Using GNU ld

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".

1. Overview
2. Invocation
3. Linker Scripts
4. Machine Dependent Features
5. BFD

6. Reporting Bugs
A. MRI Compatible Script Files
B. GNU Free Documentation License
Index

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. See section 5. 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).

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.

2.1 Command Line Options
2.2 Environment Variables

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 can not 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. Note that specifying a script in this way should only be used to augment the main linker script; if you want to use some command that logically can only appear once, such as the SECTIONS or MEMORY command, you must replace the default linker script using the `-T' option. See section 3. Linker 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 preceeded 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 (eg `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:

-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. See section ld and the Intel 960 family, 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'.) See section 5. 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. See section 2.2 Environment Variables. You can also define the input format from a script, using the command TARGET; see 3.4.3 Commands dealing with object file formats.

-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 MRI Compatible Script Files. 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. See section 3.4.4 Other linker script 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). See section 3.4.1 Setting the entry point, for a discussion of defaults and other ways of specifying the entry point.

-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.

-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.

-larchive

--library=archive

Add archive file archive to the list of files to link. This option may be used any number of times. ld will search its path-list for occurrences of libarchive.a for every archive specified.

On systems which support shared libraries, ld may also search for libraries with extensions other than .a. Specifically, on ELF and SunOS systems, ld will search a directory for a library with an extension of .so before searching for one with an extension of .a. By convention, a .so extension indicates a shared library.

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.

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. See section 2.2 Environment Variables.

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 and symbols 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.

-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. If the output format supports Unix style magic numbers, mark the output as OMAGIC.

-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.

-q

--emit-relocs

Leave relocation sections and contents in fully linked exececutables. Post link analysis and optimization tools may need this information in order to perform correct modifications of executables. This results in larger executables.

-r

--relocateable

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'.

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. You must use this option if you want to use a command which can only appear once in a linker script, such as the SECTIONS or MEMORY command. See section 3. Linker 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.

-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. For most targets, this is all local symbols whose names begin with `L'.

-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 initfirst, interpose, loadfltr, nodefaultlib, nodelete, nodlopen, nodump, now and origin. The other keywords are ignored for Solaris compatibility. initfirst marks the object to be initialized first at runtime before any other objects. interpose marks the object that its symbol table interposes before all symbols but the primary executable. loadfltr marks the object that its filtees be processed immediately at runtime. 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. now marks the object with the non-lazy runtime binding. origin marks the object may contain $ORIGIN. defs disallows undefined symbols.

-( 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.

-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. --no-undefined 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.

-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.

--check-sections

--no-check-sections

Asks the linker not to check section addresses after they have been assigned to see if there 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.

--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 (see section Assignment: Symbol Definitions). 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.

--embedded-relocs

This option is only meaningful when linking MIPS embedded PIC code, generated by the -membedded-pic option to the GNU compiler and assembler. It causes the linker to create a table which may be used at runtime to relocate any data which was statically initialized to pointer values. See the code in testsuite/ld-empic for details.

--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.

--no-gc-sections

--gc-sections

Enable garbage collection of unused input sections. It is ignored on targets that do not support this option. This option is not compatible with `-r', nor should it be used with dynamic linking. The default behaviour (of not performing this garbage collection) can be restored by specifying `--no-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

Normally when creating a non-symbolic shared library, undefined symbols are allowed and left to be resolved by the runtime loader. These options disallow such undefined symbols.

--allow-shlib-undefined

Allow undefined symbols in shared objects even when --no-undefined is set. The net result will be that undefined symbols in regular objects will still trigger an error, but undefined symbols in shared objects will be ignored. The implementation of no_undefined makes the assumption that the runtime linker will choke on undefined symbols. However there is at least one system (BeOS) where undefined symbols in shared libraries is normal since the kernel patches them at load time to select which function is most appropriate for the current architecture. I.E. dynamically select an appropriate memset function. Apparently it is also normal for HPPA shared libraries to have undefined 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-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.

--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. See section 5. BFD.

-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. See section ld and the H8/300. See section ld and the Intel 960 family.

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 filesystems.

