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This is ld.info, produced by makeinfo version 5.1 from ld.texinfo.
This file documents the GNU linker LD (GNU Binutils) version 2.24.0.
Copyright (C) 1991-2013 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
Texts. A copy of the license is included in the section entitled "GNU
Free Documentation License".
INFO-DIR-SECTION Software development
START-INFO-DIR-ENTRY
* Ld: (ld). The GNU linker.
END-INFO-DIR-ENTRY

File: ld.info, Node: Top, Next: Overview, Up: (dir)
LD
**
This file documents the GNU linker ld (GNU Binutils) version 2.24.0.
This document is distributed under the terms of the GNU Free
Documentation License version 1.3. A copy of the license is included in
the section entitled "GNU Free Documentation License".
* Menu:
* Overview:: Overview
* Invocation:: Invocation
* Scripts:: Linker Scripts
* Machine Dependent:: Machine Dependent Features
* BFD:: BFD
* Reporting Bugs:: Reporting Bugs
* MRI:: MRI Compatible Script Files
* GNU Free Documentation License:: GNU Free Documentation License
* LD Index:: LD Index

File: ld.info, Node: Overview, Next: Invocation, Prev: Top, Up: Top
1 Overview
**********
'ld' combines a number of object and archive files, relocates their data
and ties up symbol references. Usually the last step in compiling a
program is to run 'ld'.
'ld' accepts Linker Command Language files written in a superset of
AT&T's Link Editor Command Language syntax, to provide explicit and
total control over the linking process.
This version of 'ld' uses the general purpose BFD libraries to
operate on object files. This allows 'ld' to read, combine, and write
object files in many different formats--for example, COFF or 'a.out'.
Different formats may be linked together to produce any available kind
of object file. *Note BFD::, for more information.
Aside from its flexibility, the GNU linker is more helpful than other
linkers in providing diagnostic information. Many linkers abandon
execution immediately upon encountering an error; whenever possible,
'ld' continues executing, allowing you to identify other errors (or, in
some cases, to get an output file in spite of the error).

File: ld.info, Node: Invocation, Next: Scripts, Prev: Overview, Up: Top
2 Invocation
************
The GNU linker 'ld' is meant to cover a broad range of situations, and
to be as compatible as possible with other linkers. As a result, you
have many choices to control its behavior.
* Menu:
* Options:: Command Line Options
* Environment:: Environment Variables

