ADDR END_ADDR'
`append [binary] value FILENAME EXPR'
Append the contents of memory from START_ADDR to END_ADDR, or the
value of EXPR, to the file FILENAME, in raw binary form. (GDB can
only append data to files in raw binary form.)
`restore FILENAME [binary] BIAS START END'
Restore the contents of file FILENAME into memory. The `restore'
command can automatically recognize any known BFD file format,
except for raw binary. To restore a raw binary file you must
specify the optional keyword `binary' after the filename.
If BIAS is non-zero, its value will be added to the addresses
contained in the file. Binary files always start at address zero,
so they will be restored at address BIAS. Other bfd files have a
built-in location; they will be restored at offset BIAS from that
location.
If START and/or END are non-zero, then only data between file
offset START and file offset END will be restored. These offsets
are relative to the addresses in the file, before the BIAS
argument is applied.

File: gdb.info, Node: Core File Generation, Next: Character Sets, Prev: Dump/Restore Files, Up: Data
10.18 How to Produce a Core File from Your Program
==================================================
A "core file" or "core dump" is a file that records the memory image of
a running process and its process status (register values etc.). Its
primary use is post-mortem debugging of a program that crashed while it
ran outside a debugger. A program that crashes automatically produces
a core file, unless this feature is disabled by the user. *Note
Files::, for information on invoking GDB in the post-mortem debugging
mode.
Occasionally, you may wish to produce a core file of the program you
are debugging in order to preserve a snapshot of its state. GDB has a
special command for that.
`generate-core-file [FILE]'
`gcore [FILE]'
Produce a core dump of the inferior process. The optional argument
FILE specifies the file name where to put the core dump. If not
specified, the file name defaults to `core.PID', where PID is the
inferior process ID.
Note that this command is implemented only for some systems (as of
this writing, GNU/Linux, FreeBSD, Solaris, Unixware, and S390).

File: gdb.info, Node: Character Sets, Next: Caching Remote Data, Prev: Core File Generation, Up: Data
10.19 Character Sets
====================
If the program you are debugging uses a different character set to
represent characters and strings than the one GDB uses itself, GDB can
automatically translate between the character sets for you. The
character set GDB uses we call the "host character set"; the one the
inferior program uses we call the "target character set".
For example, if you are running GDB on a GNU/Linux system, which
uses the ISO Latin 1 character set, but you are using GDB's remote
protocol (*note Remote Debugging::) to debug a program running on an
IBM mainframe, which uses the EBCDIC character set, then the host
character set is Latin-1, and the target character set is EBCDIC. If
you give GDB the command `set target-charset EBCDIC-US', then GDB
translates between EBCDIC and Latin 1 as you print character or string
values, or use character and string literals in expressions.
GDB has no way to automatically recognize which character set the
inferior program uses; you must tell it, using the `set target-charset'
command, described below.
Here are the commands for controlling GDB's character set support:
`set target-charset CHARSET'
Set the current target character set to CHARSET. To display the
list of supported target character sets, type
`set target-charset <TAB><TAB>'.
`set host-charset CHARSET'
Set the current host character set to CHARSET.
By default, GDB uses a host character set appropriate to the
system it is running on; you can override that default using the
`set host-charset' command. On some systems, GDB cannot
automatically determine the appropriate host character set. In
this case, GDB uses `UTF-8'.
GDB can only use certain character sets as its host character set.
If you type `set target-charset <TAB><TAB>', GDB will list the
host character sets it supports.
`set charset CHARSET'
Set the current host and target character sets to CHARSET. As
above, if you type `set charset <TAB><TAB>', GDB will list the
names of the character sets that can be used for both host and
target.
`show charset'
Show the names of the current host and target character sets.
`show host-charset'
Show the name of the current host character set.
`show target-charset'
Show the name of the current target character set.
`set target-wide-charset CHARSET'
Set the current target's wide character set to CHARSET. This is
the character set used by the target's `wchar_t' type. To display
the list of supported wide character sets, type
`set target-wide-charset <TAB><TAB>'.
`show target-wide-charset'
Show the name of the current target's wide character set.
Here is an example of GDB's character set support in action. Assume
that the following source code has been placed in the file
`charset-test.c':
#include <stdio.h>
char ascii_hello[]
= {72, 101, 108, 108, 111, 44, 32, 119,
111, 114, 108, 100, 33, 10, 0};
char ibm1047_hello[]
= {200, 133, 147, 147, 150, 107, 64, 166,
150, 153, 147, 132, 90, 37, 0};
main ()
{
printf ("Hello, world!\n");
}
In this program, `ascii_hello' and `ibm1047_hello' are arrays
containing the string `Hello, world!' followed by a newline, encoded in
the ASCII and IBM1047 character sets.
We compile the program, and invoke the debugger on it:
$ gcc -g charset-test.c -o charset-test
$ gdb -nw charset-test
GNU gdb 2001-12-19-cvs
Copyright 2001 Free Software Foundation, Inc.
...
(gdb)
We can use the `show charset' command to see what character sets GDB
is currently using to interpret and display characters and strings:
(gdb) show charset
The current host and target character set is `ISO-8859-1'.
(gdb)
For the sake of printing this manual, let's use ASCII as our initial
character set:
(gdb) set charset ASCII
(gdb) show charset
The current host and target character set is `ASCII'.
(gdb)
Let's assume that ASCII is indeed the correct character set for our
host system -- in other words, let's assume that if GDB prints
characters using the ASCII character set, our terminal will display
them properly. Since our current target character set is also ASCII,
the contents of `ascii_hello' print legibly:
(gdb) print ascii_hello
$1 = 0x401698 "Hello, world!\n"
(gdb) print ascii_hello[0]
$2 = 72 'H'
(gdb)
GDB uses the target character set for character and string literals
you use in expressions:
(gdb) print '+'
$3 = 43 '+'
(gdb)
The ASCII character set uses the number 43 to encode the `+'
character.
GDB relies on the user to tell it which character set the target
program uses. If we print `ibm1047_hello' while our target character
set is still ASCII, we get jibberish:
(gdb) print ibm1047_hello
$4 = 0x4016a8 "\310\205\223\223\226k@\246\226\231\223\204Z%"
(gdb) print ibm1047_hello[0]
$5 = 200 '\310'
(gdb)
If we invoke the `set target-charset' followed by <TAB><TAB>, GDB
tells us the character sets it supports:
(gdb) set target-charset
ASCII EBCDIC-US IBM1047 ISO-8859-1
(gdb) set target-charset
We can select IBM1047 as our target character set, and examine the
program's strings again. Now the ASCII string is wrong, but GDB
translates the contents of `ibm1047_hello' from the target character
set, IBM1047, to the host character set, ASCII, and they display
correctly:
(gdb) set target-charset IBM1047
(gdb) show charset
The current host character set is `ASCII'.
The current target character set is `IBM1047'.
(gdb) print ascii_hello
$6 = 0x401698 "\110\145%%?\054\040\167?\162%\144\041\012"
(gdb) print ascii_hello[0]
$7 = 72 '\110'
(gdb) print ibm1047_hello
$8 = 0x4016a8 "Hello, world!\n"
(gdb) print ibm1047_hello[0]
$9 = 200 'H'
(gdb)
As above, GDB uses the target character set for character and string
literals you use in expressions:
(gdb) print '+'
$10 = 78 '+'
(gdb)
The IBM1047 character set uses the number 78 to encode the `+'
character.

File: gdb.info, Node: Caching Remote Data, Next: Searching Memory, Prev: Character Sets, Up: Data
10.20 Caching Data of Remote Targets
====================================
GDB caches data exchanged between the debugger and a remote target
(*note Remote Debugging::). Such caching generally improves
performance, because it reduces the overhead of the remote protocol by
bundling memory reads and writes into large chunks. Unfortunately,
simply caching everything would lead to incorrect results, since GDB
does not necessarily know anything about volatile values, memory-mapped
I/O addresses, etc. Furthermore, in non-stop mode (*note Non-Stop
Mode::) memory can be changed _while_ a gdb command is executing.
Therefore, by default, GDB only caches data known to be on the stack(1).
Other regions of memory can be explicitly marked as cacheable; see
*note Memory Region Attributes::.
`set remotecache on'
`set remotecache off'
This option no longer does anything; it exists for compatibility
with old scripts.
`show remotecache'
Show the current state of the obsolete remotecache flag.
`set stack-cache on'
`set stack-cache off'
Enable or disable caching of stack accesses. When `ON', use
caching. By default, this option is `ON'.
`show stack-cache'
Show the current state of data caching for memory accesses.
`info dcache [line]'
Print the information about the data cache performance. The
information displayed includes the dcache width and depth, and for
each cache line, its number, address, and how many times it was
referenced. This command is useful for debugging the data cache
operation.
If a line number is specified, the contents of that line will be
printed in hex.
---------- Footnotes ----------
(1) In non-stop mode, it is moderately rare for a running thread to
modify the stack of a stopped thread in a way that would interfere with
a backtrace, and caching of stack reads provides a significant speed up
of remote backtraces.

File: gdb.info, Node: Searching Memory, Prev: Caching Remote Data, Up: Data
10.21 Search Memory
===================
Memory can be searched for a particular sequence of bytes with the
`find' command.
`find [/SN] START_ADDR, +LEN, VAL1 [, VAL2, ...]'
`find [/SN] START_ADDR, END_ADDR, VAL1 [, VAL2, ...]'
Search memory for the sequence of bytes specified by VAL1, VAL2,
etc. The search begins at address START_ADDR and continues for
either LEN bytes or through to END_ADDR inclusive.
S and N are optional parameters. They may be specified in either
order, apart or together.
S, search query size
The size of each search query value.
`b'
bytes
`h'
halfwords (two bytes)
`w'
words (four bytes)
`g'
giant words (eight bytes)
All values are interpreted in the current language. This means,
for example, that if the current source language is C/C++ then
searching for the string "hello" includes the trailing '\0'.
If the value size is not specified, it is taken from the value's
type in the current language. This is useful when one wants to
specify the search pattern as a mixture of types. Note that this
means, for example, that in the case of C-like languages a search
for an untyped 0x42 will search for `(int) 0x42' which is
typically four bytes.
N, maximum number of finds
The maximum number of matches to print. The default is to print
all finds.
You can use strings as search values. Quote them with double-quotes
(`"'). The string value is copied into the search pattern byte by
byte, regardless of the endianness of the target and the size
specification.
The address of each match found is printed as well as a count of the
number of matches found.
The address of the last value found is stored in convenience variable
`$_'. A count of the number of matches is stored in `$numfound'.
For example, if stopped at the `printf' in this function:
void
hello ()
{
static char hello[] = "hello-hello";
static struct { char c; short s; int i; }
__attribute__ ((packed)) mixed
= { 'c', 0x1234, 0x87654321 };
printf ("%s\n", hello);
}
you get during debugging:
(gdb) find &hello[0], +sizeof(hello), "hello"
0x804956d <hello.1620+6>
1 pattern found
(gdb) find &hello[0], +sizeof(hello), 'h', 'e', 'l', 'l', 'o'
0x8049567 <hello.1620>
0x804956d <hello.1620+6>
2 patterns found
(gdb) find /b1 &hello[0], +sizeof(hello), 'h', 0x65, 'l'
0x8049567 <hello.1620>
1 pattern found
(gdb) find &mixed, +sizeof(mixed), (char) 'c', (short) 0x1234, (int) 0x87654321
0x8049560 <mixed.1625>
1 pattern found
(gdb) print $numfound
$1 = 1
(gdb) print $_
$2 = (void *) 0x8049560

File: gdb.info, Node: Optimized Code, Next: Macros, Prev: Data, Up: Top
11 Debugging Optimized Code
***************************
Almost all compilers support optimization. With optimization disabled,
the compiler generates assembly code that corresponds directly to your
source code, in a simplistic way. As the compiler applies more
powerful optimizations, the generated assembly code diverges from your
original source code. With help from debugging information generated
by the compiler, GDB can map from the running program back to
constructs from your original source.
GDB is more accurate with optimization disabled. If you can
recompile without optimization, it is easier to follow the progress of
your program during debugging. But, there are many cases where you may
need to debug an optimized version.
When you debug a program compiled with `-g -O', remember that the
optimizer has rearranged your code; the debugger shows you what is
really there. Do not be too surprised when the execution path does not
exactly match your source file! An extreme example: if you define a
variable, but never use it, GDB never sees that variable--because the
compiler optimizes it out of existence.
Some things do not work as well with `-g -O' as with just `-g',
particularly on machines with instruction scheduling. If in doubt,
recompile with `-g' alone, and if this fixes the problem, please report
it to us as a bug (including a test case!). *Note Variables::, for
more information about debugging optimized code.
* Menu:
* Inline Functions:: How GDB presents inlining

File: gdb.info, Node: Inline Functions, Up: Optimized Code
11.1 Inline Functions
=====================
"Inlining" is an optimization that inserts a copy of the function body
directly at each call site, instead of jumping to a shared routine.
GDB displays inlined functions just like non-inlined functions. They
appear in backtraces. You can view their arguments and local
variables, step into them with `step', skip them with `next', and
escape from them with `finish'. You can check whether a function was
inlined by using the `info frame' command.
For GDB to support inlined functions, the compiler must record
information about inlining in the debug information -- GCC using the
DWARF 2 format does this, and several other compilers do also. GDB
only supports inlined functions when using DWARF 2. Versions of GCC
before 4.1 do not emit two required attributes (`DW_AT_call_file' and
`DW_AT_call_line'); GDB does not display inlined function calls with
earlier versions of GCC. It instead displays the arguments and local
variables of inlined functions as local variables in the caller.
The body of an inlined function is directly included at its call
site; unlike a non-inlined function, there are no instructions devoted
to the call. GDB still pretends that the call site and the start of
the inlined function are different instructions. Stepping to the call
site shows the call site, and then stepping again shows the first line
of the inlined function, even though no additional instructions are
executed.
This makes source-level debugging much clearer; you can see both the
context of the call and then the effect of the call. Only stepping by
a single instruction using `stepi' or `nexti' does not do this; single
instruction steps always show the inlined body.
There are some ways that GDB does not pretend that inlined function
calls are the same as normal calls:
* You cannot set breakpoints on inlined functions. GDB either
reports that there is no symbol with that name, or else sets the
breakpoint only on non-inlined copies of the function. This
limitation will be removed in a future version of GDB; until then,
set a breakpoint by line number on the first line of the inlined
function instead.
* Setting breakpoints at the call site of an inlined function may not
work, because the call site does not contain any code. GDB may
incorrectly move the breakpoint to the next line of the enclosing
function, after the call. This limitation will be removed in a
future version of GDB; until then, set a breakpoint on an earlier
line or inside the inlined function instead.
* GDB cannot locate the return value of inlined calls after using
the `finish' command. This is a limitation of compiler-generated
debugging information; after `finish', you can step to the next
line and print a variable where your program stored the return
value.

File: gdb.info, Node: Macros, Next: Tracepoints, Prev: Optimized Code, Up: Top
12 C Preprocessor Macros
************************
Some languages, such as C and C++, provide a way to define and invoke
"preprocessor macros" which expand into strings of tokens. GDB can
evaluate expressions containing macro invocations, show the result of
macro expansion, and show a macro's definition, including where it was
defined.
You may need to compile your program specially to provide GDB with
information about preprocessor macros. Most compilers do not include
macros in their debugging information, even when you compile with the
`-g' flag. *Note Compilation::.
A program may define a macro at one point, remove that definition
later, and then provide a different definition after that. Thus, at
different points in the program, a macro may have different
definitions, or have no definition at all. If there is a current stack
frame, GDB uses the macros in scope at that frame's source code line.
Otherwise, GDB uses the macros in scope at the current listing location;
see *Note List::.
Whenever GDB evaluates an expression, it always expands any macro
invocations present in the expression. GDB also provides the following
commands for working with macros explicitly.
`macro expand EXPRESSION'
`macro exp EXPRESSION'
Show the results of expanding all preprocessor macro invocations in
EXPRESSION. Since GDB simply expands macros, but does not parse
the result, EXPRESSION need not be a valid expression; it can be
any string of tokens.
`macro expand-once EXPRESSION'
`macro exp1 EXPRESSION'
(This command is not yet implemented.) Show the results of
expanding those preprocessor macro invocations that appear
explicitly in EXPRESSION. Macro invocations appearing in that
expansion are left unchanged. This command allows you to see the
effect of a particular macro more clearly, without being confused
by further expansions. Since GDB simply expands macros, but does
not parse the result, EXPRESSION need not be a valid expression; it
can be any string of tokens.
`info macro MACRO'
Show the definition of the macro named MACRO, and describe the
source location or compiler command-line where that definition was
established.
`macro define MACRO REPLACEMENT-LIST'
`macro define MACRO(ARGLIST) REPLACEMENT-LIST'
Introduce a definition for a preprocessor macro named MACRO,
invocations of which are replaced by the tokens given in
REPLACEMENT-LIST. The first form of this command defines an
"object-like" macro, which takes no arguments; the second form
defines a "function-like" macro, which takes the arguments given in
ARGLIST.
A definition introduced by this command is in scope in every
expression evaluated in GDB, until it is removed with the `macro
undef' command, described below. The definition overrides all
definitions for MACRO present in the program being debugged, as
well as any previous user-supplied definition.
`macro undef MACRO'
Remove any user-supplied definition for the macro named MACRO.
This command only affects definitions provided with the `macro
define' command, described above; it cannot remove definitions
present in the program being debugged.
`macro list'
List all the macros defined using the `macro define' command.
Here is a transcript showing the above commands in action. First, we
show our source files:
$ cat sample.c
#include <stdio.h>
#include "sample.h"
#define M 42
#define ADD(x) (M + x)
main ()
{
#define N 28
printf ("Hello, world!\n");
#undef N
printf ("We're so creative.\n");
#define N 1729
printf ("Goodbye, world!\n");
}
$ cat sample.h
#define Q <
$
Now, we compile the program using the GNU C compiler, GCC. We pass
the `-gdwarf-2' and `-g3' flags to ensure the compiler includes
information about preprocessor macros in the debugging information.
$ gcc -gdwarf-2 -g3 sample.c -o sample
$
Now, we start GDB on our sample program:
$ gdb -nw sample
GNU gdb 2002-05-06-cvs
Copyright 2002 Free Software Foundation, Inc.
GDB is free software, ...
(gdb)
We can expand macros and examine their definitions, even when the
program is not running. GDB uses the current listing position to
decide which macro definitions are in scope:
(gdb) list main
3
4 #define M 42
5 #define ADD(x) (M + x)
6
7 main ()
8 {
9 #define N 28
10 printf ("Hello, world!\n");
11 #undef N
12 printf ("We're so creative.\n");
(gdb) info macro ADD
Defined at /home/jimb/gdb/macros/play/sample.c:5
#define ADD(x) (M + x)
(gdb) info macro Q
Defined at /home/jimb/gdb/macros/play/sample.h:1
included at /home/jimb/gdb/macros/play/sample.c:2
#define Q <
(gdb) macro expand ADD(1)
expands to: (42 + 1)
(gdb) macro expand-once ADD(1)
expands to: once (M + 1)
(gdb)
In the example above, note that `macro expand-once' expands only the
macro invocation explicit in the original text -- the invocation of
`ADD' -- but does not expand the invocation of the macro `M', which was
introduced by `ADD'.
Once the program is running, GDB uses the macro definitions in force
at the source line of the current stack frame:
(gdb) break main
Breakpoint 1 at 0x8048370: file sample.c, line 10.
(gdb) run
Starting program: /home/jimb/gdb/macros/play/sample
Breakpoint 1, main () at sample.c:10
10 printf ("Hello, world!\n");
(gdb)
At line 10, the definition of the macro `N' at line 9 is in force:
(gdb) info macro N
Defined at /home/jimb/gdb/macros/play/sample.c:9
#define N 28
(gdb) macro expand N Q M
expands to: 28 < 42
(gdb) print N Q M
$1 = 1
(gdb)
As we step over directives that remove `N''s definition, and then
give it a new definition, GDB finds the definition (or lack thereof) in
force at each point:
(gdb) next
Hello, world!
12 printf ("We're so creative.\n");
(gdb) info macro N
The symbol `N' has no definition as a C/C++ preprocessor macro
at /home/jimb/gdb/macros/play/sample.c:12
(gdb) next
We're so creative.
14 printf ("Goodbye, world!\n");
(gdb) info macro N
Defined at /home/jimb/gdb/macros/play/sample.c:13
#define N 1729
(gdb) macro expand N Q M
expands to: 1729 < 42
(gdb) print N Q M
$2 = 0
(gdb)
In addition to source files, macros can be defined on the
compilation command line using the `-DNAME=VALUE' syntax. For macros
defined in such a way, GDB displays the location of their definition as
line zero of the source file submitted to the compiler.
(gdb) info macro __STDC__
Defined at /home/jimb/gdb/macros/play/sample.c:0
-D__STDC__=1
(gdb)

File: gdb.info, Node: Tracepoints, Next: Overlays, Prev: Macros, Up: Top
13 Tracepoints
**************
In some applications, it is not feasible for the debugger to interrupt
the program's execution long enough for the developer to learn anything
helpful about its behavior. If the program's correctness depends on
its real-time behavior, delays introduced by a debugger might cause the
program to change its behavior drastically, or perhaps fail, even when
the code itself is correct. It is useful to be able to observe the
program's behavior without interrupting it.
Using GDB's `trace' and `collect' commands, you can specify
locations in the program, called "tracepoints", and arbitrary
expressions to evaluate when those tracepoints are reached. Later,
using the `tfind' command, you can examine the values those expressions
had when the program hit the tracepoints. The expressions may also
denote objects in memory--structures or arrays, for example--whose
values GDB should record; while visiting a particular tracepoint, you
may inspect those objects as if they were in memory at that moment.
However, because GDB records these values without interacting with you,
it can do so quickly and unobtrusively, hopefully not disturbing the
program's behavior.
The tracepoint facility is currently available only for remote
targets. *Note Targets::. In addition, your remote target must know
how to collect trace data. This functionality is implemented in the
remote stub; however, none of the stubs distributed with GDB support
tracepoints as of this writing. The format of the remote packets used
to implement tracepoints are described in *Note Tracepoint Packets::.
It is also possible to get trace data from a file, in a manner
reminiscent of corefiles; you specify the filename, and use `tfind' to
search through the file. *Note Trace Files::, for more details.
This chapter describes the tracepoint commands and features.
* Menu:
* Set Tracepoints::
* Analyze Collected Data::
* Tracepoint Variables::
* Trace Files::

File: gdb.info, Node: Set Tracepoints, Next: Analyze Collected Data, Up: Tracepoints
13.1 Commands to Set Tracepoints
================================
Before running such a "trace experiment", an arbitrary number of
tracepoints can be set. A tracepoint is actually a special type of
breakpoint (*note Set Breaks::), so you can manipulate it using
standard breakpoint commands. For instance, as with breakpoints,
tracepoint numbers are successive integers starting from one, and many
of the commands associated with tracepoints take the tracepoint number
as their argument, to identify which tracepoint to work on.
For each tracepoint, you can specify, in advance, some arbitrary set
of data that you want the target to collect in the trace buffer when it
hits that tracepoint. The collected data can include registers, local
variables, or global data. Later, you can use GDB commands to examine
the values these data had at the time the tracepoint was hit.
Tracepoints do not support every breakpoint feature. Ignore counts
on tracepoints have no effect, and tracepoints cannot run GDB commands
when they are hit. Tracepoints may not be thread-specific either.
Some targets may support "fast tracepoints", which are inserted in a
different way (such as with a jump instead of a trap), that is faster
but possibly restricted in where they may be installed.
Regular and fast tracepoints are dynamic tracing facilities, meaning
that they can be used to insert tracepoints at (almost) any location in
the target. Some targets may also support controlling "static
tracepoints" from GDB. With static tracing, a set of instrumentation
points, also known as "markers", are embedded in the target program,
and can be activated or deactivated by name or address. These are
usually placed at locations which facilitate investigating what the
target is actually doing. GDB's support for static tracing includes
being able to list instrumentation points, and attach them with GDB
defined high level tracepoints that expose the whole range of
convenience of GDB's tracepoints support. Namelly, support for
collecting registers values and values of global or local (to the
instrumentation point) variables; tracepoint conditions and trace state
variables. The act of installing a GDB static tracepoint on an
instrumentation point, or marker, is referred to as "probing" a static
tracepoint marker.
`gdbserver' supports tracepoints on some target systems. *Note
Tracepoints support in `gdbserver': Server.
This section describes commands to set tracepoints and associated
conditions and actions.
* Menu:
* Create and Delete Tracepoints::
* Enable and Disable Tracepoints::
* Tracepoint Passcounts::
* Tracepoint Conditions::
* Trace State Variables::
* Tracepoint Actions::
* Listing Tracepoints::
* Listing Static Tracepoint Markers::
* Starting and Stopping Trace Experiments::
* Tracepoint Restrictions::

File: gdb.info, Node: Create and Delete Tracepoints, Next: Enable and Disable Tracepoints, Up: Set Tracepoints
13.1.1 Create and Delete Tracepoints
------------------------------------
`trace LOCATION'
The `trace' command is very similar to the `break' command. Its
argument LOCATION can be a source line, a function name, or an
address in the target program. *Note Specify Location::. The
`trace' command defines a tracepoint, which is a point in the
target program where the debugger will briefly stop, collect some
data, and then allow the program to continue. Setting a
tracepoint or changing its actions doesn't take effect until the
next `tstart' command, and once a trace experiment is running,
further changes will not have any effect until the next trace
experiment starts.
Here are some examples of using the `trace' command:
(gdb) trace foo.c:121 // a source file and line number
(gdb) trace +2 // 2 lines forward
(gdb) trace my_function // first source line of function
(gdb) trace *my_function // EXACT start address of function
(gdb) trace *0x2117c4 // an address
You can abbreviate `trace' as `tr'.
`trace LOCATION if COND'
Set a tracepoint with condition COND; evaluate the expression COND
each time the tracepoint is reached, and collect data only if the
value is nonzero--that is, if COND evaluates as true. *Note
Tracepoint Conditions: Tracepoint Conditions, for more information
on tracepoint conditions.
`ftrace LOCATION [ if COND ]'
The `ftrace' command sets a fast tracepoint. For targets that
support them, fast tracepoints will use a more efficient but
possibly less general technique to trigger data collection, such
as a jump instruction instead of a trap, or some sort of hardware
support. It may not be possible to create a fast tracepoint at
the desired location, in which case the command will exit with an
explanatory message.
GDB handles arguments to `ftrace' exactly as for `trace'.
`strace LOCATION [ if COND ]'
The `strace' command sets a static tracepoint. For targets that
support it, setting a static tracepoint probes a static
instrumentation point, or marker, found at LOCATION. It may not
be possible to set a static tracepoint at the desired location, in
which case the command will exit with an explanatory message.
GDB handles arguments to `strace' exactly as for `trace', with the
addition that the user can also specify `-m MARKER' as LOCATION.
This probes the marker identified by the MARKER string identifier.
This identifier depends on the static tracepoint backend library
your program is using. You can find all the marker identifiers in
the `ID' field of the `info static-tracepoint-markers' command
output. *Note Listing Static Tracepoint Markers: Listing Static
Tracepoint Markers. For example, in the following small program
using the UST tracing engine:
main ()
{
trace_mark(ust, bar33, "str %s", "FOOBAZ");
}
the marker id is composed of joining the first two arguments to the
`trace_mark' call with a slash, which translates to:
(gdb) info static-tracepoint-markers
Cnt Enb ID Address What
1 n ust/bar33 0x0000000000400ddc in main at stexample.c:22
Data: "str %s"
[etc...]
so you may probe the marker above with:
(gdb) strace -m ust/bar33
Static tracepoints accept an extra collect action -- `collect
$_sdata'. This collects arbitrary user data passed in the probe
point call to the tracing library. In the UST example above,
you'll see that the third argument to `trace_mark' is a
printf-like format string. The user data is then the result of
running that formating string against the following arguments.
Note that `info static-tracepoint-markers' command output lists
that format string in the `Data:' field.
You can inspect this data when analyzing the trace buffer, by
printing the $_sdata variable like any other variable available to
GDB. *Note Tracepoint Action Lists: Tracepoint Actions.
The convenience variable `$tpnum' records the tracepoint number of
the most recently set tracepoint.
`delete tracepoint [NUM]'
Permanently delete one or more tracepoints. With no argument, the
default is to delete all tracepoints. Note that the regular
`delete' command can remove tracepoints also.
Examples:
(gdb) delete trace 1 2 3 // remove three tracepoints
(gdb) delete trace // remove all tracepoints
You can abbreviate this command as `del tr'.

