LD(7)

NAME
ld - The GNU Linker

LD
This file documents the GNU linker ld version "2.17.50 [FreeBSD] 2007-07-03".

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”.

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 “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).

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.

  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 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. Note that specifying a script in this way merely augments the main linker script; use the -T option to replace the default linker script entirely.See Section “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.

-a keyword
    This option is supported for HP/UX compatibility. The keyword argument must be one of the strings archive, shared, or default. -aarchive is functionally equivalent to -Bstatic, and the other two keywords are functionally equivalent to -Bdynamic. This option may be used any number of times.

-A architecture

--architecture= architecture
    In the current release of ld, this option is useful only for the Intel 960 family of architectures. In that ld configuration, the architecture argument identifies the particular architecture in the 960 family, enabling some safeguards and modifying the archive-library search path.See Section “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.) See Section.Dq 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.Dq Environment . You can also define the input format from a script, using the command TARGET; see 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 MRI,,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 “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).See Section “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.

-G value

--gpsize= value
    Set the maximum size of objects to be optimized using the GP register to size. This is only meaningful for object file formats such as MIPS ECOFF which supports putting large and small objects into different sections. This is ignored for other object file formats.

-h name

-soname= name
    When creating an ELF shared object, set the internal DT_SONAME field to the specified name. When an executable is linked with a shared object which has a DT_SONAME field, then when the executable is run the dynamic linker will attempt to load the shared object specified by the DT_SONAME field rather than the using the file name given to the linker.

-i  Perform an incremental link (same as option -r).

-init name
    When creating an ELF executable or shared object, call NAME when the executable or shared object is loaded, by setting DT_INIT to the address of the function. By default, the linker uses _init as the function to call.

-l namespec

--library= namespec
    Add the archive or object file specified by namespec to the list of files to link. This option may be used any number of times. If namespec is of the form: filename, ld will search the library path for a file called filename, otherise it will search the library path for a file called lib namespec.a.

    On systems which support shared libraries, ld may also search for files other than lib namespec.a. Specifically, on ELF and SunOS systems, ld will search a directory for a library called lib namespec.so before searching for one called lib namespec.a. (By convention, a .so extension indicates a shared library.) Note that this behavior does not apply to: filename, which always specifies a file called filename.

    The linker will search an archive only once, at the location where it is specified on the command line. If the archive defines a symbol which was undefined in some object which appeared before the archive on the command line, the linker will include the appropriate file(s) from the archive. However, an undefined symbol in an object appearing later on the command line will not cause the linker to search the archive again.

    See the [-(] option for a way to force the linker to search archives multiple times.

    You may list the same archive multiple times on the command line.

    This type of archive searching is standard for Unix linkers. However, if you are using ld on AIX, note that it is different from the behaviour of the AIX linker.

-L searchdir

--library-path= searchdir
    Add path searchdir to the list of paths that ld will search for archive libraries and ld control scripts. You may use this option any number of times. The directories are searched in the order in which they are specified on the command line. Directories specified on the command line are searched before the default directories. All [-L] options apply to all [-l] options, regardless of the order in which the options appear.

    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.See Section “Environment”.

    The paths can also be specified in a link script with the SEARCH_DIR command. Directories specified this way are searched at the point in which the linker script appears in the command line.

-m emulation
    Emulate the emulation linker. You can list the available emulations with the --verbose or -V options.

    If the -m option is not used, the emulation is taken from the LDEMULATION environment variable, if that is defined.

    Otherwise, the default emulation depends upon how the linker was configured.

-M

--print-map
    Print a link map to the standard output. A link map provides information about the link, including the following:

    •   Where object files are mapped into memory.

    •   How common symbols are allocated.

    •   All archive members included in the link, with a mention of the symbol which caused the archive member to be brought in.

    •   The values assigned to symbols.

        Note - symbols whose values are computed by an expression which involves a reference to a previous value of the same symbol may not have correct result displayed in the link map. This is because the linker discards intermediate results and only retains the final value of an expression. Under such circumstances the linker will display the final value enclosed by square brackets. Thus for example a linker script containing:

               foo = 1
               foo = foo * 4
               foo = foo + 8

        will produce the following output in the link map if the [-M] option is used:

               0x00000001                foo = 0x1
               [0x0000000c]                foo = (foo * 0x4)
               [0x0000000c]                foo = (foo + 0x8)

        See 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.

-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.See Section “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.See Section “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 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.See Section “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 (see Section “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
    Treat all warnings as errors.

--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. This option is not compatible with -r or --emit-relocs. The default behaviour (of not performing this garbage collection) can be restored by specifying --no-gc-sections on the command line.

--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.See Section “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.See Section “H8/300”. See Section.Dq i960 . See Section.Dq Xtensa . See Section.Dq M68HC11/68HC12 . See Section.Dq 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, 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 byte, then all the four byte, and then everything else. This is to prevent gaps between symbols due to alignment constraints.

--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.See Section “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 (see Section “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.

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 amount 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.

    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 amount 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]

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.

  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 (see Section “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.

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 “Implicit Linker Scripts”.

  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.

  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.

  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.

  Simple Linker Script Commands
In this section we describe the simple linker script commands.

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.

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.

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 Options,,Command Line Options.

    If you use INPUT (-l file), ld will transform the name to lib file.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 Options,,Command Line 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 (see Section “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 “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.

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 (see Section “BFD”). Using OUTPUT_FORMAT( bfdname) is exactly like using --oformat bfdname on the command line (see Section “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 “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 “BFD”.

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 symbol s 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.

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 “BFD”). You can see the architecture of an object file by using the objdump program with the -f option.

  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.

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.See Section “Location Counter”.

The semicolon after expression is required.

Expressions are defined below; see 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 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.

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.

PROVIDE_HIDDEN

Similar to PROVIDE. For ELF targeted ports, the symbol will be hidden and won't be exported.

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.

  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 (see Section “Entry Point”)

•   a symbol assignment (see Section “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.

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 (see Section “Assignments”)

•   an input section description (see Section “Input Section”)

•   data values to include directly (see Section “Output Section Data”)

•   a special output section keyword (see Section “Output Section Keywords”)

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; Output Section Discarding.

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; 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.

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.

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 (see Section “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)

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 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.

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) }
    }

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).

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)).

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)
        }
    }

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 (see Section “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.See Section “Output Section Fill”, for details on the fill expression.

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.

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 (see Section “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.

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.

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) : { ... }
      ...
    }

Output Section LMA

Every section has a virtual address (VMA) and a load address (LMA); see Basic Script Concepts. The address expression which may appear in an output section description sets the VMA (see Section “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.See Section “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.See Section “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;

Forced Output Alignment

You can increase an output section's alignment by using ALIGN.

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.

Output Section Region

You can assign a section to a previously defined region of memory by using > region. See Section.Dq MEMORY .

Here is a simple example:

    MEMORY { rom : ORIGIN = 0x1000, LENGTH = 0x1000 }
    SECTIONS { ROM : { *(.text) } >rom }

Output Section Phdr

You can assign a section to a previously defined program segment by using: phdr. See Section.Dq 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 }

Output Section Fill

You can set the fill pattern for an entire section by using = fillexp. fillexp is an expression (see Section “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; (see Section “Output Section Data”).

Here is a simple example:

    SECTIONS { .text : { *(.text) } =0x90909090 }

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 (see Section “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.See Section “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));

  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 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.See Section “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;

  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.See Section “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 (see Section “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
      ...
    }

  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.

  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.

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;

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.

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.

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 (see Section “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.

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)See Section “Assignments”.

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.

Some expressions, such as those depending upon the location counter., must be evaluated during section allocation.

If the result of an expression is required, but the value is not available, then an error results. For example, a script like the following

    SECTIONS
      {
        .text 9+this_isnt_constant :
          { *(.text) }
      }

will cause the error message non constant expression for initial address.

The Section of an Expression

When the linker evaluates an expression, the result is either absolute or relative to some section. A relative expression is expressed as a fixed offset from the base of a section.