File: ld.info, Node: Options, Next: Environment, Up: Invocation
2.1 Command Line Options
========================
The linker supports a plethora of command-line options, but in actual
practice few of them are used in any particular context. For instance,
a frequent use of 'ld' is to link standard Unix object files on a
standard, supported Unix system. On such a system, to link a file
'hello.o':
ld -o OUTPUT /lib/crt0.o hello.o -lc
This tells 'ld' to produce a file called OUTPUT as the result of
linking the file '/lib/crt0.o' with 'hello.o' and the library 'libc.a',
which will come from the standard search directories. (See the
discussion of the '-l' option below.)
Some of the command-line options to 'ld' may be specified at any
point in the command line. However, options which refer to files, such
as '-l' or '-T', cause the file to be read at the point at which the
option appears in the command line, relative to the object files and
other file options. Repeating non-file options with a different
argument will either have no further effect, or override prior
occurrences (those further to the left on the command line) of that
option. Options which may be meaningfully specified more than once are
noted in the descriptions below.
Non-option arguments are object files or archives which are to be
linked together. They may follow, precede, or be mixed in with
command-line options, except that an object file argument may not be
placed between an option and its argument.
Usually the linker is invoked with at least one object file, but you
can specify other forms of binary input files using '-l', '-R', and the
script command language. If _no_ binary input files at all are
specified, the linker does not produce any output, and issues the
message 'No input files'.
If the linker cannot recognize the format of an object file, it will
assume that it is a linker script. A script specified in this way
augments the main linker script used for the link (either the default
linker script or the one specified by using '-T'). This feature permits
the linker to link against a file which appears to be an object or an
archive, but actually merely defines some symbol values, or uses 'INPUT'
or 'GROUP' to load other objects. Specifying a script in this way
merely augments the main linker script, with the extra commands placed
after the main script; use the '-T' option to replace the default linker
script entirely, but note the effect of the 'INSERT' command. *Note
Scripts::.
For options whose names are a single letter, option arguments must
either follow the option letter without intervening whitespace, or be
given as separate arguments immediately following the option that
requires them.
For options whose names are multiple letters, either one dash or two
can precede the option name; for example, '-trace-symbol' and
'--trace-symbol' are equivalent. Note--there is one exception to this
rule. Multiple letter options that start with a lower case 'o' can only
be preceded by two dashes. This is to reduce confusion with the '-o'
option. So for example '-omagic' sets the output file name to 'magic'
whereas '--omagic' sets the NMAGIC flag on the output.
Arguments to multiple-letter options must either be separated from
the option name by an equals sign, or be given as separate arguments
immediately following the option that requires them. For example,
'--trace-symbol foo' and '--trace-symbol=foo' are equivalent. Unique
abbreviations of the names of multiple-letter options are accepted.
Note--if the linker is being invoked indirectly, via a compiler
driver (e.g. 'gcc') then all the linker command line options should be
prefixed by '-Wl,' (or whatever is appropriate for the particular
compiler driver) like this:
gcc -Wl,--start-group foo.o bar.o -Wl,--end-group
This is important, because otherwise the compiler driver program may
silently drop the linker options, resulting in a bad link. Confusion
may also arise when passing options that require values through a
driver, as the use of a space between option and argument acts as a
separator, and causes the driver to pass only the option to the linker
and the argument to the compiler. In this case, it is simplest to use
the joined forms of both single- and multiple-letter options, such as:
gcc foo.o bar.o -Wl,-eENTRY -Wl,-Map=a.map
Here is a table of the generic command line switches accepted by the
GNU linker:
'@FILE'
Read command-line options from FILE. The options read are inserted
in place of the original @FILE option. If FILE does not exist, or
cannot be read, then the option will be treated literally, and not
removed.
Options in FILE are separated by whitespace. A whitespace
character may be included in an option by surrounding the entire
option in either single or double quotes. Any character (including
a backslash) may be included by prefixing the character to be
included with a backslash. The FILE may itself contain additional
@FILE options; any such options will be processed recursively.
'-a KEYWORD'
This option is supported for HP/UX compatibility. The KEYWORD
argument must be one of the strings 'archive', 'shared', or
'default'. '-aarchive' is functionally equivalent to '-Bstatic',
and the other two keywords are functionally equivalent to
'-Bdynamic'. This option may be used any number of times.
'--audit AUDITLIB'
Adds AUDITLIB to the 'DT_AUDIT' entry of the dynamic section.
AUDITLIB is not checked for existence, nor will it use the
DT_SONAME specified in the library. If specified multiple times
'DT_AUDIT' will contain a colon separated list of audit interfaces
to use. If the linker finds an object with an audit entry while
searching for shared libraries, it will add a corresponding
'DT_DEPAUDIT' entry in the output file. This option is only
meaningful on ELF platforms supporting the rtld-audit interface.
'-A ARCHITECTURE'
'--architecture=ARCHITECTURE'
In the current release of 'ld', this option is useful only for the
Intel 960 family of architectures. In that 'ld' configuration, the
ARCHITECTURE argument identifies the particular architecture in the
960 family, enabling some safeguards and modifying the
archive-library search path. *Note 'ld' and the Intel 960 family:
i960, for details.
Future releases of 'ld' may support similar functionality for other
architecture families.
'-b INPUT-FORMAT'
'--format=INPUT-FORMAT'
'ld' may be configured to support more than one kind of object
file. If your 'ld' is configured this way, you can use the '-b'
option to specify the binary format for input object files that
follow this option on the command line. Even when 'ld' is
configured to support alternative object formats, you don't usually
need to specify this, as 'ld' should be configured to expect as a
default input format the most usual format on each machine.
INPUT-FORMAT is a text string, the name of a particular format
supported by the BFD libraries. (You can list the available binary
formats with 'objdump -i'.) *Note BFD::.
You may want to use this option if you are linking files with an
unusual binary format. You can also use '-b' to switch formats
explicitly (when linking object files of different formats), by
including '-b INPUT-FORMAT' before each group of object files in a
particular format.
The default format is taken from the environment variable
'GNUTARGET'. *Note Environment::. You can also define the input
format from a script, using the command 'TARGET'; see *note Format
Commands::.
'-c MRI-COMMANDFILE'
'--mri-script=MRI-COMMANDFILE'
For compatibility with linkers produced by MRI, 'ld' accepts script
files written in an alternate, restricted command language,
described in *note MRI Compatible Script Files: MRI. Introduce MRI
script files with the option '-c'; use the '-T' option to run
linker scripts written in the general-purpose 'ld' scripting
language. If MRI-CMDFILE does not exist, 'ld' looks for it in the
directories specified by any '-L' options.
'-d'
'-dc'
'-dp'
These three options are equivalent; multiple forms are supported
for compatibility with other linkers. They assign space to common
symbols even if a relocatable output file is specified (with '-r').
The script command 'FORCE_COMMON_ALLOCATION' has the same effect.
*Note Miscellaneous Commands::.
'--depaudit AUDITLIB'
'-P AUDITLIB'
Adds AUDITLIB to the 'DT_DEPAUDIT' entry of the dynamic section.
AUDITLIB is not checked for existence, nor will it use the
DT_SONAME specified in the library. If specified multiple times
'DT_DEPAUDIT' will contain a colon separated list of audit
interfaces to use. This option is only meaningful on ELF platforms
supporting the rtld-audit interface. The -P option is provided for
Solaris compatibility.
'-e ENTRY'
'--entry=ENTRY'
Use ENTRY as the explicit symbol for beginning execution of your
program, rather than the default entry point. If there is no
symbol named ENTRY, the linker will try to parse ENTRY as a number,
and use that as the entry address (the number will be interpreted
in base 10; you may use a leading '0x' for base 16, or a leading
'0' for base 8). *Note Entry Point::, for a discussion of defaults
and other ways of specifying the entry point.
'--exclude-libs LIB,LIB,...'
Specifies a list of archive libraries from which symbols should not
be automatically exported. The library names may be delimited by
commas or colons. Specifying '--exclude-libs ALL' excludes symbols
in all archive libraries from automatic export. This option is
available only for the i386 PE targeted port of the linker and for
ELF targeted ports. For i386 PE, symbols explicitly listed in a
.def file are still exported, regardless of this option. For ELF
targeted ports, symbols affected by this option will be treated as
hidden.
'--exclude-modules-for-implib MODULE,MODULE,...'
Specifies a list of object files or archive members, from which
symbols should not be automatically exported, but which should be
copied wholesale into the import library being generated during the
link. The module names may be delimited by commas or colons, and
must match exactly the filenames used by 'ld' to open the files;
for archive members, this is simply the member name, but for object
files the name listed must include and match precisely any path
used to specify the input file on the linker's command-line. This
option is available only for the i386 PE targeted port of the
linker. Symbols explicitly listed in a .def file are still
exported, regardless of this option.
'-E'
'--export-dynamic'
'--no-export-dynamic'
When creating a dynamically linked executable, using the '-E'
option or the '--export-dynamic' option causes the linker to add
all symbols to the dynamic symbol table. The dynamic symbol table
is the set of symbols which are visible from dynamic objects at run
time.
If you do not use either of these options (or use the
'--no-export-dynamic' option to restore the default behavior), the
dynamic symbol table will normally contain only those symbols which
are referenced by some dynamic object mentioned in the link.
If you use 'dlopen' to load a dynamic object which needs to refer
back to the symbols defined by the program, rather than some other
dynamic object, then you will probably need to use this option when
linking the program itself.
You can also use the dynamic list to control what symbols should be
added to the dynamic symbol table if the output format supports it.
See the description of '--dynamic-list'.
Note that this option is specific to ELF targeted ports. PE
targets support a similar function to export all symbols from a DLL
or EXE; see the description of '--export-all-symbols' below.
'-EB'
Link big-endian objects. This affects the default output format.
'-EL'
Link little-endian objects. This affects the default output
format.
'-f NAME'
'--auxiliary=NAME'
When creating an ELF shared object, set the internal DT_AUXILIARY
field to the specified name. This tells the dynamic linker that
the symbol table of the shared object should be used as an
auxiliary filter on the symbol table of the shared object NAME.
If you later link a program against this filter object, then, when
you run the program, the dynamic linker will see the DT_AUXILIARY
field. If the dynamic linker resolves any symbols from the filter
object, it will first check whether there is a definition in the
shared object NAME. If there is one, it will be used instead of
the definition in the filter object. The shared object NAME need
not exist. Thus the shared object NAME may be used to provide an
alternative implementation of certain functions, perhaps for
debugging or for machine specific performance.
This option may be specified more than once. The DT_AUXILIARY
entries will be created in the order in which they appear on the
command line.
'-F NAME'
'--filter=NAME'
When creating an ELF shared object, set the internal DT_FILTER
field to the specified name. This tells the dynamic linker that
the symbol table of the shared object which is being created should
be used as a filter on the symbol table of the shared object NAME.
If you later link a program against this filter object, then, when
you run the program, the dynamic linker will see the DT_FILTER
field. The dynamic linker will resolve symbols according to the
symbol table of the filter object as usual, but it will actually
link to the definitions found in the shared object NAME. Thus the
filter object can be used to select a subset of the symbols
provided by the object NAME.
Some older linkers used the '-F' option throughout a compilation
toolchain for specifying object-file format for both input and
output object files. The GNU linker uses other mechanisms for this
purpose: the '-b', '--format', '--oformat' options, the 'TARGET'
command in linker scripts, and the 'GNUTARGET' environment
variable. The GNU linker will ignore the '-F' option when not
creating an ELF shared object.
'-fini=NAME'
When creating an ELF executable or shared object, call NAME when
the executable or shared object is unloaded, by setting DT_FINI to
the address of the function. By default, the linker uses '_fini'
as the function to call.
'-g'
Ignored. Provided for compatibility with other tools.
'-G VALUE'
'--gpsize=VALUE'
Set the maximum size of objects to be optimized using the GP
register to SIZE. This is only meaningful for object file formats
such as MIPS ELF that support putting large and small objects into
different sections. This is ignored for other object file formats.
'-h NAME'
'-soname=NAME'
When creating an ELF shared object, set the internal DT_SONAME
field to the specified name. When an executable is linked with a
shared object which has a DT_SONAME field, then when the executable
is run the dynamic linker will attempt to load the shared object
specified by the DT_SONAME field rather than the using the file
name given to the linker.
'-i'
Perform an incremental link (same as option '-r').
'-init=NAME'
When creating an ELF executable or shared object, call NAME when
the executable or shared object is loaded, by setting DT_INIT to
the address of the function. By default, the linker uses '_init'
as the function to call.
'-l NAMESPEC'
'--library=NAMESPEC'
Add the archive or object file specified by NAMESPEC to the list of
files to link. This option may be used any number of times. If
NAMESPEC is of the form ':FILENAME', 'ld' will search the library
path for a file called FILENAME, otherwise it will search the
library path for a file called 'libNAMESPEC.a'.
On systems which support shared libraries, 'ld' may also search for
files other than 'libNAMESPEC.a'. Specifically, on ELF and SunOS
systems, 'ld' will search a directory for a library called
'libNAMESPEC.so' before searching for one called 'libNAMESPEC.a'.
(By convention, a '.so' extension indicates a shared library.)
Note that this behavior does not apply to ':FILENAME', which always
specifies a file called FILENAME.
The linker will search an archive only once, at the location where
it is specified on the command line. If the archive defines a
symbol which was undefined in some object which appeared before the
archive on the command line, the linker will include the
appropriate file(s) from the archive. However, an undefined symbol
in an object appearing later on the command line will not cause the
linker to search the archive again.
See the '-(' option for a way to force the linker to search
archives multiple times.
You may list the same archive multiple times on the command line.
This type of archive searching is standard for Unix linkers.
However, if you are using 'ld' on AIX, note that it is different
from the behaviour of the AIX linker.
'-L SEARCHDIR'
'--library-path=SEARCHDIR'
Add path SEARCHDIR to the list of paths that 'ld' will search for
archive libraries and 'ld' control scripts. You may use this
option any number of times. The directories are searched in the
order in which they are specified on the command line. Directories
specified on the command line are searched before the default
directories. All '-L' options apply to all '-l' options,
regardless of the order in which the options appear. '-L' options
do not affect how 'ld' searches for a linker script unless '-T'
option is specified.
If SEARCHDIR begins with '=', then the '=' will be replaced by the
"sysroot prefix", a path specified when the linker is configured.
The default set of paths searched (without being specified with
'-L') depends on which emulation mode 'ld' is using, and in some
cases also on how it was configured. *Note Environment::.
The paths can also be specified in a link script with the
'SEARCH_DIR' command. Directories specified this way are searched
at the point in which the linker script appears in the command
line.
'-m EMULATION'
Emulate the EMULATION linker. You can list the available
emulations with the '--verbose' or '-V' options.
If the '-m' option is not used, the emulation is taken from the
'LDEMULATION' environment variable, if that is defined.
Otherwise, the default emulation depends upon how the linker was
configured.
'-M'
'--print-map'
Print a link map to the standard output. A link map provides
information about the link, including the following:
* Where object files are mapped into memory.
* How common symbols are allocated.
* All archive members included in the link, with a mention of
the symbol which caused the archive member to be brought in.
* The values assigned to symbols.
Note - symbols whose values are computed by an expression
which involves a reference to a previous value of the same
symbol may not have correct result displayed in the link map.
This is because the linker discards intermediate results and
only retains the final value of an expression. Under such
circumstances the linker will display the final value enclosed
by square brackets. Thus for example a linker script
containing:
foo = 1
foo = foo * 4
foo = foo + 8
will produce the following output in the link map if the '-M'
option is used:
0x00000001 foo = 0x1
[0x0000000c] foo = (foo * 0x4)
[0x0000000c] foo = (foo + 0x8)
See *note Expressions:: for more information about expressions
in linker scripts.
'-n'
'--nmagic'
Turn off page alignment of sections, and disable linking against
shared libraries. If the output format supports Unix style magic
numbers, mark the output as 'NMAGIC'.
'-N'
'--omagic'
Set the text and data sections to be readable and writable. Also,
do not page-align the data segment, and disable linking against
shared libraries. If the output format supports Unix style magic
numbers, mark the output as 'OMAGIC'. Note: Although a writable
text section is allowed for PE-COFF targets, it does not conform to
the format specification published by Microsoft.
'--no-omagic'
This option negates most of the effects of the '-N' option. It
sets the text section to be read-only, and forces the data segment
to be page-aligned. Note - this option does not enable linking
against shared libraries. Use '-Bdynamic' for this.
'-o OUTPUT'
'--output=OUTPUT'
Use OUTPUT as the name for the program produced by 'ld'; if this
option is not specified, the name 'a.out' is used by default. The
script command 'OUTPUT' can also specify the output file name.
'-O LEVEL'
If LEVEL is a numeric values greater than zero 'ld' optimizes the
output. This might take significantly longer and therefore
probably should only be enabled for the final binary. At the
moment this option only affects ELF shared library generation.
Future releases of the linker may make more use of this option.
Also currently there is no difference in the linker's behaviour for
different non-zero values of this option. Again this may change
with future releases.
'-q'
'--emit-relocs'
Leave relocation sections and contents in fully linked executables.
Post link analysis and optimization tools may need this information
in order to perform correct modifications of executables. This
results in larger executables.
This option is currently only supported on ELF platforms.
'--force-dynamic'
Force the output file to have dynamic sections. This option is
specific to VxWorks targets.
'-r'
'--relocatable'
Generate relocatable output--i.e., generate an output file that can
in turn serve as input to 'ld'. This is often called "partial
linking". As a side effect, in environments that support standard
Unix magic numbers, this option also sets the output file's magic
number to 'OMAGIC'. If this option is not specified, an absolute
file is produced. When linking C++ programs, this option _will
not_ resolve references to constructors; to do that, use '-Ur'.
When an input file does not have the same format as the output
file, partial linking is only supported if that input file does not
contain any relocations. Different output formats can have further
restrictions; for example some 'a.out'-based formats do not support
partial linking with input files in other formats at all.
This option does the same thing as '-i'.
'-R FILENAME'
'--just-symbols=FILENAME'
Read symbol names and their addresses from FILENAME, but do not
relocate it or include it in the output. This allows your output
file to refer symbolically to absolute locations of memory defined
in other programs. You may use this option more than once.
For compatibility with other ELF linkers, if the '-R' option is
followed by a directory name, rather than a file name, it is
treated as the '-rpath' option.
'-s'
'--strip-all'
Omit all symbol information from the output file.
'-S'
'--strip-debug'
Omit debugger symbol information (but not all symbols) from the
output file.
'-t'
'--trace'
Print the names of the input files as 'ld' processes them.
'-T SCRIPTFILE'
'--script=SCRIPTFILE'
Use SCRIPTFILE as the linker script. This script replaces 'ld''s
default linker script (rather than adding to it), so COMMANDFILE
must specify everything necessary to describe the output file.
*Note Scripts::. If SCRIPTFILE does not exist in the current
directory, 'ld' looks for it in the directories specified by any
preceding '-L' options. Multiple '-T' options accumulate.
'-dT SCRIPTFILE'
'--default-script=SCRIPTFILE'
Use SCRIPTFILE as the default linker script. *Note Scripts::.
This option is similar to the '--script' option except that
processing of the script is delayed until after the rest of the
command line has been processed. This allows options placed after
the '--default-script' option on the command line to affect the
behaviour of the linker script, which can be important when the
linker command line cannot be directly controlled by the user. (eg
because the command line is being constructed by another tool, such
as 'gcc').
'-u SYMBOL'
'--undefined=SYMBOL'
Force SYMBOL to be entered in the output file as an undefined
symbol. Doing this may, for example, trigger linking of additional
modules from standard libraries. '-u' may be repeated with
different option arguments to enter additional undefined symbols.
This option is equivalent to the 'EXTERN' linker script command.
'-Ur'
For anything other than C++ programs, this option is equivalent to
'-r': it generates relocatable output--i.e., an output file that
can in turn serve as input to 'ld'. When linking C++ programs,
'-Ur' _does_ resolve references to constructors, unlike '-r'. It
does not work to use '-Ur' on files that were themselves linked
with '-Ur'; once the constructor table has been built, it cannot be
added to. Use '-Ur' only for the last partial link, and '-r' for
the others.
'--unique[=SECTION]'
Creates a separate output section for every input section matching
SECTION, or if the optional wildcard SECTION argument is missing,
for every orphan input section. An orphan section is one not
specifically mentioned in a linker script. You may use this option
multiple times on the command line; It prevents the normal merging
of input sections with the same name, overriding output section
assignments in a linker script.
'-v'
'--version'
'-V'
Display the version number for 'ld'. The '-V' option also lists
the supported emulations.
'-x'
'--discard-all'
Delete all local symbols.
'-X'
'--discard-locals'
Delete all temporary local symbols. (These symbols start with
system-specific local label prefixes, typically '.L' for ELF
systems or 'L' for traditional a.out systems.)
'-y SYMBOL'
'--trace-symbol=SYMBOL'
Print the name of each linked file in which SYMBOL appears. This
option may be given any number of times. On many systems it is
necessary to prepend an underscore.
This option is useful when you have an undefined symbol in your
link but don't know where the reference is coming from.
'-Y PATH'
Add PATH to the default library search path. This option exists
for Solaris compatibility.
'-z KEYWORD'
The recognized keywords are:
'combreloc'
Combines multiple reloc sections and sorts them to make
dynamic symbol lookup caching possible.
'defs'
Disallows undefined symbols in object files. Undefined
symbols in shared libraries are still allowed.
'execstack'
Marks the object as requiring executable stack.
'global'
This option is only meaningful when building a shared object.
It makes the symbols defined by this shared object available
for symbol resolution of subsequently loaded libraries.
'initfirst'
This option is only meaningful when building a shared object.
It marks the object so that its runtime initialization will
occur before the runtime initialization of any other objects
brought into the process at the same time. Similarly the
runtime finalization of the object will occur after the
runtime finalization of any other objects.
'interpose'
Marks the object that its symbol table interposes before all
symbols but the primary executable.
'lazy'
When generating an executable or shared library, mark it to
tell the dynamic linker to defer function call resolution to
the point when the function is called (lazy binding), rather
than at load time. Lazy binding is the default.
'loadfltr'
Marks the object that its filters be processed immediately at
runtime.
'muldefs'
Allows multiple definitions.
'nocombreloc'
Disables multiple reloc sections combining.
'nocopyreloc'
Disables production of copy relocs.
'nodefaultlib'
Marks the object that the search for dependencies of this
object will ignore any default library search paths.
'nodelete'
Marks the object shouldn't be unloaded at runtime.
'nodlopen'
Marks the object not available to 'dlopen'.
'nodump'
Marks the object can not be dumped by 'dldump'.
'noexecstack'
Marks the object as not requiring executable stack.
'norelro'
Don't create an ELF 'PT_GNU_RELRO' segment header in the
object.
'now'
When generating an executable or shared library, mark it to
tell the dynamic linker to resolve all symbols when the
program is started, or when the shared library is linked to
using dlopen, instead of deferring function call resolution to
the point when the function is first called.
'origin'
Marks the object may contain $ORIGIN.
'relro'
Create an ELF 'PT_GNU_RELRO' segment header in the object.
'max-page-size=VALUE'
Set the emulation maximum page size to VALUE.
'common-page-size=VALUE'
Set the emulation common page size to VALUE.
'stack-size=VALUE'
Specify a stack size for in an ELF 'PT_GNU_STACK' segment.
Specifying zero will override any default non-zero sized
'PT_GNU_STACK' segment creation.
Other keywords are ignored for Solaris compatibility.
'-( ARCHIVES -)'
'--start-group ARCHIVES --end-group'
The ARCHIVES should be a list of archive files. They may be either
explicit file names, or '-l' options.
The specified archives are searched repeatedly until no new
undefined references are created. Normally, an archive is searched
only once in the order that it is specified on the command line.
If a symbol in that archive is needed to resolve an undefined
symbol referred to by an object in an archive that appears later on
the command line, the linker would not be able to resolve that
reference. By grouping the archives, they all be searched
repeatedly until all possible references are resolved.
Using this option has a significant performance cost. It is best
to use it only when there are unavoidable circular references
between two or more archives.
'--accept-unknown-input-arch'
'--no-accept-unknown-input-arch'
Tells the linker to accept input files whose architecture cannot be
recognised. The assumption is that the user knows what they are
doing and deliberately wants to link in these unknown input files.
This was the default behaviour of the linker, before release 2.14.
The default behaviour from release 2.14 onwards is to reject such
input files, and so the '--accept-unknown-input-arch' option has
been added to restore the old behaviour.
'--as-needed'
'--no-as-needed'
This option affects ELF DT_NEEDED tags for dynamic libraries
mentioned on the command line after the '--as-needed' option.
Normally the linker will add a DT_NEEDED tag for each dynamic
library mentioned on the command line, regardless of whether the
library is actually needed or not. '--as-needed' causes a
DT_NEEDED tag to only be emitted for a library that _at that point
in the link_ satisfies a non-weak undefined symbol reference from a
regular object file or, if the library is not found in the
DT_NEEDED lists of other libraries, a non-weak undefined symbol
reference from another dynamic library. Object files or libraries
appearing on the command line _after_ the library in question do
not affect whether the library is seen as needed. This is similar
to the rules for extraction of object files from archives.
'--no-as-needed' restores the default behaviour.
'--add-needed'
'--no-add-needed'
These two options have been deprecated because of the similarity of
their names to the '--as-needed' and '--no-as-needed' options.
They have been replaced by '--copy-dt-needed-entries' and
'--no-copy-dt-needed-entries'.
'-assert KEYWORD'
This option is ignored for SunOS compatibility.
'-Bdynamic'
'-dy'
'-call_shared'
Link against dynamic libraries. This is only meaningful on
platforms for which shared libraries are supported. This option is
normally the default on such platforms. The different variants of
this option are for compatibility with various systems. You may
use this option multiple times on the command line: it affects
library searching for '-l' options which follow it.
'-Bgroup'
Set the 'DF_1_GROUP' flag in the 'DT_FLAGS_1' entry in the dynamic
section. This causes the runtime linker to handle lookups in this
object and its dependencies to be performed only inside the group.
'--unresolved-symbols=report-all' is implied. This option is only
meaningful on ELF platforms which support shared libraries.
'-Bstatic'
'-dn'
'-non_shared'
'-static'
Do not link against shared libraries. This is only meaningful on
platforms for which shared libraries are supported. The different
variants of this option are for compatibility with various systems.
You may use this option multiple times on the command line: it
affects library searching for '-l' options which follow it. This
option also implies '--unresolved-symbols=report-all'. This option
can be used with '-shared'. Doing so means that a shared library
is being created but that all of the library's external references
must be resolved by pulling in entries from static libraries.
'-Bsymbolic'
When creating a shared library, bind references to global symbols
to the definition within the shared library, if any. Normally, it
is possible for a program linked against a shared library to
override the definition within the shared library. This option is
only meaningful on ELF platforms which support shared libraries.
'-Bsymbolic-functions'
When creating a shared library, bind references to global function
symbols to the definition within the shared library, if any. This
option is only meaningful on ELF platforms which support shared
libraries.
'--dynamic-list=DYNAMIC-LIST-FILE'
Specify the name of a dynamic list file to the linker. This is
typically used when creating shared libraries to specify a list of
global symbols whose references shouldn't be bound to the
definition within the shared library, or creating dynamically
linked executables to specify a list of symbols which should be
added to the symbol table in the executable. This option is only
meaningful on ELF platforms which support shared libraries.
The format of the dynamic list is the same as the version node
without scope and node name. See *note VERSION:: for more
information.
'--dynamic-list-data'
Include all global data symbols to the dynamic list.
'--dynamic-list-cpp-new'
Provide the builtin dynamic list for C++ operator new and delete.
It is mainly useful for building shared libstdc++.
'--dynamic-list-cpp-typeinfo'
Provide the builtin dynamic list for C++ runtime type
identification.
'--check-sections'
'--no-check-sections'
Asks the linker _not_ to check section addresses after they have
been assigned to see if there are any overlaps. Normally the
linker will perform this check, and if it finds any overlaps it
will produce suitable error messages. The linker does know about,
and does make allowances for sections in overlays. The default
behaviour can be restored by using the command line switch
'--check-sections'. Section overlap is not usually checked for
relocatable links. You can force checking in that case by using
the '--check-sections' option.
'--copy-dt-needed-entries'
'--no-copy-dt-needed-entries'
This option affects the treatment of dynamic libraries referred to
by DT_NEEDED tags _inside_ ELF dynamic libraries mentioned on the
command line. Normally the linker won't add a DT_NEEDED tag to the
output binary for each library mentioned in a DT_NEEDED tag in an
input dynamic library. With '--copy-dt-needed-entries' specified
on the command line however any dynamic libraries that follow it
will have their DT_NEEDED entries added. The default behaviour can
be restored with '--no-copy-dt-needed-entries'.
This option also has an effect on the resolution of symbols in
dynamic libraries. With '--copy-dt-needed-entries' dynamic
libraries mentioned on the command line will be recursively
searched, following their DT_NEEDED tags to other libraries, in
order to resolve symbols required by the output binary. With the
default setting however the searching of dynamic libraries that
follow it will stop with the dynamic library itself. No DT_NEEDED
links will be traversed to resolve symbols.
'--cref'
Output a cross reference table. If a linker map file is being
generated, the cross reference table is printed to the map file.
Otherwise, it is printed on the standard output.
The format of the table is intentionally simple, so that it may be
easily processed by a script if necessary. The symbols are printed
out, sorted by name. For each symbol, a list of file names is
given. If the symbol is defined, the first file listed is the
location of the definition. If the symbol is defined as a common
value then any files where this happens appear next. Finally any
files that reference the symbol are listed.
'--no-define-common'
This option inhibits the assignment of addresses to common symbols.
The script command 'INHIBIT_COMMON_ALLOCATION' has the same effect.
*Note Miscellaneous Commands::.
The '--no-define-common' option allows decoupling the decision to
assign addresses to Common symbols from the choice of the output
file type; otherwise a non-Relocatable output type forces assigning
addresses to Common symbols. Using '--no-define-common' allows
Common symbols that are referenced from a shared library to be
assigned addresses only in the main program. This eliminates the
unused duplicate space in the shared library, and also prevents any
possible confusion over resolving to the wrong duplicate when there
are many dynamic modules with specialized search paths for runtime
symbol resolution.
'--defsym=SYMBOL=EXPRESSION'
Create a global symbol in the output file, containing the absolute
address given by EXPRESSION. You may use this option as many times
as necessary to define multiple symbols in the command line. A
limited form of arithmetic is supported for the EXPRESSION in this
context: you may give a hexadecimal constant or the name of an
existing symbol, or use '+' and '-' to add or subtract hexadecimal
constants or symbols. If you need more elaborate expressions,
consider using the linker command language from a script (*note
Assignment: Symbol Definitions: Assignments.). _Note:_ there
should be no white space between SYMBOL, the equals sign ("<=>"),
and EXPRESSION.
'--demangle[=STYLE]'
'--no-demangle'
These options control whether to demangle symbol names in error
messages and other output. When the linker is told to demangle, it
tries to present symbol names in a readable fashion: it strips
leading underscores if they are used by the object file format, and
converts C++ mangled symbol names into user readable names.
Different compilers have different mangling styles. The optional
demangling style argument can be used to choose an appropriate
demangling style for your compiler. The linker will demangle by
default unless the environment variable 'COLLECT_NO_DEMANGLE' is
set. These options may be used to override the default.
'-IFILE'
'--dynamic-linker=FILE'
Set the name of the dynamic linker. This is only meaningful when
generating dynamically linked ELF executables. The default dynamic
linker is normally correct; don't use this unless you know what you
are doing.
'--fatal-warnings'
'--no-fatal-warnings'
Treat all warnings as errors. The default behaviour can be
restored with the option '--no-fatal-warnings'.
'--force-exe-suffix'
Make sure that an output file has a .exe suffix.
If a successfully built fully linked output file does not have a
'.exe' or '.dll' suffix, this option forces the linker to copy the
output file to one of the same name with a '.exe' suffix. This
option is useful when using unmodified Unix makefiles on a
Microsoft Windows host, since some versions of Windows won't run an
image unless it ends in a '.exe' suffix.
'--gc-sections'
'--no-gc-sections'
Enable garbage collection of unused input sections. It is ignored
on targets that do not support this option. The default behaviour
(of not performing this garbage collection) can be restored by
specifying '--no-gc-sections' on the command line.
'--gc-sections' decides which input sections are used by examining
symbols and relocations. The section containing the entry symbol
and all sections containing symbols undefined on the command-line
will be kept, as will sections containing symbols referenced by
dynamic objects. Note that when building shared libraries, the
linker must assume that any visible symbol is referenced. Once
this initial set of sections has been determined, the linker
recursively marks as used any section referenced by their
relocations. See '--entry' and '--undefined'.
This option can be set when doing a partial link (enabled with
option '-r'). In this case the root of symbols kept must be
explicitly specified either by an '--entry' or '--undefined' option
or by a 'ENTRY' command in the linker script.
'--print-gc-sections'
'--no-print-gc-sections'
List all sections removed by garbage collection. The listing is
printed on stderr. This option is only effective if garbage
collection has been enabled via the '--gc-sections') option. The
default behaviour (of not listing the sections that are removed)
can be restored by specifying '--no-print-gc-sections' on the
command line.
'--print-output-format'
Print the name of the default output format (perhaps influenced by
other command-line options). This is the string that would appear
in an 'OUTPUT_FORMAT' linker script command (*note File
Commands::).
'--help'
Print a summary of the command-line options on the standard output
and exit.
'--target-help'
Print a summary of all target specific options on the standard
output and exit.
'-Map=MAPFILE'
Print a link map to the file MAPFILE. See the description of the
'-M' option, above.
'--no-keep-memory'
'ld' normally optimizes for speed over memory usage by caching the
symbol tables of input files in memory. This option tells 'ld' to
instead optimize for memory usage, by rereading the symbol tables
as necessary. This may be required if 'ld' runs out of memory
space while linking a large executable.
'--no-undefined'
'-z defs'
Report unresolved symbol references from regular object files.
This is done even if the linker is creating a non-symbolic shared
library. The switch '--[no-]allow-shlib-undefined' controls the
behaviour for reporting unresolved references found in shared
libraries being linked in.
'--allow-multiple-definition'
'-z muldefs'
Normally when a symbol is defined multiple times, the linker will
report a fatal error. These options allow multiple definitions and
the first definition will be used.
'--allow-shlib-undefined'
'--no-allow-shlib-undefined'
Allows or disallows undefined symbols in shared libraries. This
switch is similar to '--no-undefined' except that it determines the
behaviour when the undefined symbols are in a shared library rather
than a regular object file. It does not affect how undefined
symbols in regular object files are handled.
The default behaviour is to report errors for any undefined symbols
referenced in shared libraries if the linker is being used to
create an executable, but to allow them if the linker is being used
to create a shared library.
The reasons for allowing undefined symbol references in shared
libraries specified at link time are that:
* A shared library specified at link time may not be the same as
the one that is available at load time, so the symbol might
actually be resolvable at load time.
* There are some operating systems, eg BeOS and HPPA, where
undefined symbols in shared libraries are normal.
The BeOS kernel for example patches shared libraries at load
time to select whichever function is most appropriate for the
current architecture. This is used, for example, to
dynamically select an appropriate memset function.
'--no-undefined-version'
Normally when a symbol has an undefined version, the linker will
ignore it. This option disallows symbols with undefined version
and a fatal error will be issued instead.
'--default-symver'
Create and use a default symbol version (the soname) for
unversioned exported symbols.
'--default-imported-symver'
Create and use a default symbol version (the soname) for
unversioned imported symbols.
'--no-warn-mismatch'
Normally 'ld' will give an error if you try to link together input
files that are mismatched for some reason, perhaps because they
have been compiled for different processors or for different
endiannesses. This option tells 'ld' that it should silently
permit such possible errors. This option should only be used with
care, in cases when you have taken some special action that ensures
that the linker errors are inappropriate.
'--no-warn-search-mismatch'
Normally 'ld' will give a warning if it finds an incompatible
library during a library search. This option silences the warning.
'--no-whole-archive'
Turn off the effect of the '--whole-archive' option for subsequent
archive files.
'--noinhibit-exec'
Retain the executable output file whenever it is still usable.
Normally, the linker will not produce an output file if it
encounters errors during the link process; it exits without writing
an output file when it issues any error whatsoever.
'-nostdlib'
Only search library directories explicitly specified on the command
line. Library directories specified in linker scripts (including
linker scripts specified on the command line) are ignored.
'--oformat=OUTPUT-FORMAT'
'ld' may be configured to support more than one kind of object
file. If your 'ld' is configured this way, you can use the
'--oformat' option to specify the binary format for the output
object file. Even when 'ld' is configured to support alternative
object formats, you don't usually need to specify this, as 'ld'
should be configured to produce as a default output format the most
usual format on each machine. OUTPUT-FORMAT is a text string, the
name of a particular format supported by the BFD libraries. (You
can list the available binary formats with 'objdump -i'.) The
script command 'OUTPUT_FORMAT' can also specify the output format,
but this option overrides it. *Note BFD::.
'-pie'
'--pic-executable'
Create a position independent executable. This is currently only
supported on ELF platforms. Position independent executables are
similar to shared libraries in that they are relocated by the
dynamic linker to the virtual address the OS chooses for them
(which can vary between invocations). Like normal dynamically
linked executables they can be executed and symbols defined in the
executable cannot be overridden by shared libraries.
'-qmagic'
This option is ignored for Linux compatibility.
'-Qy'
This option is ignored for SVR4 compatibility.
'--relax'
'--no-relax'
An option with machine dependent effects. This option is only
supported on a few targets. *Note 'ld' and the H8/300: H8/300.
*Note 'ld' and the Intel 960 family: i960. *Note 'ld' and Xtensa
Processors: Xtensa. *Note 'ld' and the 68HC11 and 68HC12:
M68HC11/68HC12. *Note 'ld' and PowerPC 32-bit ELF Support: PowerPC
ELF32.
On some platforms the '--relax' option performs target specific,
global optimizations that become possible when the linker resolves
addressing in the program, such as relaxing address modes,
synthesizing new instructions, selecting shorter version of current
instructions, and combining constant values.
On some platforms these link time global optimizations may make
symbolic debugging of the resulting executable impossible. This is
known to be the case for the Matsushita MN10200 and MN10300 family
of processors.
On platforms where this is not supported, '--relax' is accepted,
but ignored.
On platforms where '--relax' is accepted the option '--no-relax'
can be used to disable the feature.
'--retain-symbols-file=FILENAME'
Retain _only_ the symbols listed in the file FILENAME, discarding
all others. FILENAME is simply a flat file, with one symbol name
per line. This option is especially useful in environments (such
as VxWorks) where a large global symbol table is accumulated
gradually, to conserve run-time memory.
'--retain-symbols-file' does _not_ discard undefined symbols, or
symbols needed for relocations.
You may only specify '--retain-symbols-file' once in the command
line. It overrides '-s' and '-S'.
'-rpath=DIR'
Add a directory to the runtime library search path. This is used
when linking an ELF executable with shared objects. All '-rpath'
arguments are concatenated and passed to the runtime linker, which
uses them to locate shared objects at runtime. The '-rpath' option
is also used when locating shared objects which are needed by
shared objects explicitly included in the link; see the description
of the '-rpath-link' option. If '-rpath' is not used when linking
an ELF executable, the contents of the environment variable
'LD_RUN_PATH' will be used if it is defined.
The '-rpath' option may also be used on SunOS. By default, on
SunOS, the linker will form a runtime search patch out of all the
'-L' options it is given. If a '-rpath' option is used, the
runtime search path will be formed exclusively using the '-rpath'
options, ignoring the '-L' options. This can be useful when using
gcc, which adds many '-L' options which may be on NFS mounted file
systems.
For compatibility with other ELF linkers, if the '-R' option is
followed by a directory name, rather than a file name, it is
treated as the '-rpath' option.
'-rpath-link=DIR'
When using ELF or SunOS, one shared library may require another.
This happens when an 'ld -shared' link includes a shared library as
one of the input files.
When the linker encounters such a dependency when doing a
non-shared, non-relocatable link, it will automatically try to
locate the required shared library and include it in the link, if
it is not included explicitly. In such a case, the '-rpath-link'
option specifies the first set of directories to search. The
'-rpath-link' option may specify a sequence of directory names
either by specifying a list of names separated by colons, or by
appearing multiple times.
This option should be used with caution as it overrides the search
path that may have been hard compiled into a shared library. In
such a case it is possible to use unintentionally a different
search path than the runtime linker would do.
The linker uses the following search paths to locate required
shared libraries:
1. Any directories specified by '-rpath-link' options.
2. Any directories specified by '-rpath' options. The difference
between '-rpath' and '-rpath-link' is that directories
specified by '-rpath' options are included in the executable
and used at runtime, whereas the '-rpath-link' option is only
effective at link time. Searching '-rpath' in this way is
only supported by native linkers and cross linkers which have
been configured with the '--with-sysroot' option.
3. On an ELF system, for native linkers, if the '-rpath' and
'-rpath-link' options were not used, search the contents of
the environment variable 'LD_RUN_PATH'.
4. On SunOS, if the '-rpath' option was not used, search any
directories specified using '-L' options.
5. For a native linker, search the contents of the environment
variable 'LD_LIBRARY_PATH'.
6. For a native ELF linker, the directories in 'DT_RUNPATH' or
'DT_RPATH' of a shared library are searched for shared
libraries needed by it. The 'DT_RPATH' entries are ignored if
'DT_RUNPATH' entries exist.
7. The default directories, normally '/lib' and '/usr/lib'.
8. For a native linker on an ELF system, if the file
'/etc/ld.so.conf' exists, the list of directories found in
that file.
If the required shared library is not found, the linker will issue
a warning and continue with the link.
'-shared'
'-Bshareable'
Create a shared library. This is currently only supported on ELF,
XCOFF and SunOS platforms. On SunOS, the linker will automatically
create a shared library if the '-e' option is not used and there
are undefined symbols in the link.
'--sort-common'
'--sort-common=ascending'
'--sort-common=descending'
This option tells 'ld' to sort the common symbols by alignment in
ascending or descending order when it places them in the
appropriate output sections. The symbol alignments considered are
sixteen-byte or larger, eight-byte, four-byte, two-byte, and
one-byte. This is to prevent gaps between symbols due to alignment
constraints. If no sorting order is specified, then descending
order is assumed.
'--sort-section=name'
This option will apply 'SORT_BY_NAME' to all wildcard section
patterns in the linker script.
'--sort-section=alignment'
This option will apply 'SORT_BY_ALIGNMENT' to all wildcard section
patterns in the linker script.
'--split-by-file[=SIZE]'
Similar to '--split-by-reloc' but creates a new output section for
each input file when SIZE is reached. SIZE defaults to a size of 1
if not given.
'--split-by-reloc[=COUNT]'
Tries to creates extra sections in the output file so that no
single output section in the file contains more than COUNT
relocations. This is useful when generating huge relocatable files
for downloading into certain real time kernels with the COFF object
file format; since COFF cannot represent more than 65535
relocations in a single section. Note that this will fail to work
with object file formats which do not support arbitrary sections.
The linker will not split up individual input sections for
redistribution, so if a single input section contains more than
COUNT relocations one output section will contain that many
relocations. COUNT defaults to a value of 32768.
'--stats'
Compute and display statistics about the operation of the linker,
such as execution time and memory usage.
'--sysroot=DIRECTORY'
Use DIRECTORY as the location of the sysroot, overriding the
configure-time default. This option is only supported by linkers
that were configured using '--with-sysroot'.
'--traditional-format'
For some targets, the output of 'ld' is different in some ways from
the output of some existing linker. This switch requests 'ld' to
use the traditional format instead.
For example, on SunOS, 'ld' combines duplicate entries in the
symbol string table. This can reduce the size of an output file
with full debugging information by over 30 percent. Unfortunately,
the SunOS 'dbx' program can not read the resulting program ('gdb'
has no trouble). The '--traditional-format' switch tells 'ld' to
not combine duplicate entries.
'--section-start=SECTIONNAME=ORG'
Locate a section in the output file at the absolute address given
by ORG. You may use this option as many times as necessary to
locate multiple sections in the command line. ORG must be a single
hexadecimal integer; for compatibility with other linkers, you may
omit the leading '0x' usually associated with hexadecimal values.
_Note:_ there should be no white space between SECTIONNAME, the
equals sign ("<=>"), and ORG.
'-Tbss=ORG'
'-Tdata=ORG'
'-Ttext=ORG'
Same as '--section-start', with '.bss', '.data' or '.text' as the
SECTIONNAME.
'-Ttext-segment=ORG'
When creating an ELF executable or shared object, it will set the
address of the first byte of the text segment.
'-Trodata-segment=ORG'
When creating an ELF executable or shared object for a target where
the read-only data is in its own segment separate from the
executable text, it will set the address of the first byte of the
read-only data segment.
'-Tldata-segment=ORG'
When creating an ELF executable or shared object for x86-64 medium
memory model, it will set the address of the first byte of the
ldata segment.
'--unresolved-symbols=METHOD'
Determine how to handle unresolved symbols. There are four
possible values for 'method':
'ignore-all'
Do not report any unresolved symbols.
'report-all'
Report all unresolved symbols. This is the default.
'ignore-in-object-files'
Report unresolved symbols that are contained in shared
libraries, but ignore them if they come from regular object
files.
'ignore-in-shared-libs'
Report unresolved symbols that come from regular object files,
but ignore them if they come from shared libraries. This can
be useful when creating a dynamic binary and it is known that
all the shared libraries that it should be referencing are
included on the linker's command line.
The behaviour for shared libraries on their own can also be
controlled by the '--[no-]allow-shlib-undefined' option.
Normally the linker will generate an error message for each
reported unresolved symbol but the option
'--warn-unresolved-symbols' can change this to a warning.
'--dll-verbose'
'--verbose[=NUMBER]'
Display the version number for 'ld' and list the linker emulations
supported. Display which input files can and cannot be opened.
Display the linker script being used by the linker. If the
optional NUMBER argument > 1, plugin symbol status will also be
displayed.
'--version-script=VERSION-SCRIPTFILE'
Specify the name of a version script to the linker. This is
typically used when creating shared libraries to specify additional
information about the version hierarchy for the library being
created. This option is only fully supported on ELF platforms
which support shared libraries; see *note VERSION::. It is
partially supported on PE platforms, which can use version scripts
to filter symbol visibility in auto-export mode: any symbols marked
'local' in the version script will not be exported. *Note WIN32::.
'--warn-common'
Warn when a common symbol is combined with another common symbol or
with a symbol definition. Unix linkers allow this somewhat sloppy
practice, but linkers on some other operating systems do not. This
option allows you to find potential problems from combining global
symbols. Unfortunately, some C libraries use this practice, so you
may get some warnings about symbols in the libraries as well as in
your programs.
There are three kinds of global symbols, illustrated here by C
examples:
'int i = 1;'
A definition, which goes in the initialized data section of
the output file.
'extern int i;'
An undefined reference, which does not allocate space. There
must be either a definition or a common symbol for the
variable somewhere.
'int i;'
A common symbol. If there are only (one or more) common
symbols for a variable, it goes in the uninitialized data area
of the output file. The linker merges multiple common symbols
for the same variable into a single symbol. If they are of
different sizes, it picks the largest size. The linker turns
a common symbol into a declaration, if there is a definition
of the same variable.
The '--warn-common' option can produce five kinds of warnings.
Each warning consists of a pair of lines: the first describes the
symbol just encountered, and the second describes the previous
symbol encountered with the same name. One or both of the two
symbols will be a common symbol.
1. Turning a common symbol into a reference, because there is
already a definition for the symbol.
FILE(SECTION): warning: common of `SYMBOL'
overridden by definition
FILE(SECTION): warning: defined here
2. Turning a common symbol into a reference, because a later
definition for the symbol is encountered. This is the same as
the previous case, except that the symbols are encountered in
a different order.
FILE(SECTION): warning: definition of `SYMBOL'
overriding common
FILE(SECTION): warning: common is here
3. Merging a common symbol with a previous same-sized common
symbol.
FILE(SECTION): warning: multiple common
of `SYMBOL'
FILE(SECTION): warning: previous common is here
4. Merging a common symbol with a previous larger common symbol.
FILE(SECTION): warning: common of `SYMBOL'
overridden by larger common
FILE(SECTION): warning: larger common is here
5. Merging a common symbol with a previous smaller common symbol.
This is the same as the previous case, except that the symbols
are encountered in a different order.
FILE(SECTION): warning: common of `SYMBOL'
overriding smaller common
FILE(SECTION): warning: smaller common is here
'--warn-constructors'
Warn if any global constructors are used. This is only useful for
a few object file formats. For formats like COFF or ELF, the
linker can not detect the use of global constructors.
'--warn-multiple-gp'
Warn if multiple global pointer values are required in the output
file. This is only meaningful for certain processors, such as the
Alpha. Specifically, some processors put large-valued constants in
a special section. A special register (the global pointer) points
into the middle of this section, so that constants can be loaded
efficiently via a base-register relative addressing mode. Since
the offset in base-register relative mode is fixed and relatively
small (e.g., 16 bits), this limits the maximum size of the constant
pool. Thus, in large programs, it is often necessary to use
multiple global pointer values in order to be able to address all
possible constants. This option causes a warning to be issued
whenever this case occurs.
'--warn-once'
Only warn once for each undefined symbol, rather than once per
module which refers to it.
'--warn-section-align'
Warn if the address of an output section is changed because of
alignment. Typically, the alignment will be set by an input
section. The address will only be changed if it not explicitly
specified; that is, if the 'SECTIONS' command does not specify a
start address for the section (*note SECTIONS::).
'--warn-shared-textrel'
Warn if the linker adds a DT_TEXTREL to a shared object.
'--warn-alternate-em'
Warn if an object has alternate ELF machine code.
'--warn-unresolved-symbols'
If the linker is going to report an unresolved symbol (see the
option '--unresolved-symbols') it will normally generate an error.
This option makes it generate a warning instead.
'--error-unresolved-symbols'
This restores the linker's default behaviour of generating errors
when it is reporting unresolved symbols.
'--whole-archive'
For each archive mentioned on the command line after the
'--whole-archive' option, include every object file in the archive
in the link, rather than searching the archive for the required
object files. This is normally used to turn an archive file into a
shared library, forcing every object to be included in the
resulting shared library. This option may be used more than once.
Two notes when using this option from gcc: First, gcc doesn't know
about this option, so you have to use '-Wl,-whole-archive'.
Second, don't forget to use '-Wl,-no-whole-archive' after your list
of archives, because gcc will add its own list of archives to your
link and you may not want this flag to affect those as well.
'--wrap=SYMBOL'
Use a wrapper function for SYMBOL. Any undefined reference to
SYMBOL will be resolved to '__wrap_SYMBOL'. Any undefined
reference to '__real_SYMBOL' will be resolved to SYMBOL.
This can be used to provide a wrapper for a system function. The
wrapper function should be called '__wrap_SYMBOL'. If it wishes to
call the system function, it should call '__real_SYMBOL'.
Here is a trivial example:
void *
__wrap_malloc (size_t c)
{
printf ("malloc called with %zu\n", c);
return __real_malloc (c);
}
If you link other code with this file using '--wrap malloc', then
all calls to 'malloc' will call the function '__wrap_malloc'
instead. The call to '__real_malloc' in '__wrap_malloc' will call
the real 'malloc' function.
You may wish to provide a '__real_malloc' function as well, so that
links without the '--wrap' option will succeed. If you do this,
you should not put the definition of '__real_malloc' in the same
file as '__wrap_malloc'; if you do, the assembler may resolve the
call before the linker has a chance to wrap it to 'malloc'.
'--eh-frame-hdr'
Request creation of '.eh_frame_hdr' section and ELF
'PT_GNU_EH_FRAME' segment header.
'--no-ld-generated-unwind-info'
Request creation of '.eh_frame' unwind info for linker generated
code sections like PLT. This option is on by default if linker
generated unwind info is supported.
'--enable-new-dtags'
'--disable-new-dtags'
This linker can create the new dynamic tags in ELF. But the older
ELF systems may not understand them. If you specify
'--enable-new-dtags', the new dynamic tags will be created as
needed and older dynamic tags will be omitted. If you specify
'--disable-new-dtags', no new dynamic tags will be created. By
default, the new dynamic tags are not created. Note that those
options are only available for ELF systems.
'--hash-size=NUMBER'
Set the default size of the linker's hash tables to a prime number
close to NUMBER. Increasing this value can reduce the length of
time it takes the linker to perform its tasks, at the expense of
increasing the linker's memory requirements. Similarly reducing
this value can reduce the memory requirements at the expense of
speed.
'--hash-style=STYLE'
Set the type of linker's hash table(s). STYLE can be either 'sysv'
for classic ELF '.hash' section, 'gnu' for new style GNU
'.gnu.hash' section or 'both' for both the classic ELF '.hash' and
new style GNU '.gnu.hash' hash tables. The default is 'sysv'.
'--reduce-memory-overheads'
This option reduces memory requirements at ld runtime, at the
expense of linking speed. This was introduced to select the old
O(n^2) algorithm for link map file generation, rather than the new
O(n) algorithm which uses about 40% more memory for symbol storage.
Another effect of the switch is to set the default hash table size
to 1021, which again saves memory at the cost of lengthening the
linker's run time. This is not done however if the '--hash-size'
switch has been used.
The '--reduce-memory-overheads' switch may be also be used to
enable other tradeoffs in future versions of the linker.
'--build-id'
'--build-id=STYLE'
Request creation of '.note.gnu.build-id' ELF note section. The
contents of the note are unique bits identifying this linked file.
STYLE can be 'uuid' to use 128 random bits, 'sha1' to use a 160-bit
SHA1 hash on the normative parts of the output contents, 'md5' to
use a 128-bit MD5 hash on the normative parts of the output
contents, or '0xHEXSTRING' to use a chosen bit string specified as
an even number of hexadecimal digits ('-' and ':' characters
between digit pairs are ignored). If STYLE is omitted, 'sha1' is
used.
The 'md5' and 'sha1' styles produces an identifier that is always
the same in an identical output file, but will be unique among all
nonidentical output files. It is not intended to be compared as a
checksum for the file's contents. A linked file may be changed
later by other tools, but the build ID bit string identifying the
original linked file does not change.
Passing 'none' for STYLE disables the setting from any '--build-id'
options earlier on the command line.
2.1.1 Options Specific to i386 PE Targets
-----------------------------------------
The i386 PE linker supports the '-shared' option, which causes the
output to be a dynamically linked library (DLL) instead of a normal
executable. You should name the output '*.dll' when you use this
option. In addition, the linker fully supports the standard '*.def'
files, which may be specified on the linker command line like an object
file (in fact, it should precede archives it exports symbols from, to
ensure that they get linked in, just like a normal object file).
In addition to the options common to all targets, the i386 PE linker
support additional command line options that are specific to the i386 PE
target. Options that take values may be separated from their values by
either a space or an equals sign.
'--add-stdcall-alias'
If given, symbols with a stdcall suffix (@NN) will be exported
as-is and also with the suffix stripped. [This option is specific
to the i386 PE targeted port of the linker]
'--base-file FILE'
Use FILE as the name of a file in which to save the base addresses
of all the relocations needed for generating DLLs with 'dlltool'.
[This is an i386 PE specific option]
'--dll'
Create a DLL instead of a regular executable. You may also use
'-shared' or specify a 'LIBRARY' in a given '.def' file. [This
option is specific to the i386 PE targeted port of the linker]
'--enable-long-section-names'
'--disable-long-section-names'
The PE variants of the Coff object format add an extension that
permits the use of section names longer than eight characters, the
normal limit for Coff. By default, these names are only allowed in
object files, as fully-linked executable images do not carry the
Coff string table required to support the longer names. As a GNU
extension, it is possible to allow their use in executable images
as well, or to (probably pointlessly!) disallow it in object
files, by using these two options. Executable images generated
with these long section names are slightly non-standard, carrying
as they do a string table, and may generate confusing output when
examined with non-GNU PE-aware tools, such as file viewers and
dumpers. However, GDB relies on the use of PE long section names
to find Dwarf-2 debug information sections in an executable image
at runtime, and so if neither option is specified on the
command-line, 'ld' will enable long section names, overriding the
default and technically correct behaviour, when it finds the
presence of debug information while linking an executable image and
not stripping symbols. [This option is valid for all PE targeted
ports of the linker]
'--enable-stdcall-fixup'
'--disable-stdcall-fixup'
If the link finds a symbol that it cannot resolve, it will attempt
to do "fuzzy linking" by looking for another defined symbol that
differs only in the format of the symbol name (cdecl vs stdcall)
and will resolve that symbol by linking to the match. For example,
the undefined symbol '_foo' might be linked to the function
'_foo@12', or the undefined symbol '_bar@16' might be linked to the
function '_bar'. When the linker does this, it prints a warning,
since it normally should have failed to link, but sometimes import
libraries generated from third-party dlls may need this feature to
be usable. If you specify '--enable-stdcall-fixup', this feature
is fully enabled and warnings are not printed. If you specify
'--disable-stdcall-fixup', this feature is disabled and such
mismatches are considered to be errors. [This option is specific
to the i386 PE targeted port of the linker]
'--leading-underscore'
'--no-leading-underscore'
For most targets default symbol-prefix is an underscore and is
defined in target's description. By this option it is possible to
disable/enable the default underscore symbol-prefix.
'--export-all-symbols'
If given, all global symbols in the objects used to build a DLL
will be exported by the DLL. Note that this is the default if there
otherwise wouldn't be any exported symbols. When symbols are
explicitly exported via DEF files or implicitly exported via
function attributes, the default is to not export anything else
unless this option is given. Note that the symbols 'DllMain@12',
'DllEntryPoint@0', 'DllMainCRTStartup@12', and 'impure_ptr' will
not be automatically exported. Also, symbols imported from other
DLLs will not be re-exported, nor will symbols specifying the DLL's
internal layout such as those beginning with '_head_' or ending
with '_iname'. In addition, no symbols from 'libgcc', 'libstd++',
'libmingw32', or 'crtX.o' will be exported. Symbols whose names
begin with '__rtti_' or '__builtin_' will not be exported, to help
with C++ DLLs. Finally, there is an extensive list of
cygwin-private symbols that are not exported (obviously, this
applies on when building DLLs for cygwin targets). These
cygwin-excludes are: '_cygwin_dll_entry@12',
'_cygwin_crt0_common@8', '_cygwin_noncygwin_dll_entry@12',
'_fmode', '_impure_ptr', 'cygwin_attach_dll', 'cygwin_premain0',
'cygwin_premain1', 'cygwin_premain2', 'cygwin_premain3', and
'environ'. [This option is specific to the i386 PE targeted port
of the linker]
'--exclude-symbols SYMBOL,SYMBOL,...'
Specifies a list of symbols which should not be automatically
exported. The symbol names may be delimited by commas or colons.
[This option is specific to the i386 PE targeted port of the
linker]
'--exclude-all-symbols'
Specifies no symbols should be automatically exported. [This
option is specific to the i386 PE targeted port of the linker]
'--file-alignment'
Specify the file alignment. Sections in the file will always begin
at file offsets which are multiples of this number. This defaults
to 512. [This option is specific to the i386 PE targeted port of
the linker]
'--heap RESERVE'
'--heap RESERVE,COMMIT'
Specify the number of bytes of memory to reserve (and optionally
commit) to be used as heap for this program. The default is 1MB
reserved, 4K committed. [This option is specific to the i386 PE
targeted port of the linker]
'--image-base VALUE'
Use VALUE as the base address of your program or dll. This is the
lowest memory location that will be used when your program or dll
is loaded. To reduce the need to relocate and improve performance
of your dlls, each should have a unique base address and not
overlap any other dlls. The default is 0x400000 for executables,
and 0x10000000 for dlls. [This option is specific to the i386 PE
targeted port of the linker]
'--kill-at'
If given, the stdcall suffixes (@NN) will be stripped from symbols
before they are exported. [This option is specific to the i386 PE
targeted port of the linker]
'--large-address-aware'
If given, the appropriate bit in the "Characteristics" field of the
COFF header is set to indicate that this executable supports
virtual addresses greater than 2 gigabytes. This should be used in
conjunction with the /3GB or /USERVA=VALUE megabytes switch in the
"[operating systems]" section of the BOOT.INI. Otherwise, this bit
has no effect. [This option is specific to PE targeted ports of
the linker]
'--disable-large-address-aware'
Reverts the effect of a previous '--large-address-aware' option.
This is useful if '--large-address-aware' is always set by the
compiler driver (e.g. Cygwin gcc) and the executable does not
support virtual addresses greater than 2 gigabytes. [This option
is specific to PE targeted ports of the linker]
'--major-image-version VALUE'
Sets the major number of the "image version". Defaults to 1.
[This option is specific to the i386 PE targeted port of the
linker]
'--major-os-version VALUE'
Sets the major number of the "os version". Defaults to 4. [This
option is specific to the i386 PE targeted port of the linker]
'--major-subsystem-version VALUE'
Sets the major number of the "subsystem version". Defaults to 4.
[This option is specific to the i386 PE targeted port of the
linker]
'--minor-image-version VALUE'
Sets the minor number of the "image version". Defaults to 0.
[This option is specific to the i386 PE targeted port of the
linker]
'--minor-os-version VALUE'
Sets the minor number of the "os version". Defaults to 0. [This
option is specific to the i386 PE targeted port of the linker]
'--minor-subsystem-version VALUE'
Sets the minor number of the "subsystem version". Defaults to 0.
[This option is specific to the i386 PE targeted port of the
linker]
'--output-def FILE'
The linker will create the file FILE which will contain a DEF file
corresponding to the DLL the linker is generating. This DEF file
(which should be called '*.def') may be used to create an import
library with 'dlltool' or may be used as a reference to
automatically or implicitly exported symbols. [This option is
specific to the i386 PE targeted port of the linker]
'--out-implib FILE'
The linker will create the file FILE which will contain an import
lib corresponding to the DLL the linker is generating. This import
lib (which should be called '*.dll.a' or '*.a' may be used to link
clients against the generated DLL; this behaviour makes it possible
to skip a separate 'dlltool' import library creation step. [This
option is specific to the i386 PE targeted port of the linker]
'--enable-auto-image-base'
Automatically choose the image base for DLLs, unless one is
specified using the '--image-base' argument. By using a hash
generated from the dllname to create unique image bases for each
DLL, in-memory collisions and relocations which can delay program
execution are avoided. [This option is specific to the i386 PE
targeted port of the linker]
'--disable-auto-image-base'
Do not automatically generate a unique image base. If there is no
user-specified image base ('--image-base') then use the platform
default. [This option is specific to the i386 PE targeted port of
the linker]
'--dll-search-prefix STRING'
When linking dynamically to a dll without an import library, search
for '<string><basename>.dll' in preference to 'lib<basename>.dll'.
This behaviour allows easy distinction between DLLs built for the
various "subplatforms": native, cygwin, uwin, pw, etc. For
instance, cygwin DLLs typically use '--dll-search-prefix=cyg'.
[This option is specific to the i386 PE targeted port of the
linker]
'--enable-auto-import'
Do sophisticated linking of '_symbol' to '__imp__symbol' for DATA
imports from DLLs, and create the necessary thunking symbols when
building the import libraries with those DATA exports. Note: Use
of the 'auto-import' extension will cause the text section of the
image file to be made writable. This does not conform to the
PE-COFF format specification published by Microsoft.
Note - use of the 'auto-import' extension will also cause read only
data which would normally be placed into the .rdata section to be
placed into the .data section instead. This is in order to work
around a problem with consts that is described here:
http://www.cygwin.com/ml/cygwin/2004-09/msg01101.html
Using 'auto-import' generally will 'just work' - but sometimes you
may see this message:
"variable '<var>' can't be auto-imported. Please read the
documentation for ld's '--enable-auto-import' for details."
This message occurs when some (sub)expression accesses an address
ultimately given by the sum of two constants (Win32 import tables
only allow one). Instances where this may occur include accesses
to member fields of struct variables imported from a DLL, as well
as using a constant index into an array variable imported from a
DLL. Any multiword variable (arrays, structs, long long, etc) may
trigger this error condition. However, regardless of the exact
data type of the offending exported variable, ld will always detect
it, issue the warning, and exit.
There are several ways to address this difficulty, regardless of
the data type of the exported variable:
One way is to use -enable-runtime-pseudo-reloc switch. This leaves
the task of adjusting references in your client code for runtime
environment, so this method works only when runtime environment
supports this feature.
A second solution is to force one of the 'constants' to be a
variable - that is, unknown and un-optimizable at compile time.
For arrays, there are two possibilities: a) make the indexee (the
array's address) a variable, or b) make the 'constant' index a
variable. Thus:
extern type extern_array[];
extern_array[1] -->
{ volatile type *t=extern_array; t[1] }
or
extern type extern_array[];
extern_array[1] -->
{ volatile int t=1; extern_array[t] }
For structs (and most other multiword data types) the only option
is to make the struct itself (or the long long, or the ...)
variable:
extern struct s extern_struct;
extern_struct.field -->
{ volatile struct s *t=&extern_struct; t->field }
or
extern long long extern_ll;
extern_ll -->
{ volatile long long * local_ll=&extern_ll; *local_ll }
A third method of dealing with this difficulty is to abandon
'auto-import' for the offending symbol and mark it with
'__declspec(dllimport)'. However, in practice that requires using
compile-time #defines to indicate whether you are building a DLL,
building client code that will link to the DLL, or merely
building/linking to a static library. In making the choice between
the various methods of resolving the 'direct address with constant
offset' problem, you should consider typical real-world usage:
Original:
--foo.h
extern int arr[];
--foo.c
#include "foo.h"
void main(int argc, char **argv){
printf("%d\n",arr[1]);
}
Solution 1:
--foo.h
extern int arr[];
--foo.c
#include "foo.h"
void main(int argc, char **argv){
/* This workaround is for win32 and cygwin; do not "optimize" */
volatile int *parr = arr;
printf("%d\n",parr[1]);
}
Solution 2:
--foo.h
/* Note: auto-export is assumed (no __declspec(dllexport)) */
#if (defined(_WIN32) || defined(__CYGWIN__)) && \
!(defined(FOO_BUILD_DLL) || defined(FOO_STATIC))
#define FOO_IMPORT __declspec(dllimport)
#else
#define FOO_IMPORT
#endif
extern FOO_IMPORT int arr[];
--foo.c
#include "foo.h"
void main(int argc, char **argv){
printf("%d\n",arr[1]);
}
A fourth way to avoid this problem is to re-code your library to
use a functional interface rather than a data interface for the
offending variables (e.g. set_foo() and get_foo() accessor
functions). [This option is specific to the i386 PE targeted port
of the linker]
'--disable-auto-import'
Do not attempt to do sophisticated linking of '_symbol' to
'__imp__symbol' for DATA imports from DLLs. [This option is
specific to the i386 PE targeted port of the linker]
'--enable-runtime-pseudo-reloc'
If your code contains expressions described in -enable-auto-import
section, that is, DATA imports from DLL with non-zero offset, this
switch will create a vector of 'runtime pseudo relocations' which
can be used by runtime environment to adjust references to such
data in your client code. [This option is specific to the i386 PE
targeted port of the linker]
'--disable-runtime-pseudo-reloc'
Do not create pseudo relocations for non-zero offset DATA imports
from DLLs. [This option is specific to the i386 PE targeted port
of the linker]
'--enable-extra-pe-debug'
Show additional debug info related to auto-import symbol thunking.
[This option is specific to the i386 PE targeted port of the
linker]
'--section-alignment'
Sets the section alignment. Sections in memory will always begin
at addresses which are a multiple of this number. Defaults to
0x1000. [This option is specific to the i386 PE targeted port of
the linker]
'--stack RESERVE'
'--stack RESERVE,COMMIT'
Specify the number of bytes of memory to reserve (and optionally
commit) to be used as stack for this program. The default is 2MB
reserved, 4K committed. [This option is specific to the i386 PE
targeted port of the linker]
'--subsystem WHICH'
'--subsystem WHICH:MAJOR'
'--subsystem WHICH:MAJOR.MINOR'
Specifies the subsystem under which your program will execute. The
legal values for WHICH are 'native', 'windows', 'console', 'posix',
and 'xbox'. You may optionally set the subsystem version also.
Numeric values are also accepted for WHICH. [This option is
specific to the i386 PE targeted port of the linker]
The following options set flags in the 'DllCharacteristics' field
of the PE file header: [These options are specific to PE targeted
ports of the linker]
'--dynamicbase'
The image base address may be relocated using address space layout
randomization (ASLR). This feature was introduced with MS Windows
Vista for i386 PE targets.
'--forceinteg'
Code integrity checks are enforced.
'--nxcompat'
The image is compatible with the Data Execution Prevention. This
feature was introduced with MS Windows XP SP2 for i386 PE targets.
'--no-isolation'
Although the image understands isolation, do not isolate the image.
'--no-seh'
The image does not use SEH. No SE handler may be called from this
image.
'--no-bind'
Do not bind this image.
'--wdmdriver'
The driver uses the MS Windows Driver Model.
'--tsaware'
The image is Terminal Server aware.
'--insert-timestamp'
Insert a real timestamp into the image, rather than the default
value of zero. This will result in a slightly different results
with each invocation, which could be helpful for distributing
unique images.
2.1.2 Options specific to C6X uClinux targets
---------------------------------------------
The C6X uClinux target uses a binary format called DSBT to support
shared libraries. Each shared library in the system needs to have a
unique index; all executables use an index of 0.
'--dsbt-size SIZE'
This option sets the number of entires in the DSBT of the current
executable or shared library to SIZE. The default is to create a
table with 64 entries.
'--dsbt-index INDEX'
This option sets the DSBT index of the current executable or shared
library to INDEX. The default is 0, which is appropriate for
generating executables. If a shared library is generated with a
DSBT index of 0, the 'R_C6000_DSBT_INDEX' relocs are copied into
the output file.
The '--no-merge-exidx-entries' switch disables the merging of
adjacent exidx entries in frame unwind info.
2.1.3 Options specific to Motorola 68HC11 and 68HC12 targets
------------------------------------------------------------
The 68HC11 and 68HC12 linkers support specific options to control the
memory bank switching mapping and trampoline code generation.
'--no-trampoline'
This option disables the generation of trampoline. By default a
trampoline is generated for each far function which is called using
a 'jsr' instruction (this happens when a pointer to a far function
is taken).
'--bank-window NAME'
This option indicates to the linker the name of the memory region
in the 'MEMORY' specification that describes the memory bank
window. The definition of such region is then used by the linker
to compute paging and addresses within the memory window.
2.1.4 Options specific to Motorola 68K target
---------------------------------------------
The following options are supported to control handling of GOT
generation when linking for 68K targets.
'--got=TYPE'
This option tells the linker which GOT generation scheme to use.
TYPE should be one of 'single', 'negative', 'multigot' or 'target'.
For more information refer to the Info entry for 'ld'.
2.1.5 Options specific to MIPS targets
--------------------------------------
The following options are supported to control microMIPS instruction
generation when linking for MIPS targets.
'--insn32'
'--no-insn32'
These options control the choice of microMIPS instructions used in
code generated by the linker, such as that in the PLT or lazy
binding stubs, or in relaxation. If '--insn32' is used, then the
linker only uses 32-bit instruction encodings. By default or if
'--no-insn32' is used, all instruction encodings are used,
including 16-bit ones where possible.