File: gdb.info, Node: Enable and Disable Tracepoints, Next: Tracepoint Passcounts, Prev: Create and Delete Tracepoints, Up: Set Tracepoints
13.1.2 Enable and Disable Tracepoints
-------------------------------------
These commands are deprecated; they are equivalent to plain `disable'
and `enable'.
`disable tracepoint [NUM]'
Disable tracepoint NUM, or all tracepoints if no argument NUM is
given. A disabled tracepoint will have no effect during the next
trace experiment, but it is not forgotten. You can re-enable a
disabled tracepoint using the `enable tracepoint' command.
`enable tracepoint [NUM]'
Enable tracepoint NUM, or all tracepoints. The enabled
tracepoints will become effective the next time a trace experiment
is run.

File: gdb.info, Node: Tracepoint Passcounts, Next: Tracepoint Conditions, Prev: Enable and Disable Tracepoints, Up: Set Tracepoints
13.1.3 Tracepoint Passcounts
----------------------------
`passcount [N [NUM]]'
Set the "passcount" of a tracepoint. The passcount is a way to
automatically stop a trace experiment. If a tracepoint's
passcount is N, then the trace experiment will be automatically
stopped on the N'th time that tracepoint is hit. If the
tracepoint number NUM is not specified, the `passcount' command
sets the passcount of the most recently defined tracepoint. If no
passcount is given, the trace experiment will run until stopped
explicitly by the user.
Examples:
(gdb) passcount 5 2 // Stop on the 5th execution of
`// tracepoint 2'
(gdb) passcount 12 // Stop on the 12th execution of the
`// most recently defined tracepoint.'
(gdb) trace foo
(gdb) pass 3
(gdb) trace bar
(gdb) pass 2
(gdb) trace baz
(gdb) pass 1 // Stop tracing when foo has been
`// executed 3 times OR when bar has'
`// been executed 2 times'
`// OR when baz has been executed 1 time.'

File: gdb.info, Node: Tracepoint Conditions, Next: Trace State Variables, Prev: Tracepoint Passcounts, Up: Set Tracepoints
13.1.4 Tracepoint Conditions
----------------------------
The simplest sort of tracepoint collects data every time your program
reaches a specified place. You can also specify a "condition" for a
tracepoint. A condition is just a Boolean expression in your
programming language (*note Expressions: Expressions.). A tracepoint
with a condition evaluates the expression each time your program
reaches it, and data collection happens only if the condition is true.
Tracepoint conditions can be specified when a tracepoint is set, by
using `if' in the arguments to the `trace' command. *Note Setting
Tracepoints: Create and Delete Tracepoints. They can also be set or
changed at any time with the `condition' command, just as with
breakpoints.
Unlike breakpoint conditions, GDB does not actually evaluate the
conditional expression itself. Instead, GDB encodes the expression
into an agent expression (*note Agent Expressions:: suitable for
execution on the target, independently of GDB. Global variables become
raw memory locations, locals become stack accesses, and so forth.
For instance, suppose you have a function that is usually called
frequently, but should not be called after an error has occurred. You
could use the following tracepoint command to collect data about calls
of that function that happen while the error code is propagating
through the program; an unconditional tracepoint could end up
collecting thousands of useless trace frames that you would have to
search through.
(gdb) trace normal_operation if errcode > 0

File: gdb.info, Node: Trace State Variables, Next: Tracepoint Actions, Prev: Tracepoint Conditions, Up: Set Tracepoints
13.1.5 Trace State Variables
----------------------------
A "trace state variable" is a special type of variable that is created
and managed by target-side code. The syntax is the same as that for
GDB's convenience variables (a string prefixed with "$"), but they are
stored on the target. They must be created explicitly, using a
`tvariable' command. They are always 64-bit signed integers.
Trace state variables are remembered by GDB, and downloaded to the
target along with tracepoint information when the trace experiment
starts. There are no intrinsic limits on the number of trace state
variables, beyond memory limitations of the target.
Although trace state variables are managed by the target, you can use
them in print commands and expressions as if they were convenience
variables; GDB will get the current value from the target while the
trace experiment is running. Trace state variables share the same
namespace as other "$" variables, which means that you cannot have
trace state variables with names like `$23' or `$pc', nor can you have
a trace state variable and a convenience variable with the same name.
`tvariable $NAME [ = EXPRESSION ]'
The `tvariable' command creates a new trace state variable named
`$NAME', and optionally gives it an initial value of EXPRESSION.
EXPRESSION is evaluated when this command is entered; the result
will be converted to an integer if possible, otherwise GDB will
report an error. A subsequent `tvariable' command specifying the
same name does not create a variable, but instead assigns the
supplied initial value to the existing variable of that name,
overwriting any previous initial value. The default initial value
is 0.
`info tvariables'
List all the trace state variables along with their initial values.
Their current values may also be displayed, if the trace
experiment is currently running.
`delete tvariable [ $NAME ... ]'
Delete the given trace state variables, or all of them if no
arguments are specified.

File: gdb.info, Node: Tracepoint Actions, Next: Listing Tracepoints, Prev: Trace State Variables, Up: Set Tracepoints
13.1.6 Tracepoint Action Lists
------------------------------
`actions [NUM]'
This command will prompt for a list of actions to be taken when the
tracepoint is hit. If the tracepoint number NUM is not specified,
this command sets the actions for the one that was most recently
defined (so that you can define a tracepoint and then say
`actions' without bothering about its number). You specify the
actions themselves on the following lines, one action at a time,
and terminate the actions list with a line containing just `end'.
So far, the only defined actions are `collect', `teval', and
`while-stepping'.
`actions' is actually equivalent to `commands' (*note Breakpoint
Command Lists: Break Commands.), except that only the defined
actions are allowed; any other GDB command is rejected.
To remove all actions from a tracepoint, type `actions NUM' and
follow it immediately with `end'.
(gdb) collect DATA // collect some data
(gdb) while-stepping 5 // single-step 5 times, collect data
(gdb) end // signals the end of actions.
In the following example, the action list begins with `collect'
commands indicating the things to be collected when the tracepoint
is hit. Then, in order to single-step and collect additional data
following the tracepoint, a `while-stepping' command is used,
followed by the list of things to be collected after each step in a
sequence of single steps. The `while-stepping' command is
terminated by its own separate `end' command. Lastly, the action
list is terminated by an `end' command.
(gdb) trace foo
(gdb) actions
Enter actions for tracepoint 1, one per line:
> collect bar,baz
> collect $regs
> while-stepping 12
> collect $pc, arr[i]
> end
end
`collect EXPR1, EXPR2, ...'
Collect values of the given expressions when the tracepoint is hit.
This command accepts a comma-separated list of any valid
expressions. In addition to global, static, or local variables,
the following special arguments are supported:
`$regs'
Collect all registers.
`$args'
Collect all function arguments.
`$locals'
Collect all local variables.
`$_sdata'
Collect static tracepoint marker specific data. Only
available for static tracepoints. *Note Tracepoint Action
Lists: Tracepoint Actions. On the UST static tracepoints
library backend, an instrumentation point resembles a
`printf' function call. The tracing library is able to
collect user specified data formatted to a character string
using the format provided by the programmer that instrumented
the program. Other backends have similar mechanisms. Here's
an example of a UST marker call:
const char master_name[] = "$your_name";
trace_mark(channel1, marker1, "hello %s", master_name)
In this case, collecting `$_sdata' collects the string `hello
$yourname'. When analyzing the trace buffer, you can inspect
`$_sdata' like any other variable available to GDB.
You can give several consecutive `collect' commands, each one with
a single argument, or one `collect' command with several arguments
separated by commas; the effect is the same.
The command `info scope' (*note info scope: Symbols.) is
particularly useful for figuring out what data to collect.
`teval EXPR1, EXPR2, ...'
Evaluate the given expressions when the tracepoint is hit. This
command accepts a comma-separated list of expressions. The results
are discarded, so this is mainly useful for assigning values to
trace state variables (*note Trace State Variables::) without
adding those values to the trace buffer, as would be the case if
the `collect' action were used.
`while-stepping N'
Perform N single-step instruction traces after the tracepoint,
collecting new data after each step. The `while-stepping' command
is followed by the list of what to collect while stepping
(followed by its own `end' command):
> while-stepping 12
> collect $regs, myglobal
> end
>
Note that `$pc' is not automatically collected by
`while-stepping'; you need to explicitly collect that register if
you need it. You may abbreviate `while-stepping' as `ws' or
`stepping'.
`set default-collect EXPR1, EXPR2, ...'
This variable is a list of expressions to collect at each
tracepoint hit. It is effectively an additional `collect' action
prepended to every tracepoint action list. The expressions are
parsed individually for each tracepoint, so for instance a
variable named `xyz' may be interpreted as a global for one
tracepoint, and a local for another, as appropriate to the
tracepoint's location.
`show default-collect'
Show the list of expressions that are collected by default at each
tracepoint hit.

File: gdb.info, Node: Listing Tracepoints, Next: Listing Static Tracepoint Markers, Prev: Tracepoint Actions, Up: Set Tracepoints
13.1.7 Listing Tracepoints
--------------------------
`info tracepoints [NUM]'
Display information about the tracepoint NUM. If you don't
specify a tracepoint number, displays information about all the
tracepoints defined so far. The format is similar to that used for
`info breakpoints'; in fact, `info tracepoints' is the same
command, simply restricting itself to tracepoints.
A tracepoint's listing may include additional information specific
to tracing:
* its passcount as given by the `passcount N' command
(gdb) info trace
Num Type Disp Enb Address What
1 tracepoint keep y 0x0804ab57 in foo() at main.cxx:7
while-stepping 20
collect globfoo, $regs
end
collect globfoo2
end
pass count 1200
(gdb)
This command can be abbreviated `info tp'.

File: gdb.info, Node: Listing Static Tracepoint Markers, Next: Starting and Stopping Trace Experiments, Prev: Listing Tracepoints, Up: Set Tracepoints
13.1.8 Listing Static Tracepoint Markers
----------------------------------------
`info static-tracepoint-markers'
Display information about all static tracepoint markers defined in
the program.
For each marker, the following columns are printed:
_Count_
An incrementing counter, output to help readability. This is
not a stable identifier.
_ID_
The marker ID, as reported by the target.
_Enabled or Disabled_
Probed markers are tagged with `y'. `n' identifies marks
that are not enabled.
_Address_
Where the marker is in your program, as a memory address.
_What_
Where the marker is in the source for your program, as a file
and line number. If the debug information included in the
program does not allow GDB to locate the source of the
marker, this column will be left blank.
In addition, the following information may be printed for each
marker:
_Data_
User data passed to the tracing library by the marker call.
In the UST backend, this is the format string passed as
argument to the marker call.
_Static tracepoints probing the marker_
The list of static tracepoints attached to the marker.
(gdb) info static-tracepoint-markers
Cnt ID Enb Address What
1 ust/bar2 y 0x0000000000400e1a in main at stexample.c:25
Data: number1 %d number2 %d
Probed by static tracepoints: #2
2 ust/bar33 n 0x0000000000400c87 in main at stexample.c:24
Data: str %s
(gdb)

File: gdb.info, Node: Starting and Stopping Trace Experiments, Next: Tracepoint Restrictions, Prev: Listing Static Tracepoint Markers, Up: Set Tracepoints
13.1.9 Starting and Stopping Trace Experiments
----------------------------------------------
`tstart'
This command takes no arguments. It starts the trace experiment,
and begins collecting data. This has the side effect of
discarding all the data collected in the trace buffer during the
previous trace experiment.
`tstop'
This command takes no arguments. It ends the trace experiment, and
stops collecting data.
*Note*: a trace experiment and data collection may stop
automatically if any tracepoint's passcount is reached (*note
Tracepoint Passcounts::), or if the trace buffer becomes full.
`tstatus'
This command displays the status of the current trace data
collection.
Here is an example of the commands we described so far:
(gdb) trace gdb_c_test
(gdb) actions
Enter actions for tracepoint #1, one per line.
> collect $regs,$locals,$args
> while-stepping 11
> collect $regs
> end
> end
(gdb) tstart
[time passes ...]
(gdb) tstop
You can choose to continue running the trace experiment even if GDB
disconnects from the target, voluntarily or involuntarily. For
commands such as `detach', the debugger will ask what you want to do
with the trace. But for unexpected terminations (GDB crash, network
outage), it would be unfortunate to lose hard-won trace data, so the
variable `disconnected-tracing' lets you decide whether the trace should
continue running without GDB.
`set disconnected-tracing on'
`set disconnected-tracing off'
Choose whether a tracing run should continue to run if GDB has
disconnected from the target. Note that `detach' or `quit' will
ask you directly what to do about a running trace no matter what
this variable's setting, so the variable is mainly useful for
handling unexpected situations, such as loss of the network.
`show disconnected-tracing'
Show the current choice for disconnected tracing.
When you reconnect to the target, the trace experiment may or may not
still be running; it might have filled the trace buffer in the
meantime, or stopped for one of the other reasons. If it is running,
it will continue after reconnection.
Upon reconnection, the target will upload information about the
tracepoints in effect. GDB will then compare that information to the
set of tracepoints currently defined, and attempt to match them up,
allowing for the possibility that the numbers may have changed due to
creation and deletion in the meantime. If one of the target's
tracepoints does not match any in GDB, the debugger will create a new
tracepoint, so that you have a number with which to specify that
tracepoint. This matching-up process is necessarily heuristic, and it
may result in useless tracepoints being created; you may simply delete
them if they are of no use.
If your target agent supports a "circular trace buffer", then you
can run a trace experiment indefinitely without filling the trace
buffer; when space runs out, the agent deletes already-collected trace
frames, oldest first, until there is enough room to continue
collecting. This is especially useful if your tracepoints are being
hit too often, and your trace gets terminated prematurely because the
buffer is full. To ask for a circular trace buffer, simply set
`circular_trace_buffer' to on. You can set this at any time, including
during tracing; if the agent can do it, it will change buffer handling
on the fly, otherwise it will not take effect until the next run.
`set circular-trace-buffer on'
`set circular-trace-buffer off'
Choose whether a tracing run should use a linear or circular buffer
for trace data. A linear buffer will not lose any trace data, but
may fill up prematurely, while a circular buffer will discard old
trace data, but it will have always room for the latest tracepoint
hits.
`show circular-trace-buffer'
Show the current choice for the trace buffer. Note that this may
not match the agent's current buffer handling, nor is it
guaranteed to match the setting that might have been in effect
during a past run, for instance if you are looking at frames from
a trace file.

File: gdb.info, Node: Tracepoint Restrictions, Prev: Starting and Stopping Trace Experiments, Up: Set Tracepoints
13.1.10 Tracepoint Restrictions
-------------------------------
There are a number of restrictions on the use of tracepoints. As
described above, tracepoint data gathering occurs on the target without
interaction from GDB. Thus the full capabilities of the debugger are
not available during data gathering, and then at data examination time,
you will be limited by only having what was collected. The following
items describe some common problems, but it is not exhaustive, and you
may run into additional difficulties not mentioned here.
* Tracepoint expressions are intended to gather objects (lvalues).
Thus the full flexibility of GDB's expression evaluator is not
available. You cannot call functions, cast objects to aggregate
types, access convenience variables or modify values (except by
assignment to trace state variables). Some language features may
implicitly call functions (for instance Objective-C fields with
accessors), and therefore cannot be collected either.
* Collection of local variables, either individually or in bulk with
`$locals' or `$args', during `while-stepping' may behave
erratically. The stepping action may enter a new scope (for
instance by stepping into a function), or the location of the
variable may change (for instance it is loaded into a register).
The tracepoint data recorded uses the location information for the
variables that is correct for the tracepoint location. When the
tracepoint is created, it is not possible, in general, to determine
where the steps of a `while-stepping' sequence will advance the
program--particularly if a conditional branch is stepped.
* Collection of an incompletely-initialized or partially-destroyed
object may result in something that GDB cannot display, or displays
in a misleading way.
* When GDB displays a pointer to character it automatically
dereferences the pointer to also display characters of the string
being pointed to. However, collecting the pointer during tracing
does not automatically collect the string. You need to explicitly
dereference the pointer and provide size information if you want to
collect not only the pointer, but the memory pointed to. For
example, `*ptr@50' can be used to collect the 50 element array
pointed to by `ptr'.
* It is not possible to collect a complete stack backtrace at a
tracepoint. Instead, you may collect the registers and a few
hundred bytes from the stack pointer with something like
`*$esp@300' (adjust to use the name of the actual stack pointer
register on your target architecture, and the amount of stack you
wish to capture). Then the `backtrace' command will show a
partial backtrace when using a trace frame. The number of stack
frames that can be examined depends on the sizes of the frames in
the collected stack. Note that if you ask for a block so large
that it goes past the bottom of the stack, the target agent may
report an error trying to read from an invalid address.
* If you do not collect registers at a tracepoint, GDB can infer
that the value of `$pc' must be the same as the address of the
tracepoint and use that when you are looking at a trace frame for
that tracepoint. However, this cannot work if the tracepoint has
multiple locations (for instance if it was set in a function that
was inlined), or if it has a `while-stepping' loop. In those cases
GDB will warn you that it can't infer `$pc', and default it to
zero.

File: gdb.info, Node: Analyze Collected Data, Next: Tracepoint Variables, Prev: Set Tracepoints, Up: Tracepoints
13.2 Using the Collected Data
=============================
After the tracepoint experiment ends, you use GDB commands for
examining the trace data. The basic idea is that each tracepoint
collects a trace "snapshot" every time it is hit and another snapshot
every time it single-steps. All these snapshots are consecutively
numbered from zero and go into a buffer, and you can examine them
later. The way you examine them is to "focus" on a specific trace
snapshot. When the remote stub is focused on a trace snapshot, it will
respond to all GDB requests for memory and registers by reading from
the buffer which belongs to that snapshot, rather than from _real_
memory or registers of the program being debugged. This means that
*all* GDB commands (`print', `info registers', `backtrace', etc.) will
behave as if we were currently debugging the program state as it was
when the tracepoint occurred. Any requests for data that are not in
the buffer will fail.
* Menu:
* tfind:: How to select a trace snapshot
* tdump:: How to display all data for a snapshot
* save tracepoints:: How to save tracepoints for a future run

File: gdb.info, Node: tfind, Next: tdump, Up: Analyze Collected Data
13.2.1 `tfind N'
----------------
The basic command for selecting a trace snapshot from the buffer is
`tfind N', which finds trace snapshot number N, counting from zero. If
no argument N is given, the next snapshot is selected.
Here are the various forms of using the `tfind' command.
`tfind start'
Find the first snapshot in the buffer. This is a synonym for
`tfind 0' (since 0 is the number of the first snapshot).
`tfind none'
Stop debugging trace snapshots, resume _live_ debugging.
`tfind end'
Same as `tfind none'.
`tfind'
No argument means find the next trace snapshot.
`tfind -'
Find the previous trace snapshot before the current one. This
permits retracing earlier steps.
`tfind tracepoint NUM'
Find the next snapshot associated with tracepoint NUM. Search
proceeds forward from the last examined trace snapshot. If no
argument NUM is given, it means find the next snapshot collected
for the same tracepoint as the current snapshot.
`tfind pc ADDR'
Find the next snapshot associated with the value ADDR of the
program counter. Search proceeds forward from the last examined
trace snapshot. If no argument ADDR is given, it means find the
next snapshot with the same value of PC as the current snapshot.
`tfind outside ADDR1, ADDR2'
Find the next snapshot whose PC is outside the given range of
addresses (exclusive).
`tfind range ADDR1, ADDR2'
Find the next snapshot whose PC is between ADDR1 and ADDR2
(inclusive).
`tfind line [FILE:]N'
Find the next snapshot associated with the source line N. If the
optional argument FILE is given, refer to line N in that source
file. Search proceeds forward from the last examined trace
snapshot. If no argument N is given, it means find the next line
other than the one currently being examined; thus saying `tfind
line' repeatedly can appear to have the same effect as stepping
from line to line in a _live_ debugging session.
The default arguments for the `tfind' commands are specifically
designed to make it easy to scan through the trace buffer. For
instance, `tfind' with no argument selects the next trace snapshot, and
`tfind -' with no argument selects the previous trace snapshot. So, by
giving one `tfind' command, and then simply hitting <RET> repeatedly
you can examine all the trace snapshots in order. Or, by saying `tfind
-' and then hitting <RET> repeatedly you can examine the snapshots in
reverse order. The `tfind line' command with no argument selects the
snapshot for the next source line executed. The `tfind pc' command with
no argument selects the next snapshot with the same program counter
(PC) as the current frame. The `tfind tracepoint' command with no
argument selects the next trace snapshot collected by the same
tracepoint as the current one.
In addition to letting you scan through the trace buffer manually,
these commands make it easy to construct GDB scripts that scan through
the trace buffer and print out whatever collected data you are
interested in. Thus, if we want to examine the PC, FP, and SP
registers from each trace frame in the buffer, we can say this:
(gdb) tfind start
(gdb) while ($trace_frame != -1)
> printf "Frame %d, PC = %08X, SP = %08X, FP = %08X\n", \
$trace_frame, $pc, $sp, $fp
> tfind
> end
Frame 0, PC = 0020DC64, SP = 0030BF3C, FP = 0030BF44
Frame 1, PC = 0020DC6C, SP = 0030BF38, FP = 0030BF44
Frame 2, PC = 0020DC70, SP = 0030BF34, FP = 0030BF44
Frame 3, PC = 0020DC74, SP = 0030BF30, FP = 0030BF44
Frame 4, PC = 0020DC78, SP = 0030BF2C, FP = 0030BF44
Frame 5, PC = 0020DC7C, SP = 0030BF28, FP = 0030BF44
Frame 6, PC = 0020DC80, SP = 0030BF24, FP = 0030BF44
Frame 7, PC = 0020DC84, SP = 0030BF20, FP = 0030BF44
Frame 8, PC = 0020DC88, SP = 0030BF1C, FP = 0030BF44
Frame 9, PC = 0020DC8E, SP = 0030BF18, FP = 0030BF44
Frame 10, PC = 00203F6C, SP = 0030BE3C, FP = 0030BF14
Or, if we want to examine the variable `X' at each source line in
the buffer:
(gdb) tfind start
(gdb) while ($trace_frame != -1)
> printf "Frame %d, X == %d\n", $trace_frame, X
> tfind line
> end
Frame 0, X = 1
Frame 7, X = 2
Frame 13, X = 255

File: gdb.info, Node: tdump, Next: save tracepoints, Prev: tfind, Up: Analyze Collected Data
13.2.2 `tdump'
--------------
This command takes no arguments. It prints all the data collected at
the current trace snapshot.
(gdb) trace 444
(gdb) actions
Enter actions for tracepoint #2, one per line:
> collect $regs, $locals, $args, gdb_long_test
> end
(gdb) tstart
(gdb) tfind line 444
#0 gdb_test (p1=0x11, p2=0x22, p3=0x33, p4=0x44, p5=0x55, p6=0x66)
at gdb_test.c:444
444 printp( "%s: arguments = 0x%X 0x%X 0x%X 0x%X 0x%X 0x%X\n", )
(gdb) tdump
Data collected at tracepoint 2, trace frame 1:
d0 0xc4aa0085 -995491707
d1 0x18 24
d2 0x80 128
d3 0x33 51
d4 0x71aea3d 119204413
d5 0x22 34
d6 0xe0 224
d7 0x380035 3670069
a0 0x19e24a 1696330
a1 0x3000668 50333288
a2 0x100 256
a3 0x322000 3284992
a4 0x3000698 50333336
a5 0x1ad3cc 1758156
fp 0x30bf3c 0x30bf3c
sp 0x30bf34 0x30bf34
ps 0x0 0
pc 0x20b2c8 0x20b2c8
fpcontrol 0x0 0
fpstatus 0x0 0
fpiaddr 0x0 0
p = 0x20e5b4 "gdb-test"
p1 = (void *) 0x11
p2 = (void *) 0x22
p3 = (void *) 0x33
p4 = (void *) 0x44
p5 = (void *) 0x55
p6 = (void *) 0x66
gdb_long_test = 17 '\021'
(gdb)
`tdump' works by scanning the tracepoint's current collection
actions and printing the value of each expression listed. So `tdump'
can fail, if after a run, you change the tracepoint's actions to
mention variables that were not collected during the run.
Also, for tracepoints with `while-stepping' loops, `tdump' uses the
collected value of `$pc' to distinguish between trace frames that were
collected at the tracepoint hit, and frames that were collected while
stepping. This allows it to correctly choose whether to display the
basic list of collections, or the collections from the body of the
while-stepping loop. However, if `$pc' was not collected, then `tdump'
will always attempt to dump using the basic collection list, and may
fail if a while-stepping frame does not include all the same data that
is collected at the tracepoint hit.

File: gdb.info, Node: save tracepoints, Prev: tdump, Up: Analyze Collected Data
13.2.3 `save tracepoints FILENAME'
----------------------------------
This command saves all current tracepoint definitions together with
their actions and passcounts, into a file `FILENAME' suitable for use
in a later debugging session. To read the saved tracepoint
definitions, use the `source' command (*note Command Files::). The
`save-tracepoints' command is a deprecated alias for `save tracepoints'

File: gdb.info, Node: Tracepoint Variables, Next: Trace Files, Prev: Analyze Collected Data, Up: Tracepoints
13.3 Convenience Variables for Tracepoints
==========================================
`(int) $trace_frame'
The current trace snapshot (a.k.a. "frame") number, or -1 if no
snapshot is selected.
`(int) $tracepoint'
The tracepoint for the current trace snapshot.
`(int) $trace_line'
The line number for the current trace snapshot.
`(char []) $trace_file'
The source file for the current trace snapshot.
`(char []) $trace_func'
The name of the function containing `$tracepoint'.
Note: `$trace_file' is not suitable for use in `printf', use
`output' instead.
Here's a simple example of using these convenience variables for
stepping through all the trace snapshots and printing some of their
data. Note that these are not the same as trace state variables, which
are managed by the target.
(gdb) tfind start
(gdb) while $trace_frame != -1
> output $trace_file
> printf ", line %d (tracepoint #%d)\n", $trace_line, $tracepoint
> tfind
> end

File: gdb.info, Node: Trace Files, Prev: Tracepoint Variables, Up: Tracepoints
13.4 Using Trace Files
======================
In some situations, the target running a trace experiment may no longer
be available; perhaps it crashed, or the hardware was needed for a
different activity. To handle these cases, you can arrange to dump the
trace data into a file, and later use that file as a source of trace
data, via the `target tfile' command.
`tsave [ -r ] FILENAME'
Save the trace data to FILENAME. By default, this command assumes
that FILENAME refers to the host filesystem, so if necessary GDB
will copy raw trace data up from the target and then save it. If
the target supports it, you can also supply the optional argument
`-r' ("remote") to direct the target to save the data directly
into FILENAME in its own filesystem, which may be more efficient
if the trace buffer is very large. (Note, however, that `target
tfile' can only read from files accessible to the host.)
`target tfile FILENAME'
Use the file named FILENAME as a source of trace data. Commands
that examine data work as they do with a live target, but it is not
possible to run any new trace experiments. `tstatus' will report
the state of the trace run at the moment the data was saved, as
well as the current trace frame you are examining. FILENAME must
be on a filesystem accessible to the host.

