\input texinfo @setfilename ld.info @c Copyright 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, @c 2001, 2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc. @syncodeindex ky cp @c man begin INCLUDE @include configdoc.texi @c (configdoc.texi is generated by the Makefile) @include ldver.texi @c man end @c @smallbook @macro gcctabopt{body} @code{\body\} @end macro @c man begin NAME @ifset man @c Configure for the generation of man pages @set UsesEnvVars @set GENERIC @set ARC @set ARM @set D10V @set D30V @set H8/300 @set H8/500 @set HPPA @set I370 @set I80386 @set I860 @set I960 @set M32R @set M68HC11 @set M680X0 @set MCORE @set MIPS @set MMIX @set MSP430 @set PDP11 @set PJ @set POWERPC @set POWERPC64 @set SH @set SPARC @set TIC54X @set V850 @set VAX @set WIN32 @set XTENSA @end ifset @c man end @ifinfo @format START-INFO-DIR-ENTRY * Ld: (ld). The GNU linker. END-INFO-DIR-ENTRY @end format @end ifinfo @ifinfo This file documents the @sc{gnu} linker LD version @value{VERSION}. Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, 2001, 2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc. @ignore Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''. Permission is granted to process this file through Tex and print the results, provided the printed document carries copying permission notice identical to this one except for the removal of this paragraph (this paragraph not being relevant to the printed manual). @end ignore @end ifinfo @iftex @finalout @setchapternewpage odd @settitle The GNU linker @titlepage @title The GNU linker @sp 1 @subtitle @code{ld} version 2 @subtitle Version @value{VERSION} @author Steve Chamberlain @author Ian Lance Taylor @page @tex {\parskip=0pt \hfill Red Hat Inc\par \hfill nickc\@credhat.com, doc\@redhat.com\par \hfill {\it The GNU linker}\par \hfill Edited by Jeffrey Osier (jeffrey\@cygnus.com)\par } \global\parindent=0pt % Steve likes it this way. @end tex @vskip 0pt plus 1filll @c man begin COPYRIGHT Copyright @copyright{} 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, 2001, 2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''. @c man end @end titlepage @end iftex @c FIXME: Talk about importance of *order* of args, cmds to linker! @ifnottex @node Top @top LD This file documents the @sc{gnu} linker ld version @value{VERSION}. This document is distributed under the terms of the GNU Free Documentation License. A copy of the license is included in the section entitled ``GNU Free Documentation License''. @menu * Overview:: Overview * Invocation:: Invocation * Scripts:: Linker Scripts @ifset GENERIC * Machine Dependent:: Machine Dependent Features @end ifset @ifclear GENERIC @ifset H8300 * H8/300:: ld and the H8/300 @end ifset @ifset Renesas * Renesas:: ld and other Renesas micros @end ifset @ifset I960 * i960:: ld and the Intel 960 family @end ifset @ifset ARM * ARM:: ld and the ARM family @end ifset @ifset HPPA * HPPA ELF32:: ld and HPPA 32-bit ELF @end ifset @ifset M68HC11 * M68HC11/68HC12:: ld and the Motorola 68HC11 and 68HC12 families @end ifset @ifset POWERPC * PowerPC ELF32:: ld and PowerPC 32-bit ELF Support @end ifset @ifset POWERPC64 * PowerPC64 ELF64:: ld and PowerPC64 64-bit ELF Support @end ifset @ifset TICOFF * TI COFF:: ld and the TI COFF @end ifset @ifset WIN32 * Win32:: ld and WIN32 (cygwin/mingw) @end ifset @ifset XTENSA * Xtensa:: ld and Xtensa Processors @end ifset @end ifclear @ifclear SingleFormat * BFD:: BFD @end ifclear @c Following blank line required for remaining bug in makeinfo conds/menus * Reporting Bugs:: Reporting Bugs * MRI:: MRI Compatible Script Files * GNU Free Documentation License:: GNU Free Documentation License * LD Index:: LD Index @end menu @end ifnottex @node Overview @chapter Overview @cindex @sc{gnu} linker @cindex what is this? @ifset man @c man begin SYNOPSIS ld [@b{options}] @var{objfile} @dots{} @c man end @c man begin SEEALSO ar(1), nm(1), objcopy(1), objdump(1), readelf(1) and the Info entries for @file{binutils} and @file{ld}. @c man end @end ifset @c man begin DESCRIPTION @command{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 @command{ld}. @command{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. @ifset man @c For the man only This man page does not describe the command language; see the @command{ld} entry in @code{info} for full details on the command language and on other aspects of the GNU linker. @end ifset @ifclear SingleFormat This version of @command{ld} uses the general purpose BFD libraries to operate on object files. This allows @command{ld} to read, combine, and write object files in many different formats---for example, COFF or @code{a.out}. Different formats may be linked together to produce any available kind of object file. @xref{BFD}, for more information. @end ifclear Aside from its flexibility, the @sc{gnu} linker is more helpful than other linkers in providing diagnostic information. Many linkers abandon execution immediately upon encountering an error; whenever possible, @command{ld} continues executing, allowing you to identify other errors (or, in some cases, to get an output file in spite of the error). @c man end @node Invocation @chapter Invocation @c man begin DESCRIPTION The @sc{gnu} linker @command{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. @c man end @ifset UsesEnvVars @menu * Options:: Command Line Options * Environment:: Environment