File: ld.info, Node: Environment, Prev: Options, Up: Invocation
2.2 Environment Variables
=========================
You can change the behaviour of 'ld' with the environment variables
'GNUTARGET', 'LDEMULATION' and 'COLLECT_NO_DEMANGLE'.
'GNUTARGET' determines the input-file object format if you don't use
'-b' (or its synonym '--format'). Its value should be one of the BFD
names for an input format (*note BFD::). If there is no 'GNUTARGET' in
the environment, 'ld' uses the natural format of the target. If
'GNUTARGET' is set to 'default' then BFD attempts to discover the input
format by examining binary input files; this method often succeeds, but
there are potential ambiguities, since there is no method of ensuring
that the magic number used to specify object-file formats is unique.
However, the configuration procedure for BFD on each system places the
conventional format for that system first in the search-list, so
ambiguities are resolved in favor of convention.
'LDEMULATION' determines the default emulation if you don't use the
'-m' option. The emulation can affect various aspects of linker
behaviour, particularly the default linker script. You can list the
available emulations with the '--verbose' or '-V' options. If the '-m'
option is not used, and the 'LDEMULATION' environment variable is not
defined, the default emulation depends upon how the linker was
configured.
Normally, the linker will default to demangling symbols. However, if
'COLLECT_NO_DEMANGLE' is set in the environment, then it will default to
not demangling symbols. This environment variable is used in a similar
fashion by the 'gcc' linker wrapper program. The default may be
overridden by the '--demangle' and '--no-demangle' options.