File: gdb.info, Node: Overlays, Next: Languages, Prev: Tracepoints, Up: Top
14 Debugging Programs That Use Overlays
***************************************
If your program is too large to fit completely in your target system's
memory, you can sometimes use "overlays" to work around this problem.
GDB provides some support for debugging programs that use overlays.
* Menu:
* How Overlays Work:: A general explanation of overlays.
* Overlay Commands:: Managing overlays in GDB.
* Automatic Overlay Debugging:: GDB can find out which overlays are
mapped by asking the inferior.
* Overlay Sample Program:: A sample program using overlays.

File: gdb.info, Node: How Overlays Work, Next: Overlay Commands, Up: Overlays
14.1 How Overlays Work
======================
Suppose you have a computer whose instruction address space is only 64
kilobytes long, but which has much more memory which can be accessed by
other means: special instructions, segment registers, or memory
management hardware, for example. Suppose further that you want to
adapt a program which is larger than 64 kilobytes to run on this system.
One solution is to identify modules of your program which are
relatively independent, and need not call each other directly; call
these modules "overlays". Separate the overlays from the main program,
and place their machine code in the larger memory. Place your main
program in instruction memory, but leave at least enough space there to
hold the largest overlay as well.
Now, to call a function located in an overlay, you must first copy
that overlay's machine code from the large memory into the space set
aside for it in the instruction memory, and then jump to its entry point
there.
Data Instruction Larger
Address Space Address Space Address Space
+-----------+ +-----------+ +-----------+
| | | | | |
+-----------+ +-----------+ +-----------+<-- overlay 1
| program | | main | .----| overlay 1 | load address
| variables | | program | | +-----------+
| and heap | | | | | |
+-----------+ | | | +-----------+<-- overlay 2
| | +-----------+ | | | load address
+-----------+ | | | .-| overlay 2 |
| | | | | |
mapped --->+-----------+ | | +-----------+
address | | | | | |
| overlay | <-' | | |
| area | <---' +-----------+<-- overlay 3
| | <---. | | load address
+-----------+ `--| overlay 3 |
| | | |
+-----------+ | |
+-----------+
| |
+-----------+
A code overlay
The diagram (*note A code overlay::) shows a system with separate
data and instruction address spaces. To map an overlay, the program
copies its code from the larger address space to the instruction
address space. Since the overlays shown here all use the same mapped
address, only one may be mapped at a time. For a system with a single
address space for data and instructions, the diagram would be similar,
except that the program variables and heap would share an address space
with the main program and the overlay area.
An overlay loaded into instruction memory and ready for use is
called a "mapped" overlay; its "mapped address" is its address in the
instruction memory. An overlay not present (or only partially present)
in instruction memory is called "unmapped"; its "load address" is its
address in the larger memory. The mapped address is also called the
"virtual memory address", or "VMA"; the load address is also called the
"load memory address", or "LMA".
Unfortunately, overlays are not a completely transparent way to
adapt a program to limited instruction memory. They introduce a new
set of global constraints you must keep in mind as you design your
program:
* Before calling or returning to a function in an overlay, your
program must make sure that overlay is actually mapped.
Otherwise, the call or return will transfer control to the right
address, but in the wrong overlay, and your program will probably
crash.
* If the process of mapping an overlay is expensive on your system,
you will need to choose your overlays carefully to minimize their
effect on your program's performance.
* The executable file you load onto your system must contain each
overlay's instructions, appearing at the overlay's load address,
not its mapped address. However, each overlay's instructions must
be relocated and its symbols defined as if the overlay were at its
mapped address. You can use GNU linker scripts to specify
different load and relocation addresses for pieces of your
program; see *Note Overlay Description: (ld.info)Overlay
Description.
* The procedure for loading executable files onto your system must
be able to load their contents into the larger address space as
well as the instruction and data spaces.
The overlay system described above is rather simple, and could be
improved in many ways:
* If your system has suitable bank switch registers or memory
management hardware, you could use those facilities to make an
overlay's load area contents simply appear at their mapped address
in instruction space. This would probably be faster than copying
the overlay to its mapped area in the usual way.
* If your overlays are small enough, you could set aside more than
one overlay area, and have more than one overlay mapped at a time.
* You can use overlays to manage data, as well as instructions. In
general, data overlays are even less transparent to your design
than code overlays: whereas code overlays only require care when
you call or return to functions, data overlays require care every
time you access the data. Also, if you change the contents of a
data overlay, you must copy its contents back out to its load
address before you can copy a different data overlay into the same
mapped area.

File: gdb.info, Node: Overlay Commands, Next: Automatic Overlay Debugging, Prev: How Overlays Work, Up: Overlays
14.2 Overlay Commands
=====================
To use GDB's overlay support, each overlay in your program must
correspond to a separate section of the executable file. The section's
virtual memory address and load memory address must be the overlay's
mapped and load addresses. Identifying overlays with sections allows
GDB to determine the appropriate address of a function or variable,
depending on whether the overlay is mapped or not.
GDB's overlay commands all start with the word `overlay'; you can
abbreviate this as `ov' or `ovly'. The commands are:
`overlay off'
Disable GDB's overlay support. When overlay support is disabled,
GDB assumes that all functions and variables are always present at
their mapped addresses. By default, GDB's overlay support is
disabled.
`overlay manual'
Enable "manual" overlay debugging. In this mode, GDB relies on
you to tell it which overlays are mapped, and which are not, using
the `overlay map-overlay' and `overlay unmap-overlay' commands
described below.
`overlay map-overlay OVERLAY'
`overlay map OVERLAY'
Tell GDB that OVERLAY is now mapped; OVERLAY must be the name of
the object file section containing the overlay. When an overlay
is mapped, GDB assumes it can find the overlay's functions and
variables at their mapped addresses. GDB assumes that any other
overlays whose mapped ranges overlap that of OVERLAY are now
unmapped.
`overlay unmap-overlay OVERLAY'
`overlay unmap OVERLAY'
Tell GDB that OVERLAY is no longer mapped; OVERLAY must be the
name of the object file section containing the overlay. When an
overlay is unmapped, GDB assumes it can find the overlay's
functions and variables at their load addresses.
`overlay auto'
Enable "automatic" overlay debugging. In this mode, GDB consults
a data structure the overlay manager maintains in the inferior to
see which overlays are mapped. For details, see *Note Automatic
Overlay Debugging::.
`overlay load-target'
`overlay load'
Re-read the overlay table from the inferior. Normally, GDB
re-reads the table GDB automatically each time the inferior stops,
so this command should only be necessary if you have changed the
overlay mapping yourself using GDB. This command is only useful
when using automatic overlay debugging.
`overlay list-overlays'
`overlay list'
Display a list of the overlays currently mapped, along with their
mapped addresses, load addresses, and sizes.
Normally, when GDB prints a code address, it includes the name of
the function the address falls in:
(gdb) print main
$3 = {int ()} 0x11a0 <main>
When overlay debugging is enabled, GDB recognizes code in unmapped
overlays, and prints the names of unmapped functions with asterisks
around them. For example, if `foo' is a function in an unmapped
overlay, GDB prints it this way:
(gdb) overlay list
No sections are mapped.
(gdb) print foo
$5 = {int (int)} 0x100000 <*foo*>
When `foo''s overlay is mapped, GDB prints the function's name
normally:
(gdb) overlay list
Section .ov.foo.text, loaded at 0x100000 - 0x100034,
mapped at 0x1016 - 0x104a
(gdb) print foo
$6 = {int (int)} 0x1016 <foo>
When overlay debugging is enabled, GDB can find the correct address
for functions and variables in an overlay, whether or not the overlay
is mapped. This allows most GDB commands, like `break' and
`disassemble', to work normally, even on unmapped code. However, GDB's
breakpoint support has some limitations:
* You can set breakpoints in functions in unmapped overlays, as long
as GDB can write to the overlay at its load address.
* GDB can not set hardware or simulator-based breakpoints in
unmapped overlays. However, if you set a breakpoint at the end of
your overlay manager (and tell GDB which overlays are now mapped,
if you are using manual overlay management), GDB will re-set its
breakpoints properly.

File: gdb.info, Node: Automatic Overlay Debugging, Next: Overlay Sample Program, Prev: Overlay Commands, Up: Overlays
14.3 Automatic Overlay Debugging
================================
GDB can automatically track which overlays are mapped and which are
not, given some simple co-operation from the overlay manager in the
inferior. If you enable automatic overlay debugging with the `overlay
auto' command (*note Overlay Commands::), GDB looks in the inferior's
memory for certain variables describing the current state of the
overlays.
Here are the variables your overlay manager must define to support
GDB's automatic overlay debugging:
`_ovly_table':
This variable must be an array of the following structures:
struct
{
/* The overlay's mapped address. */
unsigned long vma;
/* The size of the overlay, in bytes. */
unsigned long size;
/* The overlay's load address. */
unsigned long lma;
/* Non-zero if the overlay is currently mapped;
zero otherwise. */
unsigned long mapped;
}
`_novlys':
This variable must be a four-byte signed integer, holding the total
number of elements in `_ovly_table'.
To decide whether a particular overlay is mapped or not, GDB looks
for an entry in `_ovly_table' whose `vma' and `lma' members equal the
VMA and LMA of the overlay's section in the executable file. When GDB
finds a matching entry, it consults the entry's `mapped' member to
determine whether the overlay is currently mapped.
In addition, your overlay manager may define a function called
`_ovly_debug_event'. If this function is defined, GDB will silently
set a breakpoint there. If the overlay manager then calls this
function whenever it has changed the overlay table, this will enable
GDB to accurately keep track of which overlays are in program memory,
and update any breakpoints that may be set in overlays. This will
allow breakpoints to work even if the overlays are kept in ROM or other
non-writable memory while they are not being executed.

File: gdb.info, Node: Overlay Sample Program, Prev: Automatic Overlay Debugging, Up: Overlays
14.4 Overlay Sample Program
===========================
When linking a program which uses overlays, you must place the overlays
at their load addresses, while relocating them to run at their mapped
addresses. To do this, you must write a linker script (*note Overlay
Description: (ld.info)Overlay Description.). Unfortunately, since
linker scripts are specific to a particular host system, target
architecture, and target memory layout, this manual cannot provide
portable sample code demonstrating GDB's overlay support.
However, the GDB source distribution does contain an overlaid
program, with linker scripts for a few systems, as part of its test
suite. The program consists of the following files from
`gdb/testsuite/gdb.base':
`overlays.c'
The main program file.
`ovlymgr.c'
A simple overlay manager, used by `overlays.c'.
`foo.c'
`bar.c'
`baz.c'
`grbx.c'
Overlay modules, loaded and used by `overlays.c'.
`d10v.ld'
`m32r.ld'
Linker scripts for linking the test program on the `d10v-elf' and
`m32r-elf' targets.
You can build the test program using the `d10v-elf' GCC
cross-compiler like this:
$ d10v-elf-gcc -g -c overlays.c
$ d10v-elf-gcc -g -c ovlymgr.c
$ d10v-elf-gcc -g -c foo.c
$ d10v-elf-gcc -g -c bar.c
$ d10v-elf-gcc -g -c baz.c
$ d10v-elf-gcc -g -c grbx.c
$ d10v-elf-gcc -g overlays.o ovlymgr.o foo.o bar.o \
baz.o grbx.o -Wl,-Td10v.ld -o overlays
The build process is identical for any other architecture, except
that you must substitute the appropriate compiler and linker script for
the target system for `d10v-elf-gcc' and `d10v.ld'.

File: gdb.info, Node: Languages, Next: Symbols, Prev: Overlays, Up: Top
15 Using GDB with Different Languages
*************************************
Although programming languages generally have common aspects, they are
rarely expressed in the same manner. For instance, in ANSI C,
dereferencing a pointer `p' is accomplished by `*p', but in Modula-2,
it is accomplished by `p^'. Values can also be represented (and
displayed) differently. Hex numbers in C appear as `0x1ae', while in
Modula-2 they appear as `1AEH'.
Language-specific information is built into GDB for some languages,
allowing you to express operations like the above in your program's
native language, and allowing GDB to output values in a manner
consistent with the syntax of your program's native language. The
language you use to build expressions is called the "working language".
* Menu:
* Setting:: Switching between source languages
* Show:: Displaying the language
* Checks:: Type and range checks
* Supported Languages:: Supported languages
* Unsupported Languages:: Unsupported languages

File: gdb.info, Node: Setting, Next: Show, Up: Languages
15.1 Switching Between Source Languages
=======================================
There are two ways to control the working language--either have GDB set
it automatically, or select it manually yourself. You can use the `set
language' command for either purpose. On startup, GDB defaults to
setting the language automatically. The working language is used to
determine how expressions you type are interpreted, how values are
printed, etc.
In addition to the working language, every source file that GDB
knows about has its own working language. For some object file
formats, the compiler might indicate which language a particular source
file is in. However, most of the time GDB infers the language from the
name of the file. The language of a source file controls whether C++
names are demangled--this way `backtrace' can show each frame
appropriately for its own language. There is no way to set the
language of a source file from within GDB, but you can set the language
associated with a filename extension. *Note Displaying the Language:
Show.
This is most commonly a problem when you use a program, such as
`cfront' or `f2c', that generates C but is written in another language.
In that case, make the program use `#line' directives in its C output;
that way GDB will know the correct language of the source code of the
original program, and will display that source code, not the generated
C code.
* Menu:
* Filenames:: Filename extensions and languages.
* Manually:: Setting the working language manually
* Automatically:: Having GDB infer the source language

File: gdb.info, Node: Filenames, Next: Manually, Up: Setting
15.1.1 List of Filename Extensions and Languages
------------------------------------------------
If a source file name ends in one of the following extensions, then GDB
infers that its language is the one indicated.
`.ada'
`.ads'
`.adb'
`.a'
Ada source file.
`.c'
C source file
`.C'
`.cc'
`.cp'
`.cpp'
`.cxx'
`.c++'
C++ source file
`.d'
D source file
`.m'
Objective-C source file
`.f'
`.F'
Fortran source file
`.mod'
Modula-2 source file
`.s'
`.S'
Assembler source file. This actually behaves almost like C, but
GDB does not skip over function prologues when stepping.
In addition, you may set the language associated with a filename
extension. *Note Displaying the Language: Show.

File: gdb.info, Node: Manually, Next: Automatically, Prev: Filenames, Up: Setting
15.1.2 Setting the Working Language
-----------------------------------
If you allow GDB to set the language automatically, expressions are
interpreted the same way in your debugging session and your program.
If you wish, you may set the language manually. To do this, issue
the command `set language LANG', where LANG is the name of a language,
such as `c' or `modula-2'. For a list of the supported languages, type
`set language'.
Setting the language manually prevents GDB from updating the working
language automatically. This can lead to confusion if you try to debug
a program when the working language is not the same as the source
language, when an expression is acceptable to both languages--but means
different things. For instance, if the current source file were
written in C, and GDB was parsing Modula-2, a command such as:
print a = b + c
might not have the effect you intended. In C, this means to add `b'
and `c' and place the result in `a'. The result printed would be the
value of `a'. In Modula-2, this means to compare `a' to the result of
`b+c', yielding a `BOOLEAN' value.

File: gdb.info, Node: Automatically, Prev: Manually, Up: Setting
15.1.3 Having GDB Infer the Source Language
-------------------------------------------
To have GDB set the working language automatically, use `set language
local' or `set language auto'. GDB then infers the working language.
That is, when your program stops in a frame (usually by encountering a
breakpoint), GDB sets the working language to the language recorded for
the function in that frame. If the language for a frame is unknown
(that is, if the function or block corresponding to the frame was
defined in a source file that does not have a recognized extension),
the current working language is not changed, and GDB issues a warning.
This may not seem necessary for most programs, which are written
entirely in one source language. However, program modules and libraries
written in one source language can be used by a main program written in
a different source language. Using `set language auto' in this case
frees you from having to set the working language manually.

File: gdb.info, Node: Show, Next: Checks, Prev: Setting, Up: Languages
15.2 Displaying the Language
============================
The following commands help you find out which language is the working
language, and also what language source files were written in.
`show language'
Display the current working language. This is the language you
can use with commands such as `print' to build and compute
expressions that may involve variables in your program.
`info frame'
Display the source language for this frame. This language becomes
the working language if you use an identifier from this frame.
*Note Information about a Frame: Frame Info, to identify the other
information listed here.
`info source'
Display the source language of this source file. *Note Examining
the Symbol Table: Symbols, to identify the other information
listed here.
In unusual circumstances, you may have source files with extensions
not in the standard list. You can then set the extension associated
with a language explicitly:
`set extension-language EXT LANGUAGE'
Tell GDB that source files with extension EXT are to be assumed as
written in the source language LANGUAGE.
`info extensions'
List all the filename extensions and the associated languages.

File: gdb.info, Node: Checks, Next: Supported Languages, Prev: Show, Up: Languages
15.3 Type and Range Checking
============================
_Warning:_ In this release, the GDB commands for type and range
checking are included, but they do not yet have any effect. This
section documents the intended facilities.
Some languages are designed to guard you against making seemingly
common errors through a series of compile- and run-time checks. These
include checking the type of arguments to functions and operators, and
making sure mathematical overflows are caught at run time. Checks such
as these help to ensure a program's correctness once it has been
compiled by eliminating type mismatches, and providing active checks
for range errors when your program is running.
GDB can check for conditions like the above if you wish. Although
GDB does not check the statements in your program, it can check
expressions entered directly into GDB for evaluation via the `print'
command, for example. As with the working language, GDB can also
decide whether or not to check automatically based on your program's
source language. *Note Supported Languages: Supported Languages, for
the default settings of supported languages.
* Menu:
* Type Checking:: An overview of type checking
* Range Checking:: An overview of range checking

File: gdb.info, Node: Type Checking, Next: Range Checking, Up: Checks
15.3.1 An Overview of Type Checking
-----------------------------------
Some languages, such as Modula-2, are strongly typed, meaning that the
arguments to operators and functions have to be of the correct type,
otherwise an error occurs. These checks prevent type mismatch errors
from ever causing any run-time problems. For example,
1 + 2 => 3
but
error--> 1 + 2.3
The second example fails because the `CARDINAL' 1 is not
type-compatible with the `REAL' 2.3.
For the expressions you use in GDB commands, you can tell the GDB
type checker to skip checking; to treat any mismatches as errors and
abandon the expression; or to only issue warnings when type mismatches
occur, but evaluate the expression anyway. When you choose the last of
these, GDB evaluates expressions like the second example above, but
also issues a warning.
Even if you turn type checking off, there may be other reasons
related to type that prevent GDB from evaluating an expression. For
instance, GDB does not know how to add an `int' and a `struct foo'.
These particular type errors have nothing to do with the language in
use, and usually arise from expressions, such as the one described
above, which make little sense to evaluate anyway.
Each language defines to what degree it is strict about type. For
instance, both Modula-2 and C require the arguments to arithmetical
operators to be numbers. In C, enumerated types and pointers can be
represented as numbers, so that they are valid arguments to mathematical
operators. *Note Supported Languages: Supported Languages, for further
details on specific languages.
GDB provides some additional commands for controlling the type
checker:
`set check type auto'
Set type checking on or off based on the current working language.
*Note Supported Languages: Supported Languages, for the default
settings for each language.
`set check type on'
`set check type off'
Set type checking on or off, overriding the default setting for the
current working language. Issue a warning if the setting does not
match the language default. If any type mismatches occur in
evaluating an expression while type checking is on, GDB prints a
message and aborts evaluation of the expression.
`set check type warn'
Cause the type checker to issue warnings, but to always attempt to
evaluate the expression. Evaluating the expression may still be
impossible for other reasons. For example, GDB cannot add numbers
and structures.
`show type'
Show the current setting of the type checker, and whether or not
GDB is setting it automatically.

File: gdb.info, Node: Range Checking, Prev: Type Checking, Up: Checks
15.3.2 An Overview of Range Checking
------------------------------------
In some languages (such as Modula-2), it is an error to exceed the
bounds of a type; this is enforced with run-time checks. Such range
checking is meant to ensure program correctness by making sure
computations do not overflow, or indices on an array element access do
not exceed the bounds of the array.
For expressions you use in GDB commands, you can tell GDB to treat
range errors in one of three ways: ignore them, always treat them as
errors and abandon the expression, or issue warnings but evaluate the
expression anyway.
A range error can result from numerical overflow, from exceeding an
array index bound, or when you type a constant that is not a member of
any type. Some languages, however, do not treat overflows as an error.
In many implementations of C, mathematical overflow causes the result
to "wrap around" to lower values--for example, if M is the largest
integer value, and S is the smallest, then
M + 1 => S
This, too, is specific to individual languages, and in some cases
specific to individual compilers or machines. *Note Supported
Languages: Supported Languages, for further details on specific
languages.
GDB provides some additional commands for controlling the range
checker:
`set check range auto'
Set range checking on or off based on the current working language.
*Note Supported Languages: Supported Languages, for the default
settings for each language.
`set check range on'
`set check range off'
Set range checking on or off, overriding the default setting for
the current working language. A warning is issued if the setting
does not match the language default. If a range error occurs and
range checking is on, then a message is printed and evaluation of
the expression is aborted.
`set check range warn'
Output messages when the GDB range checker detects a range error,
but attempt to evaluate the expression anyway. Evaluating the
expression may still be impossible for other reasons, such as
accessing memory that the process does not own (a typical example
from many Unix systems).
`show range'
Show the current setting of the range checker, and whether or not
it is being set automatically by GDB.

File: gdb.info, Node: Supported Languages, Next: Unsupported Languages, Prev: Checks, Up: Languages
15.4 Supported Languages
========================
GDB supports C, C++, D, Objective-C, Fortran, Java, Pascal, assembly,
Modula-2, and Ada. Some GDB features may be used in expressions
regardless of the language you use: the GDB `@' and `::' operators, and
the `{type}addr' construct (*note Expressions: Expressions.) can be
used with the constructs of any supported language.
The following sections detail to what degree each source language is
supported by GDB. These sections are not meant to be language
tutorials or references, but serve only as a reference guide to what the
GDB expression parser accepts, and what input and output formats should
look like for different languages. There are many good books written
on each of these languages; please look to these for a language
reference or tutorial.
* Menu:
* C:: C and C++
* D:: D
* Objective-C:: Objective-C
* Fortran:: Fortran
* Pascal:: Pascal
* Modula-2:: Modula-2
* Ada:: Ada

File: gdb.info, Node: C, Next: D, Up: Supported Languages
15.4.1 C and C++
----------------
Since C and C++ are so closely related, many features of GDB apply to
both languages. Whenever this is the case, we discuss those languages
together.
The C++ debugging facilities are jointly implemented by the C++
compiler and GDB. Therefore, to debug your C++ code effectively, you
must compile your C++ programs with a supported C++ compiler, such as
GNU `g++', or the HP ANSI C++ compiler (`aCC').
For best results when using GNU C++, use the DWARF 2 debugging
format; if it doesn't work on your system, try the stabs+ debugging
format. You can select those formats explicitly with the `g++'
command-line options `-gdwarf-2' and `-gstabs+'. *Note Options for
Debugging Your Program or GCC: (gcc.info)Debugging Options.
* Menu:
* C Operators:: C and C++ operators
* C Constants:: C and C++ constants
* C Plus Plus Expressions:: C++ expressions
* C Defaults:: Default settings for C and C++
* C Checks:: C and C++ type and range checks
* Debugging C:: GDB and C
* Debugging C Plus Plus:: GDB features for C++
* Decimal Floating Point:: Numbers in Decimal Floating Point format

File: gdb.info, Node: C Operators, Next: C Constants, Up: C
15.4.1.1 C and C++ Operators
............................
Operators must be defined on values of specific types. For instance,
`+' is defined on numbers, but not on structures. Operators are often
defined on groups of types.
For the purposes of C and C++, the following definitions hold:
* _Integral types_ include `int' with any of its storage-class
specifiers; `char'; `enum'; and, for C++, `bool'.
* _Floating-point types_ include `float', `double', and `long
double' (if supported by the target platform).
* _Pointer types_ include all types defined as `(TYPE *)'.
* _Scalar types_ include all of the above.
The following operators are supported. They are listed here in order
of increasing precedence:
`,'
The comma or sequencing operator. Expressions in a
comma-separated list are evaluated from left to right, with the
result of the entire expression being the last expression
evaluated.
`='
Assignment. The value of an assignment expression is the value
assigned. Defined on scalar types.
`OP='
Used in an expression of the form `A OP= B', and translated to
`A = A OP B'. `OP=' and `=' have the same precedence. OP is any
one of the operators `|', `^', `&', `<<', `>>', `+', `-', `*',
`/', `%'.
`?:'
The ternary operator. `A ? B : C' can be thought of as: if A
then B else C. A should be of an integral type.
`||'
Logical OR. Defined on integral types.
`&&'
Logical AND. Defined on integral types.
`|'
Bitwise OR. Defined on integral types.
`^'
Bitwise exclusive-OR. Defined on integral types.
`&'
Bitwise AND. Defined on integral types.
`==, !='
Equality and inequality. Defined on scalar types. The value of
these expressions is 0 for false and non-zero for true.
`<, >, <=, >='
Less than, greater than, less than or equal, greater than or equal.
Defined on scalar types. The value of these expressions is 0 for
false and non-zero for true.
`<<, >>'
left shift, and right shift. Defined on integral types.
`@'
The GDB "artificial array" operator (*note Expressions:
Expressions.).
`+, -'
Addition and subtraction. Defined on integral types,
floating-point types and pointer types.
`*, /, %'
Multiplication, division, and modulus. Multiplication and
division are defined on integral and floating-point types.
Modulus is defined on integral types.
`++, --'
Increment and decrement. When appearing before a variable, the
operation is performed before the variable is used in an
expression; when appearing after it, the variable's value is used
before the operation takes place.
`*'
Pointer dereferencing. Defined on pointer types. Same precedence
as `++'.
`&'
Address operator. Defined on variables. Same precedence as `++'.
For debugging C++, GDB implements a use of `&' beyond what is
allowed in the C++ language itself: you can use `&(&REF)' to
examine the address where a C++ reference variable (declared with
`&REF') is stored.
`-'
Negative. Defined on integral and floating-point types. Same
precedence as `++'.
`!'
Logical negation. Defined on integral types. Same precedence as
`++'.
`~'
Bitwise complement operator. Defined on integral types. Same
precedence as `++'.
`., ->'
Structure member, and pointer-to-structure member. For
convenience, GDB regards the two as equivalent, choosing whether
to dereference a pointer based on the stored type information.
Defined on `struct' and `union' data.
`.*, ->*'
Dereferences of pointers to members.
`[]'
Array indexing. `A[I]' is defined as `*(A+I)'. Same precedence
as `->'.
`()'
Function parameter list. Same precedence as `->'.
`::'
C++ scope resolution operator. Defined on `struct', `union', and
`class' types.
`::'
Doubled colons also represent the GDB scope operator (*note
Expressions: Expressions.). Same precedence as `::', above.
If an operator is redefined in the user code, GDB usually attempts
to invoke the redefined version instead of using the operator's
predefined meaning.