The position of the expression within the linker script determines whether it is absolute or relative. An expression which appears within an output section definition is relative to the base of the output section. An expression which appears elsewhere will be absolute.

A symbol set to a relative expression will be relocatable if you request relocatable output using the -r option. That means that a further link operation may change the value of the symbol. The symbol's section will be the section of the relative expression.

A symbol set to an absolute expression will retain the same value through any further link operation. The symbol will be absolute, and will not have any particular associated section.

You can use the builtin function ABSOLUTE to force an expression to be absolute when it would otherwise be relative. For example, to create an absolute symbol set to the address of the end of the output section .data:

    SECTIONS
      {
        .data : { *(.data) _edata = ABSOLUTE(.); }
      }

If ABSOLUTE were not used, _edata would be relative to the .data section.

Builtin Functions

The linker script language includes a number of builtin functions for use in linker script expressions.

ABSOLUTE( exp)
    Return the absolute (non-relocatable, as opposed to non-negative) value of the expression exp. Primarily useful to assign an absolute value to a symbol within a section definition, where symbol values are normally section relative.See Section “Expression Section”.

ADDR( section)
    Return the absolute address (the VMA) of the named section. Your script must previously have defined the location of that section. In the following example, symbol_1 and symbol_2 are assigned identical values:

        SECTIONS { ...
          .output1 :
            {
            start_of_output_1 = ABSOLUTE(.);
            ...
            }
          .output :
            {
            symbol_1 = ADDR(.output1);
            symbol_2 = start_of_output_1;
            }
        ... }

ALIGN( align)

ALIGN( exp, align)
    Return the location counter (.) or arbitrary expression aligned to the next align boundary. The single operand ALIGN doesn't change the value of the location counter---it just does arithmetic on it. The two operand ALIGN allows an arbitrary expression to be aligned upwards ( ALIGN( align) is equivalent to ALIGN(., align)).

    Here is an example which aligns the output .data section to the next 0x2000 byte boundary after the preceding section and sets a variable within the section to the next 0x8000 boundary after the input sections:

        SECTIONS { ...
          .data ALIGN(0x2000): {
            *(.data)
            variable = ALIGN(0x8000);
          }
        ... }

    The first use of ALIGN in this example specifies the location of a section because it is used as the optional address attribute of a section definition (see Section “Output Section Address”). The second use of ALIGN is used to defines the value of a symbol.

    The builtin function NEXT is closely related to ALIGN.

ALIGNOF( section)
    Return the alignment in bytes of the named section, if that section has been allocated. If the section has not been allocated when this is evaluated, the linker will report an error. In the following example, the alignment of the .output section is stored as the first value in that section.

        SECTIONS{ ...
          .output {
            LONG (ALIGNOF (.output))
            ...
            }
        ... }

BLOCK( exp)
    This is a synonym for ALIGN, for compatibility with older linker scripts. It is most often seen when setting the address of an output section.

DATA_SEGMENT_ALIGN( maxpagesize, commonpagesize)
    This is equivalent to either

        (ALIGN(maxpagesize) + (. & (maxpagesize - 1)))

    or

        (ALIGN(maxpagesize) + (. & (maxpagesize - commonpagesize)))

    depending on whether the latter uses fewer commonpagesize sized pages for the data segment (area between the result of this expression and DATA_SEGMENT_END) than the former or not. If the latter form is used, it means commonpagesize bytes of runtime memory will be saved at the expense of up to commonpagesize wasted bytes in the on-disk file.

    This expression can only be used directly in SECTIONS commands, not in any output section descriptions and only once in the linker script. commonpagesize should be less or equal to maxpagesize and should be the system page size the object wants to be optimized for (while still working on system page sizes up to maxpagesize).

    Example:

          . = DATA_SEGMENT_ALIGN(0x10000, 0x2000);

DATA_SEGMENT_END( exp)
    This defines the end of data segment for DATA_SEGMENT_ALIGN evaluation purposes.

          . = DATA_SEGMENT_END(.);

DATA_SEGMENT_RELRO_END( offset, exp)
    This defines the end of the PT_GNU_RELRO segment when -z relro option is used. Second argument is returned. When -z relro option is not present, DATA_SEGMENT_RELRO_END does nothing, otherwise DATA_SEGMENT_ALIGN is padded so that exp + offset is aligned to the most commonly used page boundary for particular target. If present in the linker script, it must always come in between DATA_SEGMENT_ALIGN and DATA_SEGMENT_END.

          . = DATA_SEGMENT_RELRO_END(24, .);

DEFINED( symbol)
    Return 1 if symbol is in the linker global symbol table and is defined before the statement using DEFINED in the script, otherwise return 0. You can use this function to provide default values for symbols. For example, the following script fragment shows how to set a global symbol begin to the first location in the .text section---but if a symbol called begin already existed, its value is preserved:

        SECTIONS { ...
          .text : {
            begin = DEFINED(begin) ? begin : . ;
            ...
          }
          ...
        }

LENGTH( memory)
    Return the length of the memory region named memory.

LOADADDR( section)
    Return the absolute LMA of the named section. This is normally the same as ADDR, but it may be different if the AT attribute is used in the output section definition (see Section “Output Section LMA”).

MAX( exp1, exp2)
    Returns the maximum of exp1 and exp2.

MIN( exp1, exp2)
    Returns the minimum of exp1 and exp2.

NEXT( exp)
    Return the next unallocated address that is a multiple of exp. This function is closely related to ALIGN( exp); unless you use the MEMORY command to define discontinuous memory for the output file, the two functions are equivalent.

ORIGIN( memory)
    Return the origin of the memory region named memory.

SEGMENT_START( segment, default)
    Return the base address of the named segment. If an explicit value has been given for this segment (with a command-line -T option) that value will be returned; otherwise the value will be default. At present, the -T command-line option can only be used to set the base address for the “text”, “data”, and “bss” sections, but you use SEGMENT_START with any segment name.

SIZEOF( section)
    Return the size in bytes of the named section, if that section has been allocated. If the section has not been allocated when this is evaluated, the linker will report an error. In the following example, symbol_1 and symbol_2 are assigned identical values:

        SECTIONS{ ...
          .output {
            .start = . ;
            ...
            .end = . ;
            }
          symbol_1 = .end - .start ;
          symbol_2 = SIZEOF(.output);
        ... }

SIZEOF_HEADERS

sizeof_headers
    Return the size in bytes of the output file's headers. This is information which appears at the start of the output file. You can use this number when setting the start address of the first section, if you choose, to facilitate paging.

    When producing an ELF output file, if the linker script uses the SIZEOF_HEADERS builtin function, the linker must compute the number of program headers before it has determined all the section addresses and sizes. If the linker later discovers that it needs additional program headers, it will report an error not enough room for program headers. To avoid this error, you must avoid using the SIZEOF_HEADERS function, or you must rework your linker script to avoid forcing the linker to use additional program headers, or you must define the program headers yourself using the PHDRS command (see Section “PHDRS”).

  Implicit Linker Scripts
If you specify a linker input file which the linker can not recognize as an object file or an archive file, it will try to read the file as a linker script. If the file can not be parsed as a linker script, the linker will report an error.

An implicit linker script will not replace the default linker script.

Typically an implicit linker script would contain only symbol assignments, or the INPUT, GROUP, or VERSION commands.

Any input files read because of an implicit linker script will be read at the position in the command line where the implicit linker script was read. This can affect archive searching.