Variables @end menu @node Options @section Command Line Options @end ifset @cindex command line @cindex options @c man begin OPTIONS The linker supports a plethora of command-line options, but in actual practice few of them are used in any particular context. @cindex standard Unix system For instance, a frequent use of @command{ld} is to link standard Unix object files on a standard, supported Unix system. On such a system, to link a file @code{hello.o}: @smallexample ld -o @var{output} /lib/crt0.o hello.o -lc @end smallexample This tells @command{ld} to produce a file called @var{output} as the result of linking the file @code{/lib/crt0.o} with @code{hello.o} and the library @code{libc.a}, which will come from the standard search directories. (See the discussion of the @samp{-l} option below.) Some of the command-line options to @command{ld} may be specified at any point in the command line. However, options which refer to files, such as @samp{-l} or @samp{-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. @cindex object files 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 @samp{-l}, @samp{-R}, and the script command language. If @emph{no} binary input files at all are specified, the linker does not produce any output, and issues the message @samp{No input files}. If the linker cannot recognize the format of an object file, it will assume that it is a linker script. A script specified in this way augments the main linker script used for the link (either the default linker script or the one specified by using @samp{-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 @code{INPUT} or @code{GROUP} to load other objects. Note that specifying a script in this way merely augments the main linker script; use the @samp{-T} option to replace the default linker script entirely. @xref{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, @samp{-trace-symbol} and @samp{--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 @samp{-o} option. So for example @samp{-omagic} sets the output file name to @samp{magic} whereas @samp{--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, @samp{--trace-symbol foo} and @samp{--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. @samp{gcc}) then all the linker command line options should be prefixed by @samp{-Wl,} (or whatever is appropriate for the particular compiler driver) like this: @smallexample gcc -Wl,--startgroup foo.o bar.o -Wl,--endgroup @end smallexample 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: @table @gcctabopt @include at-file.texi @kindex -a@var{keyword} @item -a@var{keyword} This option is supported for HP/UX compatibility. The @var{keyword} argument must be one of the strings @samp{archive}, @samp{shared}, or @samp{default}. @samp{-aarchive} is functionally equivalent to @samp{-Bstatic}, and the other two keywords are functionally equivalent to @samp{-Bdynamic}. This option may be used any number of times. @ifset I960 @cindex architectures @kindex -A@var{arch} @item -A@var{architecture} @kindex --architecture=@var{arch} @itemx --architecture=@var{architecture} In the current release of @command{ld}, this option is useful only for the Intel 960 family of architectures. In that @command{ld} configuration, the @var{architecture} argument identifies the particular architecture in the 960 family, enabling some safeguards and modifying the archive-library search path. @xref{i960,,@command{ld} and the Intel 960 family}, for details. Future releases of @command{ld} may support similar functionality for other architecture families. @end ifset @ifclear SingleFormat @cindex binary input format @kindex -b @var{format} @kindex --format=@var{format} @cindex input format @cindex input format @item -b @var{input-format} @itemx --format=@var{input-format} @command{ld} may be configured to support more than one kind of object file. If your @command{ld} is configured this way, you can use the @samp{-b} option to specify the binary format for input object files that follow this option on the command line. Even when @command{ld} is configured to support alternative object formats, you don't usually need to specify this, as @command{ld} should be configured to expect as a default input format the most usual format on each machine. @var{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 @samp{objdump -i}.) @xref{BFD}. You may want to use this option if you are linking files with an unusual binary format. You can also use @samp{-b} to switch formats explicitly (when linking object files of different formats), by including @samp{-b @var{input-format}} before each group of object files in a particular format. The default format is taken from the environment variable @code{GNUTARGET}. @ifset UsesEnvVars @xref{Environment}. @end ifset You can also define the input format from a script, using the command @code{TARGET}; @ifclear man see @ref{Format Commands}. @end ifclear @end ifclear @kindex -c @var{MRI-cmdfile} @kindex --mri-script=@var{MRI-cmdfile} @cindex compatibility, MRI @item -c @var{MRI-commandfile} @itemx --mri-script=@var{MRI-commandfile} For compatibility with linkers produced by MRI, @command{ld} accepts script files written in an alternate, restricted command language, described in @ifclear man @ref{MRI,,MRI Compatible Script Files}. @end ifclear @ifset man the MRI Compatible Script Files section of GNU ld documentation. @end