File: gdb.info, Node: C Constants, Next: C Plus Plus Expressions, Prev: C Operators, Up: C
15.4.1.2 C and C++ Constants
............................
GDB allows you to express the constants of C and C++ in the following
ways:
* Integer constants are a sequence of digits. Octal constants are
specified by a leading `0' (i.e. zero), and hexadecimal constants
by a leading `0x' or `0X'. Constants may also end with a letter
`l', specifying that the constant should be treated as a `long'
value.
* Floating point constants are a sequence of digits, followed by a
decimal point, followed by a sequence of digits, and optionally
followed by an exponent. An exponent is of the form:
`e[[+]|-]NNN', where NNN is another sequence of digits. The `+'
is optional for positive exponents. A floating-point constant may
also end with a letter `f' or `F', specifying that the constant
should be treated as being of the `float' (as opposed to the
default `double') type; or with a letter `l' or `L', which
specifies a `long double' constant.
* Enumerated constants consist of enumerated identifiers, or their
integral equivalents.
* Character constants are a single character surrounded by single
quotes (`''), or a number--the ordinal value of the corresponding
character (usually its ASCII value). Within quotes, the single
character may be represented by a letter or by "escape sequences",
which are of the form `\NNN', where NNN is the octal representation
of the character's ordinal value; or of the form `\X', where `X'
is a predefined special character--for example, `\n' for newline.
* String constants are a sequence of character constants surrounded
by double quotes (`"'). Any valid character constant (as described
above) may appear. Double quotes within the string must be
preceded by a backslash, so for instance `"a\"b'c"' is a string of
five characters.
* Pointer constants are an integral value. You can also write
pointers to constants using the C operator `&'.
* Array constants are comma-separated lists surrounded by braces `{'
and `}'; for example, `{1,2,3}' is a three-element array of
integers, `{{1,2}, {3,4}, {5,6}}' is a three-by-two array, and
`{&"hi", &"there", &"fred"}' is a three-element array of pointers.

File: gdb.info, Node: C Plus Plus Expressions, Next: C Defaults, Prev: C Constants, Up: C
15.4.1.3 C++ Expressions
........................
GDB expression handling can interpret most C++ expressions.
_Warning:_ GDB can only debug C++ code if you use the proper
compiler and the proper debug format. Currently, GDB works best
when debugging C++ code that is compiled with GCC 2.95.3 or with
GCC 3.1 or newer, using the options `-gdwarf-2' or `-gstabs+'.
DWARF 2 is preferred over stabs+. Most configurations of GCC emit
either DWARF 2 or stabs+ as their default debug format, so you
usually don't need to specify a debug format explicitly. Other
compilers and/or debug formats are likely to work badly or not at
all when using GDB to debug C++ code.
1. Member function calls are allowed; you can use expressions like
count = aml->GetOriginal(x, y)
2. While a member function is active (in the selected stack frame),
your expressions have the same namespace available as the member
function; that is, GDB allows implicit references to the class
instance pointer `this' following the same rules as C++.
3. You can call overloaded functions; GDB resolves the function call
to the right definition, with some restrictions. GDB does not
perform overload resolution involving user-defined type
conversions, calls to constructors, or instantiations of templates
that do not exist in the program. It also cannot handle ellipsis
argument lists or default arguments.
It does perform integral conversions and promotions, floating-point
promotions, arithmetic conversions, pointer conversions,
conversions of class objects to base classes, and standard
conversions such as those of functions or arrays to pointers; it
requires an exact match on the number of function arguments.
Overload resolution is always performed, unless you have specified
`set overload-resolution off'. *Note GDB Features for C++:
Debugging C Plus Plus.
You must specify `set overload-resolution off' in order to use an
explicit function signature to call an overloaded function, as in
p 'foo(char,int)'('x', 13)
The GDB command-completion facility can simplify this; see *Note
Command Completion: Completion.
4. GDB understands variables declared as C++ references; you can use
them in expressions just as you do in C++ source--they are
automatically dereferenced.
In the parameter list shown when GDB displays a frame, the values
of reference variables are not displayed (unlike other variables);
this avoids clutter, since references are often used for large
structures. The _address_ of a reference variable is always
shown, unless you have specified `set print address off'.
5. GDB supports the C++ name resolution operator `::'--your
expressions can use it just as expressions in your program do.
Since one scope may be defined in another, you can use `::'
repeatedly if necessary, for example in an expression like
`SCOPE1::SCOPE2::NAME'. GDB also allows resolving name scope by
reference to source files, in both C and C++ debugging (*note
Program Variables: Variables.).
In addition, when used with HP's C++ compiler, GDB supports calling
virtual functions correctly, printing out virtual bases of objects,
calling functions in a base subobject, casting objects, and invoking
user-defined operators.

File: gdb.info, Node: C Defaults, Next: C Checks, Prev: C Plus Plus Expressions, Up: C
15.4.1.4 C and C++ Defaults
...........................
If you allow GDB to set type and range checking automatically, they
both default to `off' whenever the working language changes to C or
C++. This happens regardless of whether you or GDB selects the working
language.
If you allow GDB to set the language automatically, it recognizes
source files whose names end with `.c', `.C', or `.cc', etc, and when
GDB enters code compiled from one of these files, it sets the working
language to C or C++. *Note Having GDB Infer the Source Language:
Automatically, for further details.

File: gdb.info, Node: C Checks, Next: Debugging C, Prev: C Defaults, Up: C
15.4.1.5 C and C++ Type and Range Checks
........................................
By default, when GDB parses C or C++ expressions, type checking is not
used. However, if you turn type checking on, GDB considers two
variables type equivalent if:
* The two variables are structured and have the same structure,
union, or enumerated tag.
* The two variables have the same type name, or types that have been
declared equivalent through `typedef'.
Range checking, if turned on, is done on mathematical operations.
Array indices are not checked, since they are often used to index a
pointer that is not itself an array.

File: gdb.info, Node: Debugging C, Next: Debugging C Plus Plus, Prev: C Checks, Up: C
15.4.1.6 GDB and C
..................
The `set print union' and `show print union' commands apply to the
`union' type. When set to `on', any `union' that is inside a `struct'
or `class' is also printed. Otherwise, it appears as `{...}'.
The `@' operator aids in the debugging of dynamic arrays, formed
with pointers and a memory allocation function. *Note Expressions:
Expressions.

File: gdb.info, Node: Debugging C Plus Plus, Next: Decimal Floating Point, Prev: Debugging C, Up: C
15.4.1.7 GDB Features for C++
.............................
Some GDB commands are particularly useful with C++, and some are
designed specifically for use with C++. Here is a summary:
`breakpoint menus'
When you want a breakpoint in a function whose name is overloaded,
GDB has the capability to display a menu of possible breakpoint
locations to help you specify which function definition you want.
*Note Ambiguous Expressions: Ambiguous Expressions.
`rbreak REGEX'
Setting breakpoints using regular expressions is helpful for
setting breakpoints on overloaded functions that are not members
of any special classes. *Note Setting Breakpoints: Set Breaks.
`catch throw'
`catch catch'
Debug C++ exception handling using these commands. *Note Setting
Catchpoints: Set Catchpoints.
`ptype TYPENAME'
Print inheritance relationships as well as other information for
type TYPENAME. *Note Examining the Symbol Table: Symbols.
`set print demangle'
`show print demangle'
`set print asm-demangle'
`show print asm-demangle'
Control whether C++ symbols display in their source form, both when
displaying code as C++ source and when displaying disassemblies.
*Note Print Settings: Print Settings.
`set print object'
`show print object'
Choose whether to print derived (actual) or declared types of
objects. *Note Print Settings: Print Settings.
`set print vtbl'
`show print vtbl'
Control the format for printing virtual function tables. *Note
Print Settings: Print Settings. (The `vtbl' commands do not work
on programs compiled with the HP ANSI C++ compiler (`aCC').)
`set overload-resolution on'
Enable overload resolution for C++ expression evaluation. The
default is on. For overloaded functions, GDB evaluates the
arguments and searches for a function whose signature matches the
argument types, using the standard C++ conversion rules (see *Note
C++ Expressions: C Plus Plus Expressions, for details). If it
cannot find a match, it emits a message.
`set overload-resolution off'
Disable overload resolution for C++ expression evaluation. For
overloaded functions that are not class member functions, GDB
chooses the first function of the specified name that it finds in
the symbol table, whether or not its arguments are of the correct
type. For overloaded functions that are class member functions,
GDB searches for a function whose signature _exactly_ matches the
argument types.
`show overload-resolution'
Show the current setting of overload resolution.
`Overloaded symbol names'
You can specify a particular definition of an overloaded symbol,
using the same notation that is used to declare such symbols in
C++: type `SYMBOL(TYPES)' rather than just SYMBOL. You can also
use the GDB command-line word completion facilities to list the
available choices, or to finish the type list for you. *Note
Command Completion: Completion, for details on how to do this.

File: gdb.info, Node: Decimal Floating Point, Prev: Debugging C Plus Plus, Up: C
15.4.1.8 Decimal Floating Point format
......................................
GDB can examine, set and perform computations with numbers in decimal
floating point format, which in the C language correspond to the
`_Decimal32', `_Decimal64' and `_Decimal128' types as specified by the
extension to support decimal floating-point arithmetic.
There are two encodings in use, depending on the architecture: BID
(Binary Integer Decimal) for x86 and x86-64, and DPD (Densely Packed
Decimal) for PowerPC. GDB will use the appropriate encoding for the
configured target.
Because of a limitation in `libdecnumber', the library used by GDB
to manipulate decimal floating point numbers, it is not possible to
convert (using a cast, for example) integers wider than 32-bit to
decimal float.
In addition, in order to imitate GDB's behaviour with binary floating
point computations, error checking in decimal float operations ignores
underflow, overflow and divide by zero exceptions.
In the PowerPC architecture, GDB provides a set of pseudo-registers
to inspect `_Decimal128' values stored in floating point registers.
See *Note PowerPC: PowerPC. for more details.

File: gdb.info, Node: D, Next: Objective-C, Prev: C, Up: Supported Languages
15.4.2 D
--------
GDB can be used to debug programs written in D and compiled with GDC,
LDC or DMD compilers. Currently GDB supports only one D specific
feature -- dynamic arrays.

File: gdb.info, Node: Objective-C, Next: Fortran, Prev: D, Up: Supported Languages
15.4.3 Objective-C
------------------
This section provides information about some commands and command
options that are useful for debugging Objective-C code. See also *Note
info classes: Symbols, and *Note info selectors: Symbols, for a few
more commands specific to Objective-C support.
* Menu:
* Method Names in Commands::
* The Print Command with Objective-C::

File: gdb.info, Node: Method Names in Commands, Next: The Print Command with Objective-C, Up: Objective-C
15.4.3.1 Method Names in Commands
.................................
The following commands have been extended to accept Objective-C method
names as line specifications:
* `clear'
* `break'
* `info line'
* `jump'
* `list'
A fully qualified Objective-C method name is specified as
-[CLASS METHODNAME]
where the minus sign is used to indicate an instance method and a
plus sign (not shown) is used to indicate a class method. The class
name CLASS and method name METHODNAME are enclosed in brackets, similar
to the way messages are specified in Objective-C source code. For
example, to set a breakpoint at the `create' instance method of class
`Fruit' in the program currently being debugged, enter:
break -[Fruit create]
To list ten program lines around the `initialize' class method,
enter:
list +[NSText initialize]
In the current version of GDB, the plus or minus sign is required.
In future versions of GDB, the plus or minus sign will be optional, but
you can use it to narrow the search. It is also possible to specify
just a method name:
break create
You must specify the complete method name, including any colons. If
your program's source files contain more than one `create' method,
you'll be presented with a numbered list of classes that implement that
method. Indicate your choice by number, or type `0' to exit if none
apply.
As another example, to clear a breakpoint established at the
`makeKeyAndOrderFront:' method of the `NSWindow' class, enter:
clear -[NSWindow makeKeyAndOrderFront:]

File: gdb.info, Node: The Print Command with Objective-C, Prev: Method Names in Commands, Up: Objective-C
15.4.3.2 The Print Command With Objective-C
...........................................
The print command has also been extended to accept methods. For
example:
print -[OBJECT hash]
will tell GDB to send the `hash' message to OBJECT and print the
result. Also, an additional command has been added, `print-object' or
`po' for short, which is meant to print the description of an object.
However, this command may only work with certain Objective-C libraries
that have a particular hook function, `_NSPrintForDebugger', defined.

File: gdb.info, Node: Fortran, Next: Pascal, Prev: Objective-C, Up: Supported Languages
15.4.4 Fortran
--------------
GDB can be used to debug programs written in Fortran, but it currently
supports only the features of Fortran 77 language.
Some Fortran compilers (GNU Fortran 77 and Fortran 95 compilers
among them) append an underscore to the names of variables and
functions. When you debug programs compiled by those compilers, you
will need to refer to variables and functions with a trailing
underscore.
* Menu:
* Fortran Operators:: Fortran operators and expressions
* Fortran Defaults:: Default settings for Fortran
* Special Fortran Commands:: Special GDB commands for Fortran

File: gdb.info, Node: Fortran Operators, Next: Fortran Defaults, Up: Fortran
15.4.4.1 Fortran Operators and Expressions
..........................................
Operators must be defined on values of specific types. For instance,
`+' is defined on numbers, but not on characters or other non-
arithmetic types. Operators are often defined on groups of types.
`**'
The exponentiation operator. It raises the first operand to the
power of the second one.
`:'
The range operator. Normally used in the form of array(low:high)
to represent a section of array.
`%'
The access component operator. Normally used to access elements
in derived types. Also suitable for unions. As unions aren't
part of regular Fortran, this can only happen when accessing a
register that uses a gdbarch-defined union type.

File: gdb.info, Node: Fortran Defaults, Next: Special Fortran Commands, Prev: Fortran Operators, Up: Fortran
15.4.4.2 Fortran Defaults
.........................
Fortran symbols are usually case-insensitive, so GDB by default uses
case-insensitive matches for Fortran symbols. You can change that with
the `set case-insensitive' command, see *Note Symbols::, for the
details.

File: gdb.info, Node: Special Fortran Commands, Prev: Fortran Defaults, Up: Fortran
15.4.4.3 Special Fortran Commands
.................................
GDB has some commands to support Fortran-specific features, such as
displaying common blocks.
`info common [COMMON-NAME]'
This command prints the values contained in the Fortran `COMMON'
block whose name is COMMON-NAME. With no argument, the names of
all `COMMON' blocks visible at the current program location are
printed.

File: gdb.info, Node: Pascal, Next: Modula-2, Prev: Fortran, Up: Supported Languages
15.4.5 Pascal
-------------
Debugging Pascal programs which use sets, subranges, file variables, or
nested functions does not currently work. GDB does not support
entering expressions, printing values, or similar features using Pascal
syntax.
The Pascal-specific command `set print pascal_static-members'
controls whether static members of Pascal objects are displayed. *Note
pascal_static-members: Print Settings.

File: gdb.info, Node: Modula-2, Next: Ada, Prev: Pascal, Up: Supported Languages
15.4.6 Modula-2
---------------
The extensions made to GDB to support Modula-2 only support output from
the GNU Modula-2 compiler (which is currently being developed). Other
Modula-2 compilers are not currently supported, and attempting to debug
executables produced by them is most likely to give an error as GDB
reads in the executable's symbol table.
* Menu:
* M2 Operators:: Built-in operators
* Built-In Func/Proc:: Built-in functions and procedures
* M2 Constants:: Modula-2 constants
* M2 Types:: Modula-2 types
* M2 Defaults:: Default settings for Modula-2
* Deviations:: Deviations from standard Modula-2
* M2 Checks:: Modula-2 type and range checks
* M2 Scope:: The scope operators `::' and `.'
* GDB/M2:: GDB and Modula-2

File: gdb.info, Node: M2 Operators, Next: Built-In Func/Proc, Up: Modula-2
15.4.6.1 Operators
..................
Operators must be defined on values of specific types. For instance,
`+' is defined on numbers, but not on structures. Operators are often
defined on groups of types. For the purposes of Modula-2, the
following definitions hold:
* _Integral types_ consist of `INTEGER', `CARDINAL', and their
subranges.
* _Character types_ consist of `CHAR' and its subranges.
* _Floating-point types_ consist of `REAL'.
* _Pointer types_ consist of anything declared as `POINTER TO TYPE'.
* _Scalar types_ consist of all of the above.
* _Set types_ consist of `SET' and `BITSET' types.
* _Boolean types_ consist of `BOOLEAN'.
The following operators are supported, and appear in order of
increasing precedence:
`,'
Function argument or array index separator.
`:='
Assignment. The value of VAR `:=' VALUE is VALUE.
`<, >'
Less than, greater than on integral, floating-point, or enumerated
types.
`<=, >='
Less than or equal to, greater than or equal to on integral,
floating-point and enumerated types, or set inclusion on set
types. Same precedence as `<'.
`=, <>, #'
Equality and two ways of expressing inequality, valid on scalar
types. Same precedence as `<'. In GDB scripts, only `<>' is
available for inequality, since `#' conflicts with the script
comment character.
`IN'
Set membership. Defined on set types and the types of their
members. Same precedence as `<'.
`OR'
Boolean disjunction. Defined on boolean types.
`AND, &'
Boolean conjunction. Defined on boolean types.
`@'
The GDB "artificial array" operator (*note Expressions:
Expressions.).
`+, -'
Addition and subtraction on integral and floating-point types, or
union and difference on set types.
`*'
Multiplication on integral and floating-point types, or set
intersection on set types.
`/'
Division on floating-point types, or symmetric set difference on
set types. Same precedence as `*'.
`DIV, MOD'
Integer division and remainder. Defined on integral types. Same
precedence as `*'.
`-'
Negative. Defined on `INTEGER' and `REAL' data.
`^'
Pointer dereferencing. Defined on pointer types.
`NOT'
Boolean negation. Defined on boolean types. Same precedence as
`^'.
`.'
`RECORD' field selector. Defined on `RECORD' data. Same
precedence as `^'.
`[]'
Array indexing. Defined on `ARRAY' data. Same precedence as `^'.
`()'
Procedure argument list. Defined on `PROCEDURE' objects. Same
precedence as `^'.
`::, .'
GDB and Modula-2 scope operators.
_Warning:_ Set expressions and their operations are not yet
supported, so GDB treats the use of the operator `IN', or the use
of operators `+', `-', `*', `/', `=', , `<>', `#', `<=', and `>='
on sets as an error.

File: gdb.info, Node: Built-In Func/Proc, Next: M2 Constants, Prev: M2 Operators, Up: Modula-2
15.4.6.2 Built-in Functions and Procedures
..........................................
Modula-2 also makes available several built-in procedures and functions.
In describing these, the following metavariables are used:
A
represents an `ARRAY' variable.
C
represents a `CHAR' constant or variable.
I
represents a variable or constant of integral type.
M
represents an identifier that belongs to a set. Generally used in
the same function with the metavariable S. The type of S should
be `SET OF MTYPE' (where MTYPE is the type of M).
N
represents a variable or constant of integral or floating-point
type.
R
represents a variable or constant of floating-point type.
T
represents a type.
V
represents a variable.
X
represents a variable or constant of one of many types. See the
explanation of the function for details.
All Modula-2 built-in procedures also return a result, described
below.
`ABS(N)'
Returns the absolute value of N.
`CAP(C)'
If C is a lower case letter, it returns its upper case equivalent,
otherwise it returns its argument.
`CHR(I)'
Returns the character whose ordinal value is I.
`DEC(V)'
Decrements the value in the variable V by one. Returns the new
value.
`DEC(V,I)'
Decrements the value in the variable V by I. Returns the new
value.
`EXCL(M,S)'
Removes the element M from the set S. Returns the new set.
`FLOAT(I)'
Returns the floating point equivalent of the integer I.
`HIGH(A)'
Returns the index of the last member of A.
`INC(V)'
Increments the value in the variable V by one. Returns the new
value.
`INC(V,I)'
Increments the value in the variable V by I. Returns the new
value.
`INCL(M,S)'
Adds the element M to the set S if it is not already there.
Returns the new set.
`MAX(T)'
Returns the maximum value of the type T.
`MIN(T)'
Returns the minimum value of the type T.
`ODD(I)'
Returns boolean TRUE if I is an odd number.
`ORD(X)'
Returns the ordinal value of its argument. For example, the
ordinal value of a character is its ASCII value (on machines
supporting the ASCII character set). X must be of an ordered
type, which include integral, character and enumerated types.
`SIZE(X)'
Returns the size of its argument. X can be a variable or a type.
`TRUNC(R)'
Returns the integral part of R.
`TSIZE(X)'
Returns the size of its argument. X can be a variable or a type.
`VAL(T,I)'
Returns the member of the type T whose ordinal value is I.
_Warning:_ Sets and their operations are not yet supported, so
GDB treats the use of procedures `INCL' and `EXCL' as an error.

File: gdb.info, Node: M2 Constants, Next: M2 Types, Prev: Built-In Func/Proc, Up: Modula-2
15.4.6.3 Constants
..................
GDB allows you to express the constants of Modula-2 in the following
ways:
* Integer constants are simply a sequence of digits. When used in an
expression, a constant is interpreted to be type-compatible with
the rest of the expression. Hexadecimal integers are specified by
a trailing `H', and octal integers by a trailing `B'.
* Floating point constants appear as a sequence of digits, followed
by a decimal point and another sequence of digits. An optional
exponent can then be specified, in the form `E[+|-]NNN', where
`[+|-]NNN' is the desired exponent. All of the digits of the
floating point constant must be valid decimal (base 10) digits.
* Character constants consist of a single character enclosed by a
pair of like quotes, either single (`'') or double (`"'). They may
also be expressed by their ordinal value (their ASCII value,
usually) followed by a `C'.
* String constants consist of a sequence of characters enclosed by a
pair of like quotes, either single (`'') or double (`"'). Escape
sequences in the style of C are also allowed. *Note C and C++
Constants: C Constants, for a brief explanation of escape
sequences.
* Enumerated constants consist of an enumerated identifier.
* Boolean constants consist of the identifiers `TRUE' and `FALSE'.
* Pointer constants consist of integral values only.
* Set constants are not yet supported.

File: gdb.info, Node: M2 Types, Next: M2 Defaults, Prev: M2 Constants, Up: Modula-2
15.4.6.4 Modula-2 Types
.......................
Currently GDB can print the following data types in Modula-2 syntax:
array types, record types, set types, pointer types, procedure types,
enumerated types, subrange types and base types. You can also print
the contents of variables declared using these type. This section
gives a number of simple source code examples together with sample GDB
sessions.
The first example contains the following section of code:
VAR
s: SET OF CHAR ;
r: [20..40] ;
and you can request GDB to interrogate the type and value of `r' and
`s'.
(gdb) print s
{'A'..'C', 'Z'}
(gdb) ptype s
SET OF CHAR
(gdb) print r
21
(gdb) ptype r
[20..40]
Likewise if your source code declares `s' as:
VAR
s: SET ['A'..'Z'] ;
then you may query the type of `s' by:
(gdb) ptype s
type = SET ['A'..'Z']
Note that at present you cannot interactively manipulate set
expressions using the debugger.
The following example shows how you might declare an array in
Modula-2 and how you can interact with GDB to print its type and
contents:
VAR
s: ARRAY [-10..10] OF CHAR ;
(gdb) ptype s
ARRAY [-10..10] OF CHAR
Note that the array handling is not yet complete and although the
type is printed correctly, expression handling still assumes that all
arrays have a lower bound of zero and not `-10' as in the example above.
Here are some more type related Modula-2 examples:
TYPE
colour = (blue, red, yellow, green) ;
t = [blue..yellow] ;
VAR
s: t ;
BEGIN
s := blue ;
The GDB interaction shows how you can query the data type and value of
a variable.
(gdb) print s
$1 = blue
(gdb) ptype t
type = [blue..yellow]
In this example a Modula-2 array is declared and its contents
displayed. Observe that the contents are written in the same way as
their `C' counterparts.
VAR
s: ARRAY [1..5] OF CARDINAL ;
BEGIN
s[1] := 1 ;
(gdb) print s
$1 = {1, 0, 0, 0, 0}
(gdb) ptype s
type = ARRAY [1..5] OF CARDINAL
The Modula-2 language interface to GDB also understands pointer
types as shown in this example:
VAR
s: POINTER TO ARRAY [1..5] OF CARDINAL ;
BEGIN
NEW(s) ;
s^[1] := 1 ;
and you can request that GDB describes the type of `s'.
(gdb) ptype s
type = POINTER TO ARRAY [1..5] OF CARDINAL
GDB handles compound types as we can see in this example. Here we
combine array types, record types, pointer types and subrange types:
TYPE
foo = RECORD
f1: CARDINAL ;
f2: CHAR ;
f3: myarray ;
END ;
myarray = ARRAY myrange OF CARDINAL ;
myrange = [-2..2] ;
VAR
s: POINTER TO ARRAY myrange OF foo ;
and you can ask GDB to describe the type of `s' as shown below.
(gdb) ptype s
type = POINTER TO ARRAY [-2..2] OF foo = RECORD
f1 : CARDINAL;
f2 : CHAR;
f3 : ARRAY [-2..2] OF CARDINAL;
END

File: gdb.info, Node: M2 Defaults, Next: Deviations, Prev: M2 Types, Up: Modula-2
15.4.6.5 Modula-2 Defaults
..........................
If type and range checking are set automatically by GDB, they both
default to `on' whenever the working language changes to Modula-2.
This happens regardless of whether you or GDB selected the working
language.
If you allow GDB to set the language automatically, then entering
code compiled from a file whose name ends with `.mod' sets the working
language to Modula-2. *Note Having GDB Infer the Source Language:
Automatically, for further details.

File: gdb.info, Node: Deviations, Next: M2 Checks, Prev: M2 Defaults, Up: Modula-2
15.4.6.6 Deviations from Standard Modula-2
..........................................
A few changes have been made to make Modula-2 programs easier to debug.
This is done primarily via loosening its type strictness:
* Unlike in standard Modula-2, pointer constants can be formed by
integers. This allows you to modify pointer variables during
debugging. (In standard Modula-2, the actual address contained in
a pointer variable is hidden from you; it can only be modified
through direct assignment to another pointer variable or
expression that returned a pointer.)
* C escape sequences can be used in strings and characters to
represent non-printable characters. GDB prints out strings with
these escape sequences embedded. Single non-printable characters
are printed using the `CHR(NNN)' format.
* The assignment operator (`:=') returns the value of its right-hand
argument.
* All built-in procedures both modify _and_ return their argument.

File: gdb.info, Node: M2 Checks, Next: M2 Scope, Prev: Deviations, Up: Modula-2
15.4.6.7 Modula-2 Type and Range Checks
.......................................
_Warning:_ in this release, GDB does not yet perform type or range
checking.
GDB considers two Modula-2 variables type equivalent if:
* They are of types that have been declared equivalent via a `TYPE
T1 = T2' statement
* They have been declared on the same line. (Note: This is true of
the GNU Modula-2 compiler, but it may not be true of other
compilers.)
As long as type checking is enabled, any attempt to combine variables
whose types are not equivalent is an error.
Range checking is done on all mathematical operations, assignment,
array index bounds, and all built-in functions and procedures.

File: gdb.info, Node: M2 Scope, Next: GDB/M2, Prev: M2 Checks, Up: Modula-2
15.4.6.8 The Scope Operators `::' and `.'
.........................................
There are a few subtle differences between the Modula-2 scope operator
(`.') and the GDB scope operator (`::'). The two have similar syntax:
MODULE . ID
SCOPE :: ID
where SCOPE is the name of a module or a procedure, MODULE the name of
a module, and ID is any declared identifier within your program, except
another module.
Using the `::' operator makes GDB search the scope specified by
SCOPE for the identifier ID. If it is not found in the specified
scope, then GDB searches all scopes enclosing the one specified by
SCOPE.
Using the `.' operator makes GDB search the current scope for the
identifier specified by ID that was imported from the definition module
specified by MODULE. With this operator, it is an error if the
identifier ID was not imported from definition module MODULE, or if ID
is not an identifier in MODULE.