Machine Dependent Features
ld has additional features on some platforms; the following sections describe them. Machines where ld has no additional functionality are not listed.

  ld and the H8/300
For the H8/300, ld can perform these global optimizations when you specify the --relax command-line option.

relaxing address modes
    ld finds all jsr and jmp instructions whose targets are within eight bits, and turns them into eight-bit program-counter relative bsr and bra instructions, respectively.

synthesizing instructions
    ld finds all mov.b instructions which use the sixteen-bit absolute address form, but refer to the top page of memory, and changes them to use the eight-bit address form. (That is: the linker turns mov.b @ aa:16 into mov.b @ aa:8 whenever the address aa is in the top page of memory).

bit manipulation instructions
    ld finds all bit manipulation instructions like band, bclr, biand, bild, bior, bist, bixor, bld, bnot, bor, bset, bst, btst, bxor which use 32 bit and 16 bit absolute address form, but refer to the top page of memory, and changes them to use the 8 bit address form. (That is: the linker turns bset #xx:3, @ aa:32 into bset #xx:3, @ aa:8 whenever the address aa is in the top page of memory).

system control instructions
    ld finds all ldc.w, stc.w instructions which use the 32 bit absolute address form, but refer to the top page of memory, and changes them to use 16 bit address form. (That is: the linker turns ldc.w @ aa:32,ccr into ldc.w @ aa:16,ccr whenever the address aa is in the top page of memory).

  ld and the Intel 960 Family
You can use the -A architecture command line option to specify one of the two-letter names identifying members of the 960 family; the option specifies the desired output target, and warns of any incompatible instructions in the input files. It also modifies the linker's search strategy for archive libraries, to support the use of libraries specific to each particular architecture, by including in the search loop names suffixed with the string identifying the architecture.

For example, if your ld command line included -ACA as well as -ltry , the linker would look (in its built-in search paths, and in any paths you specify with -L) for a library with the names

    try
    libtry.a
    tryca
    libtryca.a

The first two possibilities would be considered in any event; the last two are due to the use of -ACA .

You can meaningfully use -A more than once on a command line, since the 960 architecture family allows combination of target architectures; each use will add another pair of name variants to search for when -l specifies a library.

ld supports the --relax option for the i960 family. If you specify --relax, ld finds all balx and calx instructions whose targets are within 24 bits, and turns them into 24-bit program-counter relative bal and cal instructions, respectively. ld also turns cal instructions into bal instructions when it determines that the target subroutine is a leaf routine (that is, the target subroutine does not itself call any subroutines).

  ld and the Motorola 68HC11 and 68HC12 families
Linker Relaxation

For the Motorola 68HC11, ld can perform these global optimizations when you specify the --relax command-line option.

relaxing address modes
    ld finds all jsr and jmp instructions whose targets are within eight bits, and turns them into eight-bit program-counter relative bsr and bra instructions, respectively.

    ld also looks at all 16-bit extended addressing modes and transforms them in a direct addressing mode when the address is in page 0 (between 0 and 0x0ff).

relaxing gcc instruction group
    When gcc is called with [-mrelax], it can emit group of instructions that the linker can optimize to use a 68HC11 direct addressing mode. These instructions consists of bclr or bset instructions.

Trampoline Generation

For 68HC11 and 68HC12, ld can generate trampoline code to call a far function using a normal jsr instruction. The linker will also change the relocation to some far function to use the trampoline address instead of the function address. This is typically the case when a pointer to a function is taken. The pointer will in fact point to the function trampoline.

The --pic-veneer switch makes the linker use PIC sequences for ARM/Thumb interworking veneers, even if the rest of the binary is not PIC. This avoids problems on uClinux targets where --emit-relocs is used to generate relocatable binaries.

  ld and the ARM family
For the ARM, ld will generate code stubs to allow functions calls between ARM and Thumb code. These stubs only work with code that has been compiled and assembled with the -mthumb-interwork command line option. If it is necessary to link with old ARM object files or libraries, which have not been compiled with the -mthumb-interwork option then the --support-old-code command line switch should be given to the linker. This will make it generate larger stub functions which will work with non-interworking aware ARM code. Note, however, the linker does not support generating stubs for function calls to non-interworking aware Thumb code.

The --thumb-entry switch is a duplicate of the generic --entry switch, in that it sets the program's starting address. But it also sets the bottom bit of the address, so that it can be branched to using a BX instruction, and the program will start executing in Thumb mode straight away.

The --be8 switch instructs ld to generate BE8 format executables. This option is only valid when linking big-endian objects. The resulting image will contain big-endian data and little-endian code.

The R_ARM_TARGET1 relocation is typically used for entries in the .init_array section. It is interpreted as either R_ARM_REL32 or R_ARM_ABS32, depending on the target. The --target1-rel and --target1-abs switches override the default.

The --target2=type switch overrides the default definition of the R_ARM_TARGET2 relocation. Valid values for type, their meanings, and target defaults are as follows:

rel R_ARM_REL32 (arm*-*-elf, arm*-*-eabi)

abs R_ARM_ABS32 (arm*-*-symbianelf)

got-rel
    R_ARM_GOT_PREL (arm*-*-linux, arm*-*-*bsd)

The R_ARM_V4BX relocation (defined by the ARM AAELF specification) enables objects compiled for the ARMv4 architecture to be interworking-safe when linked with other objects compiled for ARMv4t, but also allows pure ARMv4 binaries to be built from the same ARMv4 objects.

In the latter case, the switch [--fix-v4bx] must be passed to the linker, which causes v4t BX rM instructions to be rewritten as MOV PC,rM, since v4 processors do not have a BX instruction.

In the former case, the switch should not be used, and R_ARM_V4BX relocations are ignored.

The --use-blx switch enables the linker to use ARM/Thumb BLX instructions (available on ARMv5t and above) in various situations. Currently it is used to perform calls via the PLT from Thumb code using BLX rather than using BX and a mode-switching stub before each PLT entry. This should lead to such calls executing slightly faster.

This option is enabled implicitly for SymbianOS, so there is no need to specify it if you are using that target.

The --vfp11-denorm-fix switch enables a link-time workaround for a bug in certain VFP11 coprocessor hardware, which sometimes allows instructions with denorm operands (which must be handled by support code) to have those operands overwritten by subsequent instructions before the support code can read the intended values.

The bug may be avoided in scalar mode if you allow at least one intervening instruction between a VFP11 instruction which uses a register and another instruction which writes to the same register, or at least two intervening instructions if vector mode is in use. The bug only affects full-compliance floating-point mode: you do not need this workaround if you are using "runfast" mode. Please contact ARM for further details.

If you know you are using buggy VFP11 hardware, you can enable this workaround by specifying the linker option --vfp-denorm-fix=scalar if you are using the VFP11 scalar mode only, or --vfp-denorm-fix=vector if you are using vector mode (the latter also works for scalar code). The default is --vfp-denorm-fix=none.

If the workaround is enabled, instructions are scanned for potentially-troublesome sequences, and a veneer is created for each such sequence which may trigger the erratum. The veneer consists of the first instruction of the sequence and a branch back to the subsequent instruction. The original instruction is then replaced with a branch to the veneer. The extra cycles required to call and return from the veneer are sufficient to avoid the erratum in both the scalar and vector cases.

The --no-enum-size-warning switch prevents the linker from warning when linking object files that specify incompatible EABI enumeration size attributes. For example, with this switch enabled, linking of an object file using 32-bit enumeration values with another using enumeration values fitted into the smallest possible space will not be diagnosed.

  ld and HPPA 32-bit ELF Support
When generating a shared library, ld will by default generate import stubs suitable for use with a single sub-space application. The --multi-subspace switch causes ld to generate export stubs, and different (larger) import stubs suitable for use with multiple sub-spaces.

Long branch stubs and import/export stubs are placed by ld in stub sections located between groups of input sections. --stub-group-size specifies the maximum size of a group of input sections handled by one stub section. Since branch offsets are signed, a stub section may serve two groups of input sections, one group before the stub section, and one group after it. However, when using conditional branches that require stubs, it may be better (for branch prediction) that stub sections only serve one group of input sections. A negative value for N chooses this scheme, ensuring that branches to stubs always use a negative offset. Two special values of N are recognized, 1 and -1. These both instruct ld to automatically size input section groups for the branch types detected, with the same behaviour regarding stub placement as other positive or negative values of N respectively.