ifset Introduce MRI script files with the option @samp{-c}; use the @samp{-T} option to run linker scripts written in the general-purpose @command{ld} scripting language. If @var{MRI-cmdfile} does not exist, @command{ld} looks for it in the directories specified by any @samp{-L} options. @cindex common allocation @kindex -d @kindex -dc @kindex -dp @item -d @itemx -dc @itemx -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 @samp{-r}). The script command @code{FORCE_COMMON_ALLOCATION} has the same effect. @xref{Miscellaneous Commands}. @cindex entry point, from command line @kindex -e @var{entry} @kindex --entry=@var{entry} @item -e @var{entry} @itemx --entry=@var{entry} Use @var{entry} as the explicit symbol for beginning execution of your program, rather than the default entry point. If there is no symbol named @var{entry}, the linker will try to parse @var{entry} as a number, and use that as the entry address (the number will be interpreted in base 10; you may use a leading @samp{0x} for base 16, or a leading @samp{0} for base 8). @xref{Entry Point}, for a discussion of defaults and other ways of specifying the entry point. @kindex --exclude-libs @item --exclude-libs @var{lib},@var{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 @code{--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. @cindex dynamic symbol table @kindex -E @kindex --export-dynamic @item -E @itemx --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 @code{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 @samp{--dynamic-list}. @ifclear SingleFormat @cindex big-endian objects @cindex endianness @kindex -EB @item -EB Link big-endian objects. This affects the default output format. @cindex little-endian objects @kindex -EL @item -EL Link little-endian objects. This affects the default output format. @end ifclear @kindex -f @kindex --auxiliary @item -f @itemx --auxiliary @var{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 @var{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 @var{name}. If there is one, it will be used instead of the definition in the filter object. The shared object @var{name} need not exist. Thus the shared object @var{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. @kindex -F @kindex --filter @item -F @var{name} @itemx --filter @var{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 @var{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 @var{name}. Thus the filter object can be used to select a subset of the symbols provided by the object @var{name}. Some older linkers used the @option{-F} option throughout a compilation toolchain for specifying object-file format for both input and output object files. @ifclear SingleFormat The @sc{gnu} linker uses other mechanisms for this purpose: the @option{-b}, @option{--format}, @option{--oformat} options, the @code{TARGET} command in linker scripts, and the @code{GNUTARGET} environment variable. @end ifclear The @sc{gnu} linker will ignore the @option{-F} option when not creating an ELF shared object. @cindex finalization function @kindex -fini @item -fini @var{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 @code{_fini} as the function to call. @kindex -g @item -g Ignored. Provided for compatibility with other tools. @kindex -G @kindex --gpsize @cindex object size @item -G@var{value} @itemx --gpsize=@var{value} Set the maximum size of objects to be optimized using the GP register to @var{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. @cindex runtime library name @kindex -h@var{name} @kindex -soname=@var{name} @item -h@var{name} @itemx -soname=@var{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. @kindex -i @cindex incremental link @item -i Perform an incremental link (same as option @samp{-r}). @cindex initialization function @kindex -init @item -init @var{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 @code{_init} as the function to call. @cindex archive files, from cmd line @kindex -l@var{archive} @kindex --library=@var{archive} @item -l@var{archive} @itemx --library=@var{archive} Add archive file @var{archive} to the list of files to link. This option may be used any number of times. @command{ld} will search its path-list for occurrences of @code{lib@var{archive}.a} for every @var{archive} specified. On systems which support shared libraries, @command{ld} may also search for libraries with extensions other than @code{.a}. Specifically, on ELF and SunOS systems, @command{ld} will search a directory for a library with an extension of @code{.so} before searching for one with an extension of @code{.a}. By convention, a @code{.so} extension indicates a shared library. The linker will search an archive only once, at the location where it is specified on the command line. If the archive defines a symbol which was undefined in some object which appeared before the archive on the command line, the linker will include the appropriate file(s) from the archive. However, an undefined symbol in an object appearing later on the command line will not cause the linker to search the archive again. See the @option{-(} 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. @ifset GENERIC This type of archive searching is standard for Unix linkers. However, if you are using @command{ld} on AIX, note that it is different from the behaviour of the AIX linker. @end ifset @cindex search directory, from