File: gdb.info, Node: GDB/M2, Prev: M2 Scope, Up: Modula-2
15.4.6.9 GDB and Modula-2
.........................
Some GDB commands have little use when debugging Modula-2 programs.
Five subcommands of `set print' and `show print' apply specifically to
C and C++: `vtbl', `demangle', `asm-demangle', `object', and `union'.
The first four apply to C++, and the last to the C `union' type, which
has no direct analogue in Modula-2.
The `@' operator (*note Expressions: Expressions.), while available
with any language, is not useful with Modula-2. Its intent is to aid
the debugging of "dynamic arrays", which cannot be created in Modula-2
as they can in C or C++. However, because an address can be specified
by an integral constant, the construct `{TYPE}ADREXP' is still useful.
In GDB scripts, the Modula-2 inequality operator `#' is interpreted
as the beginning of a comment. Use `<>' instead.

File: gdb.info, Node: Ada, Prev: Modula-2, Up: Supported Languages
15.4.7 Ada
----------
The extensions made to GDB for Ada only support output from the GNU Ada
(GNAT) compiler. Other Ada compilers are not currently supported, and
attempting to debug executables produced by them is most likely to be
difficult.
* Menu:
* Ada Mode Intro:: General remarks on the Ada syntax
and semantics supported by Ada mode
in GDB.
* Omissions from Ada:: Restrictions on the Ada expression syntax.
* Additions to Ada:: Extensions of the Ada expression syntax.
* Stopping Before Main Program:: Debugging the program during elaboration.
* Ada Tasks:: Listing and setting breakpoints in tasks.
* Ada Tasks and Core Files:: Tasking Support when Debugging Core Files
* Ada Glitches:: Known peculiarities of Ada mode.

File: gdb.info, Node: Ada Mode Intro, Next: Omissions from Ada, Up: Ada
15.4.7.1 Introduction
.....................
The Ada mode of GDB supports a fairly large subset of Ada expression
syntax, with some extensions. The philosophy behind the design of this
subset is
* That GDB should provide basic literals and access to operations for
arithmetic, dereferencing, field selection, indexing, and
subprogram calls, leaving more sophisticated computations to
subprograms written into the program (which therefore may be
called from GDB).
* That type safety and strict adherence to Ada language restrictions
are not particularly important to the GDB user.
* That brevity is important to the GDB user.
Thus, for brevity, the debugger acts as if all names declared in
user-written packages are directly visible, even if they are not visible
according to Ada rules, thus making it unnecessary to fully qualify most
names with their packages, regardless of context. Where this causes
ambiguity, GDB asks the user's intent.
The debugger will start in Ada mode if it detects an Ada main
program. As for other languages, it will enter Ada mode when stopped
in a program that was translated from an Ada source file.
While in Ada mode, you may use `-' for comments. This is useful
mostly for documenting command files. The standard GDB comment (`#')
still works at the beginning of a line in Ada mode, but not in the
middle (to allow based literals).
The debugger supports limited overloading. Given a subprogram call
in which the function symbol has multiple definitions, it will use the
number of actual parameters and some information about their types to
attempt to narrow the set of definitions. It also makes very limited
use of context, preferring procedures to functions in the context of
the `call' command, and functions to procedures elsewhere.

File: gdb.info, Node: Omissions from Ada, Next: Additions to Ada, Prev: Ada Mode Intro, Up: Ada
15.4.7.2 Omissions from Ada
...........................
Here are the notable omissions from the subset:
* Only a subset of the attributes are supported:
- 'First, 'Last, and 'Length on array objects (not on types
and subtypes).
- 'Min and 'Max.
- 'Pos and 'Val.
- 'Tag.
- 'Range on array objects (not subtypes), but only as the right
operand of the membership (`in') operator.
- 'Access, 'Unchecked_Access, and 'Unrestricted_Access (a GNAT
extension).
- 'Address.
* The names in `Characters.Latin_1' are not available and
concatenation is not implemented. Thus, escape characters in
strings are not currently available.
* Equality tests (`=' and `/=') on arrays test for bitwise equality
of representations. They will generally work correctly for
strings and arrays whose elements have integer or enumeration
types. They may not work correctly for arrays whose element types
have user-defined equality, for arrays of real values (in
particular, IEEE-conformant floating point, because of negative
zeroes and NaNs), and for arrays whose elements contain unused
bits with indeterminate values.
* The other component-by-component array operations (`and', `or',
`xor', `not', and relational tests other than equality) are not
implemented.
* There is limited support for array and record aggregates. They are
permitted only on the right sides of assignments, as in these
examples:
(gdb) set An_Array := (1, 2, 3, 4, 5, 6)
(gdb) set An_Array := (1, others => 0)
(gdb) set An_Array := (0|4 => 1, 1..3 => 2, 5 => 6)
(gdb) set A_2D_Array := ((1, 2, 3), (4, 5, 6), (7, 8, 9))
(gdb) set A_Record := (1, "Peter", True);
(gdb) set A_Record := (Name => "Peter", Id => 1, Alive => True)
Changing a discriminant's value by assigning an aggregate has an
undefined effect if that discriminant is used within the record.
However, you can first modify discriminants by directly assigning
to them (which normally would not be allowed in Ada), and then
performing an aggregate assignment. For example, given a variable
`A_Rec' declared to have a type such as:
type Rec (Len : Small_Integer := 0) is record
Id : Integer;
Vals : IntArray (1 .. Len);
end record;
you can assign a value with a different size of `Vals' with two
assignments:
(gdb) set A_Rec.Len := 4
(gdb) set A_Rec := (Id => 42, Vals => (1, 2, 3, 4))
As this example also illustrates, GDB is very loose about the usual
rules concerning aggregates. You may leave out some of the
components of an array or record aggregate (such as the `Len'
component in the assignment to `A_Rec' above); they will retain
their original values upon assignment. You may freely use dynamic
values as indices in component associations. You may even use
overlapping or redundant component associations, although which
component values are assigned in such cases is not defined.
* Calls to dispatching subprograms are not implemented.
* The overloading algorithm is much more limited (i.e., less
selective) than that of real Ada. It makes only limited use of
the context in which a subexpression appears to resolve its
meaning, and it is much looser in its rules for allowing type
matches. As a result, some function calls will be ambiguous, and
the user will be asked to choose the proper resolution.
* The `new' operator is not implemented.
* Entry calls are not implemented.
* Aside from printing, arithmetic operations on the native VAX
floating-point formats are not supported.
* It is not possible to slice a packed array.
* The names `True' and `False', when not part of a qualified name,
are interpreted as if implicitly prefixed by `Standard',
regardless of context. Should your program redefine these names
in a package or procedure (at best a dubious practice), you will
have to use fully qualified names to access their new definitions.

File: gdb.info, Node: Additions to Ada, Next: Stopping Before Main Program, Prev: Omissions from Ada, Up: Ada
15.4.7.3 Additions to Ada
.........................
As it does for other languages, GDB makes certain generic extensions to
Ada (*note Expressions::):
* If the expression E is a variable residing in memory (typically a
local variable or array element) and N is a positive integer, then
`E@N' displays the values of E and the N-1 adjacent variables
following it in memory as an array. In Ada, this operator is
generally not necessary, since its prime use is in displaying
parts of an array, and slicing will usually do this in Ada.
However, there are occasional uses when debugging programs in
which certain debugging information has been optimized away.
* `B::VAR' means "the variable named VAR that appears in function or
file B." When B is a file name, you must typically surround it in
single quotes.
* The expression `{TYPE} ADDR' means "the variable of type TYPE that
appears at address ADDR."
* A name starting with `$' is a convenience variable (*note
Convenience Vars::) or a machine register (*note Registers::).
In addition, GDB provides a few other shortcuts and outright
additions specific to Ada:
* The assignment statement is allowed as an expression, returning
its right-hand operand as its value. Thus, you may enter
(gdb) set x := y + 3
(gdb) print A(tmp := y + 1)
* The semicolon is allowed as an "operator," returning as its value
the value of its right-hand operand. This allows, for example,
complex conditional breaks:
(gdb) break f
(gdb) condition 1 (report(i); k += 1; A(k) > 100)
* Rather than use catenation and symbolic character names to
introduce special characters into strings, one may instead use a
special bracket notation, which is also used to print strings. A
sequence of characters of the form `["XX"]' within a string or
character literal denotes the (single) character whose numeric
encoding is XX in hexadecimal. The sequence of characters `["""]'
also denotes a single quotation mark in strings. For example,
"One line.["0a"]Next line.["0a"]"
contains an ASCII newline character (`Ada.Characters.Latin_1.LF')
after each period.
* The subtype used as a prefix for the attributes 'Pos, 'Min, and
'Max is optional (and is ignored in any case). For example, it is
valid to write
(gdb) print 'max(x, y)
* When printing arrays, GDB uses positional notation when the array
has a lower bound of 1, and uses a modified named notation
otherwise. For example, a one-dimensional array of three integers
with a lower bound of 3 might print as
(3 => 10, 17, 1)
That is, in contrast to valid Ada, only the first component has a
`=>' clause.
* You may abbreviate attributes in expressions with any unique,
multi-character subsequence of their names (an exact match gets
preference). For example, you may use a'len, a'gth, or a'lh in
place of a'length.
* Since Ada is case-insensitive, the debugger normally maps
identifiers you type to lower case. The GNAT compiler uses
upper-case characters for some of its internal identifiers, which
are normally of no interest to users. For the rare occasions when
you actually have to look at them, enclose them in angle brackets
to avoid the lower-case mapping. For example,
(gdb) print <JMPBUF_SAVE>[0]
* Printing an object of class-wide type or dereferencing an
access-to-class-wide value will display all the components of the
object's specific type (as indicated by its run-time tag).
Likewise, component selection on such a value will operate on the
specific type of the object.

File: gdb.info, Node: Stopping Before Main Program, Next: Ada Tasks, Prev: Additions to Ada, Up: Ada
15.4.7.4 Stopping at the Very Beginning
.......................................
It is sometimes necessary to debug the program during elaboration, and
before reaching the main procedure. As defined in the Ada Reference
Manual, the elaboration code is invoked from a procedure called
`adainit'. To run your program up to the beginning of elaboration,
simply use the following two commands: `tbreak adainit' and `run'.

File: gdb.info, Node: Ada Tasks, Next: Ada Tasks and Core Files, Prev: Stopping Before Main Program, Up: Ada
15.4.7.5 Extensions for Ada Tasks
.................................
Support for Ada tasks is analogous to that for threads (*note
Threads::). GDB provides the following task-related commands:
`info tasks'
This command shows a list of current Ada tasks, as in the
following example:
(gdb) info tasks
ID TID P-ID Pri State Name
1 8088000 0 15 Child Activation Wait main_task
2 80a4000 1 15 Accept Statement b
3 809a800 1 15 Child Activation Wait a
* 4 80ae800 3 15 Runnable c
In this listing, the asterisk before the last task indicates it to
be the task currently being inspected.
ID
Represents GDB's internal task number.
TID
The Ada task ID.
P-ID
The parent's task ID (GDB's internal task number).
Pri
The base priority of the task.
State
Current state of the task.
`Unactivated'
The task has been created but has not been activated.
It cannot be executing.
`Runnable'
The task is not blocked for any reason known to Ada.
(It may be waiting for a mutex, though.) It is
conceptually "executing" in normal mode.
`Terminated'
The task is terminated, in the sense of ARM 9.3 (5).
Any dependents that were waiting on terminate
alternatives have been awakened and have terminated
themselves.
`Child Activation Wait'
The task is waiting for created tasks to complete
activation.
`Accept Statement'
The task is waiting on an accept or selective wait
statement.
`Waiting on entry call'
The task is waiting on an entry call.
`Async Select Wait'
The task is waiting to start the abortable part of an
asynchronous select statement.
`Delay Sleep'
The task is waiting on a select statement with only a
delay alternative open.
`Child Termination Wait'
The task is sleeping having completed a master within
itself, and is waiting for the tasks dependent on that
master to become terminated or waiting on a terminate
Phase.
`Wait Child in Term Alt'
The task is sleeping waiting for tasks on terminate
alternatives to finish terminating.
`Accepting RV with TASKNO'
The task is accepting a rendez-vous with the task TASKNO.
Name
Name of the task in the program.
`info task TASKNO'
This command shows detailled informations on the specified task,
as in the following example:
(gdb) info tasks
ID TID P-ID Pri State Name
1 8077880 0 15 Child Activation Wait main_task
* 2 807c468 1 15 Runnable task_1
(gdb) info task 2
Ada Task: 0x807c468
Name: task_1
Thread: 0x807f378
Parent: 1 (main_task)
Base Priority: 15
State: Runnable
`task'
This command prints the ID of the current task.
(gdb) info tasks
ID TID P-ID Pri State Name
1 8077870 0 15 Child Activation Wait main_task
* 2 807c458 1 15 Runnable t
(gdb) task
[Current task is 2]
`task TASKNO'
This command is like the `thread THREADNO' command (*note
Threads::). It switches the context of debugging from the current
task to the given task.
(gdb) info tasks
ID TID P-ID Pri State Name
1 8077870 0 15 Child Activation Wait main_task
* 2 807c458 1 15 Runnable t
(gdb) task 1
[Switching to task 1]
#0 0x8067726 in pthread_cond_wait ()
(gdb) bt
#0 0x8067726 in pthread_cond_wait ()
#1 0x8056714 in system.os_interface.pthread_cond_wait ()
#2 0x805cb63 in system.task_primitives.operations.sleep ()
#3 0x806153e in system.tasking.stages.activate_tasks ()
#4 0x804aacc in un () at un.adb:5
`break LINESPEC task TASKNO'
`break LINESPEC task TASKNO if ...'
These commands are like the `break ... thread ...' command (*note
Thread Stops::). LINESPEC specifies source lines, as described in
*Note Specify Location::.
Use the qualifier `task TASKNO' with a breakpoint command to
specify that you only want GDB to stop the program when a
particular Ada task reaches this breakpoint. TASKNO is one of the
numeric task identifiers assigned by GDB, shown in the first
column of the `info tasks' display.
If you do not specify `task TASKNO' when you set a breakpoint, the
breakpoint applies to _all_ tasks of your program.
You can use the `task' qualifier on conditional breakpoints as
well; in this case, place `task TASKNO' before the breakpoint
condition (before the `if').
For example,
(gdb) info tasks
ID TID P-ID Pri State Name
1 140022020 0 15 Child Activation Wait main_task
2 140045060 1 15 Accept/Select Wait t2
3 140044840 1 15 Runnable t1
* 4 140056040 1 15 Runnable t3
(gdb) b 15 task 2
Breakpoint 5 at 0x120044cb0: file test_task_debug.adb, line 15.
(gdb) cont
Continuing.
task # 1 running
task # 2 running
Breakpoint 5, test_task_debug () at test_task_debug.adb:15
15 flush;
(gdb) info tasks
ID TID P-ID Pri State Name
1 140022020 0 15 Child Activation Wait main_task
* 2 140045060 1 15 Runnable t2
3 140044840 1 15 Runnable t1
4 140056040 1 15 Delay Sleep t3

File: gdb.info, Node: Ada Tasks and Core Files, Next: Ada Glitches, Prev: Ada Tasks, Up: Ada
15.4.7.6 Tasking Support when Debugging Core Files
..................................................
When inspecting a core file, as opposed to debugging a live program,
tasking support may be limited or even unavailable, depending on the
platform being used. For instance, on x86-linux, the list of tasks is
available, but task switching is not supported. On Tru64, however,
task switching will work as usual.
On certain platforms, including Tru64, the debugger needs to perform
some memory writes in order to provide Ada tasking support. When
inspecting a core file, this means that the core file must be opened
with read-write privileges, using the command `"set write on"' (*note
Patching::). Under these circumstances, you should make a backup copy
of the core file before inspecting it with GDB.

File: gdb.info, Node: Ada Glitches, Prev: Ada Tasks and Core Files, Up: Ada
15.4.7.7 Known Peculiarities of Ada Mode
........................................
Besides the omissions listed previously (*note Omissions from Ada::),
we know of several problems with and limitations of Ada mode in GDB,
some of which will be fixed with planned future releases of the debugger
and the GNU Ada compiler.
* Currently, the debugger has insufficient information to determine
whether certain pointers represent pointers to objects or the
objects themselves. Thus, the user may have to tack an extra
`.all' after an expression to get it printed properly.
* Static constants that the compiler chooses not to materialize as
objects in storage are invisible to the debugger.
* Named parameter associations in function argument lists are
ignored (the argument lists are treated as positional).
* Many useful library packages are currently invisible to the
debugger.
* Fixed-point arithmetic, conversions, input, and output is carried
out using floating-point arithmetic, and may give results that
only approximate those on the host machine.
* The GNAT compiler never generates the prefix `Standard' for any of
the standard symbols defined by the Ada language. GDB knows about
this: it will strip the prefix from names when you use it, and
will never look for a name you have so qualified among local
symbols, nor match against symbols in other packages or
subprograms. If you have defined entities anywhere in your
program other than parameters and local variables whose simple
names match names in `Standard', GNAT's lack of qualification here
can cause confusion. When this happens, you can usually resolve
the confusion by qualifying the problematic names with package
`Standard' explicitly.
Older versions of the compiler sometimes generate erroneous debugging
information, resulting in the debugger incorrectly printing the value
of affected entities. In some cases, the debugger is able to work
around an issue automatically. In other cases, the debugger is able to
work around the issue, but the work-around has to be specifically
enabled.
`set ada trust-PAD-over-XVS on'
Configure GDB to strictly follow the GNAT encoding when computing
the value of Ada entities, particularly when `PAD' and `PAD___XVS'
types are involved (see `ada/exp_dbug.ads' in the GCC sources for
a complete description of the encoding used by the GNAT compiler).
This is the default.
`set ada trust-PAD-over-XVS off'
This is related to the encoding using by the GNAT compiler. If
GDB sometimes prints the wrong value for certain entities,
changing `ada trust-PAD-over-XVS' to `off' activates a work-around
which may fix the issue. It is always safe to set `ada
trust-PAD-over-XVS' to `off', but this incurs a slight performance
penalty, so it is recommended to leave this setting to `on' unless
necessary.

File: gdb.info, Node: Unsupported Languages, Prev: Supported Languages, Up: Languages
15.5 Unsupported Languages
==========================
In addition to the other fully-supported programming languages, GDB
also provides a pseudo-language, called `minimal'. It does not
represent a real programming language, but provides a set of
capabilities close to what the C or assembly languages provide. This
should allow most simple operations to be performed while debugging an
application that uses a language currently not supported by GDB.
If the language is set to `auto', GDB will automatically select this
language if the current frame corresponds to an unsupported language.

File: gdb.info, Node: Symbols, Next: Altering, Prev: Languages, Up: Top
16 Examining the Symbol Table
*****************************
The commands described in this chapter allow you to inquire about the
symbols (names of variables, functions and types) defined in your
program. This information is inherent in the text of your program and
does not change as your program executes. GDB finds it in your
program's symbol table, in the file indicated when you started GDB
(*note Choosing Files: File Options.), or by one of the file-management
commands (*note Commands to Specify Files: Files.).
Occasionally, you may need to refer to symbols that contain unusual
characters, which GDB ordinarily treats as word delimiters. The most
frequent case is in referring to static variables in other source files
(*note Program Variables: Variables.). File names are recorded in
object files as debugging symbols, but GDB would ordinarily parse a
typical file name, like `foo.c', as the three words `foo' `.' `c'. To
allow GDB to recognize `foo.c' as a single symbol, enclose it in single
quotes; for example,
p 'foo.c'::x
looks up the value of `x' in the scope of the file `foo.c'.
`set case-sensitive on'
`set case-sensitive off'
`set case-sensitive auto'
Normally, when GDB looks up symbols, it matches their names with
case sensitivity determined by the current source language.
Occasionally, you may wish to control that. The command `set
case-sensitive' lets you do that by specifying `on' for
case-sensitive matches or `off' for case-insensitive ones. If you
specify `auto', case sensitivity is reset to the default suitable
for the source language. The default is case-sensitive matches
for all languages except for Fortran, for which the default is
case-insensitive matches.
`show case-sensitive'
This command shows the current setting of case sensitivity for
symbols lookups.
`info address SYMBOL'
Describe where the data for SYMBOL is stored. For a register
variable, this says which register it is kept in. For a
non-register local variable, this prints the stack-frame offset at
which the variable is always stored.
Note the contrast with `print &SYMBOL', which does not work at all
for a register variable, and for a stack local variable prints the
exact address of the current instantiation of the variable.
`info symbol ADDR'
Print the name of a symbol which is stored at the address ADDR.
If no symbol is stored exactly at ADDR, GDB prints the nearest
symbol and an offset from it:
(gdb) info symbol 0x54320
_initialize_vx + 396 in section .text
This is the opposite of the `info address' command. You can use
it to find out the name of a variable or a function given its
address.
For dynamically linked executables, the name of executable or
shared library containing the symbol is also printed:
(gdb) info symbol 0x400225
_start + 5 in section .text of /tmp/a.out
(gdb) info symbol 0x2aaaac2811cf
__read_nocancel + 6 in section .text of /usr/lib64/libc.so.6
`whatis [ARG]'
Print the data type of ARG, which can be either an expression or a
data type. With no argument, print the data type of `$', the last
value in the value history. If ARG is an expression, it is not
actually evaluated, and any side-effecting operations (such as
assignments or function calls) inside it do not take place. If
ARG is a type name, it may be the name of a type or typedef, or
for C code it may have the form `class CLASS-NAME', `struct
STRUCT-TAG', `union UNION-TAG' or `enum ENUM-TAG'. *Note
Expressions: Expressions.
`ptype [ARG]'
`ptype' accepts the same arguments as `whatis', but prints a
detailed description of the type, instead of just the name of the
type. *Note Expressions: Expressions.
For example, for this variable declaration:
struct complex {double real; double imag;} v;
the two commands give this output:
(gdb) whatis v
type = struct complex
(gdb) ptype v
type = struct complex {
double real;
double imag;
}
As with `whatis', using `ptype' without an argument refers to the
type of `$', the last value in the value history.
Sometimes, programs use opaque data types or incomplete
specifications of complex data structure. If the debug
information included in the program does not allow GDB to display
a full declaration of the data type, it will say `<incomplete
type>'. For example, given these declarations:
struct foo;
struct foo *fooptr;
but no definition for `struct foo' itself, GDB will say:
(gdb) ptype foo
$1 = <incomplete type>
"Incomplete type" is C terminology for data types that are not
completely specified.
`info types REGEXP'
`info types'
Print a brief description of all types whose names match the
regular expression REGEXP (or all types in your program, if you
supply no argument). Each complete typename is matched as though
it were a complete line; thus, `i type value' gives information on
all types in your program whose names include the string `value',
but `i type ^value$' gives information only on types whose complete
name is `value'.
This command differs from `ptype' in two ways: first, like
`whatis', it does not print a detailed description; second, it
lists all source files where a type is defined.
`info scope LOCATION'
List all the variables local to a particular scope. This command
accepts a LOCATION argument--a function name, a source line, or an
address preceded by a `*', and prints all the variables local to
the scope defined by that location. (*Note Specify Location::, for
details about supported forms of LOCATION.) For example:
(gdb) info scope command_line_handler
Scope for command_line_handler:
Symbol rl is an argument at stack/frame offset 8, length 4.
Symbol linebuffer is in static storage at address 0x150a18, length 4.
Symbol linelength is in static storage at address 0x150a1c, length 4.
Symbol p is a local variable in register $esi, length 4.
Symbol p1 is a local variable in register $ebx, length 4.
Symbol nline is a local variable in register $edx, length 4.
Symbol repeat is a local variable at frame offset -8, length 4.
This command is especially useful for determining what data to
collect during a "trace experiment", see *Note collect: Tracepoint
Actions.
`info source'
Show information about the current source file--that is, the
source file for the function containing the current point of
execution:
* the name of the source file, and the directory containing it,
* the directory it was compiled in,
* its length, in lines,
* which programming language it is written in,
* whether the executable includes debugging information for
that file, and if so, what format the information is in
(e.g., STABS, Dwarf 2, etc.), and
* whether the debugging information includes information about
preprocessor macros.
`info sources'
Print the names of all source files in your program for which
there is debugging information, organized into two lists: files
whose symbols have already been read, and files whose symbols will
be read when needed.
`info functions'
Print the names and data types of all defined functions.
`info functions REGEXP'
Print the names and data types of all defined functions whose
names contain a match for regular expression REGEXP. Thus, `info
fun step' finds all functions whose names include `step'; `info
fun ^step' finds those whose names start with `step'. If a
function name contains characters that conflict with the regular
expression language (e.g. `operator*()'), they may be quoted with
a backslash.
`info variables'
Print the names and data types of all variables that are defined
outside of functions (i.e. excluding local variables).
`info variables REGEXP'
Print the names and data types of all variables (except for local
variables) whose names contain a match for regular expression
REGEXP.
`info classes'
`info classes REGEXP'
Display all Objective-C classes in your program, or (with the
REGEXP argument) all those matching a particular regular
expression.
`info selectors'
`info selectors REGEXP'
Display all Objective-C selectors in your program, or (with the
REGEXP argument) all those matching a particular regular
expression.
Some systems allow individual object files that make up your
program to be replaced without stopping and restarting your
program. For example, in VxWorks you can simply recompile a
defective object file and keep on running. If you are running on
one of these systems, you can allow GDB to reload the symbols for
automatically relinked modules:
`set symbol-reloading on'
Replace symbol definitions for the corresponding source file
when an object file with a particular name is seen again.
`set symbol-reloading off'
Do not replace symbol definitions when encountering object
files of the same name more than once. This is the default
state; if you are not running on a system that permits
automatic relinking of modules, you should leave
`symbol-reloading' off, since otherwise GDB may discard
symbols when linking large programs, that may contain several
modules (from different directories or libraries) with the
same name.
`show symbol-reloading'
Show the current `on' or `off' setting.
`set opaque-type-resolution on'
Tell GDB to resolve opaque types. An opaque type is a type
declared as a pointer to a `struct', `class', or `union'--for
example, `struct MyType *'--that is used in one source file
although the full declaration of `struct MyType' is in another
source file. The default is on.
A change in the setting of this subcommand will not take effect
until the next time symbols for a file are loaded.
`set opaque-type-resolution off'
Tell GDB not to resolve opaque types. In this case, the type is
printed as follows:
{<no data fields>}
`show opaque-type-resolution'
Show whether opaque types are resolved or not.
`maint print symbols FILENAME'
`maint print psymbols FILENAME'
`maint print msymbols FILENAME'
Write a dump of debugging symbol data into the file FILENAME.
These commands are used to debug the GDB symbol-reading code. Only
symbols with debugging data are included. If you use `maint print
symbols', GDB includes all the symbols for which it has already
collected full details: that is, FILENAME reflects symbols for
only those files whose symbols GDB has read. You can use the
command `info sources' to find out which files these are. If you
use `maint print psymbols' instead, the dump shows information
about symbols that GDB only knows partially--that is, symbols
defined in files that GDB has skimmed, but not yet read
completely. Finally, `maint print msymbols' dumps just the
minimal symbol information required for each object file from
which GDB has read some symbols. *Note Commands to Specify Files:
Files, for a discussion of how GDB reads symbols (in the
description of `symbol-file').
`maint info symtabs [ REGEXP ]'
`maint info psymtabs [ REGEXP ]'
List the `struct symtab' or `struct partial_symtab' structures
whose names match REGEXP. If REGEXP is not given, list them all.
The output includes expressions which you can copy into a GDB
debugging this one to examine a particular structure in more
detail. For example:
(gdb) maint info psymtabs dwarf2read
{ objfile /home/gnu/build/gdb/gdb
((struct objfile *) 0x82e69d0)
{ psymtab /home/gnu/src/gdb/dwarf2read.c
((struct partial_symtab *) 0x8474b10)
readin no
fullname (null)
text addresses 0x814d3c8 -- 0x8158074
globals (* (struct partial_symbol **) 0x8507a08 @ 9)
statics (* (struct partial_symbol **) 0x40e95b78 @ 2882)
dependencies (none)
}
}
(gdb) maint info symtabs
(gdb)
We see that there is one partial symbol table whose filename
contains the string `dwarf2read', belonging to the `gdb'
executable; and we see that GDB has not read in any symtabs yet at
all. If we set a breakpoint on a function, that will cause GDB to
read the symtab for the compilation unit containing that function:
(gdb) break dwarf2_psymtab_to_symtab
Breakpoint 1 at 0x814e5da: file /home/gnu/src/gdb/dwarf2read.c,
line 1574.
(gdb) maint info symtabs
{ objfile /home/gnu/build/gdb/gdb
((struct objfile *) 0x82e69d0)
{ symtab /home/gnu/src/gdb/dwarf2read.c
((struct symtab *) 0x86c1f38)
dirname (null)
fullname (null)
blockvector ((struct blockvector *) 0x86c1bd0) (primary)
linetable ((struct linetable *) 0x8370fa0)
debugformat DWARF 2
}
}
(gdb)