Note that --stub-group-size does not split input sections. A single input section larger than the group size specified will of course create a larger group (of one section). If input sections are too large, it may not be possible for a branch to reach its stub.

  ld and MMIX
For MMIX, there is a choice of generating ELF object files or mmo object files when linking. The simulator mmix understands the mmo format. The binutils objcopy utility can translate between the two formats.

There is one special section, the .MMIX.reg_contents section. Contents in this section is assumed to correspond to that of global registers, and symbols referring to it are translated to special symbols, equal to registers. In a final link, the start address of the .MMIX.reg_contents section corresponds to the first allocated global register multiplied by 8. Register $255 is not included in this section; it is always set to the program entry, which is at the symbol Main for mmo files.

Symbols with the prefix __.MMIX.start., for example __.MMIX.start..text and __.MMIX.start..data are special; there must be only one each, even if they are local. The default linker script uses these to set the default start address of a section.

Initial and trailing multiples of zero-valued 32-bit words in a section, are left out from an mmo file.

  ld and MSP430
For the MSP430 it is possible to select the MPU architecture. The flag -m [mpu type] will select an appropriate linker script for selected MPU type. (To get a list of known MPUs just pass -m help option to the linker).

The linker will recognize some extra sections which are MSP430 specific:

.vectors
    Defines a portion of ROM where interrupt vectors located.

.bootloader
    Defines the bootloader portion of the ROM (if applicable). Any code in this section will be uploaded to the MPU.

.infomem
    Defines an information memory section (if applicable). Any code in this section will be uploaded to the MPU.

.infomemnobits
    This is the same as the .infomem section except that any code in this section will not be uploaded to the MPU.

.noinit
    Denotes a portion of RAM located above .bss section.

    The last two sections are used by gcc.

  ld and PowerPC 32-bit ELF Support
Branches on PowerPC processors are limited to a signed 26-bit displacement, which may result in ld giving relocation truncated to fit errors with very large programs. --relax enables the generation of trampolines that can access the entire 32-bit address space. These trampolines are inserted at section boundaries, so may not themselves be reachable if an input section exceeds 33M in size.

--bss-plt
    Current PowerPC GCC accepts a -msecure-plt option that generates code capable of using a newer PLT and GOT layout that has the security advantage of no executable section ever needing to be writable and no writable section ever being executable. PowerPC ld will generate this layout, including stubs to access the PLT, if all input files (including startup and static libraries) were compiled with -msecure-plt. --bss-plt forces the old BSS PLT (and GOT layout) which can give slightly better performance.

--secure-plt
    ld will use the new PLT and GOT layout if it is linking new -fpic or -fPIC code, but does not do so automatically when linking non-PIC code. This option requests the new PLT and GOT layout. A warning will be given if some object file requires the old style BSS PLT.

--sdata-got
    The new secure PLT and GOT are placed differently relative to other sections compared to older BSS PLT and GOT placement. The location of .plt must change because the new secure PLT is an initialized section while the old PLT is uninitialized. The reason for the .got change is more subtle: The new placement allows .got to be read-only in applications linked with -z relro -z now. However, this placement means that .sdata cannot always be used in shared libraries, because the PowerPC ABI accesses .sdata in shared libraries from the GOT pointer. --sdata-got forces the old GOT placement. PowerPC GCC doesn't use .sdata in shared libraries, so this option is really only useful for other compilers that may do so.

--emit-stub-syms
    This option causes ld to label linker stubs with a local symbol that encodes the stub type and destination.

--no-tls-optimize
    PowerPC ld normally performs some optimization of code sequences used to access Thread-Local Storage. Use this option to disable the optimization.

  ld and PowerPC64 64-bit ELF Support

--stub-group-size
    Long branch stubs, PLT call stubs and TOC adjusting stubs are placed by ld in stub sections located between groups of input sections. --stub-group-size specifies the maximum size of a group of input sections handled by one stub section. Since branch offsets are signed, a stub section may serve two groups of input sections, one group before the stub section, and one group after it. However, when using conditional branches that require stubs, it may be better (for branch prediction) that stub sections only serve one group of input sections. A negative value for N chooses this scheme, ensuring that branches to stubs always use a negative offset. Two special values of N are recognized, 1 and -1. These both instruct ld to automatically size input section groups for the branch types detected, with the same behaviour regarding stub placement as other positive or negative values of N respectively.

    Note that --stub-group-size does not split input sections. A single input section larger than the group size specified will of course create a larger group (of one section). If input sections are too large, it may not be possible for a branch to reach its stub.

--emit-stub-syms
    This option causes ld to label linker stubs with a local symbol that encodes the stub type and destination.

--dotsyms, --no-dotsyms
    These two options control how ld interprets version patterns in a version script. Older PowerPC64 compilers emitted both a function descriptor symbol with the same name as the function, and a code entry symbol with the name prefixed by a dot ( .). To properly version a function foo, the version script thus needs to control both foo and .foo. The option --dotsyms, on by default, automatically adds the required dot-prefixed patterns. Use --no-dotsyms to disable this feature.

--no-tls-optimize
    PowerPC64 ld normally performs some optimization of code sequences used to access Thread-Local Storage. Use this option to disable the optimization.

--no-opd-optimize
    PowerPC64 ld normally removes .opd section entries corresponding to deleted link-once functions, or functions removed by the action of --gc-sections or linker scrip /DISCARD/. Use this option to disable .opd optimization.

--non-overlapping-opd
    Some PowerPC64 compilers have an option to generate compressed .opd entries spaced 16 bytes apart, overlapping the third word, the static chain pointer (unused in C) with the first word of the next entry. This option expands such entries to the full 24 bytes.

--no-toc-optimize
    PowerPC64 ld normally removes unused .toc section entries. Such entries are detected by examining relocations that reference the TOC in code sections. A reloc in a deleted code section marks a TOC word as unneeded, while a reloc in a kept code section marks a TOC word as needed. Since the TOC may reference itself, TOC relocs are also examined. TOC words marked as both needed and unneeded will of course be kept. TOC words without any referencing reloc are assumed to be part of a multi-word entry, and are kept or discarded as per the nearest marked preceding word. This works reliably for compiler generated code, but may be incorrect if assembly code is used to insert TOC entries. Use this option to disable the optimization.

--no-multi-toc
    By default, PowerPC64 GCC generates code for a TOC model where TOC entries are accessed with a 16-bit offset from r2. This limits the total TOC size to 64K. PowerPC64 ld extends this limit by grouping code sections such that each group uses less than 64K for its TOC entries, then inserts r2 adjusting stubs between inter-group calls. ld does not split apart input sections, so cannot help if a single input file has a .toc section that exceeds 64K, most likely from linking multiple files with ld(-r). Use this option to turn off this feature.

  ld and SPU ELF Support

--plugin
    This option marks an executable as a PIC plugin module.

--no-overlays
    Normally, ld recognizes calls to functions within overlay regions, and redirects such calls to an overlay manager via a stub. ld also provides a built-in overlay manager. This option turns off all this special overlay handling.

--emit-stub-syms
    This option causes ld to label overlay stubs with a local symbol that encodes the stub type and destination.

--extra-overlay-stubs
    This option causes ld to add overlay call stubs on all function calls out of overlay regions. Normally stubs are not added on calls to non-overlay regions.

--local-store=lo:hi
    ld usually checks that a final executable for SPU fits in the address range 0 to 256k. This option may be used to change the range. Disable the check entirely with [--local-store=0:0].

--stack-analysis
    SPU local store space is limited. Over-allocation of stack space unnecessarily limits space available for code and data, while under-allocation results in runtime failures. If given this option, ld will provide an estimate of maximum stack usage. ld does this by examining symbols in code sections to determine the extents of functions, and looking at function prologues for stack adjusting instructions. A call-graph is created by looking for relocations on branch instructions. The graph is then searched for the maximum stack usage path. Note that this analysis does not find calls made via function pointers, and does not handle recursion and other cycles in the call graph. Stack usage may be under-estimated if your code makes such calls. Also, stack usage for dynamic allocation, e.g. alloca, will not be detected. If a link map is requested, detailed information about each function's stack usage and calls will be given.