File: ld.info, Node: Scripts, Next: Machine Dependent, Prev: Invocation, Up: Top
3 Linker Scripts
****************
Every link is controlled by a "linker script". This script is written
in the linker command language.
The main purpose of the linker script is to describe how the sections
in the input files should be mapped into the output file, and to control
the memory layout of the output file. Most linker scripts do nothing
more than this. However, when necessary, the linker script can also
direct the linker to perform many other operations, using the commands
described below.
The linker always uses a linker script. If you do not supply one
yourself, the linker will use a default script that is compiled into the
linker executable. You can use the '--verbose' command line option to
display the default linker script. Certain command line options, such
as '-r' or '-N', will affect the default linker script.
You may supply your own linker script by using the '-T' command line
option. When you do this, your linker script will replace the default
linker script.
You may also use linker scripts implicitly by naming them as input
files to the linker, as though they were files to be linked. *Note
Implicit Linker Scripts::.
* Menu:
* Basic Script Concepts:: Basic Linker Script Concepts
* Script Format:: Linker Script Format
* Simple Example:: Simple Linker Script Example
* Simple Commands:: Simple Linker Script Commands
* Assignments:: Assigning Values to Symbols
* SECTIONS:: SECTIONS Command
* MEMORY:: MEMORY Command
* PHDRS:: PHDRS Command
* VERSION:: VERSION Command
* Expressions:: Expressions in Linker Scripts
* Implicit Linker Scripts:: Implicit Linker Scripts