cmd line @kindex -L@var{dir} @kindex --library-path=@var{dir} @item -L@var{searchdir} @itemx --library-path=@var{searchdir} Add path @var{searchdir} to the list of paths that @command{ld} will search for archive libraries and @command{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 @option{-L} options apply to all @option{-l} options, regardless of the order in which the options appear. If @var{searchdir} begins with @code{=}, then the @code{=} will be replaced by the @dfn{sysroot prefix}, a path specified when the linker is configured. @ifset UsesEnvVars The default set of paths searched (without being specified with @samp{-L}) depends on which emulation mode @command{ld} is using, and in some cases also on how it was configured. @xref{Environment}. @end ifset The paths can also be specified in a link script with the @code{SEARCH_DIR} command. Directories specified this way are searched at the point in which the linker script appears in the command line. @cindex emulation @kindex -m @var{emulation} @item -m@var{emulation} Emulate the @var{emulation} linker. You can list the available emulations with the @samp{--verbose} or @samp{-V} options. If the @samp{-m} option is not used, the emulation is taken from the @code{LDEMULATION} environment variable, if that is defined. Otherwise, the default emulation depends upon how the linker was configured. @cindex link map @kindex -M @kindex --print-map @item -M @itemx --print-map Print a link map to the standard output. A link map provides information about the link, including the following: @itemize @bullet @item Where object files are mapped into memory. @item How common symbols are allocated. @item All archive members included in the link, with a mention of the symbol which caused the archive member to be brought in. @item 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: @smallexample foo = 1 foo = foo * 4 foo = foo + 8 @end smallexample will produce the following output in the link map if the @option{-M} option is used: @smallexample 0x00000001 foo = 0x1 [0x0000000c] foo = (foo * 0x4) [0x0000000c] foo = (foo + 0x8) @end smallexample See @ref{Expressions} for more information about expressions in linker scripts. @end itemize @kindex -n @cindex read-only text @cindex NMAGIC @kindex --nmagic @item -n @itemx --nmagic Turn off page alignment of sections, and mark the output as @code{NMAGIC} if possible. @kindex -N @kindex --omagic @cindex read/write from cmd line @cindex OMAGIC @item -N @itemx --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 @code{OMAGIC}. Note: Although a writable text section is allowed for PE-COFF targets, it does not conform to the format specification published by Microsoft. @kindex --no-omagic @cindex OMAGIC @item --no-omagic This option negates most of the effects of the @option{-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 @option{-Bdynamic} for this. @kindex -o @var{output} @kindex --output=@var{output} @cindex naming the output file @item -o @var{output} @itemx --output=@var{output} Use @var{output} as the name for the program produced by @command{ld}; if this option is not specified, the name @file{a.out} is used by default. The script command @code{OUTPUT} can also specify the output file name. @kindex -O @var{level} @cindex generating optimized output @item -O @var{level} If @var{level} is a numeric values greater than zero @command{ld} optimizes the output. This might take significantly longer and therefore probably should only be enabled for the final binary. @kindex -q @kindex --emit-relocs @cindex retain relocations in final executable @item -q @itemx --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. @kindex --force-dynamic @cindex forcing the creation of dynamic sections @item --force-dynamic Force the output file to have dynamic sections. This option is specific to VxWorks targets. @cindex partial link @cindex relocatable output @kindex -r @kindex --relocatable @item -r @itemx --relocatable Generate relocatable output---i.e., generate an output file that can in turn serve as input to @command{ld}. This is often called @dfn{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 @code{OMAGIC}. @c ; see @option{-N}. If this option is not specified, an absolute file is produced. When linking C++ programs, this option @emph{will not} resolve references to constructors; to do that, use @samp{-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 @code{a.out}-based formats do not support partial linking with input files in other formats at all. This option does the same thing as @samp{-i}. @kindex -R @var{file} @kindex --just-symbols=@var{file} @cindex symbol-only input @item -R @var{filename} @itemx --just-symbols=@var{filename} Read symbol names and their addresses from @var{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 @option{-R} option is followed by a directory name, rather than a file name, it is treated as the @option{-rpath} option. @kindex -s @kindex --strip-all @cindex strip all symbols @item -s @itemx --strip-all Omit all symbol information from the output file. @kindex -S @kindex --strip-debug @cindex strip debugger symbols @item -S @itemx --strip-debug Omit debugger symbol information (but not all symbols) from the output file. @kindex -t @kindex --trace @cindex input files, displaying @item -t @itemx --trace Print the names of the input files as @command{ld} processes