--emit-stack-syms
    This option, if given along with [--stack-analysis] will result in ld emitting stack sizing symbols for each function. These take the form __stack_<function_name> for global functions, and __stack_<number>_<function_name> for static functions. <number> is the section id in hex. The value of such symbols is the stack requirement for the corresponding function. The symbol size will be zero, type STT_NOTYPE, binding STB_LOCAL, and section SHN_ABS.

  ld's Support for Various TI COFF Versions
The --format switch allows selection of one of the various TI COFF versions. The latest of this writing is 2; versions 0 and 1 are also supported. The TI COFF versions also vary in header byte-order format; ld will read any version or byte order, but the output header format depends on the default specified by the specific target.

  ld and WIN32 (cygwin/mingw)
This section describes some of the win32 specific ld issues. See Options,,Command Line Options for detailed description of the command line options mentioned here.

import libraries
    The standard Windows linker creates and uses so-called import libraries, which contains information for linking to dll's. They are regular static archives and are handled as any other static archive. The cygwin and mingw ports of ld have specific support for creating such libraries provided with the --out-implib command line option.

exporting DLL symbols
    The cygwin/mingw ld has several ways to export symbols for dll's.

    using auto-export functionality
        By default ld exports symbols with the auto-export functionality, which is controlled by the following command line options:

        •   --export-all-symbols [This is the default]

        •   --exclude-symbols

        •   --exclude-libs

        If, however, --export-all-symbols is not given explicitly on the command line, then the default auto-export behavior will be disabled if either of the following are true:

        •   A DEF file is used.

        •   Any symbol in any object file was marked with the __declspec(dllexport) attribute.

    using a DEF file
        Another way of exporting symbols is using a DEF file. A DEF file is an ASCII file containing definitions of symbols which should be exported when a dll is created. Usually it is named <dll name>.def and is added as any other object file to the linker's command line. The file's name must end in .def or .DEF.

            gcc -o <output> <objectfiles> <dll name>.def

        Using a DEF file turns off the normal auto-export behavior, unless the --export-all-symbols option is also used.

        Here is an example of a DEF file for a shared library called xyz.dll:

            LIBRARY "xyz.dll" BASE=0x20000000
            EXPORTS
            foo
            bar
            _bar = bar
            another_foo = abc.dll.afoo
            var1 DATA

        This example defines a DLL with a non-default base address and five symbols in the export table. The third exported symbol _bar is an alias for the second. The fourth symbol, another_foo is resolved by "forwarding" to another module and treating it as an alias for afoo exported from the DLL abc.dll. The final symbol var1 is declared to be a data object.

        The optional LIBRARY <name> command indicates the internal name of the output DLL. If <name> does not include a suffix, the default library suffix, .DLL is appended.

        When the .DEF file is used to build an application, rather than a library, the NAME <name> command should be used instead of LIBRARY. If <name> does not include a suffix, the default executable suffix, .EXE is appended.

        With either LIBRARY <name> or NAME <name> the optional specification BASE = <number> may be used to specify a non-default base address for the image.

        If neither LIBRARY <name> nor NAME <name> is specified, or they specify an empty string, the internal name is the same as the filename specified on the command line.

        The complete specification of an export symbol is:

            EXPORTS
              ( (  ( <name1> [ = <name2> ] )
                 | ( <name1> = <module-name> . <external-name>))
              [ @ <integer> ] [NONAME] [DATA] [CONSTANT] [PRIVATE] ) *

        Declares <name1> as an exported symbol from the DLL, or declares <name1> as an exported alias for <name2>; or declares <name1> as a "forward" alias for the symbol <external-name> in the DLL <module-name>. Optionally, the symbol may be exported by the specified ordinal <integer> alias.

        The optional keywords that follow the declaration indicate:

        NONAME: Do not put the symbol name in the DLL's export table. It will still be exported by its ordinal alias (either the value specified by the .def specification or, otherwise, the value assigned by the linker). The symbol name, however, does remain visible in the import library (if any), unless PRIVATE is also specified.

        DATA: The symbol is a variable or object, rather than a function. The import lib will export only an indirect reference to foo as the symbol _imp__foo (ie, foo must be resolved as *_imp__foo).

        CONSTANT: Like DATA, but put the undecorated foo as well as _imp__foo into the import library. Both refer to the read-only import address table's pointer to the variable, not to the variable itself. This can be dangerous. If the user code fails to add the dllimport attribute and also fails to explicitly add the extra indirection that the use of the attribute enforces, the application will behave unexpectedly.

        PRIVATE: Put the symbol in the DLL's export table, but do not put it into the static import library used to resolve imports at link time. The symbol can still be imported using the LoadLibrary/GetProcAddress API at runtime or by by using the GNU ld extension of linking directly to the DLL without an import library. See ld/deffilep.y in the binutils sources for the full specification of other DEF file statements

        While linking a shared dll, ld is able to create a DEF file with the --output-def <file> command line option.

    Using decorations
        Another way of marking symbols for export is to modify the source code itself, so that when building the DLL each symbol to be exported is declared as:

            __declspec(dllexport) int a_variable
            __declspec(dllexport) void a_function(int with_args)

        All such symbols will be exported from the DLL. If, however, any of the object files in the DLL contain symbols decorated in this way, then the normal auto-export behavior is disabled, unless the --export-all-symbols option is also used.

        Note that object files that wish to access these symbols must not decorate them with dllexport. Instead, they should use dllimport, instead:

            __declspec(dllimport) int a_variable
            __declspec(dllimport) void a_function(int with_args)

        This complicates the structure of library header files, because when included by the library itself the header must declare the variables and functions as dllexport, but when included by client code the header must declare them as dllimport. There are a number of idioms that are typically used to do this; often client code can omit the __declspec() declaration completely. See --enable-auto-import and automatic data imports for more information.

automatic data imports
    The standard Windows dll format supports data imports from dlls only by adding special decorations (dllimport/dllexport), which let the compiler produce specific assembler instructions to deal with this issue. This increases the effort necessary to port existing Un*x code to these platforms, especially for large c++ libraries and applications. The auto-import feature, which was initially provided by Paul Sokolovsky, allows one to omit the decorations to achieve a behavior that conforms to that on POSIX/Un*x platforms. This feature is enabled with the --enable-auto-import command-line option, although it is enabled by default on cygwin/mingw. The --enable-auto-import option itself now serves mainly to suppress any warnings that are ordinarily emitted when linked objects trigger the feature's use.

    auto-import of variables does not always work flawlessly without additional assistance. Sometimes, you will see this message

    "variable '<var>' can't be auto-imported. Please read the documentation for ld's --enable-auto-import for details."

    The --enable-auto-import documentation explains why this error occurs, and several methods that can be used to overcome this difficulty. One of these methods is the runtime pseudo-relocs feature, described below.

    For complex variables imported from DLLs (such as structs or classes), object files typically contain a base address for the variable and an offset ( addend) within the variable--to specify a particular field or public member, for instance. Unfortunately, the runtime loader used in win32 environments is incapable of fixing these references at runtime without the additional information supplied by dllimport/dllexport decorations. The standard auto-import feature described above is unable to resolve these references.

    The --enable-runtime-pseudo-relocs switch allows these references to be resolved without error, while leaving the task of adjusting the references themselves (with their non-zero addends) to specialized code provided by the runtime environment. Recent versions of the cygwin and mingw environments and compilers provide this runtime support; older versions do not. However, the support is only necessary on the developer's platform; the compiled result will run without error on an older system.

    --enable-runtime-pseudo-relocs is not the default; it must be explicitly enabled as needed.

direct linking to a dll
    The cygwin/mingw ports of ld support the direct linking, including data symbols, to a dll without the usage of any import libraries. This is much faster and uses much less memory than does the traditional import library method, especially when linking large libraries or applications. When ld creates an import lib, each function or variable exported from the dll is stored in its own bfd, even though a single bfd could contain many exports. The overhead involved in storing, loading, and processing so many bfd's is quite large, and explains the tremendous time, memory, and storage needed to link against particularly large or complex libraries when using import libs.