File: ld.info, Node: Basic Script Concepts, Next: Script Format, Up: Scripts
3.1 Basic Linker Script Concepts
================================
We need to define some basic concepts and vocabulary in order to
describe the linker script language.
The linker combines input files into a single output file. The
output file and each input file are in a special data format known as an
"object file format". Each file is called an "object file". The output
file is often called an "executable", but for our purposes we will also
call it an object file. Each object file has, among other things, a
list of "sections". We sometimes refer to a section in an input file as
an "input section"; similarly, a section in the output file is an
"output section".
Each section in an object file has a name and a size. Most sections
also have an associated block of data, known as the "section contents".
A section may be marked as "loadable", which means that the contents
should be loaded into memory when the output file is run. A section
with no contents may be "allocatable", which means that an area in
memory should be set aside, but nothing in particular should be loaded
there (in some cases this memory must be zeroed out). A section which
is neither loadable nor allocatable typically contains some sort of
debugging information.
Every loadable or allocatable output section has two addresses. The
first is the "VMA", or virtual memory address. This is the address the
section will have when the output file is run. The second is the "LMA",
or load memory address. This is the address at which the section will
be loaded. In most cases the two addresses will be the same. An
example of when they might be different is when a data section is loaded
into ROM, and then copied into RAM when the program starts up (this
technique is often used to initialize global variables in a ROM based
system). In this case the ROM address would be the LMA, and the RAM
address would be the VMA.
You can see the sections in an object file by using the 'objdump'
program with the '-h' option.
Every object file also has a list of "symbols", known as the "symbol
table". A symbol may be defined or undefined. Each symbol has a name,
and each defined symbol has an address, among other information. If you
compile a C or C++ program into an object file, you will get a defined
symbol for every defined function and global or static variable. Every
undefined function or global variable which is referenced in the input
file will become an undefined symbol.
You can see the symbols in an object file by using the 'nm' program,
or by using the 'objdump' program with the '-t' option.

File: ld.info, Node: Script Format, Next: Simple Example, Prev: Basic Script Concepts, Up: Scripts
3.2 Linker Script Format
========================
Linker scripts are text files.
You write a linker script as a series of commands. Each command is
either a keyword, possibly followed by arguments, or an assignment to a
symbol. You may separate commands using semicolons. Whitespace is
generally ignored.
Strings such as file or format names can normally be entered
directly. If the file name contains a character such as a comma which
would otherwise serve to separate file names, you may put the file name
in double quotes. There is no way to use a double quote character in a
file name.
You may include comments in linker scripts just as in C, delimited by
'/*' and '*/'. As in C, comments are syntactically equivalent to
whitespace.

File: ld.info, Node: Simple Example, Next: Simple Commands, Prev: Script Format, Up: Scripts
3.3 Simple Linker Script Example
================================
Many linker scripts are fairly simple.
The simplest possible linker script has just one command: 'SECTIONS'.
You use the 'SECTIONS' command to describe the memory layout of the
output file.
The 'SECTIONS' command is a powerful command. Here we will describe
a simple use of it. Let's assume your program consists only of code,
initialized data, and uninitialized data. These will be in the '.text',
'.data', and '.bss' sections, respectively. Let's assume further that
these are the only sections which appear in your input files.
For this example, let's say that the code should be loaded at address
0x10000, and that the data should start at address 0x8000000. Here is a
linker script which will do that:
SECTIONS
{
. = 0x10000;
.text : { *(.text) }
. = 0x8000000;
.data : { *(.data) }
.bss : { *(.bss) }
}
You write the 'SECTIONS' command as the keyword 'SECTIONS', followed
by a series of symbol assignments and output section descriptions
enclosed in curly braces.
The first line inside the 'SECTIONS' command of the above example
sets the value of the special symbol '.', which is the location counter.
If you do not specify the address of an output section in some other way
(other ways are described later), the address is set from the current
value of the location counter. The location counter is then incremented
by the size of the output section. At the start of the 'SECTIONS'
command, the location counter has the value '0'.
The second line defines an output section, '.text'. The colon is
required syntax which may be ignored for now. Within the curly braces
after the output section name, you list the names of the input sections
which should be placed into this output section. The '*' is a wildcard
which matches any file name. The expression '*(.text)' means all
'.text' input sections in all input files.
Since the location counter is '0x10000' when the output section
'.text' is defined, the linker will set the address of the '.text'
section in the output file to be '0x10000'.
The remaining lines define the '.data' and '.bss' sections in the
output file. The linker will place the '.data' output section at
address '0x8000000'. After the linker places the '.data' output
section, the value of the location counter will be '0x8000000' plus the
size of the '.data' output section. The effect is that the linker will
place the '.bss' output section immediately after the '.data' output
section in memory.
The linker will ensure that each output section has the required
alignment, by increasing the location counter if necessary. In this
example, the specified addresses for the '.text' and '.data' sections
will probably satisfy any alignment constraints, but the linker may have
to create a small gap between the '.data' and '.bss' sections.
That's it! That's a simple and complete linker script.

File: ld.info, Node: Simple Commands, Next: Assignments, Prev: Simple Example, Up: Scripts
3.4 Simple Linker Script Commands
=================================
In this section we describe the simple linker script commands.
* Menu:
* Entry Point:: Setting the entry point
* File Commands:: Commands dealing with files
* Format Commands:: Commands dealing with object file formats
* REGION_ALIAS:: Assign alias names to memory regions
* Miscellaneous Commands:: Other linker script commands

File: ld.info, Node: Entry Point, Next: File Commands, Up: Simple Commands
3.4.1 Setting the Entry Point
-----------------------------
The first instruction to execute in a program is called the "entry
point". You can use the 'ENTRY' linker script command to set the entry
point. The argument is a symbol name:
ENTRY(SYMBOL)
There are several ways to set the entry point. The linker will set
the entry point by trying each of the following methods in order, and
stopping when one of them succeeds:
* the '-e' ENTRY command-line option;
* the 'ENTRY(SYMBOL)' command in a linker script;
* the value of a target specific symbol, if it is defined; For many
targets this is 'start', but PE and BeOS based systems for example
check a list of possible entry symbols, matching the first one
found.
* the address of the first byte of the '.text' section, if present;
* The address '0'.

File: ld.info, Node: File Commands, Next: Format Commands, Prev: Entry Point, Up: Simple Commands
3.4.2 Commands Dealing with Files
---------------------------------
Several linker script commands deal with files.
'INCLUDE FILENAME'
Include the linker script FILENAME at this point. The file will be
searched for in the current directory, and in any directory
specified with the '-L' option. You can nest calls to 'INCLUDE' up
to 10 levels deep.
You can place 'INCLUDE' directives at the top level, in 'MEMORY' or
'SECTIONS' commands, or in output section descriptions.
'INPUT(FILE, FILE, ...)'
'INPUT(FILE FILE ...)'
The 'INPUT' command directs the linker to include the named files
in the link, as though they were named on the command line.
For example, if you always want to include 'subr.o' any time you do
a link, but you can't be bothered to put it on every link command
line, then you can put 'INPUT (subr.o)' in your linker script.
In fact, if you like, you can list all of your input files in the
linker script, and then invoke the linker with nothing but a '-T'
option.
In case a "sysroot prefix" is configured, and the filename starts
with the '/' character, and the script being processed was located
inside the "sysroot prefix", the filename will be looked for in the
"sysroot prefix". Otherwise, the linker will try to open the file
in the current directory. If it is not found, the linker will
search through the archive library search path. See the
description of '-L' in *note Command Line Options: Options.
If you use 'INPUT (-lFILE)', 'ld' will transform the name to
'libFILE.a', as with the command line argument '-l'.
When you use the 'INPUT' command in an implicit linker script, the
files will be included in the link at the point at which the linker
script file is included. This can affect archive searching.
'GROUP(FILE, FILE, ...)'
'GROUP(FILE FILE ...)'
The 'GROUP' command is like 'INPUT', except that the named files
should all be archives, and they are searched repeatedly until no
new undefined references are created. See the description of '-('
in *note Command Line Options: Options.
'AS_NEEDED(FILE, FILE, ...)'
'AS_NEEDED(FILE FILE ...)'
This construct can appear only inside of the 'INPUT' or 'GROUP'
commands, among other filenames. The files listed will be handled
as if they appear directly in the 'INPUT' or 'GROUP' commands, with
the exception of ELF shared libraries, that will be added only when
they are actually needed. This construct essentially enables
'--as-needed' option for all the files listed inside of it and
restores previous '--as-needed' resp. '--no-as-needed' setting
afterwards.
'OUTPUT(FILENAME)'
The 'OUTPUT' command names the output file. Using
'OUTPUT(FILENAME)' in the linker script is exactly like using '-o
FILENAME' on the command line (*note Command Line Options:
Options.). If both are used, the command line option takes
precedence.
You can use the 'OUTPUT' command to define a default name for the
output file other than the usual default of 'a.out'.
'SEARCH_DIR(PATH)'
The 'SEARCH_DIR' command adds PATH to the list of paths where 'ld'
looks for archive libraries. Using 'SEARCH_DIR(PATH)' is exactly
like using '-L PATH' on the command line (*note Command Line
Options: Options.). If both are used, then the linker will search
both paths. Paths specified using the command line option are
searched first.
'STARTUP(FILENAME)'
The 'STARTUP' command is just like the 'INPUT' command, except that
FILENAME will become the first input file to be linked, as though
it were specified first on the command line. This may be useful
when using a system in which the entry point is always the start of
the first file.

File: ld.info, Node: Format Commands, Next: REGION_ALIAS, Prev: File Commands, Up: Simple Commands
3.4.3 Commands Dealing with Object File Formats
-----------------------------------------------
A couple of linker script commands deal with object file formats.
'OUTPUT_FORMAT(BFDNAME)'
'OUTPUT_FORMAT(DEFAULT, BIG, LITTLE)'
The 'OUTPUT_FORMAT' command names the BFD format to use for the
output file (*note BFD::). Using 'OUTPUT_FORMAT(BFDNAME)' is
exactly like using '--oformat BFDNAME' on the command line (*note
Command Line Options: Options.). If both are used, the command
line option takes precedence.
You can use 'OUTPUT_FORMAT' with three arguments to use different
formats based on the '-EB' and '-EL' command line options. This
permits the linker script to set the output format based on the
desired endianness.
If neither '-EB' nor '-EL' are used, then the output format will be
the first argument, DEFAULT. If '-EB' is used, the output format
will be the second argument, BIG. If '-EL' is used, the output
format will be the third argument, LITTLE.
For example, the default linker script for the MIPS ELF target uses
this command:
OUTPUT_FORMAT(elf32-bigmips, elf32-bigmips, elf32-littlemips)
This says that the default format for the output file is
'elf32-bigmips', but if the user uses the '-EL' command line
option, the output file will be created in the 'elf32-littlemips'
format.
'TARGET(BFDNAME)'
The 'TARGET' command names the BFD format to use when reading input
files. It affects subsequent 'INPUT' and 'GROUP' commands. This
command is like using '-b BFDNAME' on the command line (*note
Command Line Options: Options.). If the 'TARGET' command is used
but 'OUTPUT_FORMAT' is not, then the last 'TARGET' command is also
used to set the format for the output file. *Note BFD::.