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. It is for the native linker only.
3. On an ELF system, if the -rpath and rpath-link options were not used, search the contents of the environment variable LD_RUN_PATH. It is for the native linker only.
4. On SunOS, if the -rpath option was not used, search any directories specified using -L options.
5. For a native linker, 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

This option tells ld to sort the common symbols by size when it places them in the appropriate output sections. First come all the one byte symbols, then all the two bytes, then all the four bytes, and then everything else. This is to prevent gaps between symbols due to alignment constraints.

--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.

--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. If you need more elaborate expressions, consider using the linker command language from a script. Note: there should be no white space between sectionname, the equals sign ("="), and org.

-Tbss org

-Tdata org

-Ttext org

Use org as the starting address for--respectively--the bss, data, or the text segment of the output file. org must be a single hexadecimal integer; for compatibility with other linkers, you may omit the leading `0x' usually associated with hexadecimal values.

--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 if using a default builtin script.

--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 heirarchy for the library being created. This option is only meaningful on ELF platforms which support shared libraries. See section 3.9 VERSION Command.

--warn-common

Warn when a common symbol is combined with another common symbol or with a symbol definition. Unix linkers allow this somewhat sloppy practice, but linkers on some other operating systems do not. This option allows you to find potential problems from combining global symbols. Unfortunately, some C libraries use this practice, so you may get some warnings about symbols in the libraries as well as in your programs.

There are three kinds of global symbols, illustrated here by C examples:

`int i = 1;'

A definition, which goes in the initialized data section of the output file.

`extern int i;'

An undefined reference, which does not allocate space. There must be either a definition or a common symbol for the variable somewhere.

`int i;'

A common symbol. If there are only (one or more) common symbols for a variable, it goes in the uninitialized data area of the output file. The linker merges multiple common symbols for the same variable into a single symbol. If they are of different sizes, it picks the largest size. The linker turns a common symbol into a declaration, if there is a definition of the same variable.

The `--warn-common' option can produce five kinds of warnings. Each warning consists of a pair of lines: the first describes the symbol just encountered, and the second describes the previous symbol encountered with the same name. One or both of the two symbols will be a common symbol.

1. Turning a common symbol into a reference, because there is already a definition for the symbol.

   file(section): warning: common of `symbol' overridden by definition file(section): warning: defined here

2. Turning a common symbol into a reference, because a later definition for the symbol is encountered. This is the same as the previous case, except that the symbols are encountered in a different order.

   file(section): warning: definition of `symbol' overriding common file(section): warning: common is here

3. Merging a common symbol with a previous same-sized common symbol.

   file(section): warning: multiple common of `symbol' file(section): warning: previous common is here

4. Merging a common symbol with a previous larger common symbol.

   file(section): warning: common of `symbol' overridden by larger common file(section): warning: larger common is here

5. Merging a common symbol with a previous smaller common symbol. This is the same as the previous case, except that the symbols are encountered in a different order.

   file(section): warning: common of `symbol' overriding smaller common file(section): warning: smaller common is here

--warn-constructors

Warn if any global constructors are used. This is only useful for a few object file formats. For formats like COFF or ELF, the linker can not detect the use of global constructors.

--warn-multiple-gp

Warn if multiple global pointer values are required in the output file. This is only meaningful for certain processors, such as the Alpha. Specifically, some processors put large-valued constants in a special section. A special register (the global pointer) points into the middle of this section, so that constants can be loaded efficiently via a base-register relative addressing mode. Since the offset in base-register relative mode is fixed and relatively small (e.g., 16 bits), this limits the maximum size of the constant pool. Thus, in large programs, it is often necessary to use multiple global pointer values in order to be able to address all possible constants. This option causes a warning to be issued whenever this case occurs.

--warn-once

Only warn once for each undefined symbol, rather than once per module which refers to it.

--warn-section-align

Warn if the address of an output section is changed because of alignment. Typically, the alignment will be set by an input section. The address will only be changed if it not explicitly specified; that is, if the SECTIONS command does not specify a start address for the section (see section 3.6 SECTIONS command).

--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 (int c) { printf ("malloc called with %ld\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.

--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.

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.

--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'.

--dll

Create a DLL instead of a regular executable. You may also use -shared or specify a LIBRARY in a given .def file.

--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.

--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, and impure_ptr will not be automatically exported.

--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.

--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.

--heap reserve

--heap reserve,commit

Specify the amount of memory to reserve (and optionally commit) to be used as heap for this program. The default is 1Mb reserved, 4K committed.