File: gdb.info, Node: Altering, Next: GDB Files, Prev: Symbols, Up: Top
17 Altering Execution
*********************
Once you think you have found an error in your program, you might want
to find out for certain whether correcting the apparent error would
lead to correct results in the rest of the run. You can find the
answer by experiment, using the GDB features for altering execution of
the program.
For example, you can store new values into variables or memory
locations, give your program a signal, restart it at a different
address, or even return prematurely from a function.
* Menu:
* Assignment:: Assignment to variables
* Jumping:: Continuing at a different address
* Signaling:: Giving your program a signal
* Returning:: Returning from a function
* Calling:: Calling your program's functions
* Patching:: Patching your program

File: gdb.info, Node: Assignment, Next: Jumping, Up: Altering
17.1 Assignment to Variables
============================
To alter the value of a variable, evaluate an assignment expression.
*Note Expressions: Expressions. For example,
print x=4
stores the value 4 into the variable `x', and then prints the value of
the assignment expression (which is 4). *Note Using GDB with Different
Languages: Languages, for more information on operators in supported
languages.
If you are not interested in seeing the value of the assignment, use
the `set' command instead of the `print' command. `set' is really the
same as `print' except that the expression's value is not printed and
is not put in the value history (*note Value History: Value History.).
The expression is evaluated only for its effects.
If the beginning of the argument string of the `set' command appears
identical to a `set' subcommand, use the `set variable' command instead
of just `set'. This command is identical to `set' except for its lack
of subcommands. For example, if your program has a variable `width',
you get an error if you try to set a new value with just `set
width=13', because GDB has the command `set width':
(gdb) whatis width
type = double
(gdb) p width
$4 = 13
(gdb) set width=47
Invalid syntax in expression.
The invalid expression, of course, is `=47'. In order to actually set
the program's variable `width', use
(gdb) set var width=47
Because the `set' command has many subcommands that can conflict
with the names of program variables, it is a good idea to use the `set
variable' command instead of just `set'. For example, if your program
has a variable `g', you run into problems if you try to set a new value
with just `set g=4', because GDB has the command `set gnutarget',
abbreviated `set g':
(gdb) whatis g
type = double
(gdb) p g
$1 = 1
(gdb) set g=4
(gdb) p g
$2 = 1
(gdb) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/smith/cc_progs/a.out
"/home/smith/cc_progs/a.out": can't open to read symbols:
Invalid bfd target.
(gdb) show g
The current BFD target is "=4".
The program variable `g' did not change, and you silently set the
`gnutarget' to an invalid value. In order to set the variable `g', use
(gdb) set var g=4
GDB allows more implicit conversions in assignments than C; you can
freely store an integer value into a pointer variable or vice versa,
and you can convert any structure to any other structure that is the
same length or shorter.
To store values into arbitrary places in memory, use the `{...}'
construct to generate a value of specified type at a specified address
(*note Expressions: Expressions.). For example, `{int}0x83040' refers
to memory location `0x83040' as an integer (which implies a certain size
and representation in memory), and
set {int}0x83040 = 4
stores the value 4 into that memory location.

File: gdb.info, Node: Jumping, Next: Signaling, Prev: Assignment, Up: Altering
17.2 Continuing at a Different Address
======================================
Ordinarily, when you continue your program, you do so at the place where
it stopped, with the `continue' command. You can instead continue at
an address of your own choosing, with the following commands:
`jump LINESPEC'
`jump LOCATION'
Resume execution at line LINESPEC or at address given by LOCATION.
Execution stops again immediately if there is a breakpoint there.
*Note Specify Location::, for a description of the different
forms of LINESPEC and LOCATION. It is common practice to use the
`tbreak' command in conjunction with `jump'. *Note Setting
Breakpoints: Set Breaks.
The `jump' command does not change the current stack frame, or the
stack pointer, or the contents of any memory location or any
register other than the program counter. If line LINESPEC is in a
different function from the one currently executing, the results
may be bizarre if the two functions expect different patterns of
arguments or of local variables. For this reason, the `jump'
command requests confirmation if the specified line is not in the
function currently executing. However, even bizarre results are
predictable if you are well acquainted with the machine-language
code of your program.
On many systems, you can get much the same effect as the `jump'
command by storing a new value into the register `$pc'. The difference
is that this does not start your program running; it only changes the
address of where it _will_ run when you continue. For example,
set $pc = 0x485
makes the next `continue' command or stepping command execute at
address `0x485', rather than at the address where your program stopped.
*Note Continuing and Stepping: Continuing and Stepping.
The most common occasion to use the `jump' command is to back
up--perhaps with more breakpoints set--over a portion of a program that
has already executed, in order to examine its execution in more detail.

File: gdb.info, Node: Signaling, Next: Returning, Prev: Jumping, Up: Altering
17.3 Giving your Program a Signal
=================================
`signal SIGNAL'
Resume execution where your program stopped, but immediately give
it the signal SIGNAL. SIGNAL can be the name or the number of a
signal. For example, on many systems `signal 2' and `signal
SIGINT' are both ways of sending an interrupt signal.
Alternatively, if SIGNAL is zero, continue execution without
giving a signal. This is useful when your program stopped on
account of a signal and would ordinary see the signal when resumed
with the `continue' command; `signal 0' causes it to resume
without a signal.
`signal' does not repeat when you press <RET> a second time after
executing the command.
Invoking the `signal' command is not the same as invoking the `kill'
utility from the shell. Sending a signal with `kill' causes GDB to
decide what to do with the signal depending on the signal handling
tables (*note Signals::). The `signal' command passes the signal
directly to your program.

File: gdb.info, Node: Returning, Next: Calling, Prev: Signaling, Up: Altering
17.4 Returning from a Function
==============================
`return'
`return EXPRESSION'
You can cancel execution of a function call with the `return'
command. If you give an EXPRESSION argument, its value is used as
the function's return value.
When you use `return', GDB discards the selected stack frame (and
all frames within it). You can think of this as making the discarded
frame return prematurely. If you wish to specify a value to be
returned, give that value as the argument to `return'.
This pops the selected stack frame (*note Selecting a Frame:
Selection.), and any other frames inside of it, leaving its caller as
the innermost remaining frame. That frame becomes selected. The
specified value is stored in the registers used for returning values of
functions.
The `return' command does not resume execution; it leaves the
program stopped in the state that would exist if the function had just
returned. In contrast, the `finish' command (*note Continuing and
Stepping: Continuing and Stepping.) resumes execution until the
selected stack frame returns naturally.
GDB needs to know how the EXPRESSION argument should be set for the
inferior. The concrete registers assignment depends on the OS ABI and
the type being returned by the selected stack frame. For example it is
common for OS ABI to return floating point values in FPU registers
while integer values in CPU registers. Still some ABIs return even
floating point values in CPU registers. Larger integer widths (such as
`long long int') also have specific placement rules. GDB already knows
the OS ABI from its current target so it needs to find out also the
type being returned to make the assignment into the right register(s).
Normally, the selected stack frame has debug info. GDB will always
use the debug info instead of the implicit type of EXPRESSION when the
debug info is available. For example, if you type `return -1', and the
function in the current stack frame is declared to return a `long long
int', GDB transparently converts the implicit `int' value of -1 into a
`long long int':
Breakpoint 1, func () at gdb.base/return-nodebug.c:29
29 return 31;
(gdb) return -1
Make func return now? (y or n) y
#0 0x004004f6 in main () at gdb.base/return-nodebug.c:43
43 printf ("result=%lld\n", func ());
(gdb)
However, if the selected stack frame does not have a debug info,
e.g., if the function was compiled without debug info, GDB has to find
out the type to return from user. Specifying a different type by
mistake may set the value in different inferior registers than the
caller code expects. For example, typing `return -1' with its implicit
type `int' would set only a part of a `long long int' result for a
debug info less function (on 32-bit architectures). Therefore the user
is required to specify the return type by an appropriate cast
explicitly:
Breakpoint 2, 0x0040050b in func ()
(gdb) return -1
Return value type not available for selected stack frame.
Please use an explicit cast of the value to return.
(gdb) return (long long int) -1
Make selected stack frame return now? (y or n) y
#0 0x00400526 in main ()
(gdb)

File: gdb.info, Node: Calling, Next: Patching, Prev: Returning, Up: Altering
17.5 Calling Program Functions
==============================
`print EXPR'
Evaluate the expression EXPR and display the resulting value.
EXPR may include calls to functions in the program being debugged.
`call EXPR'
Evaluate the expression EXPR without displaying `void' returned
values.
You can use this variant of the `print' command if you want to
execute a function from your program that does not return anything
(a.k.a. "a void function"), but without cluttering the output with
`void' returned values that GDB will otherwise print. If the
result is not void, it is printed and saved in the value history.
It is possible for the function you call via the `print' or `call'
command to generate a signal (e.g., if there's a bug in the function,
or if you passed it incorrect arguments). What happens in that case is
controlled by the `set unwindonsignal' command.
Similarly, with a C++ program it is possible for the function you
call via the `print' or `call' command to generate an exception that is
not handled due to the constraints of the dummy frame. In this case,
any exception that is raised in the frame, but has an out-of-frame
exception handler will not be found. GDB builds a dummy-frame for the
inferior function call, and the unwinder cannot seek for exception
handlers outside of this dummy-frame. What happens in that case is
controlled by the `set unwind-on-terminating-exception' command.
`set unwindonsignal'
Set unwinding of the stack if a signal is received while in a
function that GDB called in the program being debugged. If set to
on, GDB unwinds the stack it created for the call and restores the
context to what it was before the call. If set to off (the
default), GDB stops in the frame where the signal was received.
`show unwindonsignal'
Show the current setting of stack unwinding in the functions
called by GDB.
`set unwind-on-terminating-exception'
Set unwinding of the stack if a C++ exception is raised, but left
unhandled while in a function that GDB called in the program being
debugged. If set to on (the default), GDB unwinds the stack it
created for the call and restores the context to what it was before
the call. If set to off, GDB the exception is delivered to the
default C++ exception handler and the inferior terminated.
`show unwind-on-terminating-exception'
Show the current setting of stack unwinding in the functions
called by GDB.
Sometimes, a function you wish to call is actually a "weak alias"
for another function. In such case, GDB might not pick up the type
information, including the types of the function arguments, which
causes GDB to call the inferior function incorrectly. As a result, the
called function will function erroneously and may even crash. A
solution to that is to use the name of the aliased function instead.

File: gdb.info, Node: Patching, Prev: Calling, Up: Altering
17.6 Patching Programs
======================
By default, GDB opens the file containing your program's executable
code (or the corefile) read-only. This prevents accidental alterations
to machine code; but it also prevents you from intentionally patching
your program's binary.
If you'd like to be able to patch the binary, you can specify that
explicitly with the `set write' command. For example, you might want
to turn on internal debugging flags, or even to make emergency repairs.
`set write on'
`set write off'
If you specify `set write on', GDB opens executable and core files
for both reading and writing; if you specify `set write off' (the
default), GDB opens them read-only.
If you have already loaded a file, you must load it again (using
the `exec-file' or `core-file' command) after changing `set
write', for your new setting to take effect.
`show write'
Display whether executable files and core files are opened for
writing as well as reading.

File: gdb.info, Node: GDB Files, Next: Targets, Prev: Altering, Up: Top
18 GDB Files
************
GDB needs to know the file name of the program to be debugged, both in
order to read its symbol table and in order to start your program. To
debug a core dump of a previous run, you must also tell GDB the name of
the core dump file.
* Menu:
* Files:: Commands to specify files
* Separate Debug Files:: Debugging information in separate files
* Symbol Errors:: Errors reading symbol files
* Data Files:: GDB data files

File: gdb.info, Node: Files, Next: Separate Debug Files, Up: GDB Files
18.1 Commands to Specify Files
==============================
You may want to specify executable and core dump file names. The usual
way to do this is at start-up time, using the arguments to GDB's
start-up commands (*note Getting In and Out of GDB: Invocation.).
Occasionally it is necessary to change to a different file during a
GDB session. Or you may run GDB and forget to specify a file you want
to use. Or you are debugging a remote target via `gdbserver' (*note
file: Server.). In these situations the GDB commands to specify new
files are useful.
`file FILENAME'
Use FILENAME as the program to be debugged. It is read for its
symbols and for the contents of pure memory. It is also the
program executed when you use the `run' command. If you do not
specify a directory and the file is not found in the GDB working
directory, GDB uses the environment variable `PATH' as a list of
directories to search, just as the shell does when looking for a
program to run. You can change the value of this variable, for
both GDB and your program, using the `path' command.
You can load unlinked object `.o' files into GDB using the `file'
command. You will not be able to "run" an object file, but you
can disassemble functions and inspect variables. Also, if the
underlying BFD functionality supports it, you could use `gdb
-write' to patch object files using this technique. Note that GDB
can neither interpret nor modify relocations in this case, so
branches and some initialized variables will appear to go to the
wrong place. But this feature is still handy from time to time.
`file'
`file' with no argument makes GDB discard any information it has
on both executable file and the symbol table.
`exec-file [ FILENAME ]'
Specify that the program to be run (but not the symbol table) is
found in FILENAME. GDB searches the environment variable `PATH'
if necessary to locate your program. Omitting FILENAME means to
discard information on the executable file.
`symbol-file [ FILENAME ]'
Read symbol table information from file FILENAME. `PATH' is
searched when necessary. Use the `file' command to get both symbol
table and program to run from the same file.
`symbol-file' with no argument clears out GDB information on your
program's symbol table.
The `symbol-file' command causes GDB to forget the contents of
some breakpoints and auto-display expressions. This is because
they may contain pointers to the internal data recording symbols
and data types, which are part of the old symbol table data being
discarded inside GDB.
`symbol-file' does not repeat if you press <RET> again after
executing it once.
When GDB is configured for a particular environment, it
understands debugging information in whatever format is the
standard generated for that environment; you may use either a GNU
compiler, or other compilers that adhere to the local conventions.
Best results are usually obtained from GNU compilers; for example,
using `GCC' you can generate debugging information for optimized
code.
For most kinds of object files, with the exception of old SVR3
systems using COFF, the `symbol-file' command does not normally
read the symbol table in full right away. Instead, it scans the
symbol table quickly to find which source files and which symbols
are present. The details are read later, one source file at a
time, as they are needed.
The purpose of this two-stage reading strategy is to make GDB
start up faster. For the most part, it is invisible except for
occasional pauses while the symbol table details for a particular
source file are being read. (The `set verbose' command can turn
these pauses into messages if desired. *Note Optional Warnings
and Messages: Messages/Warnings.)
We have not implemented the two-stage strategy for COFF yet. When
the symbol table is stored in COFF format, `symbol-file' reads the
symbol table data in full right away. Note that "stabs-in-COFF"
still does the two-stage strategy, since the debug info is actually
in stabs format.
`symbol-file [ -readnow ] FILENAME'
`file [ -readnow ] FILENAME'
You can override the GDB two-stage strategy for reading symbol
tables by using the `-readnow' option with any of the commands that
load symbol table information, if you want to be sure GDB has the
entire symbol table available.
`core-file [FILENAME]'
`core'
Specify the whereabouts of a core dump file to be used as the
"contents of memory". Traditionally, core files contain only some
parts of the address space of the process that generated them; GDB
can access the executable file itself for other parts.
`core-file' with no argument specifies that no core file is to be
used.
Note that the core file is ignored when your program is actually
running under GDB. So, if you have been running your program and
you wish to debug a core file instead, you must kill the
subprocess in which the program is running. To do this, use the
`kill' command (*note Killing the Child Process: Kill Process.).
`add-symbol-file FILENAME ADDRESS'
`add-symbol-file FILENAME ADDRESS [ -readnow ]'
`add-symbol-file FILENAME -sSECTION ADDRESS ...'
The `add-symbol-file' command reads additional symbol table
information from the file FILENAME. You would use this command
when FILENAME has been dynamically loaded (by some other means)
into the program that is running. ADDRESS should be the memory
address at which the file has been loaded; GDB cannot figure this
out for itself. You can additionally specify an arbitrary number
of `-sSECTION ADDRESS' pairs, to give an explicit section name and
base address for that section. You can specify any ADDRESS as an
expression.
The symbol table of the file FILENAME is added to the symbol table
originally read with the `symbol-file' command. You can use the
`add-symbol-file' command any number of times; the new symbol data
thus read keeps adding to the old. To discard all old symbol data
instead, use the `symbol-file' command without any arguments.
Although FILENAME is typically a shared library file, an
executable file, or some other object file which has been fully
relocated for loading into a process, you can also load symbolic
information from relocatable `.o' files, as long as:
* the file's symbolic information refers only to linker symbols
defined in that file, not to symbols defined by other object
files,
* every section the file's symbolic information refers to has
actually been loaded into the inferior, as it appears in the
file, and
* you can determine the address at which every section was
loaded, and provide these to the `add-symbol-file' command.
Some embedded operating systems, like Sun Chorus and VxWorks, can
load relocatable files into an already running program; such
systems typically make the requirements above easy to meet.
However, it's important to recognize that many native systems use
complex link procedures (`.linkonce' section factoring and C++
constructor table assembly, for example) that make the
requirements difficult to meet. In general, one cannot assume
that using `add-symbol-file' to read a relocatable object file's
symbolic information will have the same effect as linking the
relocatable object file into the program in the normal way.
`add-symbol-file' does not repeat if you press <RET> after using
it.
`add-symbol-file-from-memory ADDRESS'
Load symbols from the given ADDRESS in a dynamically loaded object
file whose image is mapped directly into the inferior's memory.
For example, the Linux kernel maps a `syscall DSO' into each
process's address space; this DSO provides kernel-specific code for
some system calls. The argument can be any expression whose
evaluation yields the address of the file's shared object file
header. For this command to work, you must have used
`symbol-file' or `exec-file' commands in advance.
`add-shared-symbol-files LIBRARY-FILE'
`assf LIBRARY-FILE'
The `add-shared-symbol-files' command can currently be used only
in the Cygwin build of GDB on MS-Windows OS, where it is an alias
for the `dll-symbols' command (*note Cygwin Native::). GDB
automatically looks for shared libraries, however if GDB does not
find yours, you can invoke `add-shared-symbol-files'. It takes
one argument: the shared library's file name. `assf' is a
shorthand alias for `add-shared-symbol-files'.
`section SECTION ADDR'
The `section' command changes the base address of the named
SECTION of the exec file to ADDR. This can be used if the exec
file does not contain section addresses, (such as in the `a.out'
format), or when the addresses specified in the file itself are
wrong. Each section must be changed separately. The `info files'
command, described below, lists all the sections and their
addresses.
`info files'
`info target'
`info files' and `info target' are synonymous; both print the
current target (*note Specifying a Debugging Target: Targets.),
including the names of the executable and core dump files
currently in use by GDB, and the files from which symbols were
loaded. The command `help target' lists all possible targets
rather than current ones.
`maint info sections'
Another command that can give you extra information about program
sections is `maint info sections'. In addition to the section
information displayed by `info files', this command displays the
flags and file offset of each section in the executable and core
dump files. In addition, `maint info sections' provides the
following command options (which may be arbitrarily combined):
`ALLOBJ'
Display sections for all loaded object files, including
shared libraries.
`SECTIONS'
Display info only for named SECTIONS.
`SECTION-FLAGS'
Display info only for sections for which SECTION-FLAGS are
true. The section flags that GDB currently knows about are:
`ALLOC'
Section will have space allocated in the process when
loaded. Set for all sections except those containing
debug information.
`LOAD'
Section will be loaded from the file into the child
process memory. Set for pre-initialized code and data,
clear for `.bss' sections.
`RELOC'
Section needs to be relocated before loading.
`READONLY'
Section cannot be modified by the child process.
`CODE'
Section contains executable code only.
`DATA'
Section contains data only (no executable code).
`ROM'
Section will reside in ROM.
`CONSTRUCTOR'
Section contains data for constructor/destructor lists.
`HAS_CONTENTS'
Section is not empty.
`NEVER_LOAD'
An instruction to the linker to not output the section.
`COFF_SHARED_LIBRARY'
A notification to the linker that the section contains
COFF shared library information.
`IS_COMMON'
Section contains common symbols.
`set trust-readonly-sections on'
Tell GDB that readonly sections in your object file really are
read-only (i.e. that their contents will not change). In that
case, GDB can fetch values from these sections out of the object
file, rather than from the target program. For some targets
(notably embedded ones), this can be a significant enhancement to
debugging performance.
The default is off.
`set trust-readonly-sections off'
Tell GDB not to trust readonly sections. This means that the
contents of the section might change while the program is running,
and must therefore be fetched from the target when needed.
`show trust-readonly-sections'
Show the current setting of trusting readonly sections.
All file-specifying commands allow both absolute and relative file
names as arguments. GDB always converts the file name to an absolute
file name and remembers it that way.
GDB supports GNU/Linux, MS-Windows, HP-UX, SunOS, SVr4, Irix, and
IBM RS/6000 AIX shared libraries.
On MS-Windows GDB must be linked with the Expat library to support
shared libraries. *Note Expat::.
GDB automatically loads symbol definitions from shared libraries
when you use the `run' command, or when you examine a core file.
(Before you issue the `run' command, GDB does not understand references
to a function in a shared library, however--unless you are debugging a
core file).
On HP-UX, if the program loads a library explicitly, GDB
automatically loads the symbols at the time of the `shl_load' call.
There are times, however, when you may wish to not automatically load
symbol definitions from shared libraries, such as when they are
particularly large or there are many of them.
To control the automatic loading of shared library symbols, use the
commands:
`set auto-solib-add MODE'
If MODE is `on', symbols from all shared object libraries will be
loaded automatically when the inferior begins execution, you
attach to an independently started inferior, or when the dynamic
linker informs GDB that a new library has been loaded. If MODE is
`off', symbols must be loaded manually, using the `sharedlibrary'
command. The default value is `on'.
If your program uses lots of shared libraries with debug info that
takes large amounts of memory, you can decrease the GDB memory
footprint by preventing it from automatically loading the symbols
from shared libraries. To that end, type `set auto-solib-add off'
before running the inferior, then load each library whose debug
symbols you do need with `sharedlibrary REGEXP', where REGEXP is a
regular expression that matches the libraries whose symbols you
want to be loaded.
`show auto-solib-add'
Display the current autoloading mode.
To explicitly load shared library symbols, use the `sharedlibrary'
command:
`info share REGEX'
`info sharedlibrary REGEX'
Print the names of the shared libraries which are currently loaded
that match REGEX. If REGEX is omitted then print all shared
libraries that are loaded.
`sharedlibrary REGEX'
`share REGEX'
Load shared object library symbols for files matching a Unix
regular expression. As with files loaded automatically, it only
loads shared libraries required by your program for a core file or
after typing `run'. If REGEX is omitted all shared libraries
required by your program are loaded.
`nosharedlibrary'
Unload all shared object library symbols. This discards all
symbols that have been loaded from all shared libraries. Symbols
from shared libraries that were loaded by explicit user requests
are not discarded.
Sometimes you may wish that GDB stops and gives you control when any
of shared library events happen. Use the `set stop-on-solib-events'
command for this:
`set stop-on-solib-events'
This command controls whether GDB should give you control when the
dynamic linker notifies it about some shared library event. The
most common event of interest is loading or unloading of a new
shared library.
`show stop-on-solib-events'
Show whether GDB stops and gives you control when shared library
events happen.
Shared libraries are also supported in many cross or remote debugging
configurations. GDB needs to have access to the target's libraries;
this can be accomplished either by providing copies of the libraries on
the host system, or by asking GDB to automatically retrieve the
libraries from the target. If copies of the target libraries are
provided, they need to be the same as the target libraries, although the
copies on the target can be stripped as long as the copies on the host
are not.
For remote debugging, you need to tell GDB where the target
libraries are, so that it can load the correct copies--otherwise, it
may try to load the host's libraries. GDB has two variables to specify
the search directories for target libraries.
`set sysroot PATH'
Use PATH as the system root for the program being debugged. Any
absolute shared library paths will be prefixed with PATH; many
runtime loaders store the absolute paths to the shared library in
the target program's memory. If you use `set sysroot' to find
shared libraries, they need to be laid out in the same way that
they are on the target, with e.g. a `/lib' and `/usr/lib' hierarchy
under PATH.
If PATH starts with the sequence `remote:', GDB will retrieve the
target libraries from the remote system. This is only supported
when using a remote target that supports the `remote get' command
(*note Sending files to a remote system: File Transfer.). The
part of PATH following the initial `remote:' (if present) is used
as system root prefix on the remote file system. (1)
For targets with an MS-DOS based filesystem, such as MS-Windows and
SymbianOS, GDB tries prefixing a few variants of the target
absolute file name with PATH. But first, on Unix hosts, GDB
converts all backslash directory separators into forward slashes,
because the backslash is not a directory separator on Unix:
c:\foo\bar.dll => c:/foo/bar.dll
Then, GDB attempts prefixing the target file name with PATH, and
looks for the resulting file name in the host file system:
c:/foo/bar.dll => /path/to/sysroot/c:/foo/bar.dll
If that does not find the shared library, GDB tries removing the
`:' character from the drive spec, both for convenience, and, for
the case of the host file system not supporting file names with
colons:
c:/foo/bar.dll => /path/to/sysroot/c/foo/bar.dll
This makes it possible to have a system root that mirrors a target
with more than one drive. E.g., you may want to setup your local
copies of the target system shared libraries like so (note `c' vs
`z'):
`/path/to/sysroot/c/sys/bin/foo.dll'
`/path/to/sysroot/c/sys/bin/bar.dll'
`/path/to/sysroot/z/sys/bin/bar.dll'
and point the system root at `/path/to/sysroot', so that GDB can
find the correct copies of both `c:\sys\bin\foo.dll', and
`z:\sys\bin\bar.dll'.
If that still does not find the shared library, GDB tries removing
the whole drive spec from the target file name:
c:/foo/bar.dll => /path/to/sysroot/foo/bar.dll
This last lookup makes it possible to not care about the drive
name, if you don't want or need to.
The `set solib-absolute-prefix' command is an alias for `set
sysroot'.
You can set the default system root by using the configure-time
`--with-sysroot' option. If the system root is inside GDB's
configured binary prefix (set with `--prefix' or `--exec-prefix'),
then the default system root will be updated automatically if the
installed GDB is moved to a new location.
`show sysroot'
Display the current shared library prefix.
`set solib-search-path PATH'
If this variable is set, PATH is a colon-separated list of
directories to search for shared libraries. `solib-search-path'
is used after `sysroot' fails to locate the library, or if the
path to the library is relative instead of absolute. If you want
to use `solib-search-path' instead of `sysroot', be sure to set
`sysroot' to a nonexistent directory to prevent GDB from finding
your host's libraries. `sysroot' is preferred; setting it to a
nonexistent directory may interfere with automatic loading of
shared library symbols.
`show solib-search-path'
Display the current shared library search path.
`set target-file-system-kind KIND'
Set assumed file system kind for target reported file names.
Shared library file names as reported by the target system may not
make sense as is on the system GDB is running on. For example,
when remote debugging a target that has MS-DOS based file system
semantics, from a Unix host, the target may be reporting to GDB a
list of loaded shared libraries with file names such as
`c:\Windows\kernel32.dll'. On Unix hosts, there's no concept of
drive letters, so the `c:\' prefix is not normally understood as
indicating an absolute file name, and neither is the backslash
normally considered a directory separator character. In that case,
the native file system would interpret this whole absolute file
name as a relative file name with no directory components. This
would make it impossible to point GDB at a copy of the remote
target's shared libraries on the host using `set sysroot', and
impractical with `set solib-search-path'. Setting
`target-file-system-kind' to `dos-based' tells GDB to interpret
such file names similarly to how the target would, and to map them
to file names valid on GDB's native file system semantics. The
value of KIND can be `"auto"', in addition to one of the supported
file system kinds. In that case, GDB tries to determine the
appropriate file system variant based on the current target's
operating system (*note Configuring the Current ABI: ABI.). The
supported file system settings are:
`unix'
Instruct GDB to assume the target file system is of Unix
kind. Only file names starting the forward slash (`/')
character are considered absolute, and the directory
separator character is also the forward slash.
`dos-based'
Instruct GDB to assume the target file system is DOS based.
File names starting with either a forward slash, or a drive
letter followed by a colon (e.g., `c:'), are considered
absolute, and both the slash (`/') and the backslash (`\\')
characters are considered directory separators.
`auto'
Instruct GDB to use the file system kind associated with the
target operating system (*note Configuring the Current ABI:
ABI.). This is the default.
---------- Footnotes ----------
(1) If you want to specify a local system root using a directory
that happens to be named `remote:', you need to use some equivalent
variant of the name like `./remote:'.