File: ld.info, Node: REGION_ALIAS, Next: Miscellaneous Commands, Prev: Format Commands, Up: Simple Commands
3.4.4 Assign alias names to memory regions
------------------------------------------
Alias names can be added to existing memory regions created with the
*note MEMORY:: command. Each name corresponds to at most one memory
region.
REGION_ALIAS(ALIAS, REGION)
The 'REGION_ALIAS' function creates an alias name ALIAS for the
memory region REGION. This allows a flexible mapping of output sections
to memory regions. An example follows.
Suppose we have an application for embedded systems which come with
various memory storage devices. All have a general purpose, volatile
memory 'RAM' that allows code execution or data storage. Some may have
a read-only, non-volatile memory 'ROM' that allows code execution and
read-only data access. The last variant is a read-only, non-volatile
memory 'ROM2' with read-only data access and no code execution
capability. We have four output sections:
* '.text' program code;
* '.rodata' read-only data;
* '.data' read-write initialized data;
* '.bss' read-write zero initialized data.
The goal is to provide a linker command file that contains a system
independent part defining the output sections and a system dependent
part mapping the output sections to the memory regions available on the
system. Our embedded systems come with three different memory setups
'A', 'B' and 'C':
Section Variant A Variant B Variant C
.text RAM ROM ROM
.rodata RAM ROM ROM2
.data RAM RAM/ROM RAM/ROM2
.bss RAM RAM RAM
The notation 'RAM/ROM' or 'RAM/ROM2' means that this section is
loaded into region 'ROM' or 'ROM2' respectively. Please note that the
load address of the '.data' section starts in all three variants at the
end of the '.rodata' section.
The base linker script that deals with the output sections follows.
It includes the system dependent 'linkcmds.memory' file that describes
the memory layout:
INCLUDE linkcmds.memory
SECTIONS
{
.text :
{
*(.text)
} > REGION_TEXT
.rodata :
{
*(.rodata)
rodata_end = .;
} > REGION_RODATA
.data : AT (rodata_end)
{
data_start = .;
*(.data)
} > REGION_DATA
data_size = SIZEOF(.data);
data_load_start = LOADADDR(.data);
.bss :
{
*(.bss)
} > REGION_BSS
}
Now we need three different 'linkcmds.memory' files to define memory
regions and alias names. The content of 'linkcmds.memory' for the three
variants 'A', 'B' and 'C':
'A'
Here everything goes into the 'RAM'.
MEMORY
{
RAM : ORIGIN = 0, LENGTH = 4M
}
REGION_ALIAS("REGION_TEXT", RAM);
REGION_ALIAS("REGION_RODATA", RAM);
REGION_ALIAS("REGION_DATA", RAM);
REGION_ALIAS("REGION_BSS", RAM);
'B'
Program code and read-only data go into the 'ROM'. Read-write data
goes into the 'RAM'. An image of the initialized data is loaded
into the 'ROM' and will be copied during system start into the
'RAM'.
MEMORY
{
ROM : ORIGIN = 0, LENGTH = 3M
RAM : ORIGIN = 0x10000000, LENGTH = 1M
}
REGION_ALIAS("REGION_TEXT", ROM);
REGION_ALIAS("REGION_RODATA", ROM);
REGION_ALIAS("REGION_DATA", RAM);
REGION_ALIAS("REGION_BSS", RAM);
'C'
Program code goes into the 'ROM'. Read-only data goes into the
'ROM2'. Read-write data goes into the 'RAM'. An image of the
initialized data is loaded into the 'ROM2' and will be copied
during system start into the 'RAM'.
MEMORY
{
ROM : ORIGIN = 0, LENGTH = 2M
ROM2 : ORIGIN = 0x10000000, LENGTH = 1M
RAM : ORIGIN = 0x20000000, LENGTH = 1M
}
REGION_ALIAS("REGION_TEXT", ROM);
REGION_ALIAS("REGION_RODATA", ROM2);
REGION_ALIAS("REGION_DATA", RAM);
REGION_ALIAS("REGION_BSS", RAM);
It is possible to write a common system initialization routine to
copy the '.data' section from 'ROM' or 'ROM2' into the 'RAM' if
necessary:
#include <string.h>
extern char data_start [];
extern char data_size [];
extern char data_load_start [];
void copy_data(void)
{
if (data_start != data_load_start)
{
memcpy(data_start, data_load_start, (size_t) data_size);
}
}

File: ld.info, Node: Miscellaneous Commands, Prev: REGION_ALIAS, Up: Simple Commands
3.4.5 Other Linker Script Commands
----------------------------------
There are a few other linker scripts commands.
'ASSERT(EXP, MESSAGE)'
Ensure that EXP is non-zero. If it is zero, then exit the linker
with an error code, and print MESSAGE.
'EXTERN(SYMBOL SYMBOL ...)'
Force SYMBOL to be entered in the output file as an undefined
symbol. Doing this may, for example, trigger linking of additional
modules from standard libraries. You may list several SYMBOLs for
each 'EXTERN', and you may use 'EXTERN' multiple times. This
command has the same effect as the '-u' command-line option.
'FORCE_COMMON_ALLOCATION'
This command has the same effect as the '-d' command-line option:
to make 'ld' assign space to common symbols even if a relocatable
output file is specified ('-r').
'INHIBIT_COMMON_ALLOCATION'
This command has the same effect as the '--no-define-common'
command-line option: to make 'ld' omit the assignment of addresses
to common symbols even for a non-relocatable output file.
'INSERT [ AFTER | BEFORE ] OUTPUT_SECTION'
This command is typically used in a script specified by '-T' to
augment the default 'SECTIONS' with, for example, overlays. It
inserts all prior linker script statements after (or before)
OUTPUT_SECTION, and also causes '-T' to not override the default
linker script. The exact insertion point is as for orphan
sections. *Note Location Counter::. The insertion happens after
the linker has mapped input sections to output sections. Prior to
the insertion, since '-T' scripts are parsed before the default
linker script, statements in the '-T' script occur before the
default linker script statements in the internal linker
representation of the script. In particular, input section
assignments will be made to '-T' output sections before those in
the default script. Here is an example of how a '-T' script using
'INSERT' might look:
SECTIONS
{
OVERLAY :
{
.ov1 { ov1*(.text) }
.ov2 { ov2*(.text) }
}
}
INSERT AFTER .text;
'NOCROSSREFS(SECTION SECTION ...)'
This command may be used to tell 'ld' to issue an error about any
references among certain output sections.
In certain types of programs, particularly on embedded systems when
using overlays, when one section is loaded into memory, another
section will not be. Any direct references between the two
sections would be errors. For example, it would be an error if
code in one section called a function defined in the other section.
The 'NOCROSSREFS' command takes a list of output section names. If
'ld' detects any cross references between the sections, it reports
an error and returns a non-zero exit status. Note that the
'NOCROSSREFS' command uses output section names, not input section
names.
'OUTPUT_ARCH(BFDARCH)'
Specify a particular output machine architecture. The argument is
one of the names used by the BFD library (*note BFD::). You can
see the architecture of an object file by using the 'objdump'
program with the '-f' option.
'LD_FEATURE(STRING)'
This command may be used to modify 'ld' behavior. If STRING is
'"SANE_EXPR"' then absolute symbols and numbers in a script are
simply treated as numbers everywhere. *Note Expression Section::.

File: ld.info, Node: Assignments, Next: SECTIONS, Prev: Simple Commands, Up: Scripts
3.5 Assigning Values to Symbols
===============================
You may assign a value to a symbol in a linker script. This will define
the symbol and place it into the symbol table with a global scope.
* Menu:
* Simple Assignments:: Simple Assignments
* HIDDEN:: HIDDEN
* PROVIDE:: PROVIDE
* PROVIDE_HIDDEN:: PROVIDE_HIDDEN
* Source Code Reference:: How to use a linker script defined symbol in source code

File: ld.info, Node: Simple Assignments, Next: HIDDEN, Up: Assignments
3.5.1 Simple Assignments
------------------------
You may assign to a symbol using any of the C assignment operators:
'SYMBOL = EXPRESSION ;'
'SYMBOL += EXPRESSION ;'
'SYMBOL -= EXPRESSION ;'
'SYMBOL *= EXPRESSION ;'
'SYMBOL /= EXPRESSION ;'
'SYMBOL <<= EXPRESSION ;'
'SYMBOL >>= EXPRESSION ;'
'SYMBOL &= EXPRESSION ;'
'SYMBOL |= EXPRESSION ;'
The first case will define SYMBOL to the value of EXPRESSION. In the
other cases, SYMBOL must already be defined, and the value will be
adjusted accordingly.
The special symbol name '.' indicates the location counter. You may
only use this within a 'SECTIONS' command. *Note Location Counter::.
The semicolon after EXPRESSION is required.
Expressions are defined below; see *note Expressions::.
You may write symbol assignments as commands in their own right, or
as statements within a 'SECTIONS' command, or as part of an output
section description in a 'SECTIONS' command.
The section of the symbol will be set from the section of the
expression; for more information, see *note Expression Section::.
Here is an example showing the three different places that symbol
assignments may be used:
floating_point = 0;
SECTIONS
{
.text :
{
*(.text)
_etext = .;
}
_bdata = (. + 3) & ~ 3;
.data : { *(.data) }
}
In this example, the symbol 'floating_point' will be defined as zero.
The symbol '_etext' will be defined as the address following the last
'.text' input section. The symbol '_bdata' will be defined as the
address following the '.text' output section aligned upward to a 4 byte
boundary.

File: ld.info, Node: HIDDEN, Next: PROVIDE, Prev: Simple Assignments, Up: Assignments
3.5.2 HIDDEN
------------
For ELF targeted ports, define a symbol that will be hidden and won't be
exported. The syntax is 'HIDDEN(SYMBOL = EXPRESSION)'.
Here is the example from *note Simple Assignments::, rewritten to use
'HIDDEN':
HIDDEN(floating_point = 0);
SECTIONS
{
.text :
{
*(.text)
HIDDEN(_etext = .);
}
HIDDEN(_bdata = (. + 3) & ~ 3);
.data : { *(.data) }
}
In this case none of the three symbols will be visible outside this
module.

File: ld.info, Node: PROVIDE, Next: PROVIDE_HIDDEN, Prev: HIDDEN, Up: Assignments
3.5.3 PROVIDE
-------------
In some cases, it is desirable for a linker script to define a symbol
only if it is referenced and is not defined by any object included in
the link. For example, traditional linkers defined the symbol 'etext'.
However, ANSI C requires that the user be able to use 'etext' as a
function name without encountering an error. The 'PROVIDE' keyword may
be used to define a symbol, such as 'etext', only if it is referenced
but not defined. The syntax is 'PROVIDE(SYMBOL = EXPRESSION)'.
Here is an example of using 'PROVIDE' to define 'etext':
SECTIONS
{
.text :
{
*(.text)
_etext = .;
PROVIDE(etext = .);
}
}
In this example, if the program defines '_etext' (with a leading
underscore), the linker will give a multiple definition error. If, on
the other hand, the program defines 'etext' (with no leading
underscore), the linker will silently use the definition in the program.
If the program references 'etext' but does not define it, the linker
will use the definition in the linker script.

File: ld.info, Node: PROVIDE_HIDDEN, Next: Source Code Reference, Prev: PROVIDE, Up: Assignments
3.5.4 PROVIDE_HIDDEN
--------------------
Similar to 'PROVIDE'. For ELF targeted ports, the symbol will be hidden
and won't be exported.

File: ld.info, Node: Source Code Reference, Prev: PROVIDE_HIDDEN, Up: Assignments
3.5.5 Source Code Reference
---------------------------
Accessing a linker script defined variable from source code is not
intuitive. In particular a linker script symbol is not equivalent to a
variable declaration in a high level language, it is instead a symbol
that does not have a value.
Before going further, it is important to note that compilers often
transform names in the source code into different names when they are
stored in the symbol table. For example, Fortran compilers commonly
prepend or append an underscore, and C++ performs extensive 'name
mangling'. Therefore there might be a discrepancy between the name of a
variable as it is used in source code and the name of the same variable
as it is defined in a linker script. For example in C a linker script
variable might be referred to as:
extern int foo;
But in the linker script it might be defined as:
_foo = 1000;
In the remaining examples however it is assumed that no name
transformation has taken place.
When a symbol is declared in a high level language such as C, two
things happen. The first is that the compiler reserves enough space in
the program's memory to hold the _value_ of the symbol. The second is
that the compiler creates an entry in the program's symbol table which
holds the symbol's _address_. ie the symbol table contains the address
of the block of memory holding the symbol's value. So for example the
following C declaration, at file scope:
int foo = 1000;
creates an entry called 'foo' in the symbol table. This entry holds
the address of an 'int' sized block of memory where the number 1000 is
initially stored.
When a program references a symbol the compiler generates code that
first accesses the symbol table to find the address of the symbol's
memory block and then code to read the value from that memory block.
So:
foo = 1;
looks up the symbol 'foo' in the symbol table, gets the address
associated with this symbol and then writes the value 1 into that
address. Whereas:
int * a = & foo;
looks up the symbol 'foo' in the symbol table, gets its address and
then copies this address into the block of memory associated with the
variable 'a'.
Linker scripts symbol declarations, by contrast, create an entry in
the symbol table but do not assign any memory to them. Thus they are an
address without a value. So for example the linker script definition:
foo = 1000;
creates an entry in the symbol table called 'foo' which holds the
address of memory location 1000, but nothing special is stored at
address 1000. This means that you cannot access the _value_ of a linker
script defined symbol - it has no value - all you can do is access the
_address_ of a linker script defined symbol.
Hence when you are using a linker script defined symbol in source
code you should always take the address of the symbol, and never attempt
to use its value. For example suppose you want to copy the contents of
a section of memory called .ROM into a section called .FLASH and the
linker script contains these declarations:
start_of_ROM = .ROM;
end_of_ROM = .ROM + sizeof (.ROM) - 1;
start_of_FLASH = .FLASH;
Then the C source code to perform the copy would be:
extern char start_of_ROM, end_of_ROM, start_of_FLASH;
memcpy (& start_of_FLASH, & start_of_ROM, & end_of_ROM - & start_of_ROM);
Note the use of the '&' operators. These are correct.

File: ld.info, Node: SECTIONS, Next: MEMORY, Prev: Assignments, Up: Scripts
3.6 SECTIONS Command
====================
The 'SECTIONS' command tells the linker how to map input sections into
output sections, and how to place the output sections in memory.
The format of the 'SECTIONS' command is:
SECTIONS
{
SECTIONS-COMMAND
SECTIONS-COMMAND
...
}
Each SECTIONS-COMMAND may of be one of the following:
* an 'ENTRY' command (*note Entry command: Entry Point.)
* a symbol assignment (*note Assignments::)
* an output section description
* an overlay description
The 'ENTRY' command and symbol assignments are permitted inside the
'SECTIONS' command for convenience in using the location counter in
those commands. This can also make the linker script easier to
understand because you can use those commands at meaningful points in
the layout of the output file.
Output section descriptions and overlay descriptions are described
below.
If you do not use a 'SECTIONS' command in your linker script, the
linker will place each input section into an identically named output
section in the order that the sections are first encountered in the
input files. If all input sections are present in the first file, for
example, the order of sections in the output file will match the order
in the first input file. The first section will be at address zero.
* Menu:
* Output Section Description:: Output section description
* Output Section Name:: Output section name
* Output Section Address:: Output section address
* Input Section:: Input section description
* Output Section Data:: Output section data
* Output Section Keywords:: Output section keywords
* Output Section Discarding:: Output section discarding
* Output Section Attributes:: Output section attributes
* Overlay Description:: Overlay description

File: ld.info, Node: Output Section Description, Next: Output Section Name, Up: SECTIONS
3.6.1 Output Section Description
--------------------------------
The full description of an output section looks like this:
SECTION [ADDRESS] [(TYPE)] :
[AT(LMA)]
[ALIGN(SECTION_ALIGN) | ALIGN_WITH_INPUT]
[SUBALIGN(SUBSECTION_ALIGN)]
[CONSTRAINT]
{
OUTPUT-SECTION-COMMAND
OUTPUT-SECTION-COMMAND
...
} [>REGION] [AT>LMA_REGION] [:PHDR :PHDR ...] [=FILLEXP]
Most output sections do not use most of the optional section
attributes.
The whitespace around SECTION is required, so that the section name
is unambiguous. The colon and the curly braces are also required. The
line breaks and other white space are optional.
Each OUTPUT-SECTION-COMMAND may be one of the following:
* a symbol assignment (*note Assignments::)
* an input section description (*note Input Section::)
* data values to include directly (*note Output Section Data::)
* a special output section keyword (*note Output Section Keywords::)

File: ld.info, Node: Output Section Name, Next: Output Section Address, Prev: Output Section Description, Up: SECTIONS
3.6.2 Output Section Name
-------------------------
The name of the output section is SECTION. SECTION must meet the
constraints of your output format. In formats which only support a
limited number of sections, such as 'a.out', the name must be one of the
names supported by the format ('a.out', for example, allows only
'.text', '.data' or '.bss'). If the output format supports any number
of sections, but with numbers and not names (as is the case for Oasys),
the name should be supplied as a quoted numeric string. A section name
may consist of any sequence of characters, but a name which contains any
unusual characters such as commas must be quoted.
The output section name '/DISCARD/' is special; *note Output Section
Discarding::.