--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.

--kill-at

If given, the stdcall suffixes (@nn) will be stripped from symbols before they are exported.

--major-image-version value

Sets the major number of the "image version". Defaults to 1.

--major-os-version value

Sets the major number of the "os version". Defaults to 4.

--major-subsystem-version value

Sets the major number of the "subsystem version". Defaults to 4.

--minor-image-version value

Sets the minor number of the "image version". Defaults to 0.

--minor-os-version value

Sets the minor number of the "os version". Defaults to 0.

--minor-subsystem-version value

Sets the minor number of the "subsystem version". Defaults to 0.

--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.

--section-alignment

Sets the section alignment. Sections in memory will always begin at addresses which are a multiple of this number. Defaults to 0x1000.

--stack reserve

--stack reserve,commit

Specify the amount of memory to reserve (and optionally commit) to be used as stack for this program. The default is 32Mb reserved, 4K committed.

--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, and posix. You may optionally set the subsystem version also.

2.2 Environment Variables

You can change the behavior 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 (see section 5. 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.

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. See section 3.11 Implicit Linker Scripts.

3.1 Basic Linker Script Concepts
3.2 Linker Script Format
3.3 Simple Linker Script Example
3.4 Simple Linker Script Commands
3.5 Assigning Values to Symbols
3.6 SECTIONS command		SECTIONS Command
3.7 MEMORY command		MEMORY Command
3.8 PHDRS Command
3.9 VERSION Command
3.10 Expressions in Linker Scripts
3.11 Implicit Linker Scripts

3.1 Basic Linker Script Concepts

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.

3.2 Linker Script Format

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.

3.3 Simple Linker Script Example

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.

3.4 Simple Linker Script Commands

3.4.1 Setting the entry point
3.4.2 Commands dealing with files
3.4.3 Commands dealing with object file formats

3.4.4 Other linker script commands

3.4.1 Setting the entry point

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.

3.4.2 Commands dealing 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.

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.

The linker will first 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 Command Line 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 Command Line Options.

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 (see section Command Line 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 (see section Command Line 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.

3.4.3 Commands dealing 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 (see section 5. BFD). Using OUTPUT_FORMAT(bfdname) is exactly like using `-oformat bfdname' on the command line (see section Command Line 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 (see section Command Line 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. See section 5. BFD.

3.4.4 Other linker script 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').

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 (see section 5. BFD). You can see the architecture of an object file by using the objdump program with the `-f' option.

3.5 Assigning Values to Symbols

3.5.1 Simple Assignments
3.5.2 PROVIDE

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.

The semicolon after expression is required.

Expressions are defined below; see 3.10 Expressions in Linker Scripts.

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 3.10.6 The Section of an Expression.

Here is an example showing the three different places that symbol assignments may be used:

floating_point = 0; SECTIONS { .text : { *(.text) _etext = .; } _bdata = (. + 3) & ~ 4; .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.

3.5.2 PROVIDE

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.

3.6 SECTIONS command

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 (see section Entry command)
• a symbol assignment (see section 3.5 Assigning Values to Symbols)
• 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.

3.6.1 Output section description
3.6.2 Output section name
3.6.3 Output section address
3.6.4 Input section description
3.6.5 Output section data
3.6.6 Output section keywords
3.6.7 Output section discarding
3.6.8 Output section attributes
3.6.9 Overlay description

3.6.1 Output section description

section [address] [(type)] : [AT(lma)]
  {
    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 (see section 3.5 Assigning Values to Symbols)
• an input section description (see section 3.6.4 Input section description)
• data values to include directly (see section 3.6.5 Output section data)
• a special output section keyword (see section 3.6.6 Output section keywords)

3.6.2 Output section name

The output section name `/DISCARD/' is special; 3.6.7 Output section discarding.

3.6.3 Output section address

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; 3.10 Expressions in Linker Scripts. 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.

3.6.4 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.

3.6.4.1 Input section basics
3.6.4.2 Input section wildcard patterns
3.6.4.3 Input section for common symbols
3.6.4.4 Input section and garbage collection
3.6.4.5 Input section example

3.6.4.1 Input section basics

The file name and the section name may be wildcard patterns, which we describe further below (see section 3.6.4.2 Input section wildcard patterns).

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. 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(.d