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This is ld.info, produced by makeinfo version 4.8 from ld.texinfo.
START-INFO-DIR-ENTRY
* Ld: (ld). The GNU linker.
END-INFO-DIR-ENTRY
This file documents the GNU linker LD (GNU Binutils) version 2.18.90.
Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, 2001,
2002, 2003, 2004, 2005, 2006, 2007 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 1.1 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".

File: ld.info, Node: Top, Next: Overview, Up: (dir)
LD
**
This file documents the GNU linker ld (GNU Binutils) version 2.18.90.
This document is distributed under the terms of the GNU Free
Documentation License. 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,--startgroup foo.o bar.o -Wl,--endgroup
This is important, because otherwise the compiler driver program may
silently drop the linker options, resulting in a bad link.
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.
`-aKEYWORD'
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.
`-AARCHITECTURE'
`--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::.
`-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.
`-E'
`--export-dynamic'
When creating a dynamically linked executable, 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 this option, 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'.
`-EB'
Link big-endian objects. This affects the default output format.
`-EL'
Link little-endian objects. This affects the default output
format.
`-f'
`--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.
`-GVALUE'
`--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 ECOFF which supports putting large and small objects
into different sections. This is ignored for other object file
formats.
`-hNAME'
`-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.
`-lNAMESPEC'
`--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, otherise 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.
`-LSEARCHDIR'
`--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.
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.
`-mEMULATION'
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 mark the output as
`NMAGIC' if possible.
`-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.
`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.
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. `--as-needed' causes DT_NEEDED tags
to only be emitted for libraries that satisfy some symbol
reference from regular objects which is undefined at the point
that the library was linked. `--no-as-needed' restores the
default behaviour.
`--add-needed'
`--no-add-needed'
This option affects the treatment of dynamic libraries from ELF
DT_NEEDED tags in dynamic libraries mentioned on the command line
after the `--no-add-needed' option. Normally, the linker will add
a DT_NEEDED tag for each dynamic library from DT_NEEDED tags.
`--no-add-needed' causes DT_NEEDED tags will never be emitted for
those libraries from DT_NEEDED tags. `--add-needed' restores the
default behaviour.
`-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'.
`--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. The remaining files contain
references to the symbol.
`--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.
`--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
explicitely 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.
`--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 (the default) 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 reason that `--allow-shlib-undefined' is the default is that
the shared library being specified at link time may not be the
same as the one that is available at load time, so the symbols
might actually be resolvable at load time. Plus there are some
systems, (eg BeOS) where undefined symbols in shared libraries is
normal. (The kernel patches them at load time to select which
function is most appropriate for the current architecture. This
is used for example to dynamically select an appropriate memset
function). Apparently it is also normal for HPPA shared libraries
to have undefined symbols.
`--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'
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 global
optimizations that become possible when the linker resolves
addressing in the program, such as relaxing address modes and
synthesizing new instructions in the output object file.
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.
`--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, the 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 [= ascending | 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.
`--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'
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.
`--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 meaningful on ELF platforms
which support shared libraries. *Note VERSION::.
`--warn-common'
Warn when a common symbol is combined with another common symbol
or with a symbol definition. Unix linkers allow this somewhat
sloppy practise, 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 practise, 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-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.
`--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 dynamic tags will be created as needed.
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-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]
`--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]
`--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]
`--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 practise 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 is the default. [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]
2.1.2 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.
2.1.3 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'.

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 mean 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
* 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 the symbol `start', if defined;
* 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: Miscellaneous Commands, 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: Miscellaneous Commands, Prev: Format Commands, Up: Simple Commands
3.4.4 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.

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
* 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: PROVIDE, 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: PROVIDE, Next: PROVIDE_HIDDEN, Prev: Simple Assignments, Up: Assignments
3.5.2 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
3.5.3 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
3.5.4 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 a 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 it 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)] [SUBALIGN(SUBSECTION_ALIGN)]
{
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. If you do not provide ADDRESS, the linker will
set it based on REGION if present, or otherwise based on the current
value of the location counter.
If you provide ADDRESS, the address of the output section will be
set to precisely that. If you provide neither ADDRESS nor REGION, then
the address of the output section will be set to the current value of
the location counter aligned to the alignment requirements of the
output section. The alignment requirement of the output section is the
strictest alignment of any input section contained within the output
section.
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 a `.text' input section.
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.