File: gdb.info, Node: Separate Debug Files, Next: Symbol Errors, Prev: Files, Up: GDB Files
18.2 Debugging Information in Separate Files
============================================
GDB allows you to put a program's debugging information in a file
separate from the executable itself, in a way that allows GDB to find
and load the debugging information automatically. Since debugging
information can be very large--sometimes larger than the executable
code itself--some systems distribute debugging information for their
executables in separate files, which users can install only when they
need to debug a problem.
GDB supports two ways of specifying the separate debug info file:
* The executable contains a "debug link" that specifies the name of
the separate debug info file. The separate debug file's name is
usually `EXECUTABLE.debug', where EXECUTABLE is the name of the
corresponding executable file without leading directories (e.g.,
`ls.debug' for `/usr/bin/ls'). In addition, the debug link
specifies a 32-bit "Cyclic Redundancy Check" (CRC) checksum for
the debug file, which GDB uses to validate that the executable and
the debug file came from the same build.
* The executable contains a "build ID", a unique bit string that is
also present in the corresponding debug info file. (This is
supported only on some operating systems, notably those which use
the ELF format for binary files and the GNU Binutils.) For more
details about this feature, see the description of the `--build-id'
command-line option in *Note Command Line Options:
(ld.info)Options. The debug info file's name is not specified
explicitly by the build ID, but can be computed from the build ID,
see below.
Depending on the way the debug info file is specified, GDB uses two
different methods of looking for the debug file:
* For the "debug link" method, GDB looks up the named file in the
directory of the executable file, then in a subdirectory of that
directory named `.debug', and finally under the global debug
directory, in a subdirectory whose name is identical to the leading
directories of the executable's absolute file name.
* For the "build ID" method, GDB looks in the `.build-id'
subdirectory of the global debug directory for a file named
`NN/NNNNNNNN.debug', where NN are the first 2 hex characters of
the build ID bit string, and NNNNNNNN are the rest of the bit
string. (Real build ID strings are 32 or more hex characters, not
10.)
So, for example, suppose you ask GDB to debug `/usr/bin/ls', which
has a debug link that specifies the file `ls.debug', and a build ID
whose value in hex is `abcdef1234'. If the global debug directory is
`/usr/lib/debug', then GDB will look for the following debug
information files, in the indicated order:
- `/usr/lib/debug/.build-id/ab/cdef1234.debug'
- `/usr/bin/ls.debug'
- `/usr/bin/.debug/ls.debug'
- `/usr/lib/debug/usr/bin/ls.debug'.
You can set the global debugging info directory's name, and view the
name GDB is currently using.
`set debug-file-directory DIRECTORIES'
Set the directories which GDB searches for separate debugging
information files to DIRECTORY. Multiple directory components can
be set concatenating them by a directory separator.
`show debug-file-directory'
Show the directories GDB searches for separate debugging
information files.
A debug link is a special section of the executable file named
`.gnu_debuglink'. The section must contain:
* A filename, with any leading directory components removed,
followed by a zero byte,
* zero to three bytes of padding, as needed to reach the next
four-byte boundary within the section, and
* a four-byte CRC checksum, stored in the same endianness used for
the executable file itself. The checksum is computed on the
debugging information file's full contents by the function given
below, passing zero as the CRC argument.
Any executable file format can carry a debug link, as long as it can
contain a section named `.gnu_debuglink' with the contents described
above.
The build ID is a special section in the executable file (and in
other ELF binary files that GDB may consider). This section is often
named `.note.gnu.build-id', but that name is not mandatory. It
contains unique identification for the built files--the ID remains the
same across multiple builds of the same build tree. The default
algorithm SHA1 produces 160 bits (40 hexadecimal characters) of the
content for the build ID string. The same section with an identical
value is present in the original built binary with symbols, in its
stripped variant, and in the separate debugging information file.
The debugging information file itself should be an ordinary
executable, containing a full set of linker symbols, sections, and
debugging information. The sections of the debugging information file
should have the same names, addresses, and sizes as the original file,
but they need not contain any data--much like a `.bss' section in an
ordinary executable.
The GNU binary utilities (Binutils) package includes the `objcopy'
utility that can produce the separated executable / debugging
information file pairs using the following commands:
objcopy --only-keep-debug foo foo.debug
strip -g foo
These commands remove the debugging information from the executable
file `foo' and place it in the file `foo.debug'. You can use the
first, second or both methods to link the two files:
* The debug link method needs the following additional command to
also leave behind a debug link in `foo':
objcopy --add-gnu-debuglink=foo.debug foo
Ulrich Drepper's `elfutils' package, starting with version 0.53,
contains a version of the `strip' command such that the command
`strip foo -f foo.debug' has the same functionality as the two
`objcopy' commands and the `ln -s' command above, together.
* Build ID gets embedded into the main executable using `ld
--build-id' or the GCC counterpart `gcc -Wl,--build-id'. Build ID
support plus compatibility fixes for debug files separation are
present in GNU binary utilities (Binutils) package since version
2.18.
The CRC used in `.gnu_debuglink' is the CRC-32 defined in IEEE 802.3
using the polynomial:
x^32 + x^26 + x^23 + x^22 + x^16 + x^12 + x^11
+ x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1
The function is computed byte at a time, taking the least
significant bit of each byte first. The initial pattern `0xffffffff'
is used, to ensure leading zeros affect the CRC and the final result is
inverted to ensure trailing zeros also affect the CRC.
_Note:_ This is the same CRC polynomial as used in handling the
"Remote Serial Protocol" `qCRC' packet (*note GDB Remote Serial
Protocol: Remote Protocol.). However in the case of the Remote Serial
Protocol, the CRC is computed _most_ significant bit first, and the
result is not inverted, so trailing zeros have no effect on the CRC
value.
To complete the description, we show below the code of the function
which produces the CRC used in `.gnu_debuglink'. Inverting the
initially supplied `crc' argument means that an initial call to this
function passing in zero will start computing the CRC using
`0xffffffff'.
unsigned long
gnu_debuglink_crc32 (unsigned long crc,
unsigned char *buf, size_t len)
{
static const unsigned long crc32_table[256] =
{
0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419,
0x706af48f, 0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4,
0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07,
0x90bf1d91, 0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7, 0x136c9856,
0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4,
0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3,
0x45df5c75, 0xdcd60dcf, 0xabd13d59, 0x26d930ac, 0x51de003a,
0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599,
0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190,
0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f,
0x9fbfe4a5, 0xe8b8d433, 0x7807c9a2, 0x0f00f934, 0x9609a88e,
0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed,
0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3,
0xfbd44c65, 0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a,
0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5,
0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa, 0xbe0b1010,
0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17,
0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6,
0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615,
0x73dc1683, 0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1, 0xf00f9344,
0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a,
0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1,
0xa6bc5767, 0x3fb506dd, 0x48b2364b, 0xd80d2bda, 0xaf0a1b4c,
0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef,
0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe,
0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31,
0x2cd99e8b, 0x5bdeae1d, 0x9b64c2b0, 0xec63f226, 0x756aa39c,
0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b,
0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1,
0x18b74777, 0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45, 0xa00ae278,
0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7,
0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc, 0x40df0b66,
0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605,
0xcdd70693, 0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8,
0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b,
0x2d02ef8d
};
unsigned char *end;
crc = ~crc & 0xffffffff;
for (end = buf + len; buf < end; ++buf)
crc = crc32_table[(crc ^ *buf) & 0xff] ^ (crc >> 8);
return ~crc & 0xffffffff;
}
This computation does not apply to the "build ID" method.

File: gdb.info, Node: Symbol Errors, Next: Data Files, Prev: Separate Debug Files, Up: GDB Files
18.3 Errors Reading Symbol Files
================================
While reading a symbol file, GDB occasionally encounters problems, such
as symbol types it does not recognize, or known bugs in compiler
output. By default, GDB does not notify you of such problems, since
they are relatively common and primarily of interest to people
debugging compilers. If you are interested in seeing information about
ill-constructed symbol tables, you can either ask GDB to print only one
message about each such type of problem, no matter how many times the
problem occurs; or you can ask GDB to print more messages, to see how
many times the problems occur, with the `set complaints' command (*note
Optional Warnings and Messages: Messages/Warnings.).
The messages currently printed, and their meanings, include:
`inner block not inside outer block in SYMBOL'
The symbol information shows where symbol scopes begin and end
(such as at the start of a function or a block of statements).
This error indicates that an inner scope block is not fully
contained in its outer scope blocks.
GDB circumvents the problem by treating the inner block as if it
had the same scope as the outer block. In the error message,
SYMBOL may be shown as "`(don't know)'" if the outer block is not a
function.
`block at ADDRESS out of order'
The symbol information for symbol scope blocks should occur in
order of increasing addresses. This error indicates that it does
not do so.
GDB does not circumvent this problem, and has trouble locating
symbols in the source file whose symbols it is reading. (You can
often determine what source file is affected by specifying `set
verbose on'. *Note Optional Warnings and Messages:
Messages/Warnings.)
`bad block start address patched'
The symbol information for a symbol scope block has a start address
smaller than the address of the preceding source line. This is
known to occur in the SunOS 4.1.1 (and earlier) C compiler.
GDB circumvents the problem by treating the symbol scope block as
starting on the previous source line.
`bad string table offset in symbol N'
Symbol number N contains a pointer into the string table which is
larger than the size of the string table.
GDB circumvents the problem by considering the symbol to have the
name `foo', which may cause other problems if many symbols end up
with this name.
`unknown symbol type `0xNN''
The symbol information contains new data types that GDB does not
yet know how to read. `0xNN' is the symbol type of the
uncomprehended information, in hexadecimal.
GDB circumvents the error by ignoring this symbol information.
This usually allows you to debug your program, though certain
symbols are not accessible. If you encounter such a problem and
feel like debugging it, you can debug `gdb' with itself, breakpoint
on `complain', then go up to the function `read_dbx_symtab' and
examine `*bufp' to see the symbol.
`stub type has NULL name'
GDB could not find the full definition for a struct or class.
`const/volatile indicator missing (ok if using g++ v1.x), got...'
The symbol information for a C++ member function is missing some
information that recent versions of the compiler should have
output for it.
`info mismatch between compiler and debugger'
GDB could not parse a type specification output by the compiler.

File: gdb.info, Node: Data Files, Prev: Symbol Errors, Up: GDB Files
18.4 GDB Data Files
===================
GDB will sometimes read an auxiliary data file. These files are kept
in a directory known as the "data directory".
You can set the data directory's name, and view the name GDB is
currently using.
`set data-directory DIRECTORY'
Set the directory which GDB searches for auxiliary data files to
DIRECTORY.
`show data-directory'
Show the directory GDB searches for auxiliary data files.
You can set the default data directory by using the configure-time
`--with-gdb-datadir' option. If the data directory is inside GDB's
configured binary prefix (set with `--prefix' or `--exec-prefix'), then
the default data directory will be updated automatically if the
installed GDB is moved to a new location.

File: gdb.info, Node: Targets, Next: Remote Debugging, Prev: GDB Files, Up: Top
19 Specifying a Debugging Target
********************************
A "target" is the execution environment occupied by your program.
Often, GDB runs in the same host environment as your program; in
that case, the debugging target is specified as a side effect when you
use the `file' or `core' commands. When you need more flexibility--for
example, running GDB on a physically separate host, or controlling a
standalone system over a serial port or a realtime system over a TCP/IP
connection--you can use the `target' command to specify one of the
target types configured for GDB (*note Commands for Managing Targets:
Target Commands.).
It is possible to build GDB for several different "target
architectures". When GDB is built like that, you can choose one of the
available architectures with the `set architecture' command.
`set architecture ARCH'
This command sets the current target architecture to ARCH. The
value of ARCH can be `"auto"', in addition to one of the supported
architectures.
`show architecture'
Show the current target architecture.
`set processor'
`processor'
These are alias commands for, respectively, `set architecture' and
`show architecture'.
* Menu:
* Active Targets:: Active targets
* Target Commands:: Commands for managing targets
* Byte Order:: Choosing target byte order

File: gdb.info, Node: Active Targets, Next: Target Commands, Up: Targets
19.1 Active Targets
===================
There are three classes of targets: processes, core files, and
executable files. GDB can work concurrently on up to three active
targets, one in each class. This allows you to (for example) start a
process and inspect its activity without abandoning your work on a core
file.
For example, if you execute `gdb a.out', then the executable file
`a.out' is the only active target. If you designate a core file as
well--presumably from a prior run that crashed and coredumped--then GDB
has two active targets and uses them in tandem, looking first in the
corefile target, then in the executable file, to satisfy requests for
memory addresses. (Typically, these two classes of target are
complementary, since core files contain only a program's read-write
memory--variables and so on--plus machine status, while executable
files contain only the program text and initialized data.)
When you type `run', your executable file becomes an active process
target as well. When a process target is active, all GDB commands
requesting memory addresses refer to that target; addresses in an
active core file or executable file target are obscured while the
process target is active.
Use the `core-file' and `exec-file' commands to select a new core
file or executable target (*note Commands to Specify Files: Files.).
To specify as a target a process that is already running, use the
`attach' command (*note Debugging an Already-running Process: Attach.).

File: gdb.info, Node: Target Commands, Next: Byte Order, Prev: Active Targets, Up: Targets
19.2 Commands for Managing Targets
==================================
`target TYPE PARAMETERS'
Connects the GDB host environment to a target machine or process.
A target is typically a protocol for talking to debugging
facilities. You use the argument TYPE to specify the type or
protocol of the target machine.
Further PARAMETERS are interpreted by the target protocol, but
typically include things like device names or host names to connect
with, process numbers, and baud rates.
The `target' command does not repeat if you press <RET> again
after executing the command.
`help target'
Displays the names of all targets available. To display targets
currently selected, use either `info target' or `info files'
(*note Commands to Specify Files: Files.).
`help target NAME'
Describe a particular target, including any parameters necessary to
select it.
`set gnutarget ARGS'
GDB uses its own library BFD to read your files. GDB knows
whether it is reading an "executable", a "core", or a ".o" file;
however, you can specify the file format with the `set gnutarget'
command. Unlike most `target' commands, with `gnutarget' the
`target' refers to a program, not a machine.
_Warning:_ To specify a file format with `set gnutarget', you
must know the actual BFD name.
*Note Commands to Specify Files: Files.
`show gnutarget'
Use the `show gnutarget' command to display what file format
`gnutarget' is set to read. If you have not set `gnutarget', GDB
will determine the file format for each file automatically, and
`show gnutarget' displays `The current BDF target is "auto"'.
Here are some common targets (available, or not, depending on the GDB
configuration):
`target exec PROGRAM'
An executable file. `target exec PROGRAM' is the same as
`exec-file PROGRAM'.
`target core FILENAME'
A core dump file. `target core FILENAME' is the same as
`core-file FILENAME'.
`target remote MEDIUM'
A remote system connected to GDB via a serial line or network
connection. This command tells GDB to use its own remote protocol
over MEDIUM for debugging. *Note Remote Debugging::.
For example, if you have a board connected to `/dev/ttya' on the
machine running GDB, you could say:
target remote /dev/ttya
`target remote' supports the `load' command. This is only useful
if you have some other way of getting the stub to the target
system, and you can put it somewhere in memory where it won't get
clobbered by the download.
`target sim [SIMARGS] ...'
Builtin CPU simulator. GDB includes simulators for most
architectures. In general,
target sim
load
run
works; however, you cannot assume that a specific memory map,
device drivers, or even basic I/O is available, although some
simulators do provide these. For info about any
processor-specific simulator details, see the appropriate section
in *Note Embedded Processors: Embedded Processors.
Some configurations may include these targets as well:
`target nrom DEV'
NetROM ROM emulator. This target only supports downloading.
Different targets are available on different configurations of GDB;
your configuration may have more or fewer targets.
Many remote targets require you to download the executable's code
once you've successfully established a connection. You may wish to
control various aspects of this process.
`set hash'
This command controls whether a hash mark `#' is displayed while
downloading a file to the remote monitor. If on, a hash mark is
displayed after each S-record is successfully downloaded to the
monitor.
`show hash'
Show the current status of displaying the hash mark.
`set debug monitor'
Enable or disable display of communications messages between GDB
and the remote monitor.
`show debug monitor'
Show the current status of displaying communications between GDB
and the remote monitor.
`load FILENAME'
Depending on what remote debugging facilities are configured into
GDB, the `load' command may be available. Where it exists, it is
meant to make FILENAME (an executable) available for debugging on
the remote system--by downloading, or dynamic linking, for example.
`load' also records the FILENAME symbol table in GDB, like the
`add-symbol-file' command.
If your GDB does not have a `load' command, attempting to execute
it gets the error message "`You can't do that when your target is
...'"
The file is loaded at whatever address is specified in the
executable. For some object file formats, you can specify the
load address when you link the program; for other formats, like
a.out, the object file format specifies a fixed address.
Depending on the remote side capabilities, GDB may be able to load
programs into flash memory.
`load' does not repeat if you press <RET> again after using it.

File: gdb.info, Node: Byte Order, Prev: Target Commands, Up: Targets
19.3 Choosing Target Byte Order
===============================
Some types of processors, such as the MIPS, PowerPC, and Renesas SH,
offer the ability to run either big-endian or little-endian byte
orders. Usually the executable or symbol will include a bit to
designate the endian-ness, and you will not need to worry about which
to use. However, you may still find it useful to adjust GDB's idea of
processor endian-ness manually.
`set endian big'
Instruct GDB to assume the target is big-endian.
`set endian little'
Instruct GDB to assume the target is little-endian.
`set endian auto'
Instruct GDB to use the byte order associated with the executable.
`show endian'
Display GDB's current idea of the target byte order.
Note that these commands merely adjust interpretation of symbolic
data on the host, and that they have absolutely no effect on the target
system.

File: gdb.info, Node: Remote Debugging, Next: Configurations, Prev: Targets, Up: Top
20 Debugging Remote Programs
****************************
If you are trying to debug a program running on a machine that cannot
run GDB in the usual way, it is often useful to use remote debugging.
For example, you might use remote debugging on an operating system
kernel, or on a small system which does not have a general purpose
operating system powerful enough to run a full-featured debugger.
Some configurations of GDB have special serial or TCP/IP interfaces
to make this work with particular debugging targets. In addition, GDB
comes with a generic serial protocol (specific to GDB, but not specific
to any particular target system) which you can use if you write the
remote stubs--the code that runs on the remote system to communicate
with GDB.
Other remote targets may be available in your configuration of GDB;
use `help target' to list them.
* Menu:
* Connecting:: Connecting to a remote target
* File Transfer:: Sending files to a remote system
* Server:: Using the gdbserver program
* Remote Configuration:: Remote configuration
* Remote Stub:: Implementing a remote stub

File: gdb.info, Node: Connecting, Next: File Transfer, Up: Remote Debugging
20.1 Connecting to a Remote Target
==================================
On the GDB host machine, you will need an unstripped copy of your
program, since GDB needs symbol and debugging information. Start up
GDB as usual, using the name of the local copy of your program as the
first argument.
GDB can communicate with the target over a serial line, or over an
IP network using TCP or UDP. In each case, GDB uses the same protocol
for debugging your program; only the medium carrying the debugging
packets varies. The `target remote' command establishes a connection
to the target. Its arguments indicate which medium to use:
`target remote SERIAL-DEVICE'
Use SERIAL-DEVICE to communicate with the target. For example, to
use a serial line connected to the device named `/dev/ttyb':
target remote /dev/ttyb
If you're using a serial line, you may want to give GDB the
`--baud' option, or use the `set remotebaud' command (*note set
remotebaud: Remote Configuration.) before the `target' command.
`target remote `HOST:PORT''
`target remote `tcp:HOST:PORT''
Debug using a TCP connection to PORT on HOST. The HOST may be
either a host name or a numeric IP address; PORT must be a decimal
number. The HOST could be the target machine itself, if it is
directly connected to the net, or it might be a terminal server
which in turn has a serial line to the target.
For example, to connect to port 2828 on a terminal server named
`manyfarms':
target remote manyfarms:2828
If your remote target is actually running on the same machine as
your debugger session (e.g. a simulator for your target running on
the same host), you can omit the hostname. For example, to
connect to port 1234 on your local machine:
target remote :1234
Note that the colon is still required here.
`target remote `udp:HOST:PORT''
Debug using UDP packets to PORT on HOST. For example, to connect
to UDP port 2828 on a terminal server named `manyfarms':
target remote udp:manyfarms:2828
When using a UDP connection for remote debugging, you should keep
in mind that the `U' stands for "Unreliable". UDP can silently
drop packets on busy or unreliable networks, which will cause
havoc with your debugging session.
`target remote | COMMAND'
Run COMMAND in the background and communicate with it using a
pipe. The COMMAND is a shell command, to be parsed and expanded
by the system's command shell, `/bin/sh'; it should expect remote
protocol packets on its standard input, and send replies on its
standard output. You could use this to run a stand-alone simulator
that speaks the remote debugging protocol, to make net connections
using programs like `ssh', or for other similar tricks.
If COMMAND closes its standard output (perhaps by exiting), GDB
will try to send it a `SIGTERM' signal. (If the program has
already exited, this will have no effect.)
Once the connection has been established, you can use all the usual
commands to examine and change data. The remote program is already
running; you can use `step' and `continue', and you do not need to use
`run'.
Whenever GDB is waiting for the remote program, if you type the
interrupt character (often `Ctrl-c'), GDB attempts to stop the program.
This may or may not succeed, depending in part on the hardware and the
serial drivers the remote system uses. If you type the interrupt
character once again, GDB displays this prompt:
Interrupted while waiting for the program.
Give up (and stop debugging it)? (y or n)
If you type `y', GDB abandons the remote debugging session. (If you
decide you want to try again later, you can use `target remote' again
to connect once more.) If you type `n', GDB goes back to waiting.
`detach'
When you have finished debugging the remote program, you can use
the `detach' command to release it from GDB control. Detaching
from the target normally resumes its execution, but the results
will depend on your particular remote stub. After the `detach'
command, GDB is free to connect to another target.
`disconnect'
The `disconnect' command behaves like `detach', except that the
target is generally not resumed. It will wait for GDB (this
instance or another one) to connect and continue debugging. After
the `disconnect' command, GDB is again free to connect to another
target.
`monitor CMD'
This command allows you to send arbitrary commands directly to the
remote monitor. Since GDB doesn't care about the commands it
sends like this, this command is the way to extend GDB--you can
add new commands that only the external monitor will understand
and implement.

File: gdb.info, Node: File Transfer, Next: Server, Prev: Connecting, Up: Remote Debugging
20.2 Sending files to a remote system
=====================================
Some remote targets offer the ability to transfer files over the same
connection used to communicate with GDB. This is convenient for
targets accessible through other means, e.g. GNU/Linux systems running
`gdbserver' over a network interface. For other targets, e.g. embedded
devices with only a single serial port, this may be the only way to
upload or download files.
Not all remote targets support these commands.
`remote put HOSTFILE TARGETFILE'
Copy file HOSTFILE from the host system (the machine running GDB)
to TARGETFILE on the target system.
`remote get TARGETFILE HOSTFILE'
Copy file TARGETFILE from the target system to HOSTFILE on the
host system.
`remote delete TARGETFILE'
Delete TARGETFILE from the target system.