    Linking directly to a dll uses no extra command-line switches other than -L and -l, because ld already searches for a number of names to match each library. All that is needed from the developer's perspective is an understanding of this search, in order to force ld to select the dll instead of an import library.

    For instance, when ld is called with the argument -lxxx it will attempt to find, in the first directory of its search path,

        libxxx.dll.a
        xxx.dll.a
        libxxx.a
        xxx.lib
        cygxxx.dll (*)
        libxxx.dll
        xxx.dll

    before moving on to the next directory in the search path.

    (*) Actually, this is not cygxxx.dll but in fact is <prefix>xxx.dll, where <prefix> is set by the ld option --dll-search-prefix=<prefix>. In the case of cygwin, the standard gcc spec file includes --dll-search-prefix=cyg, so in effect we actually search for cygxxx.dll.

    Other win32-based unix environments, such as mingw or pw32, may use other <prefix> es, although at present only cygwin makes use of this feature. It was originally intended to help avoid name conflicts among dll's built for the various win32/un*x environments, so that (for example) two versions of a zlib dll could coexist on the same machine.

    The generic cygwin/mingw path layout uses a bin directory for applications and dll's and a lib directory for the import libraries (using cygwin nomenclature):

        bin/
        	cygxxx.dll
        lib/
        	libxxx.dll.a   (in case of dll's)
        	libxxx.a       (in case of static archive)

    Linking directly to a dll without using the import library can be done two ways:

    1. Use the dll directly by adding the bin path to the link line

        gcc -Wl,-verbose  -o a.exe -L../bin/ -lxxx

    However, as the dll's often have version numbers appended to their names ( cygncurses-5.dll) this will often fail, unless one specifies -L../bin -lncurses-5 to include the version. Import libs are generally not versioned, and do not have this difficulty.

    2. Create a symbolic link from the dll to a file in the lib directory according to the above mentioned search pattern. This should be used to avoid unwanted changes in the tools needed for making the app/dll.

        ln -s bin/cygxxx.dll lib/[cyg|lib|]xxx.dll[.a]

    Then you can link without any make environment changes.

        gcc -Wl,-verbose  -o a.exe -L../lib/ -lxxx

    This technique also avoids the version number problems, because the following is perfectly legal

        bin/
        	cygxxx-5.dll
        lib/
        	libxxx.dll.a -> ../bin/cygxxx-5.dll

    Linking directly to a dll without using an import lib will work even when auto-import features are exercised, and even when --enable-runtime-pseudo-relocs is used.

    Given the improvements in speed and memory usage, one might justifiably wonder why import libraries are used at all. There are three reasons:

    1. Until recently, the link-directly-to-dll functionality did not work with auto-imported data.

    2. Sometimes it is necessary to include pure static objects within the import library (which otherwise contains only bfd's for indirection symbols that point to the exports of a dll). Again, the import lib for the cygwin kernel makes use of this ability, and it is not possible to do this without an import lib.

    3. Symbol aliases can only be resolved using an import lib. This is critical when linking against OS-supplied dll's (eg, the win32 API) in which symbols are usually exported as undecorated aliases of their stdcall-decorated assembly names.

    So, import libs are not going away. But the ability to replace true import libs with a simple symbolic link to (or a copy of) a dll, in many cases, is a useful addition to the suite of tools binutils makes available to the win32 developer. Given the massive improvements in memory requirements during linking, storage requirements, and linking speed, we expect that many developers will soon begin to use this feature whenever possible.

symbol aliasing

    adding additional names
        Sometimes, it is useful to export symbols with additional names. A symbol foo will be exported as foo, but it can also be exported as _foo by using special directives in the DEF file when creating the dll. This will affect also the optional created import library. Consider the following DEF file:

            LIBRARY "xyz.dll" BASE=0x61000000
            EXPORTS
            foo
            _foo = foo

        The line _foo = foo maps the symbol foo to _foo.

        Another method for creating a symbol alias is to create it in the source code using the "weak" attribute:

            void foo () { /* Do something.  */; }
            void _foo () __attribute__ ((weak, alias ("foo")));

        See the gcc manual for more information about attributes and weak symbols.

    renaming symbols
        Sometimes it is useful to rename exports. For instance, the cygwin kernel does this regularly. A symbol _foo can be exported as foo but not as _foo by using special directives in the DEF file. (This will also affect the import library, if it is created). In the following example:

            LIBRARY "xyz.dll" BASE=0x61000000
            EXPORTS
            _foo = foo

        The line _foo = foo maps the exported symbol foo to _foo.

    Note: using a DEF file disables the default auto-export behavior, unless the --export-all-symbols command line option is used. If, however, you are trying to rename symbols, then you should list all desired exports in the DEF file, including the symbols that are not being renamed, and do not use the --export-all-symbols option. If you list only the renamed symbols in the DEF file, and use --export-all-symbols to handle the other symbols, then the both the new names and the original names for the renamed symbols will be exported. In effect, you'd be aliasing those symbols, not renaming them, which is probably not what you wanted.

weak externals
    The Windows object format, PE, specifies a form of weak symbols called weak externals. When a weak symbol is linked and the symbol is not defined, the weak symbol becomes an alias for some other symbol. There are three variants of weak externals:

    •   Definition is searched for in objects and libraries, historically called lazy externals.

    •   Definition is searched for only in other objects, not in libraries. This form is not presently implemented.

    •   No search; the symbol is an alias. This form is not presently implemented.
    As a GNU extension, weak symbols that do not specify an alternate symbol are supported. If the symbol is undefined when linking, the symbol uses a default value.

  ld and Xtensa Processors
The default ld behavior for Xtensa processors is to interpret SECTIONS commands so that lists of explicitly named sections in a specification with a wildcard file will be interleaved when necessary to keep literal pools within the range of PC-relative load offsets. For example, with the command:

    SECTIONS
    {
      .text : {
        *(.literal .text)
      }
    }

ld may interleave some of the .literal and .text sections from different object files to ensure that the literal pools are within the range of PC-relative load offsets. A valid interleaving might place the .literal sections from an initial group of files followed by the .text sections of that group of files. Then, the .literal sections from the rest of the files and the .text sections from the rest of the files would follow.

Relaxation is enabled by default for the Xtensa version of ld and provides two important link-time optimizations. The first optimization is to combine identical literal values to reduce code size. A redundant literal will be removed and all the L32R instructions that use it will be changed to reference an identical literal, as long as the location of the replacement literal is within the offset range of all the L32R instructions. The second optimization is to remove unnecessary overhead from assembler-generated “longcall” sequences of L32R / CALLX n when the target functions are within range of direct CALL n instructions.

For each of these cases where an indirect call sequence can be optimized to a direct call, the linker will change the CALLX n instruction to a CALL n instruction, remove the L32R instruction, and remove the literal referenced by the L32R instruction if it is not used for anything else. Removing the L32R instruction always reduces code size but can potentially hurt performance by changing the alignment of subsequent branch targets. By default, the linker will always preserve alignments, either by switching some instructions between 24-bit encodings and the equivalent density instructions or by inserting a no-op in place of the L32R instruction that was removed. If code size is more important than performance, the [--size-opt] option can be used to prevent the linker from widening density instructions or inserting no-ops, except in a few cases where no-ops are required for correctness.

The following Xtensa-specific command-line options can be used to control the linker:

--no-relax
    Since the Xtensa version of ld enables the [--relax] option by default, the [--no-relax] option is provided to disable relaxation.

--size-opt
    When optimizing indirect calls to direct calls, optimize for code size more than performance. With this option, the linker will not insert no-ops or widen density instructions to preserve branch target alignment. There may still be some cases where no-ops are required to preserve the correctness of the code.

BFD
The linker accesses object and archive files using the BFD libraries. These libraries allow the linker to use the same routines to operate on object files whatever the object file format. A different object file format can be supported simply by creating a new BFD back end and adding it to the library. To conserve runtime memory, however, the linker and associated tools are usually configured to support only a subset of the object file formats available. You can use objdump -i (see Section “objdump”) to list all the formats available for your configuration.