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)
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 ascending
order by alignment 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 2 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 2 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.
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)]
{
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 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. The contents of the `ROM' section will
appear in the linker output file as usual.
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 address expression which may appear
in an output section description sets the VMA (*note Output Section
Address::).
The expression LMA that follows the `AT' keyword specifies the load
address of the section.
Alternatively, with `AT>LMA_REGION' expression, you may specify a
memory region for the section's load address. *Note MEMORY::. Note
that if the section has not had a VMA assigned to it then the linker
will use the LMA_REGION as the VMA region as well.
If neither `AT' nor `AT>' is specified for an allocatable section,
the linker will set the LMA such that the difference between VMA and
LMA for the section is the same as the preceding output section in the
same region. If there is no preceding output section or the section is
not allocatable, the linker will set the LMA equal to the VMA. *Note
Output Section Region::.
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.

File: ld.info, Node: Forced Input Alignment, Next: Output Section Region, 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 Region, Next: Output Section Phdr, Prev: Forced Input Alignment, Up: Output Section Attributes
3.6.8.5 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
3.6.8.6 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
3.6.8.7 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' contruct (*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 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.
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 preceding attributes
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.
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 appear 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.
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.
`PT_INTERP' (3)
Indicates a segment where the name of the program interpreter may
be found.
`PT_NOTE' (4)
Indicates a segment holding note information.
`PT_SHLIB' (5)
A reserved program header type, defined but not specified by the
ELF ABI.
`PT_PHDR' (6)
Indicates a segment where the program headers may be found.
EXPRESSION
An expression giving the numeric type of the program header. This
may be used for types not defined above.
You can specify that a segment should be loaded at a particular
address in memory by using an `AT' expression. This is identical to the
`AT' command used as an output section attribute (*note Output Section
LMA::). The `AT' command for a program header overrides the output
section attribute.
The linker will normally set the segment flags based on the sections
which comprise the segment. You may use the `FLAGS' keyword to
explicitly specify the segment flags. The value of FLAGS must be an
integer. It is used to set the `p_flags' field of the program header.
Here is an example of `PHDRS'. This shows a typical set of program
headers used on a native ELF system.
PHDRS
{
headers PT_PHDR PHDRS ;
interp PT_INTERP ;
text PT_LOAD FILEHDR PHDRS ;
data PT_LOAD ;
dynamic PT_DYNAMIC ;
}
SECTIONS
{
. = SIZEOF_HEADERS;
.interp : { *(.interp) } :text :interp
.text : { *(.text) } :text
.rodata : { *(.rodata) } /* defaults to :text */
...
. = . + 0x1000; /* move to a new page in memory */
.data : { *(.data) } :data
.dynamic : { *(.dynamic) } :data :dynamic
...
}