File: ld.info, Node: Output Section Address, Next: Input Section, Prev: Output Section Name, Up: SECTIONS
3.6.3 Output Section Address
----------------------------
The ADDRESS is an expression for the VMA (the virtual memory address) of
the output section. This address is optional, but if it is provided
then the output address will be set exactly as specified.
If the output address is not specified then one will be chosen for
the section, based on the heuristic below. This address will be
adjusted to fit the alignment requirement of the output section. The
alignment requirement is the strictest alignment of any input section
contained within the output section.
The output section address heuristic is as follows:
* If an output memory REGION is set for the section then it is added
to this region and its address will be the next free address in
that region.
* If the MEMORY command has been used to create a list of memory
regions then the first region which has attributes compatible with
the section is selected to contain it. The section's output
address will be the next free address in that region; *note
MEMORY::.
* If no memory regions were specified, or none match the section then
the output address will be based on the current value of the
location counter.
For example:
.text . : { *(.text) }
and
.text : { *(.text) }
are subtly different. The first will set the address of the '.text'
output section to the current value of the location counter. The second
will set it to the current value of the location counter aligned to the
strictest alignment of any of the '.text' input sections.
The ADDRESS may be an arbitrary expression; *note Expressions::. For
example, if you want to align the section on a 0x10 byte boundary, so
that the lowest four bits of the section address are zero, you could do
something like this:
.text ALIGN(0x10) : { *(.text) }
This works because 'ALIGN' returns the current location counter aligned
upward to the specified value.
Specifying ADDRESS for a section will change the value of the
location counter, provided that the section is non-empty. (Empty
sections are ignored).

File: ld.info, Node: Input Section, Next: Output Section Data, Prev: Output Section Address, Up: SECTIONS
3.6.4 Input Section Description
-------------------------------
The most common output section command is an input section description.
The input section description is the most basic linker script
operation. You use output sections to tell the linker how to lay out
your program in memory. You use input section descriptions to tell the
linker how to map the input files into your memory layout.
* Menu:
* Input Section Basics:: Input section basics
* Input Section Wildcards:: Input section wildcard patterns
* Input Section Common:: Input section for common symbols
* Input Section Keep:: Input section and garbage collection
* Input Section Example:: Input section example

File: ld.info, Node: Input Section Basics, Next: Input Section Wildcards, Up: Input Section
3.6.4.1 Input Section Basics
............................
An input section description consists of a file name optionally followed
by a list of section names in parentheses.
The file name and the section name may be wildcard patterns, which we
describe further below (*note Input Section Wildcards::).
The most common input section description is to include all input
sections with a particular name in the output section. For example, to
include all input '.text' sections, you would write:
*(.text)
Here the '*' is a wildcard which matches any file name. To exclude a
list of files from matching the file name wildcard, EXCLUDE_FILE may be
used to match all files except the ones specified in the EXCLUDE_FILE
list. For example:
*(EXCLUDE_FILE (*crtend.o *otherfile.o) .ctors)
will cause all .ctors sections from all files except 'crtend.o' and
'otherfile.o' to be included.
There are two ways to include more than one section:
*(.text .rdata)
*(.text) *(.rdata)
The difference between these is the order in which the '.text' and
'.rdata' input sections will appear in the output section. In the first
example, they will be intermingled, appearing in the same order as they
are found in the linker input. In the second example, all '.text' input
sections will appear first, followed by all '.rdata' input sections.
You can specify a file name to include sections from a particular
file. You would do this if one or more of your files contain special
data that needs to be at a particular location in memory. For example:
data.o(.data)
To refine the sections that are included based on the section flags
of an input section, INPUT_SECTION_FLAGS may be used.
Here is a simple example for using Section header flags for ELF
sections:
SECTIONS {
.text : { INPUT_SECTION_FLAGS (SHF_MERGE & SHF_STRINGS) *(.text) }
.text2 : { INPUT_SECTION_FLAGS (!SHF_WRITE) *(.text) }
}
In this example, the output section '.text' will be comprised of any
input section matching the name *(.text) whose section header flags
'SHF_MERGE' and 'SHF_STRINGS' are set. The output section '.text2' will
be comprised of any input section matching the name *(.text) whose
section header flag 'SHF_WRITE' is clear.
You can also specify files within archives by writing a pattern
matching the archive, a colon, then the pattern matching the file, with
no whitespace around the colon.
'archive:file'
matches file within archive
'archive:'
matches the whole archive
':file'
matches file but not one in an archive
Either one or both of 'archive' and 'file' can contain shell
wildcards. On DOS based file systems, the linker will assume that a
single letter followed by a colon is a drive specifier, so 'c:myfile.o'
is a simple file specification, not 'myfile.o' within an archive called
'c'. 'archive:file' filespecs may also be used within an 'EXCLUDE_FILE'
list, but may not appear in other linker script contexts. For instance,
you cannot extract a file from an archive by using 'archive:file' in an
'INPUT' command.
If you use a file name without a list of sections, then all sections
in the input file will be included in the output section. This is not
commonly done, but it may by useful on occasion. For example:
data.o
When you use a file name which is not an 'archive:file' specifier and
does not contain any wild card characters, the linker will first see if
you also specified the file name on the linker command line or in an
'INPUT' command. If you did not, the linker will attempt to open the
file as an input file, as though it appeared on the command line. Note
that this differs from an 'INPUT' command, because the linker will not
search for the file in the archive search path.

File: ld.info, Node: Input Section Wildcards, Next: Input Section Common, Prev: Input Section Basics, Up: Input Section
3.6.4.2 Input Section Wildcard Patterns
.......................................
In an input section description, either the file name or the section
name or both may be wildcard patterns.
The file name of '*' seen in many examples is a simple wildcard
pattern for the file name.
The wildcard patterns are like those used by the Unix shell.
'*'
matches any number of characters
'?'
matches any single character
'[CHARS]'
matches a single instance of any of the CHARS; the '-' character
may be used to specify a range of characters, as in '[a-z]' to
match any lower case letter
'\'
quotes the following character
When a file name is matched with a wildcard, the wildcard characters
will not match a '/' character (used to separate directory names on
Unix). A pattern consisting of a single '*' character is an exception;
it will always match any file name, whether it contains a '/' or not.
In a section name, the wildcard characters will match a '/' character.
File name wildcard patterns only match files which are explicitly
specified on the command line or in an 'INPUT' command. The linker does
not search directories to expand wildcards.
If a file name matches more than one wildcard pattern, or if a file
name appears explicitly and is also matched by a wildcard pattern, the
linker will use the first match in the linker script. For example, this
sequence of input section descriptions is probably in error, because the
'data.o' rule will not be used:
.data : { *(.data) }
.data1 : { data.o(.data) }
Normally, the linker will place files and sections matched by
wildcards in the order in which they are seen during the link. You can
change this by using the 'SORT_BY_NAME' keyword, which appears before a
wildcard pattern in parentheses (e.g., 'SORT_BY_NAME(.text*)'). When
the 'SORT_BY_NAME' keyword is used, the linker will sort the files or
sections into ascending order by name before placing them in the output
file.
'SORT_BY_ALIGNMENT' is very similar to 'SORT_BY_NAME'. The
difference is 'SORT_BY_ALIGNMENT' will sort sections into descending
order by alignment before placing them in the output file. Larger
alignments are placed before smaller alignments in order to reduce the
amount of padding necessary.
'SORT_BY_INIT_PRIORITY' is very similar to 'SORT_BY_NAME'. The
difference is 'SORT_BY_INIT_PRIORITY' will sort sections into ascending
order by numerical value of the GCC init_priority attribute encoded in
the section name before placing them in the output file.
'SORT' is an alias for 'SORT_BY_NAME'.
When there are nested section sorting commands in linker script,
there can be at most 1 level of nesting for section sorting commands.
1. 'SORT_BY_NAME' ('SORT_BY_ALIGNMENT' (wildcard section pattern)).
It will sort the input sections by name first, then by alignment if
two sections have the same name.
2. 'SORT_BY_ALIGNMENT' ('SORT_BY_NAME' (wildcard section pattern)).
It will sort the input sections by alignment first, then by name if
two sections have the same alignment.
3. 'SORT_BY_NAME' ('SORT_BY_NAME' (wildcard section pattern)) is
treated the same as 'SORT_BY_NAME' (wildcard section pattern).
4. 'SORT_BY_ALIGNMENT' ('SORT_BY_ALIGNMENT' (wildcard section
pattern)) is treated the same as 'SORT_BY_ALIGNMENT' (wildcard
section pattern).
5. All other nested section sorting commands are invalid.
When both command line section sorting option and linker script
section sorting command are used, section sorting command always takes
precedence over the command line option.
If the section sorting command in linker script isn't nested, the
command line option will make the section sorting command to be treated
as nested sorting command.
1. 'SORT_BY_NAME' (wildcard section pattern ) with '--sort-sections
alignment' is equivalent to 'SORT_BY_NAME' ('SORT_BY_ALIGNMENT'
(wildcard section pattern)).
2. 'SORT_BY_ALIGNMENT' (wildcard section pattern) with '--sort-section
name' is equivalent to 'SORT_BY_ALIGNMENT' ('SORT_BY_NAME'
(wildcard section pattern)).
If the section sorting command in linker script is nested, the
command line option will be ignored.
'SORT_NONE' disables section sorting by ignoring the command line
section sorting option.
If you ever get confused about where input sections are going, use
the '-M' linker option to generate a map file. The map file shows
precisely how input sections are mapped to output sections.
This example shows how wildcard patterns might be used to partition
files. This linker script directs the linker to place all '.text'
sections in '.text' and all '.bss' sections in '.bss'. The linker will
place the '.data' section from all files beginning with an upper case
character in '.DATA'; for all other files, the linker will place the
'.data' section in '.data'.
SECTIONS {
.text : { *(.text) }
.DATA : { [A-Z]*(.data) }
.data : { *(.data) }
.bss : { *(.bss) }
}

File: ld.info, Node: Input Section Common, Next: Input Section Keep, Prev: Input Section Wildcards, Up: Input Section
3.6.4.3 Input Section for Common Symbols
........................................
A special notation is needed for common symbols, because in many object
file formats common symbols do not have a particular input section. The
linker treats common symbols as though they are in an input section
named 'COMMON'.
You may use file names with the 'COMMON' section just as with any
other input sections. You can use this to place common symbols from a
particular input file in one section while common symbols from other
input files are placed in another section.
In most cases, common symbols in input files will be placed in the
'.bss' section in the output file. For example:
.bss { *(.bss) *(COMMON) }
Some object file formats have more than one type of common symbol.
For example, the MIPS ELF object file format distinguishes standard
common symbols and small common symbols. In this case, the linker will
use a different special section name for other types of common symbols.
In the case of MIPS ELF, the linker uses 'COMMON' for standard common
symbols and '.scommon' for small common symbols. This permits you to
map the different types of common symbols into memory at different
locations.
You will sometimes see '[COMMON]' in old linker scripts. This
notation is now considered obsolete. It is equivalent to '*(COMMON)'.

File: ld.info, Node: Input Section Keep, Next: Input Section Example, Prev: Input Section Common, Up: Input Section
3.6.4.4 Input Section and Garbage Collection
............................................
When link-time garbage collection is in use ('--gc-sections'), it is
often useful to mark sections that should not be eliminated. This is
accomplished by surrounding an input section's wildcard entry with
'KEEP()', as in 'KEEP(*(.init))' or 'KEEP(SORT_BY_NAME(*)(.ctors))'.

File: ld.info, Node: Input Section Example, Prev: Input Section Keep, Up: Input Section
3.6.4.5 Input Section Example
.............................
The following example is a complete linker script. It tells the linker
to read all of the sections from file 'all.o' and place them at the
start of output section 'outputa' which starts at location '0x10000'.
All of section '.input1' from file 'foo.o' follows immediately, in the
same output section. All of section '.input2' from 'foo.o' goes into
output section 'outputb', followed by section '.input1' from 'foo1.o'.
All of the remaining '.input1' and '.input2' sections from any files are
written to output section 'outputc'.
SECTIONS {
outputa 0x10000 :
{
all.o
foo.o (.input1)
}
outputb :
{
foo.o (.input2)
foo1.o (.input1)
}
outputc :
{
*(.input1)
*(.input2)
}
}

File: ld.info, Node: Output Section Data, Next: Output Section Keywords, Prev: Input Section, Up: SECTIONS
3.6.5 Output Section Data
-------------------------
You can include explicit bytes of data in an output section by using
'BYTE', 'SHORT', 'LONG', 'QUAD', or 'SQUAD' as an output section
command. Each keyword is followed by an expression in parentheses
providing the value to store (*note Expressions::). The value of the
expression is stored at the current value of the location counter.
The 'BYTE', 'SHORT', 'LONG', and 'QUAD' commands store one, two,
four, and eight bytes (respectively). After storing the bytes, the
location counter is incremented by the number of bytes stored.
For example, this will store the byte 1 followed by the four byte
value of the symbol 'addr':
BYTE(1)
LONG(addr)
When using a 64 bit host or target, 'QUAD' and 'SQUAD' are the same;
they both store an 8 byte, or 64 bit, value. When both host and target
are 32 bits, an expression is computed as 32 bits. In this case 'QUAD'
stores a 32 bit value zero extended to 64 bits, and 'SQUAD' stores a 32
bit value sign extended to 64 bits.
If the object file format of the output file has an explicit
endianness, which is the normal case, the value will be stored in that
endianness. When the object file format does not have an explicit
endianness, as is true of, for example, S-records, the value will be
stored in the endianness of the first input object file.
Note--these commands only work inside a section description and not
between them, so the following will produce an error from the linker:
SECTIONS { .text : { *(.text) } LONG(1) .data : { *(.data) } }
whereas this will work:
SECTIONS { .text : { *(.text) ; LONG(1) } .data : { *(.data) } }
You may use the 'FILL' command to set the fill pattern for the
current section. It is followed by an expression in parentheses. Any
otherwise unspecified regions of memory within the section (for example,
gaps left due to the required alignment of input sections) are filled
with the value of the expression, repeated as necessary. A 'FILL'
statement covers memory locations after the point at which it occurs in
the section definition; by including more than one 'FILL' statement, you
can have different fill patterns in different parts of an output
section.
This example shows how to fill unspecified regions of memory with the
value '0x90':
FILL(0x90909090)
The 'FILL' command is similar to the '=FILLEXP' output section
attribute, but it only affects the part of the section following the
'FILL' command, rather than the entire section. If both are used, the
'FILL' command takes precedence. *Note Output Section Fill::, for
details on the fill expression.

File: ld.info, Node: Output Section Keywords, Next: Output Section Discarding, Prev: Output Section Data, Up: SECTIONS
3.6.6 Output Section Keywords
-----------------------------
There are a couple of keywords which can appear as output section
commands.
'CREATE_OBJECT_SYMBOLS'
The command tells the linker to create a symbol for each input
file. The name of each symbol will be the name of the
corresponding input file. The section of each symbol will be the
output section in which the 'CREATE_OBJECT_SYMBOLS' command
appears.
This is conventional for the a.out object file format. It is not
normally used for any other object file format.
'CONSTRUCTORS'
When linking using the a.out object file format, the linker uses an
unusual set construct to support C++ global constructors and
destructors. When linking object file formats which do not support
arbitrary sections, such as ECOFF and XCOFF, the linker will
automatically recognize C++ global constructors and destructors by
name. For these object file formats, the 'CONSTRUCTORS' command
tells the linker to place constructor information in the output
section where the 'CONSTRUCTORS' command appears. The
'CONSTRUCTORS' command is ignored for other object file formats.
The symbol '__CTOR_LIST__' marks the start of the global
constructors, and the symbol '__CTOR_END__' marks the end.
Similarly, '__DTOR_LIST__' and '__DTOR_END__' mark the start and
end of the global destructors. The first word in the list is the
number of entries, followed by the address of each constructor or
destructor, followed by a zero word. The compiler must arrange to
actually run the code. For these object file formats GNU C++
normally calls constructors from a subroutine '__main'; a call to
'__main' is automatically inserted into the startup code for
'main'. GNU C++ normally runs destructors either by using
'atexit', or directly from the function 'exit'.
For object file formats such as 'COFF' or 'ELF' which support
arbitrary section names, GNU C++ will normally arrange to put the
addresses of global constructors and destructors into the '.ctors'
and '.dtors' sections. Placing the following sequence into your
linker script will build the sort of table which the GNU C++
runtime code expects to see.
__CTOR_LIST__ = .;
LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2)
*(.ctors)
LONG(0)
__CTOR_END__ = .;
__DTOR_LIST__ = .;
LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2)
*(.dtors)
LONG(0)
__DTOR_END__ = .;
If you are using the GNU C++ support for initialization priority,
which provides some control over the order in which global
constructors are run, you must sort the constructors at link time
to ensure that they are executed in the correct order. When using
the 'CONSTRUCTORS' command, use 'SORT_BY_NAME(CONSTRUCTORS)'
instead. When using the '.ctors' and '.dtors' sections, use
'*(SORT_BY_NAME(.ctors))' and '*(SORT_BY_NAME(.dtors))' instead of
just '*(.ctors)' and '*(.dtors)'.
Normally the compiler and linker will handle these issues
automatically, and you will not need to concern yourself with them.
However, you may need to consider this if you are using C++ and
writing your own linker scripts.

File: ld.info, Node: Output Section Discarding, Next: Output Section Attributes, Prev: Output Section Keywords, Up: SECTIONS
3.6.7 Output Section Discarding
-------------------------------
The linker will not create output sections with no contents. This is
for convenience when referring to input sections that may or may not be
present in any of the input files. For example:
.foo : { *(.foo) }
will only create a '.foo' section in the output file if there is a
'.foo' section in at least one input file, and if the input sections are
not all empty. Other link script directives that allocate space in an
output section will also create the output section.
The linker will ignore address assignments (*note Output Section
Address::) on discarded output sections, except when the linker script
defines symbols in the output section. In that case the linker will
obey the address assignments, possibly advancing dot even though the
section is discarded.
The special output section name '/DISCARD/' may be used to discard
input sections. Any input sections which are assigned to an output
section named '/DISCARD/' are not included in the output file.