As with most implementations, BFD is a compromise between several conflicting requirements. The major factor influencing BFD design was efficiency: any time used converting between formats is time which would not have been spent had BFD not been involved. This is partly offset by abstraction payback; since BFD simplifies applications and back ends, more time and care may be spent optimizing algorithms for a greater speed.

One minor artifact of the BFD solution which you should bear in mind is the potential for information loss. There are two places where useful information can be lost using the BFD mechanism: during conversion and during output.See Section “BFD information loss”.

  How It Works: Outline of BFD
When an object file is opened, BFD subroutines automatically determine the format of the input object file. They then build a descriptor in memory with pointers to routines that will be used to access elements of the object file's data structures.

As different information from the object files is required, BFD reads from different sections of the file and processes them. For example, a very common operation for the linker is processing symbol tables. Each BFD back end provides a routine for converting between the object file's representation of symbols and an internal canonical format. When the linker asks for the symbol table of an object file, it calls through a memory pointer to the routine from the relevant BFD back end which reads and converts the table into a canonical form. The linker then operates upon the canonical form. When the link is finished and the linker writes the output file's symbol table, another BFD back end routine is called to take the newly created symbol table and convert it into the chosen output format.

Information Loss

Information can be lost during output. The output formats supported by BFD do not provide identical facilities, and information which can be described in one form has nowhere to go in another format. One example of this is alignment information in b.out. There is nowhere in an a.out format file to store alignment information on the contained data, so when a file is linked from b.out and an a.out image is produced, alignment information will not propagate to the output file. (The linker will still use the alignment information internally, so the link is performed correctly).

Another example is COFF section names. COFF files may contain an unlimited number of sections, each one with a textual section name. If the target of the link is a format which does not have many sections (e.g., a.out) or has sections without names (e.g., the Oasys format), the link cannot be done simply. You can circumvent this problem by describing the desired input-to-output section mapping with the linker command language.

Information can be lost during canonicalization. The BFD internal canonical form of the external formats is not exhaustive; there are structures in input formats for which there is no direct representation internally. This means that the BFD back ends cannot maintain all possible data richness through the transformation between external to internal and back to external formats.

This limitation is only a problem when an application reads one format and writes another. Each BFD back end is responsible for maintaining as much data as possible, and the internal BFD canonical form has structures which are opaque to the BFD core, and exported only to the back ends. When a file is read in one format, the canonical form is generated for BFD and the application. At the same time, the back end saves away any information which may otherwise be lost. If the data is then written back in the same format, the back end routine will be able to use the canonical form provided by the BFD core as well as the information it prepared earlier. Since there is a great deal of commonality between back ends, there is no information lost when linking or copying big endian COFF to little endian COFF, or a.out to b.out. When a mixture of formats is linked, the information is only lost from the files whose format differs from the destination.

The BFD canonical object-file format

The greatest potential for loss of information occurs when there is the least overlap between the information provided by the source format, that stored by the canonical format, and that needed by the destination format. A brief description of the canonical form may help you understand which kinds of data you can count on preserving across conversions.

files
    Information stored on a per-file basis includes target machine architecture, particular implementation format type, a demand pageable bit, and a write protected bit. Information like Unix magic numbers is not stored here---only the magic numbers' meaning, so a ZMAGIC file would have both the demand pageable bit and the write protected text bit set. The byte order of the target is stored on a per-file basis, so that big- and little-endian object files may be used with one another.

sections
    Each section in the input file contains the name of the section, the section's original address in the object file, size and alignment information, various flags, and pointers into other BFD data structures.

symbols
    Each symbol contains a pointer to the information for the object file which originally defined it, its name, its value, and various flag bits. When a BFD back end reads in a symbol table, it relocates all symbols to make them relative to the base of the section where they were defined. Doing this ensures that each symbol points to its containing section. Each symbol also has a varying amount of hidden private data for the BFD back end. Since the symbol points to the original file, the private data format for that symbol is accessible. ld can operate on a collection of symbols of wildly different formats without problems.

    Normal global and simple local symbols are maintained on output, so an output file (no matter its format) will retain symbols pointing to functions and to global, static, and common variables. Some symbol information is not worth retaining; in a.out, type information is stored in the symbol table as long symbol names. This information would be useless to most COFF debuggers; the linker has command line switches to allow users to throw it away.

    There is one word of type information within the symbol, so if the format supports symbol type information within symbols (for example, COFF, IEEE, Oasys) and the type is simple enough to fit within one word (nearly everything but aggregates), the information will be preserved.

relocation level
    Each canonical BFD relocation record contains a pointer to the symbol to relocate to, the offset of the data to relocate, the section the data is in, and a pointer to a relocation type descriptor. Relocation is performed by passing messages through the relocation type descriptor and the symbol pointer. Therefore, relocations can be performed on output data using a relocation method that is only available in one of the input formats. For instance, Oasys provides a byte relocation format. A relocation record requesting this relocation type would point indirectly to a routine to perform this, so the relocation may be performed on a byte being written to a 68k COFF file, even though 68k COFF has no such relocation type.

line numbers
    Object formats can contain, for debugging purposes, some form of mapping between symbols, source line numbers, and addresses in the output file. These addresses have to be relocated along with the symbol information. Each symbol with an associated list of line number records points to the first record of the list. The head of a line number list consists of a pointer to the symbol, which allows finding out the address of the function whose line number is being described. The rest of the list is made up of pairs: offsets into the section and line numbers. Any format which can simply derive this information can pass it successfully between formats (COFF, IEEE and Oasys).

Reporting Bugs
Your bug reports play an essential role in making ld reliable.

Reporting a bug may help you by bringing a solution to your problem, or it may not. But in any case the principal function of a bug report is to help the entire community by making the next version of ld work better. Bug reports are your contribution to the maintenance of ld.

In order for a bug report to serve its purpose, you must include the information that enables us to fix the bug.

  Have You Found a Bug?
If you are not sure whether you have found a bug, here are some guidelines:

•   If the linker gets a fatal signal, for any input whatever, that is a ld bug. Reliable linkers never crash.

•   If ld produces an error message for valid input, that is a bug.

•   If ld does not produce an error message for invalid input, that may be a bug. In the general case, the linker can not verify that object files are correct.

•   If you are an experienced user of linkers, your suggestions for improvement of ld are welcome in any case.

  How to Report Bugs
A number of companies and individuals offer support for GNU products. If you obtained ld from a support organization, we recommend you contact that organization first.

You can find contact information for many support companies and individuals in the file etc/SERVICE in the GNU Emacs distribution.

The fundamental principle of reporting bugs usefully is this: report all the facts. If you are not sure whether to state a fact or leave it out, state it!

Often people omit facts because they think they know what causes the problem and assume that some details do not matter. Thus, you might assume that the name of a symbol you use in an example does not matter. Well, probably it does not, but one cannot be sure. Perhaps the bug is a stray memory reference which happens to fetch from the location where that name is stored in memory; perhaps, if the name were different, the contents of that location would fool the linker into doing the right thing despite the bug. Play it safe and give a specific, complete example. That is the easiest thing for you to do, and the most helpful.

Keep in mind that the purpose of a bug report is to enable us to fix the bug if it is new to us. Therefore, always write your bug reports on the assumption that the bug has not been reported previously.

Sometimes people give a few sketchy facts and ask, “Does this ring a bell?” This cannot help us fix a bug, so it is basically useless. We respond by asking for enough details to enable us to investigate. You might as well expedite matters by sending them to begin with.

To enable us to fix the bug, you should include all these things:

•   The version of ld. ld announces it if you start it with the --version argument.

    Without this, we will not know whether there is any point in looking for the bug in the current version of ld.

•   Any patches you may have applied to the ld source, including any patches made to the BFD library.

•   The type of machine you are using, and the operating system name and version number.