File: ld.info, Node: VERSION, Next: Expressions, Prev: PHDRS, Up: Scripts
3.9 VERSION Command
===================
The linker supports symbol versions when using ELF. Symbol versions are
only useful when using shared libraries. The dynamic linker can use
symbol versions to select a specific version of a function when it runs
a program that may have been linked against an earlier version of the
shared library.
You can include a version script directly in the main linker script,
or you can supply the version script as an implicit linker script. You
can also use the `--version-script' linker option.
The syntax of the `VERSION' command is simply
VERSION { version-script-commands }
The format of the version script commands is identical to that used
by Sun's linker in Solaris 2.5. The version script defines a tree of
version nodes. You specify the node names and interdependencies in the
version script. You can specify which symbols are bound to which
version nodes, and you can reduce a specified set of symbols to local
scope so that they are not globally visible outside of the shared
library.
The easiest way to demonstrate the version script language is with a
few examples.
VERS_1.1 {
global:
foo1;
local:
old*;
original*;
new*;
};
VERS_1.2 {
foo2;
} VERS_1.1;
VERS_2.0 {
bar1; bar2;
extern "C++" {
ns::*;
"int f(int, double)";
}
} VERS_1.2;
This example version script defines three version nodes. The first
version node defined is `VERS_1.1'; it has no other dependencies. The
script binds the symbol `foo1' to `VERS_1.1'. It reduces a number of
symbols to local scope so that they are not visible outside of the
shared library; this is done using wildcard patterns, so that any
symbol whose name begins with `old', `original', or `new' is matched.
The wildcard patterns available are the same as those used in the shell
when matching filenames (also known as "globbing"). However, if you
specify the symbol name inside double quotes, then the name is treated
as literal, rather than as a glob pattern.
Next, the version script defines node `VERS_1.2'. This node depends
upon `VERS_1.1'. The script binds the symbol `foo2' to the version
node `VERS_1.2'.
Finally, the version script defines node `VERS_2.0'. This node
depends upon `VERS_1.2'. The scripts binds the symbols `bar1' and
`bar2' are bound to the version node `VERS_2.0'.
When the linker finds a symbol defined in a library which is not
specifically bound to a version node, it will effectively bind it to an
unspecified base version of the library. You can bind all otherwise
unspecified symbols to a given version node by using `global: *;'
somewhere in the version script.
The names of the version nodes have no specific meaning other than
what they might suggest to the person reading them. The `2.0' version
could just as well have appeared in between `1.1' and `1.2'. However,
this would be a confusing way to write a version script.
Node name can be omitted, provided it is the only version node in
the version script. Such version script doesn't assign any versions to
symbols, only selects which symbols will be globally visible out and
which won't.
{ global: foo; bar; local: *; };
When you link an application against a shared library that has
versioned symbols, the application itself knows which version of each
symbol it requires, and it also knows which version nodes it needs from
each shared library it is linked against. Thus at runtime, the dynamic
loader can make a quick check to make sure that the libraries you have
linked against do in fact supply all of the version nodes that the
application will need to resolve all of the dynamic symbols. In this
way it is possible for the dynamic linker to know with certainty that
all external symbols that it needs will be resolvable without having to
search for each symbol reference.
The symbol versioning is in effect a much more sophisticated way of
doing minor version checking that SunOS does. The fundamental problem
that is being addressed here is that typically references to external
functions are bound on an as-needed basis, and are not all bound when
the application starts up. If a shared library is out of date, a
required interface may be missing; when the application tries to use
that interface, it may suddenly and unexpectedly fail. With symbol
versioning, the user will get a warning when they start their program if
the libraries being used with the application are too old.
There are several GNU extensions to Sun's versioning approach. The
first of these is the ability to bind a symbol to a version node in the
source file where the symbol is defined instead of in the versioning
script. This was done mainly to reduce the burden on the library
maintainer. You can do this by putting something like:
__asm__(".symver original_foo,foo@VERS_1.1");
in the C source file. This renames the function `original_foo' to
be an alias for `foo' bound to the version node `VERS_1.1'. The
`local:' directive can be used to prevent the symbol `original_foo'
from being exported. A `.symver' directive takes precedence over a
version script.
The second GNU extension is to allow multiple versions of the same
function to appear in a given shared library. In this way you can make
an incompatible change to an interface without increasing the major
version number of the shared library, while still allowing applications
linked against the old interface to continue to function.
To do this, you must use multiple `.symver' directives in the source
file. Here is an example:
__asm__(".symver original_foo,foo@");
__asm__(".symver old_foo,foo@VERS_1.1");
__asm__(".symver old_foo1,foo@VERS_1.2");
__asm__(".symver new_foo,foo@@VERS_2.0");
In this example, `foo@' represents the symbol `foo' bound to the
unspecified base version of the symbol. The source file that contains
this example would define 4 C functions: `original_foo', `old_foo',
`old_foo1', and `new_foo'.
When you have multiple definitions of a given symbol, there needs to
be some way to specify a default version to which external references to
this symbol will be bound. You can do this with the `foo@@VERS_2.0'
type of `.symver' directive. You can only declare one version of a
symbol as the default in this manner; otherwise you would effectively
have multiple definitions of the same symbol.
If you wish to bind a reference to a specific version of the symbol
within the shared library, you can use the aliases of convenience
(i.e., `old_foo'), or you can use the `.symver' directive to
specifically bind to an external version of the function in question.
You can also specify the language in the version script:
VERSION extern "lang" { version-script-commands }
The supported `lang's are `C', `C++', and `Java'. The linker will
iterate over the list of symbols at the link time and demangle them
according to `lang' before matching them to the patterns specified in
`version-script-commands'.
Demangled names may contains spaces and other special characters. As
described above, you can use a glob pattern to match demangled names,
or you can use a double-quoted string to match the string exactly. In
the latter case, be aware that minor differences (such as differing
whitespace) between the version script and the demangler output will
cause a mismatch. As the exact string generated by the demangler might
change in the future, even if the mangled name does not, you should
check that all of your version directives are behaving as you expect
when you upgrade.