File: gdb.info, Node: Server, Next: Remote Configuration, Prev: File Transfer, Up: Remote Debugging
20.3 Using the `gdbserver' Program
==================================
`gdbserver' is a control program for Unix-like systems, which allows
you to connect your program with a remote GDB via `target remote'--but
without linking in the usual debugging stub.
`gdbserver' is not a complete replacement for the debugging stubs,
because it requires essentially the same operating-system facilities
that GDB itself does. In fact, a system that can run `gdbserver' to
connect to a remote GDB could also run GDB locally! `gdbserver' is
sometimes useful nevertheless, because it is a much smaller program
than GDB itself. It is also easier to port than all of GDB, so you may
be able to get started more quickly on a new system by using
`gdbserver'. Finally, if you develop code for real-time systems, you
may find that the tradeoffs involved in real-time operation make it
more convenient to do as much development work as possible on another
system, for example by cross-compiling. You can use `gdbserver' to
make a similar choice for debugging.
GDB and `gdbserver' communicate via either a serial line or a TCP
connection, using the standard GDB remote serial protocol.
_Warning:_ `gdbserver' does not have any built-in security. Do
not run `gdbserver' connected to any public network; a GDB
connection to `gdbserver' provides access to the target system
with the same privileges as the user running `gdbserver'.
20.3.1 Running `gdbserver'
--------------------------
Run `gdbserver' on the target system. You need a copy of the program
you want to debug, including any libraries it requires. `gdbserver'
does not need your program's symbol table, so you can strip the program
if necessary to save space. GDB on the host system does all the symbol
handling.
To use the server, you must tell it how to communicate with GDB; the
name of your program; and the arguments for your program. The usual
syntax is:
target> gdbserver COMM PROGRAM [ ARGS ... ]
COMM is either a device name (to use a serial line) or a TCP
hostname and portnumber. For example, to debug Emacs with the argument
`foo.txt' and communicate with GDB over the serial port `/dev/com1':
target> gdbserver /dev/com1 emacs foo.txt
`gdbserver' waits passively for the host GDB to communicate with it.
To use a TCP connection instead of a serial line:
target> gdbserver host:2345 emacs foo.txt
The only difference from the previous example is the first argument,
specifying that you are communicating with the host GDB via TCP. The
`host:2345' argument means that `gdbserver' is to expect a TCP
connection from machine `host' to local TCP port 2345. (Currently, the
`host' part is ignored.) You can choose any number you want for the
port number as long as it does not conflict with any TCP ports already
in use on the target system (for example, `23' is reserved for
`telnet').(1) You must use the same port number with the host GDB
`target remote' command.
20.3.1.1 Attaching to a Running Program
.......................................
On some targets, `gdbserver' can also attach to running programs. This
is accomplished via the `--attach' argument. The syntax is:
target> gdbserver --attach COMM PID
PID is the process ID of a currently running process. It isn't
necessary to point `gdbserver' at a binary for the running process.
You can debug processes by name instead of process ID if your target
has the `pidof' utility:
target> gdbserver --attach COMM `pidof PROGRAM`
In case more than one copy of PROGRAM is running, or PROGRAM has
multiple threads, most versions of `pidof' support the `-s' option to
only return the first process ID.
20.3.1.2 Multi-Process Mode for `gdbserver'
...........................................
When you connect to `gdbserver' using `target remote', `gdbserver'
debugs the specified program only once. When the program exits, or you
detach from it, GDB closes the connection and `gdbserver' exits.
If you connect using `target extended-remote', `gdbserver' enters
multi-process mode. When the debugged program exits, or you detach
from it, GDB stays connected to `gdbserver' even though no program is
running. The `run' and `attach' commands instruct `gdbserver' to run
or attach to a new program. The `run' command uses `set remote
exec-file' (*note set remote exec-file::) to select the program to run.
Command line arguments are supported, except for wildcard expansion
and I/O redirection (*note Arguments::).
To start `gdbserver' without supplying an initial command to run or
process ID to attach, use the `--multi' command line option. Then you
can connect using `target extended-remote' and start the program you
want to debug.
`gdbserver' does not automatically exit in multi-process mode. You
can terminate it by using `monitor exit' (*note Monitor Commands for
gdbserver::).
20.3.1.3 Other Command-Line Arguments for `gdbserver'
.....................................................
The `--debug' option tells `gdbserver' to display extra status
information about the debugging process. The `--remote-debug' option
tells `gdbserver' to display remote protocol debug output. These
options are intended for `gdbserver' development and for bug reports to
the developers.
The `--wrapper' option specifies a wrapper to launch programs for
debugging. The option should be followed by the name of the wrapper,
then any command-line arguments to pass to the wrapper, then `--'
indicating the end of the wrapper arguments.
`gdbserver' runs the specified wrapper program with a combined
command line including the wrapper arguments, then the name of the
program to debug, then any arguments to the program. The wrapper runs
until it executes your program, and then GDB gains control.
You can use any program that eventually calls `execve' with its
arguments as a wrapper. Several standard Unix utilities do this, e.g.
`env' and `nohup'. Any Unix shell script ending with `exec "$@"' will
also work.
For example, you can use `env' to pass an environment variable to
the debugged program, without setting the variable in `gdbserver''s
environment:
$ gdbserver --wrapper env LD_PRELOAD=libtest.so -- :2222 ./testprog
20.3.2 Connecting to `gdbserver'
--------------------------------
Run GDB on the host system.
First make sure you have the necessary symbol files. Load symbols
for your application using the `file' command before you connect. Use
`set sysroot' to locate target libraries (unless your GDB was compiled
with the correct sysroot using `--with-sysroot').
The symbol file and target libraries must exactly match the
executable and libraries on the target, with one exception: the files
on the host system should not be stripped, even if the files on the
target system are. Mismatched or missing files will lead to confusing
results during debugging. On GNU/Linux targets, mismatched or missing
files may also prevent `gdbserver' from debugging multi-threaded
programs.
Connect to your target (*note Connecting to a Remote Target:
Connecting.). For TCP connections, you must start up `gdbserver' prior
to using the `target remote' command. Otherwise you may get an error
whose text depends on the host system, but which usually looks
something like `Connection refused'. Don't use the `load' command in
GDB when using `gdbserver', since the program is already on the target.
20.3.3 Monitor Commands for `gdbserver'
---------------------------------------
During a GDB session using `gdbserver', you can use the `monitor'
command to send special requests to `gdbserver'. Here are the
available commands.
`monitor help'
List the available monitor commands.
`monitor set debug 0'
`monitor set debug 1'
Disable or enable general debugging messages.
`monitor set remote-debug 0'
`monitor set remote-debug 1'
Disable or enable specific debugging messages associated with the
remote protocol (*note Remote Protocol::).
`monitor set libthread-db-search-path [PATH]'
When this command is issued, PATH is a colon-separated list of
directories to search for `libthread_db' (*note set
libthread-db-search-path: Threads.). If you omit PATH,
`libthread-db-search-path' will be reset to an empty list.
`monitor exit'
Tell gdbserver to exit immediately. This command should be
followed by `disconnect' to close the debugging session.
`gdbserver' will detach from any attached processes and kill any
processes it created. Use `monitor exit' to terminate `gdbserver'
at the end of a multi-process mode debug session.
20.3.4 Tracepoints support in `gdbserver'
-----------------------------------------
On some targets, `gdbserver' supports tracepoints, fast tracepoints and
static tracepoints.
For fast or static tracepoints to work, a special library called the
"in-process agent" (IPA), must be loaded in the inferior process. This
library is built and distributed as an integral part of `gdbserver'.
In addition, support for static tracepoints requires building the
in-process agent library with static tracepoints support. At present,
the UST (LTTng Userspace Tracer, `http://lttng.org/ust') tracing engine
is supported. This support is automatically available if UST
development headers are found in the standard include path when
`gdbserver' is built, or if `gdbserver' was explicitly configured using
`--with-ust' to point at such headers. You can explicitly disable the
support using `--with-ust=no'.
There are several ways to load the in-process agent in your program:
`Specifying it as dependency at link time'
You can link your program dynamically with the in-process agent
library. On most systems, this is accomplished by adding
`-linproctrace' to the link command.
`Using the system's preloading mechanisms'
You can force loading the in-process agent at startup time by using
your system's support for preloading shared libraries. Many Unixes
support the concept of preloading user defined libraries. In most
cases, you do that by specifying `LD_PRELOAD=libinproctrace.so' in
the environment. See also the description of `gdbserver''s
`--wrapper' command line option.
`Using GDB to force loading the agent at run time'
On some systems, you can force the inferior to load a shared
library, by calling a dynamic loader function in the inferior that
takes care of dynamically looking up and loading a shared library.
On most Unix systems, the function is `dlopen'. You'll use the
`call' command for that. For example:
(gdb) call dlopen ("libinproctrace.so", ...)
Note that on most Unix systems, for the `dlopen' function to be
available, the program needs to be linked with `-ldl'.
On systems that have a userspace dynamic loader, like most Unix
systems, when you connect to `gdbserver' using `target remote', you'll
find that the program is stopped at the dynamic loader's entry point,
and no shared library has been loaded in the program's address space
yet, including the in-process agent. In that case, before being able
to use any of the fast or static tracepoints features, you need to let
the loader run and load the shared libraries. The simplest way to do
that is to run the program to the main procedure. E.g., if debugging a
C or C++ program, start `gdbserver' like so:
$ gdbserver :9999 myprogram
Start GDB and connect to `gdbserver' like so, and run to main:
$ gdb myprogram
(gdb) target remote myhost:9999
0x00007f215893ba60 in ?? () from /lib64/ld-linux-x86-64.so.2
(gdb) b main
(gdb) continue
The in-process tracing agent library should now be loaded into the
process; you can confirm it with the `info sharedlibrary' command,
which will list `libinproctrace.so' as loaded in the process. You are
now ready to install fast tracepoints, list static tracepoint markers,
probe static tracepoints markers, and start tracing.
---------- Footnotes ----------
(1) If you choose a port number that conflicts with another service,
`gdbserver' prints an error message and exits.

File: gdb.info, Node: Remote Configuration, Next: Remote Stub, Prev: Server, Up: Remote Debugging
20.4 Remote Configuration
=========================
This section documents the configuration options available when
debugging remote programs. For the options related to the File I/O
extensions of the remote protocol, see *Note system-call-allowed:
system.
`set remoteaddresssize BITS'
Set the maximum size of address in a memory packet to the specified
number of bits. GDB will mask off the address bits above that
number, when it passes addresses to the remote target. The
default value is the number of bits in the target's address.
`show remoteaddresssize'
Show the current value of remote address size in bits.
`set remotebaud N'
Set the baud rate for the remote serial I/O to N baud. The value
is used to set the speed of the serial port used for debugging
remote targets.
`show remotebaud'
Show the current speed of the remote connection.
`set remotebreak'
If set to on, GDB sends a `BREAK' signal to the remote when you
type `Ctrl-c' to interrupt the program running on the remote. If
set to off, GDB sends the `Ctrl-C' character instead. The default
is off, since most remote systems expect to see `Ctrl-C' as the
interrupt signal.
`show remotebreak'
Show whether GDB sends `BREAK' or `Ctrl-C' to interrupt the remote
program.
`set remoteflow on'
`set remoteflow off'
Enable or disable hardware flow control (`RTS'/`CTS') on the
serial port used to communicate to the remote target.
`show remoteflow'
Show the current setting of hardware flow control.
`set remotelogbase BASE'
Set the base (a.k.a. radix) of logging serial protocol
communications to BASE. Supported values of BASE are: `ascii',
`octal', and `hex'. The default is `ascii'.
`show remotelogbase'
Show the current setting of the radix for logging remote serial
protocol.
`set remotelogfile FILE'
Record remote serial communications on the named FILE. The
default is not to record at all.
`show remotelogfile.'
Show the current setting of the file name on which to record the
serial communications.
`set remotetimeout NUM'
Set the timeout limit to wait for the remote target to respond to
NUM seconds. The default is 2 seconds.
`show remotetimeout'
Show the current number of seconds to wait for the remote target
responses.
`set remote hardware-watchpoint-limit LIMIT'
`set remote hardware-breakpoint-limit LIMIT'
Restrict GDB to using LIMIT remote hardware breakpoint or
watchpoints. A limit of -1, the default, is treated as unlimited.
`set remote exec-file FILENAME'
`show remote exec-file'
Select the file used for `run' with `target extended-remote'.
This should be set to a filename valid on the target system. If
it is not set, the target will use a default filename (e.g. the
last program run).
`set remote interrupt-sequence'
Allow the user to select one of `Ctrl-C', a `BREAK' or `BREAK-g'
as the sequence to the remote target in order to interrupt the
execution. `Ctrl-C' is a default. Some system prefers `BREAK'
which is high level of serial line for some certain time. Linux
kernel prefers `BREAK-g', a.k.a Magic SysRq g. It is `BREAK'
signal followed by character `g'.
`show interrupt-sequence'
Show which of `Ctrl-C', `BREAK' or `BREAK-g' is sent by GDB to
interrupt the remote program. `BREAK-g' is BREAK signal followed
by `g' and also known as Magic SysRq g.
`set remote interrupt-on-connect'
Specify whether interrupt-sequence is sent to remote target when
GDB connects to it. This is mostly needed when you debug Linux
kernel. Linux kernel expects `BREAK' followed by `g' which is
known as Magic SysRq g in order to connect GDB.
`show interrupt-on-connect'
Show whether interrupt-sequence is sent to remote target when GDB
connects to it.
`set tcp auto-retry on'
Enable auto-retry for remote TCP connections. This is useful if
the remote debugging agent is launched in parallel with GDB; there
is a race condition because the agent may not become ready to
accept the connection before GDB attempts to connect. When
auto-retry is enabled, if the initial attempt to connect fails,
GDB reattempts to establish the connection using the timeout
specified by `set tcp connect-timeout'.
`set tcp auto-retry off'
Do not auto-retry failed TCP connections.
`show tcp auto-retry'
Show the current auto-retry setting.
`set tcp connect-timeout SECONDS'
Set the timeout for establishing a TCP connection to the remote
target to SECONDS. The timeout affects both polling to retry
failed connections (enabled by `set tcp auto-retry on') and
waiting for connections that are merely slow to complete, and
represents an approximate cumulative value.
`show tcp connect-timeout'
Show the current connection timeout setting.
The GDB remote protocol autodetects the packets supported by your
debugging stub. If you need to override the autodetection, you can use
these commands to enable or disable individual packets. Each packet
can be set to `on' (the remote target supports this packet), `off' (the
remote target does not support this packet), or `auto' (detect remote
target support for this packet). They all default to `auto'. For more
information about each packet, see *Note Remote Protocol::.
During normal use, you should not have to use any of these commands.
If you do, that may be a bug in your remote debugging stub, or a bug in
GDB. You may want to report the problem to the GDB developers.
For each packet NAME, the command to enable or disable the packet is
`set remote NAME-packet'. The available settings are:
Command Name Remote Packet Related Features
`fetch-register' `p' `info registers'
`set-register' `P' `set'
`binary-download' `X' `load', `set'
`read-aux-vector' `qXfer:auxv:read' `info auxv'
`symbol-lookup' `qSymbol' Detecting
multiple threads
`attach' `vAttach' `attach'
`verbose-resume' `vCont' Stepping or
resuming multiple
threads
`run' `vRun' `run'
`software-breakpoint'`Z0' `break'
`hardware-breakpoint'`Z1' `hbreak'
`write-watchpoint' `Z2' `watch'
`read-watchpoint' `Z3' `rwatch'
`access-watchpoint' `Z4' `awatch'
`target-features' `qXfer:features:read' `set architecture'
`library-info' `qXfer:libraries:read' `info
sharedlibrary'
`memory-map' `qXfer:memory-map:read' `info mem'
`read-sdata-object' `qXfer:sdata:read' `print $_sdata'
`read-spu-object' `qXfer:spu:read' `info spu'
`write-spu-object' `qXfer:spu:write' `info spu'
`read-siginfo-object'`qXfer:siginfo:read' `print $_siginfo'
`write-siginfo-object'`qXfer:siginfo:write' `set $_siginfo'
`threads' `qXfer:threads:read' `info threads'
`get-thread-local- `qGetTLSAddr' Displaying
storage-address' `__thread'
variables
`get-thread-information-block-address'`qGetTIBAddr' Display
MS-Windows Thread
Information Block.
`search-memory' `qSearch:memory' `find'
`supported-packets' `qSupported' Remote
communications
parameters
`pass-signals' `QPassSignals' `handle SIGNAL'
`hostio-close-packet'`vFile:close' `remote get',
`remote put'
`hostio-open-packet' `vFile:open' `remote get',
`remote put'
`hostio-pread-packet'`vFile:pread' `remote get',
`remote put'
`hostio-pwrite-packet'`vFile:pwrite' `remote get',
`remote put'
`hostio-unlink-packet'`vFile:unlink' `remote delete'
`noack-packet' `QStartNoAckMode' Packet
acknowledgment
`osdata' `qXfer:osdata:read' `info os'
`query-attached' `qAttached' Querying remote
process attach
state.

File: gdb.info, Node: Remote Stub, Prev: Remote Configuration, Up: Remote Debugging
20.5 Implementing a Remote Stub
===============================
The stub files provided with GDB implement the target side of the
communication protocol, and the GDB side is implemented in the GDB
source file `remote.c'. Normally, you can simply allow these
subroutines to communicate, and ignore the details. (If you're
implementing your own stub file, you can still ignore the details: start
with one of the existing stub files. `sparc-stub.c' is the best
organized, and therefore the easiest to read.)
To debug a program running on another machine (the debugging
"target" machine), you must first arrange for all the usual
prerequisites for the program to run by itself. For example, for a C
program, you need:
1. A startup routine to set up the C runtime environment; these
usually have a name like `crt0'. The startup routine may be
supplied by your hardware supplier, or you may have to write your
own.
2. A C subroutine library to support your program's subroutine calls,
notably managing input and output.
3. A way of getting your program to the other machine--for example, a
download program. These are often supplied by the hardware
manufacturer, but you may have to write your own from hardware
documentation.
The next step is to arrange for your program to use a serial port to
communicate with the machine where GDB is running (the "host" machine).
In general terms, the scheme looks like this:
_On the host,_
GDB already understands how to use this protocol; when everything
else is set up, you can simply use the `target remote' command
(*note Specifying a Debugging Target: Targets.).
_On the target,_
you must link with your program a few special-purpose subroutines
that implement the GDB remote serial protocol. The file
containing these subroutines is called a "debugging stub".
On certain remote targets, you can use an auxiliary program
`gdbserver' instead of linking a stub into your program. *Note
Using the `gdbserver' Program: Server, for details.
The debugging stub is specific to the architecture of the remote
machine; for example, use `sparc-stub.c' to debug programs on SPARC
boards.
These working remote stubs are distributed with GDB:
`i386-stub.c'
For Intel 386 and compatible architectures.
`m68k-stub.c'
For Motorola 680x0 architectures.
`sh-stub.c'
For Renesas SH architectures.
`sparc-stub.c'
For SPARC architectures.
`sparcl-stub.c'
For Fujitsu SPARCLITE architectures.
The `README' file in the GDB distribution may list other recently
added stubs.
* Menu:
* Stub Contents:: What the stub can do for you
* Bootstrapping:: What you must do for the stub
* Debug Session:: Putting it all together

File: gdb.info, Node: Stub Contents, Next: Bootstrapping, Up: Remote Stub
20.5.1 What the Stub Can Do for You
-----------------------------------
The debugging stub for your architecture supplies these three
subroutines:
`set_debug_traps'
This routine arranges for `handle_exception' to run when your
program stops. You must call this subroutine explicitly near the
beginning of your program.
`handle_exception'
This is the central workhorse, but your program never calls it
explicitly--the setup code arranges for `handle_exception' to run
when a trap is triggered.
`handle_exception' takes control when your program stops during
execution (for example, on a breakpoint), and mediates
communications with GDB on the host machine. This is where the
communications protocol is implemented; `handle_exception' acts as
the GDB representative on the target machine. It begins by
sending summary information on the state of your program, then
continues to execute, retrieving and transmitting any information
GDB needs, until you execute a GDB command that makes your program
resume; at that point, `handle_exception' returns control to your
own code on the target machine.
`breakpoint'
Use this auxiliary subroutine to make your program contain a
breakpoint. Depending on the particular situation, this may be
the only way for GDB to get control. For instance, if your target
machine has some sort of interrupt button, you won't need to call
this; pressing the interrupt button transfers control to
`handle_exception'--in effect, to GDB. On some machines, simply
receiving characters on the serial port may also trigger a trap;
again, in that situation, you don't need to call `breakpoint' from
your own program--simply running `target remote' from the host GDB
session gets control.
Call `breakpoint' if none of these is true, or if you simply want
to make certain your program stops at a predetermined point for the
start of your debugging session.

File: gdb.info, Node: Bootstrapping, Next: Debug Session, Prev: Stub Contents, Up: Remote Stub
20.5.2 What You Must Do for the Stub
------------------------------------
The debugging stubs that come with GDB are set up for a particular chip
architecture, but they have no information about the rest of your
debugging target machine.
First of all you need to tell the stub how to communicate with the
serial port.
`int getDebugChar()'
Write this subroutine to read a single character from the serial
port. It may be identical to `getchar' for your target system; a
different name is used to allow you to distinguish the two if you
wish.
`void putDebugChar(int)'
Write this subroutine to write a single character to the serial
port. It may be identical to `putchar' for your target system; a
different name is used to allow you to distinguish the two if you
wish.
If you want GDB to be able to stop your program while it is running,
you need to use an interrupt-driven serial driver, and arrange for it
to stop when it receives a `^C' (`\003', the control-C character).
That is the character which GDB uses to tell the remote system to stop.
Getting the debugging target to return the proper status to GDB
probably requires changes to the standard stub; one quick and dirty way
is to just execute a breakpoint instruction (the "dirty" part is that
GDB reports a `SIGTRAP' instead of a `SIGINT').
Other routines you need to supply are:
`void exceptionHandler (int EXCEPTION_NUMBER, void *EXCEPTION_ADDRESS)'
Write this function to install EXCEPTION_ADDRESS in the exception
handling tables. You need to do this because the stub does not
have any way of knowing what the exception handling tables on your
target system are like (for example, the processor's table might
be in ROM, containing entries which point to a table in RAM).
EXCEPTION_NUMBER is the exception number which should be changed;
its meaning is architecture-dependent (for example, different
numbers might represent divide by zero, misaligned access, etc).
When this exception occurs, control should be transferred directly
to EXCEPTION_ADDRESS, and the processor state (stack, registers,
and so on) should be just as it is when a processor exception
occurs. So if you want to use a jump instruction to reach
EXCEPTION_ADDRESS, it should be a simple jump, not a jump to
subroutine.
For the 386, EXCEPTION_ADDRESS should be installed as an interrupt
gate so that interrupts are masked while the handler runs. The
gate should be at privilege level 0 (the most privileged level).
The SPARC and 68k stubs are able to mask interrupts themselves
without help from `exceptionHandler'.
`void flush_i_cache()'
On SPARC and SPARCLITE only, write this subroutine to flush the
instruction cache, if any, on your target machine. If there is no
instruction cache, this subroutine may be a no-op.
On target machines that have instruction caches, GDB requires this
function to make certain that the state of your program is stable.
You must also make sure this library routine is available:
`void *memset(void *, int, int)'
This is the standard library function `memset' that sets an area of
memory to a known value. If you have one of the free versions of
`libc.a', `memset' can be found there; otherwise, you must either
obtain it from your hardware manufacturer, or write your own.
If you do not use the GNU C compiler, you may need other standard
library subroutines as well; this varies from one stub to another, but
in general the stubs are likely to use any of the common library
subroutines which `GCC' generates as inline code.

File: gdb.info, Node: Debug Session, Prev: Bootstrapping, Up: Remote Stub
20.5.3 Putting it All Together
------------------------------
In summary, when your program is ready to debug, you must follow these
steps.
1. Make sure you have defined the supporting low-level routines
(*note What You Must Do for the Stub: Bootstrapping.):
`getDebugChar', `putDebugChar',
`flush_i_cache', `memset', `exceptionHandler'.
2. Insert these lines near the top of your program:
set_debug_traps();
breakpoint();
3. For the 680x0 stub only, you need to provide a variable called
`exceptionHook'. Normally you just use:
void (*exceptionHook)() = 0;
but if before calling `set_debug_traps', you set it to point to a
function in your program, that function is called when `GDB'
continues after stopping on a trap (for example, bus error). The
function indicated by `exceptionHook' is called with one
parameter: an `int' which is the exception number.
4. Compile and link together: your program, the GDB debugging stub for
your target architecture, and the supporting subroutines.
5. Make sure you have a serial connection between your target machine
and the GDB host, and identify the serial port on the host.
6. Download your program to your target machine (or get it there by
whatever means the manufacturer provides), and start it.
7. Start GDB on the host, and connect to the target (*note Connecting
to a Remote Target: Connecting.).

File: gdb.info, Node: Configurations, Next: Controlling GDB, Prev: Remote Debugging, Up: Top
21 Configuration-Specific Information
*************************************
While nearly all GDB commands are available for all native and cross
versions of the debugger, there are some exceptions. This chapter
describes things that are only available in certain configurations.
There are three major categories of configurations: native
configurations, where the host and target are the same, embedded
operating system configurations, which are usually the same for several
different processor architectures, and bare embedded processors, which
are quite different from each other.
* Menu:
* Native::
* Embedded OS::
* Embedded Processors::
* Architectures::

File: gdb.info, Node: Native, Next: Embedded OS, Up: Configurations
21.1 Native
===========
This section describes details specific to particular native
configurations.
* Menu:
* HP-UX:: HP-UX
* BSD libkvm Interface:: Debugging BSD kernel memory images
* SVR4 Process Information:: SVR4 process information
* DJGPP Native:: Features specific to the DJGPP port
* Cygwin Native:: Features specific to the Cygwin port
* Hurd Native:: Features specific to GNU Hurd
* Neutrino:: Features specific to QNX Neutrino
* Darwin:: Features specific to Darwin

File: gdb.info, Node: HP-UX, Next: BSD libkvm Interface, Up: Native
21.1.1 HP-UX
------------
On HP-UX systems, if you refer to a function or variable name that
begins with a dollar sign, GDB searches for a user or system name
first, before it searches for a convenience variable.

File: gdb.info, Node: BSD libkvm Interface, Next: SVR4 Process Information, Prev: HP-UX, Up: Native
21.1.2 BSD libkvm Interface
---------------------------
BSD-derived systems (FreeBSD/NetBSD/OpenBSD) have a kernel memory
interface that provides a uniform interface for accessing kernel virtual
memory images, including live systems and crash dumps. GDB uses this
interface to allow you to debug live kernels and kernel crash dumps on
many native BSD configurations. This is implemented as a special `kvm'
debugging target. For debugging a live system, load the currently
running kernel into GDB and connect to the `kvm' target:
(gdb) target kvm
For debugging crash dumps, provide the file name of the crash dump
as an argument:
(gdb) target kvm /var/crash/bsd.0
Once connected to the `kvm' target, the following commands are
available:
`kvm pcb'
Set current context from the "Process Control Block" (PCB) address.
`kvm proc'
Set current context from proc address. This command isn't
available on modern FreeBSD systems.

File: gdb.info, Node: SVR4 Process Information, Next: DJGPP Native, Prev: BSD libkvm Interface, Up: Native
21.1.3 SVR4 Process Information
-------------------------------
Many versions of SVR4 and compatible systems provide a facility called
`/proc' that can be used to examine the image of a running process
using file-system subroutines. If GDB is configured for an operating
system with this facility, the command `info proc' is available to
report information about the process running your program, or about any
process running on your system. `info proc' works only on SVR4 systems
that include the `procfs' code. This includes, as of this writing,
GNU/Linux, OSF/1 (Digital Unix), Solaris, Irix, and Unixware, but not
HP-UX, for example.
`info proc'
`info proc PROCESS-ID'
Summarize available information about any running process. If a
process ID is specified by PROCESS-ID, display information about
that process; otherwise display information about the program being
debugged. The summary includes the debugged process ID, the
command line used to invoke it, its current working directory, and
its executable file's absolute file name.
On some systems, PROCESS-ID can be of the form `[PID]/TID' which
specifies a certain thread ID within a process. If the optional
PID part is missing, it means a thread from the process being
debugged (the leading `/' still needs to be present, or else GDB
will interpret the number as a process ID rather than a thread ID).
`info proc mappings'
Report the memory address space ranges accessible in the program,
with information on whether the process has read, write, or
execute access rights to each range. On GNU/Linux systems, each
memory range includes the object file which is mapped to that
range, instead of the memory access rights to that range.
`info proc stat'
`info proc status'
These subcommands are specific to GNU/Linux systems. They show
the process-related information, including the user ID and group
ID; how many threads are there in the process; its virtual memory
usage; the signals that are pending, blocked, and ignored; its
TTY; its consumption of system and user time; its stack size; its
`nice' value; etc. For more information, see the `proc' man page
(type `man 5 proc' from your shell prompt).
`info proc all'
Show all the information about the process described under all of
the above `info proc' subcommands.
`set procfs-trace'
This command enables and disables tracing of `procfs' API calls.
`show procfs-trace'
Show the current state of `procfs' API call tracing.
`set procfs-file FILE'
Tell GDB to write `procfs' API trace to the named FILE. GDB
appends the trace info to the previous contents of the file. The
default is to display the trace on the standard output.
`show procfs-file'
Show the file to which `procfs' API trace is written.
`proc-trace-entry'
`proc-trace-exit'
`proc-untrace-entry'
`proc-untrace-exit'
These commands enable and disable tracing of entries into and exits
from the `syscall' interface.
`info pidlist'
For QNX Neutrino only, this command displays the list of all the
processes and all the threads within each process.
`info meminfo'
For QNX Neutrino only, this command displays the list of all
mapinfos.