•   What compiler (and its version) was used to compile ld ---e.g. “ gcc-2.7 ”.

•   The command arguments you gave the linker to link your example and observe the bug. To guarantee you will not omit something important, list them all. A copy of the Makefile (or the output from make) is sufficient.

    If we were to try to guess the arguments, we would probably guess wrong and then we might not encounter the bug.

•   A complete input file, or set of input files, that will reproduce the bug. It is generally most helpful to send the actual object files provided that they are reasonably small. Say no more than 10K. For bigger files you can either make them available by FTP or HTTP or else state that you are willing to send the object file(s) to whomever requests them. (Note - your email will be going to a mailing list, so we do not want to clog it up with large attachments). But small attachments are best.

    If the source files were assembled using gas or compiled using gcc, then it may be OK to send the source files rather than the object files. In this case, be sure to say exactly what version of gas or gcc was used to produce the object files. Also say how gas or gcc were configured.

•   A description of what behavior you observe that you believe is incorrect. For example, “It gets a fatal signal.”

    Of course, if the bug is that ld gets a fatal signal, then we will certainly notice it. But if the bug is incorrect output, we might not notice unless it is glaringly wrong. You might as well not give us a chance to make a mistake.

    Even if the problem you experience is a fatal signal, you should still say so explicitly. Suppose something strange is going on, such as, your copy of ld is out of sync, or you have encountered a bug in the C library on your system. (This has happened!) Your copy might crash and ours would not. If you told us to expect a crash, then when ours fails to crash, we would know that the bug was not happening for us. If you had not told us to expect a crash, then we would not be able to draw any conclusion from our observations.

•   If you wish to suggest changes to the ld source, send us context diffs, as generated by diff with the -u, -c, or -p option. Always send diffs from the old file to the new file. If you even discuss something in the ld source, refer to it by context, not by line number.

    The line numbers in our development sources will not match those in your sources. Your line numbers would convey no useful information to us.

Here are some things that are not necessary:

•   A description of the envelope of the bug.

    Often people who encounter a bug spend a lot of time investigating which changes to the input file will make the bug go away and which changes will not affect it.

    This is often time consuming and not very useful, because the way we will find the bug is by running a single example under the debugger with breakpoints, not by pure deduction from a series of examples. We recommend that you save your time for something else.

    Of course, if you can find a simpler example to report instead of the original one, that is a convenience for us. Errors in the output will be easier to spot, running under the debugger will take less time, and so on.

    However, simplification is not vital; if you do not want to do this, report the bug anyway and send us the entire test case you used.

•   A patch for the bug.

    A patch for the bug does help us if it is a good one. But do not omit the necessary information, such as the test case, on the assumption that a patch is all we need. We might see problems with your patch and decide to fix the problem another way, or we might not understand it at all.

    Sometimes with a program as complicated as ld it is very hard to construct an example that will make the program follow a certain path through the code. If you do not send us the example, we will not be able to construct one, so we will not be able to verify that the bug is fixed.

    And if we cannot understand what bug you are trying to fix, or why your patch should be an improvement, we will not install it. A test case will help us to understand.

•   A guess about what the bug is or what it depends on.

    Such guesses are usually wrong. Even we cannot guess right about such things without first using the debugger to find the facts.

MRI Compatible Script Files
To aid users making the transition to GNU ld from the MRI linker, ld can use MRI compatible linker scripts as an alternative to the more general-purpose linker scripting language described in Scripts. MRI compatible linker scripts have a much simpler command set than the scripting language otherwise used with ld. GNU ld supports the most commonly used MRI linker commands; these commands are described here.

In general, MRI scripts aren't of much use with the a.out object file format, since it only has three sections and MRI scripts lack some features to make use of them.

You can specify a file containing an MRI-compatible script using the -c command-line option.

Each command in an MRI-compatible script occupies its own line; each command line starts with the keyword that identifies the command (though blank lines are also allowed for punctuation). If a line of an MRI-compatible script begins with an unrecognized keyword, ld issues a warning message, but continues processing the script.

Lines beginning with * are comments.

You can write these commands using all upper-case letters, or all lower case; for example, chip is the same as CHIP. The following list shows only the upper-case form of each command.

ABSOLUTE secname

ABSOLUTE secname, secname, ... secname
    Normally, ld includes in the output file all sections from all the input files. However, in an MRI-compatible script, you can use the ABSOLUTE command to restrict the sections that will be present in your output program. If the ABSOLUTE command is used at all in a script, then only the sections named explicitly in ABSOLUTE commands will appear in the linker output. You can still use other input sections (whatever you select on the command line, or using LOAD) to resolve addresses in the output file.

ALIAS out-secname, in-secname
    Use this command to place the data from input section in-secname in a section called out-secname in the linker output file.

    in-secname may be an integer.

ALIGN secname = expression
    Align the section called secname to expression. The expression should be a power of two.

BASE expression
    Use the value of expression as the lowest address (other than absolute addresses) in the output file.

CHIP expression

CHIP expression, expression
    This command does nothing; it is accepted only for compatibility.

END This command does nothing whatever; it's only accepted for compatibility.

FORMAT output-format
    Similar to the OUTPUT_FORMAT command in the more general linker language, but restricted to one of these output formats:

    1.  S-records, if output-format is S

    2.  IEEE, if output-format is IEEE

    3.  COFF (the coff-m68k variant in BFD), if output-format is COFF

LIST anything...
    Print (to the standard output file) a link map, as produced by the ld command-line option -M.

    The keyword LIST may be followed by anything on the same line, with no change in its effect.

LOAD filename

LOAD filename, filename, ... filename
    Include one or more object file filename in the link; this has the same effect as specifying filename directly on the ld command line.

NAME output-name
    output-name is the name for the program produced by ld; the MRI-compatible command NAME is equivalent to the command-line option -o or the general script language command OUTPUT.

ORDER secname, secname, ... secname

ORDER secname secname secname
    Normally, ld orders the sections in its output file in the order in which they first appear in the input files. In an MRI-compatible script, you can override this ordering with the ORDER command. The sections you list with ORDER will appear first in your output file, in the order specified.

PUBLIC name= expression

PUBLIC name, expression

PUBLIC name expression
    Supply a value ( expression) for external symbol name used in the linker input files.

SECT secname, expression

SECT secname= expression

SECT secname expression
    You can use any of these three forms of the SECT command to specify the start address ( expression) for section secname. If you have more than one SECT statement for the same secname, only the first sets the start address.

GNU Free Documentation License

Copyright (C) 2000, 2003 Free Software Foundation, Inc. 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA

Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.

1.  PREAMBLE

    The purpose of this License is to make a manual, textbook, or other written document “free” in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

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2.  APPLICABILITY AND DEFINITIONS

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5.  MODIFICATIONS

    You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

    A. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission. B. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has less than five). C. State on the Title page the name of the publisher of the Modified Version, as the publisher. D. Preserve all the copyright notices of the Document. E. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices. F. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below. G. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document's license notice. H. Include an unaltered copy of this License. I. Preserve the section entitled “History”, and its title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section entitled “History” in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence. J. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the “History” section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission. K. In any section entitled “Acknowledgements” or “Dedications”, preserve the section's title, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein. L. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles. M. Delete any section entitled “Endorsements.” Such a section may not be included in the Modified Version. N. Do not retitle any existing section as “Endorsements” or to conflict in title with any Invariant Section.

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    You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

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11. FUTURE REVISIONS OF THIS LICENSE

    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

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  ADDENDUM: How to use this License for your documents
To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

    Copyright (C)  year  your name.
    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 the Invariant Sections being list their titles, with the
    Front-Cover Texts being list, and with the Back-Cover Texts being list.
    A copy of the license is included in the section entitled "GNU
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If you have no Invariant Sections, write “with no Invariant Sections” instead of saying which ones are invariant. If you have no Front-Cover Texts, write “no Front-Cover Texts” instead of “Front-Cover Texts being list ”; likewise for Back-Cover Texts.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

LD Index