File: ld.info, Node: Expressions, Next: Implicit Linker Scripts, Prev: VERSION, Up: Scripts
3.10 Expressions in Linker Scripts
==================================
The syntax for expressions in the linker script language is identical to
that of C expressions. All expressions are evaluated as integers. All
expressions are evaluated in the same size, which is 32 bits if both the
host and target are 32 bits, and is otherwise 64 bits.
You can use and set symbol values in expressions.
The linker defines several special purpose builtin functions for use
in expressions.
* Menu:
* Constants:: Constants
* Symbols:: Symbol Names
* Orphan Sections:: Orphan Sections
* Location Counter:: The Location Counter
* Operators:: Operators
* Evaluation:: Evaluation
* Expression Section:: The Section of an Expression
* Builtin Functions:: Builtin Functions

File: ld.info, Node: Constants, Next: Symbols, Up: Expressions
3.10.1 Constants
----------------
All constants are integers.
As in C, the linker considers an integer beginning with `0' to be
octal, and an integer beginning with `0x' or `0X' to be hexadecimal.
The linker considers other integers to be decimal.
In addition, you can use the suffixes `K' and `M' to scale a
constant by `1024' or `1024*1024' respectively. For example, the
following all refer to the same quantity:
_fourk_1 = 4K;
_fourk_2 = 4096;
_fourk_3 = 0x1000;

File: ld.info, Node: Symbols, Next: Orphan Sections, Prev: Constants, Up: Expressions
3.10.2 Symbol Names
-------------------
Unless quoted, symbol names start with a letter, underscore, or period
and may include letters, digits, underscores, periods, and hyphens.
Unquoted symbol names must not conflict with any keywords. You can
specify a symbol which contains odd characters or has the same name as a
keyword by surrounding the symbol name in double quotes:
"SECTION" = 9;
"with a space" = "also with a space" + 10;
Since symbols can contain many non-alphabetic characters, it is
safest to delimit symbols with spaces. For example, `A-B' is one
symbol, whereas `A - B' is an expression involving subtraction.

File: ld.info, Node: Orphan Sections, Next: Location Counter, Prev: Symbols, Up: Expressions
3.10.3 Orphan Sections
----------------------
Orphan sections are sections present in the input files which are not
explicitly placed into the output file by the linker script. The
linker will still copy these sections into the output file, but it has
to guess as to where they should be placed. The linker uses a simple
heuristic to do this. It attempts to place orphan sections after
non-orphan sections of the same attribute, such as code vs data,
loadable vs non-loadable, etc. If there is not enough room to do this
then it places at the end of the file.
For ELF targets, the attribute of the section includes section type
as well as section flag.
If an orphaned section's name is representable as a C identifier then
the linker will automatically *note PROVIDE:: two symbols:
__start_SECNAME and __end_SECNAME, where SECNAME is the name of the
section. These indicate the start address and end address of the
orphaned section respectively. Note: most section names are not
representable as C identifiers because they contain a `.' character.