File: ld.info, Node: Output Section Attributes, Next: Overlay Description, Prev: Output Section Discarding, Up: SECTIONS
3.6.8 Output Section Attributes
-------------------------------
We showed above that the full description of an output section looked
like this:
SECTION [ADDRESS] [(TYPE)] :
[AT(LMA)]
[ALIGN(SECTION_ALIGN)]
[SUBALIGN(SUBSECTION_ALIGN)]
[CONSTRAINT]
{
OUTPUT-SECTION-COMMAND
OUTPUT-SECTION-COMMAND
...
} [>REGION] [AT>LMA_REGION] [:PHDR :PHDR ...] [=FILLEXP]
We've already described SECTION, ADDRESS, and OUTPUT-SECTION-COMMAND.
In this section we will describe the remaining section attributes.
* Menu:
* Output Section Type:: Output section type
* Output Section LMA:: Output section LMA
* Forced Output Alignment:: Forced Output Alignment
* Forced Input Alignment:: Forced Input Alignment
* Output Section Constraint:: Output section constraint
* Output Section Region:: Output section region
* Output Section Phdr:: Output section phdr
* Output Section Fill:: Output section fill

File: ld.info, Node: Output Section Type, Next: Output Section LMA, Up: Output Section Attributes
3.6.8.1 Output Section Type
...........................
Each output section may have a type. The type is a keyword in
parentheses. The following types are defined:
'NOLOAD'
The section should be marked as not loadable, so that it will not
be loaded into memory when the program is run.
'DSECT'
'COPY'
'INFO'
'OVERLAY'
These type names are supported for backward compatibility, and are
rarely used. They all have the same effect: the section should be
marked as not allocatable, so that no memory is allocated for the
section when the program is run.
The linker normally sets the attributes of an output section based on
the input sections which map into it. You can override this by using
the section type. For example, in the script sample below, the 'ROM'
section is addressed at memory location '0' and does not need to be
loaded when the program is run.
SECTIONS {
ROM 0 (NOLOAD) : { ... }
...
}

File: ld.info, Node: Output Section LMA, Next: Forced Output Alignment, Prev: Output Section Type, Up: Output Section Attributes
3.6.8.2 Output Section LMA
..........................
Every section has a virtual address (VMA) and a load address (LMA); see
*note Basic Script Concepts::. The virtual address is specified by the
*note Output Section Address:: described earlier. The load address is
specified by the 'AT' or 'AT>' keywords. Specifying a load address is
optional.
The 'AT' keyword takes an expression as an argument. This specifies
the exact load address of the section. The 'AT>' keyword takes the name
of a memory region as an argument. *Note MEMORY::. The load address of
the section is set to the next free address in the region, aligned to
the section's alignment requirements.
If neither 'AT' nor 'AT>' is specified for an allocatable section,
the linker will use the following heuristic to determine the load
address:
* If the section has a specific VMA address, then this is used as the
LMA address as well.
* If the section is not allocatable then its LMA is set to its VMA.
* Otherwise if a memory region can be found that is compatible with
the current section, and this region contains at least one section,
then the LMA is set so the difference between the VMA and LMA is
the same as the difference between the VMA and LMA of the last
section in the located region.
* If no memory regions have been declared then a default region that
covers the entire address space is used in the previous step.
* If no suitable region could be found, or there was no previous
section then the LMA is set equal to the VMA.
This feature is designed to make it easy to build a ROM image. For
example, the following linker script creates three output sections: one
called '.text', which starts at '0x1000', one called '.mdata', which is
loaded at the end of the '.text' section even though its VMA is
'0x2000', and one called '.bss' to hold uninitialized data at address
'0x3000'. The symbol '_data' is defined with the value '0x2000', which
shows that the location counter holds the VMA value, not the LMA value.
SECTIONS
{
.text 0x1000 : { *(.text) _etext = . ; }
.mdata 0x2000 :
AT ( ADDR (.text) + SIZEOF (.text) )
{ _data = . ; *(.data); _edata = . ; }
.bss 0x3000 :
{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;}
}
The run-time initialization code for use with a program generated
with this linker script would include something like the following, to
copy the initialized data from the ROM image to its runtime address.
Notice how this code takes advantage of the symbols defined by the
linker script.
extern char _etext, _data, _edata, _bstart, _bend;
char *src = &_etext;
char *dst = &_data;
/* ROM has data at end of text; copy it. */
while (dst < &_edata)
*dst++ = *src++;
/* Zero bss. */
for (dst = &_bstart; dst< &_bend; dst++)
*dst = 0;

File: ld.info, Node: Forced Output Alignment, Next: Forced Input Alignment, Prev: Output Section LMA, Up: Output Section Attributes
3.6.8.3 Forced Output Alignment
...............................
You can increase an output section's alignment by using ALIGN. As an
alternative you can force the output section alignment to the maximum
alignment of all its input sections with ALIGN_WITH_INPUT. The alignment
forced by ALIGN_WITH_INPUT is used even in case the load and virtual
memory regions are different.

File: ld.info, Node: Forced Input Alignment, Next: Output Section Constraint, Prev: Forced Output Alignment, Up: Output Section Attributes
3.6.8.4 Forced Input Alignment
..............................
You can force input section alignment within an output section by using
SUBALIGN. The value specified overrides any alignment given by input
sections, whether larger or smaller.

File: ld.info, Node: Output Section Constraint, Next: Output Section Region, Prev: Forced Input Alignment, Up: Output Section Attributes
3.6.8.5 Output Section Constraint
.................................
You can specify that an output section should only be created if all of
its input sections are read-only or all of its input sections are
read-write by using the keyword 'ONLY_IF_RO' and 'ONLY_IF_RW'
respectively.

File: ld.info, Node: Output Section Region, Next: Output Section Phdr, Prev: Output Section Constraint, Up: Output Section Attributes
3.6.8.6 Output Section Region
.............................
You can assign a section to a previously defined region of memory by
using '>REGION'. *Note MEMORY::.
Here is a simple example:
MEMORY { rom : ORIGIN = 0x1000, LENGTH = 0x1000 }
SECTIONS { ROM : { *(.text) } >rom }

File: ld.info, Node: Output Section Phdr, Next: Output Section Fill, Prev: Output Section Region, Up: Output Section Attributes
3.6.8.7 Output Section Phdr
...........................
You can assign a section to a previously defined program segment by
using ':PHDR'. *Note PHDRS::. If a section is assigned to one or more
segments, then all subsequent allocated sections will be assigned to
those segments as well, unless they use an explicitly ':PHDR' modifier.
You can use ':NONE' to tell the linker to not put the section in any
segment at all.
Here is a simple example:
PHDRS { text PT_LOAD ; }
SECTIONS { .text : { *(.text) } :text }

File: ld.info, Node: Output Section Fill, Prev: Output Section Phdr, Up: Output Section Attributes
3.6.8.8 Output Section Fill
...........................
You can set the fill pattern for an entire section by using '=FILLEXP'.
FILLEXP is an expression (*note Expressions::). Any otherwise
unspecified regions of memory within the output section (for example,
gaps left due to the required alignment of input sections) will be
filled with the value, repeated as necessary. If the fill expression is
a simple hex number, ie. a string of hex digit starting with '0x' and
without a trailing 'k' or 'M', then an arbitrarily long sequence of hex
digits can be used to specify the fill pattern; Leading zeros become
part of the pattern too. For all other cases, including extra
parentheses or a unary '+', the fill pattern is the four least
significant bytes of the value of the expression. In all cases, the
number is big-endian.
You can also change the fill value with a 'FILL' command in the
output section commands; (*note Output Section Data::).
Here is a simple example:
SECTIONS { .text : { *(.text) } =0x90909090 }

File: ld.info, Node: Overlay Description, Prev: Output Section Attributes, Up: SECTIONS
3.6.9 Overlay Description
-------------------------
An overlay description provides an easy way to describe sections which
are to be loaded as part of a single memory image but are to be run at
the same memory address. At run time, some sort of overlay manager will
copy the overlaid sections in and out of the runtime memory address as
required, perhaps by simply manipulating addressing bits. This approach
can be useful, for example, when a certain region of memory is faster
than another.
Overlays are described using the 'OVERLAY' command. The 'OVERLAY'
command is used within a 'SECTIONS' command, like an output section
description. The full syntax of the 'OVERLAY' command is as follows:
OVERLAY [START] : [NOCROSSREFS] [AT ( LDADDR )]
{
SECNAME1
{
OUTPUT-SECTION-COMMAND
OUTPUT-SECTION-COMMAND
...
} [:PHDR...] [=FILL]
SECNAME2
{
OUTPUT-SECTION-COMMAND
OUTPUT-SECTION-COMMAND
...
} [:PHDR...] [=FILL]
...
} [>REGION] [:PHDR...] [=FILL]
Everything is optional except 'OVERLAY' (a keyword), and each section
must have a name (SECNAME1 and SECNAME2 above). The section definitions
within the 'OVERLAY' construct are identical to those within the general
'SECTIONS' construct (*note SECTIONS::), except that no addresses and no
memory regions may be defined for sections within an 'OVERLAY'.
The sections are all defined with the same starting address. The
load addresses of the sections are arranged such that they are
consecutive in memory starting at the load address used for the
'OVERLAY' as a whole (as with normal section definitions, the load
address is optional, and defaults to the start address; the start
address is also optional, and defaults to the current value of the
location counter).
If the 'NOCROSSREFS' keyword is used, and there are any references
among the sections, the linker will report an error. Since the sections
all run at the same address, it normally does not make sense for one
section to refer directly to another. *Note NOCROSSREFS: Miscellaneous
Commands.
For each section within the 'OVERLAY', the linker automatically
provides two symbols. The symbol '__load_start_SECNAME' is defined as
the starting load address of the section. The symbol
'__load_stop_SECNAME' is defined as the final load address of the
section. Any characters within SECNAME which are not legal within C
identifiers are removed. C (or assembler) code may use these symbols to
move the overlaid sections around as necessary.
At the end of the overlay, the value of the location counter is set
to the start address of the overlay plus the size of the largest
section.
Here is an example. Remember that this would appear inside a
'SECTIONS' construct.
OVERLAY 0x1000 : AT (0x4000)
{
.text0 { o1/*.o(.text) }
.text1 { o2/*.o(.text) }
}
This will define both '.text0' and '.text1' to start at address 0x1000.
'.text0' will be loaded at address 0x4000, and '.text1' will be loaded
immediately after '.text0'. The following symbols will be defined if
referenced: '__load_start_text0', '__load_stop_text0',
'__load_start_text1', '__load_stop_text1'.
C code to copy overlay '.text1' into the overlay area might look like
the following.
extern char __load_start_text1, __load_stop_text1;
memcpy ((char *) 0x1000, &__load_start_text1,
&__load_stop_text1 - &__load_start_text1);
Note that the 'OVERLAY' command is just syntactic sugar, since
everything it does can be done using the more basic commands. The above
example could have been written identically as follows.
.text0 0x1000 : AT (0x4000) { o1/*.o(.text) }
PROVIDE (__load_start_text0 = LOADADDR (.text0));
PROVIDE (__load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0));
.text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) { o2/*.o(.text) }
PROVIDE (__load_start_text1 = LOADADDR (.text1));
PROVIDE (__load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1));
. = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1));

File: ld.info, Node: MEMORY, Next: PHDRS, Prev: SECTIONS, Up: Scripts
3.7 MEMORY Command
==================
The linker's default configuration permits allocation of all available
memory. You can override this by using the 'MEMORY' command.
The 'MEMORY' command describes the location and size of blocks of
memory in the target. You can use it to describe which memory regions
may be used by the linker, and which memory regions it must avoid. You
can then assign sections to particular memory regions. The linker will
set section addresses based on the memory regions, and will warn about
regions that become too full. The linker will not shuffle sections
around to fit into the available regions.
A linker script may contain at most one use of the 'MEMORY' command.
However, you can define as many blocks of memory within it as you wish.
The syntax is:
MEMORY
{
NAME [(ATTR)] : ORIGIN = ORIGIN, LENGTH = LEN
...
}
The NAME is a name used in the linker script to refer to the region.
The region name has no meaning outside of the linker script. Region
names are stored in a separate name space, and will not conflict with
symbol names, file names, or section names. Each memory region must
have a distinct name within the 'MEMORY' command. However you can add
later alias names to existing memory regions with the *note
REGION_ALIAS:: command.
The ATTR string is an optional list of attributes that specify
whether to use a particular memory region for an input section which is
not explicitly mapped in the linker script. As described in *note
SECTIONS::, if you do not specify an output section for some input
section, the linker will create an output section with the same name as
the input section. If you define region attributes, the linker will use
them to select the memory region for the output section that it creates.
The ATTR string must consist only of the following characters:
'R'
Read-only section
'W'
Read/write section
'X'
Executable section
'A'
Allocatable section
'I'
Initialized section
'L'
Same as 'I'
'!'
Invert the sense of any of the attributes that follow
If a unmapped section matches any of the listed attributes other than
'!', it will be placed in the memory region. The '!' attribute reverses
this test, so that an unmapped section will be placed in the memory
region only if it does not match any of the listed attributes.
The ORIGIN is an numerical expression for the start address of the
memory region. The expression must evaluate to a constant and it cannot
involve any symbols. The keyword 'ORIGIN' may be abbreviated to 'org'
or 'o' (but not, for example, 'ORG').
The LEN is an expression for the size in bytes of the memory region.
As with the ORIGIN expression, the expression must be numerical only and
must evaluate to a constant. The keyword 'LENGTH' may be abbreviated to
'len' or 'l'.
In the following example, we specify that there are two memory
regions available for allocation: one starting at '0' for 256 kilobytes,
and the other starting at '0x40000000' for four megabytes. The linker
will place into the 'rom' memory region every section which is not
explicitly mapped into a memory region, and is either read-only or
executable. The linker will place other sections which are not
explicitly mapped into a memory region into the 'ram' memory region.
MEMORY
{
rom (rx) : ORIGIN = 0, LENGTH = 256K
ram (!rx) : org = 0x40000000, l = 4M
}
Once you define a memory region, you can direct the linker to place
specific output sections into that memory region by using the '>REGION'
output section attribute. For example, if you have a memory region
named 'mem', you would use '>mem' in the output section definition.
*Note Output Section Region::. If no address was specified for the
output section, the linker will set the address to the next available
address within the memory region. If the combined output sections
directed to a memory region are too large for the region, the linker
will issue an error message.
It is possible to access the origin and length of a memory in an
expression via the 'ORIGIN(MEMORY)' and 'LENGTH(MEMORY)' functions:
_fstack = ORIGIN(ram) + LENGTH(ram) - 4;

File: ld.info, Node: PHDRS, Next: VERSION, Prev: MEMORY, Up: Scripts
3.8 PHDRS Command
=================
The ELF object file format uses "program headers", also knows as
"segments". The program headers describe how the program should be
loaded into memory. You can print them out by using the 'objdump'
program with the '-p' option.
When you run an ELF program on a native ELF system, the system loader
reads the program headers in order to figure out how to load the
program. This will only work if the program headers are set correctly.
This manual does not describe the details of how the system loader
interprets program headers; for more information, see the ELF ABI.
The linker will create reasonable program headers by default.
However, in some cases, you may need to specify the program headers more
precisely. You may use the 'PHDRS' command for this purpose. When the
linker sees the 'PHDRS' command in the linker script, it will not create
any program headers other than the ones specified.
The linker only pays attention to the 'PHDRS' command when generating
an ELF output file. In other cases, the linker will simply ignore
'PHDRS'.
This is the syntax of the 'PHDRS' command. The words 'PHDRS',
'FILEHDR', 'AT', and 'FLAGS' are keywords.
PHDRS
{
NAME TYPE [ FILEHDR ] [ PHDRS ] [ AT ( ADDRESS ) ]
[ FLAGS ( FLAGS ) ] ;
}
The NAME is used only for reference in the 'SECTIONS' command of the
linker script. It is not put into the output file. Program header
names are stored in a separate name space, and will not conflict with
symbol names, file names, or section names. Each program header must
have a distinct name. The headers are processed in order and it is
usual for them to map to sections in ascending load address order.
Certain program header types describe segments of memory which the
system loader will load from the file. In the linker script, you
specify the contents of these segments by placing allocatable output
sections in the segments. You use the ':PHDR' output section attribute
to place a section in a particular segment. *Note Output Section
Phdr::.
It is normal to put certain sections in more than one segment. This
merely implies that one segment of memory contains another. You may
repeat ':PHDR', using it once for each segment which should contain the
section.
If you place a section in one or more segments using ':PHDR', then
the linker will place all subsequent allocatable sections which do not
specify ':PHDR' in the same segments. This is for convenience, since
generally a whole set of contiguous sections will be placed in a single
segment. You can use ':NONE' to override the default segment and tell
the linker to not put the section in any segment at all.
You may use the 'FILEHDR' and 'PHDRS' keywords after the program
header type to further describe the contents of the segment. The
'FILEHDR' keyword means that the segment should include the ELF file
header. The 'PHDRS' keyword means that the segment should include the
ELF program headers themselves. If applied to a loadable segment
('PT_LOAD'), all prior loadable segments must have one of these
keywords.
The TYPE may be one of the following. The numbers indicate the value
of the keyword.
'PT_NULL' (0)
Indicates an unused program header.
'PT_LOAD' (1)
Indicates that this program header describes a segment to be loaded
from the file.
'PT_DYNAMIC' (2)
Indicates a segment where dynamic linking information can be found.