File: ld.info, Node: Location Counter, Next: Operators, Prev: Orphan Sections, Up: Expressions
3.10.4 The Location Counter
---------------------------
The special linker variable "dot" `.' always contains the current
output location counter. Since the `.' always refers to a location in
an output section, it may only appear in an expression within a
`SECTIONS' command. The `.' symbol may appear anywhere that an
ordinary symbol is allowed in an expression.
Assigning a value to `.' will cause the location counter to be
moved. This may be used to create holes in the output section. The
location counter may not be moved backwards inside an output section,
and may not be moved backwards outside of an output section if so doing
creates areas with overlapping LMAs.
SECTIONS
{
output :
{
file1(.text)
. = . + 1000;
file2(.text)
. += 1000;
file3(.text)
} = 0x12345678;
}
In the previous example, the `.text' section from `file1' is located
at the beginning of the output section `output'. It is followed by a
1000 byte gap. Then the `.text' section from `file2' appears, also
with a 1000 byte gap following before the `.text' section from `file3'.
The notation `= 0x12345678' specifies what data to write in the gaps
(*note Output Section Fill::).
Note: `.' actually refers to the byte offset from the start of the
current containing object. Normally this is the `SECTIONS' statement,
whose start address is 0, hence `.' can be used as an absolute address.
If `.' is used inside a section description however, it refers to the
byte offset from the start of that section, not an absolute address.
Thus in a script like this:
SECTIONS
{
. = 0x100
.text: {
*(.text)
. = 0x200
}
. = 0x500
.data: {
*(.data)
. += 0x600
}
}
The `.text' section will be assigned a starting address of 0x100 and
a size of exactly 0x200 bytes, even if there is not enough data in the
`.text' input sections to fill this area. (If there is too much data,
an error will be produced because this would be an attempt to move `.'
backwards). The `.data' section will start at 0x500 and it will have
an extra 0x600 bytes worth of space after the end of the values from
the `.data' input sections and before the end of the `.data' output
section itself.
Setting symbols to the value of the location counter outside of an
output section statement can result in unexpected values if the linker
needs to place orphan sections. For example, given the following:
SECTIONS
{
start_of_text = . ;
.text: { *(.text) }
end_of_text = . ;
start_of_data = . ;
.data: { *(.data) }
end_of_data = . ;
}
If the linker needs to place some input section, e.g. `.rodata', not
mentioned in the script, it might choose to place that section between
`.text' and `.data'. You might think the linker should place `.rodata'
on the blank line in the above script, but blank lines are of no
particular significance to the linker. As well, the linker doesn't
associate the above symbol names with their sections. Instead, it
assumes that all assignments or other statements belong to the previous
output section, except for the special case of an assignment to `.'.
I.e., the linker will place the orphan `.rodata' section as if the
script was written as follows:
SECTIONS
{
start_of_text = . ;
.text: { *(.text) }
end_of_text = . ;
start_of_data = . ;
.rodata: { *(.rodata) }
.data: { *(.data) }
end_of_data = . ;
}
This may or may not be the script author's intention for the value of
`start_of_data'. One way to influence the orphan section placement is
to assign the location counter to itself, as the linker assumes that an
assignment to `.' is setting the start address of a following output
section and thus should be grouped with that section. So you could
write:
SECTIONS
{
start_of_text = . ;
.text: { *(.text) }
end_of_text = . ;
. = . ;
start_of_data = . ;
.data: { *(.data) }
end_of_data = . ;
}
Now, the orphan `.rodata' section will be placed between
`end_of_text' and `start_of_data'.

File: ld.info, Node: Operators, Next: Evaluation, Prev: Location Counter, Up: Expressions
3.10.5 Operators
----------------
The linker recognizes the standard C set of arithmetic operators, with
the standard bindings and precedence levels:
precedence associativity Operators Notes
(highest)
1 left ! - ~ (1)
2 left * / %
3 left + -
4 left >> <<
5 left == != > < <= >=
6 left &
7 left |
8 left &&
9 left ||
10 right ? :
11 right &= += -= *= /= (2)
(lowest)
Notes: (1) Prefix operators (2) *Note Assignments::.

File: ld.info, Node: Evaluation, Next: Expression Section, Prev: Operators, Up: Expressions
3.10.6 Evaluation
-----------------
The linker evaluates expressions lazily. It only computes the value of
an expression when absolutely necessary.
The linker needs some information, such as the value of the start
address of the first section, and the origins and lengths of memory
regions, in order to do any linking at all. These values are computed
as soon as possible when the linker reads in the linker script.
However, other values (such as symbol values) are not known or needed
until after storage allocation. Such values are evaluated later, when
other information (such as the sizes of output sections) is available
for use in the symbol assignment expression.
The sizes of sections cannot be known until after allocation, so
assignments dependent upon these are not performed until after
allocation.