them. @kindex -T @var{script} @kindex --script=@var{script} @cindex script files @item -T @var{scriptfile} @itemx --script=@var{scriptfile} Use @var{scriptfile} as the linker script. This script replaces @command{ld}'s default linker script (rather than adding to it), so @var{commandfile} must specify everything necessary to describe the output file. @xref{Scripts}. If @var{scriptfile} does not exist in the current directory, @code{ld} looks for it in the directories specified by any preceding @samp{-L} options. Multiple @samp{-T} options accumulate. @kindex -u @var{symbol} @kindex --undefined=@var{symbol} @cindex undefined symbol @item -u @var{symbol} @itemx --undefined=@var{symbol} Force @var{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. @samp{-u} may be repeated with different option arguments to enter additional undefined symbols. This option is equivalent to the @code{EXTERN} linker script command. @kindex -Ur @cindex constructors @item -Ur For anything other than C++ programs, this option is equivalent to @samp{-r}: it generates relocatable output---i.e., an output file that can in turn serve as input to @command{ld}. When linking C++ programs, @samp{-Ur} @emph{does} resolve references to constructors, unlike @samp{-r}. It does not work to use @samp{-Ur} on files that were themselves linked with @samp{-Ur}; once the constructor table has been built, it cannot be added to. Use @samp{-Ur} only for the last partial link, and @samp{-r} for the others. @kindex --unique[=@var{SECTION}] @item --unique[=@var{SECTION}] Creates a separate output section for every input section matching @var{SECTION}, or if the optional wildcard @var{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. @kindex -v @kindex -V @kindex --version @cindex version @item -v @itemx --version @itemx -V Display the version number for @command{ld}. The @option{-V} option also lists the supported emulations. @kindex -x @kindex --discard-all @cindex deleting local symbols @item -x @itemx --discard-all Delete all local symbols. @kindex -X @kindex --discard-locals @cindex local symbols, deleting @item -X @itemx --discard-locals Delete all temporary local symbols. (These symbols start with system-specific local label prefixes, typically @samp{.L} for ELF systems or @samp{L} for traditional a.out systems.) @kindex -y @var{symbol} @kindex --trace-symbol=@var{symbol} @cindex symbol tracing @item -y @var{symbol} @itemx --trace-symbol=@var{symbol} Print the name of each linked file in which @var{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. @kindex -Y @var{path} @item -Y @var{path} Add @var{path} to the default library search path. This option exists for Solaris compatibility. @kindex -z @var{keyword} @item -z @var{keyword} The recognized keywords are: @table @samp @item combreloc Combines multiple reloc sections and sorts them to make dynamic symbol lookup caching possible. @item defs Disallows undefined symbols in object files. Undefined symbols in shared libraries are still allowed. @item execstack Marks the object as requiring executable stack. @item 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. @item interpose Marks the object that its symbol table interposes before all symbols but the primary executable. @item 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. @item loadfltr Marks the object that its filters be processed immediately at runtime. @item muldefs Allows multiple definitions. @item nocombreloc Disables multiple reloc sections combining. @item nocopyreloc Disables production of copy relocs. @item nodefaultlib Marks the object that the search for dependencies of this object will ignore any default library search paths. @item nodelete Marks the object shouldn't be unloaded at runtime. @item nodlopen Marks the object not available to @code{dlopen}. @item nodump Marks the object can not be dumped by @code{dldump}. @item noexecstack Marks the object as not requiring executable stack. @item norelro Don't create an ELF @code{PT_GNU_RELRO} segment header in the object. @item 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. @item origin Marks the object may contain $ORIGIN. @item relro Create an ELF @code{PT_GNU_RELRO} segment header in the object. @item max-page-size=@var{value} Set the emulation maximum page size to @var{value}. @item common-page-size=@var{value} Set the emulation common page size to @var{value}. @end table Other keywords are ignored for Solaris compatibility. @kindex -( @cindex groups of archives @item -( @var{archives} -) @itemx --start-group @var{archives} --end-group The @var{archives} should be a list of archive files. They may be either explicit file names, or @samp{-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. @kindex --accept-unknown-input-arch @kindex --no-accept-unknown-input-arch @item --accept-unknown-input-arch @itemx --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 @samp{--accept-unknown-input-arch} option has been added to restore the old behaviour. @kindex --as-needed @kindex --no-as-needed @item --as-needed @itemx --no-as-needed This option affects ELF DT_NEEDED tags for dynamic libraries mentioned on the command line after the @option{--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. @option{--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. @option{--no-as-needed} restores the default behaviour. @kindex --add-needed @kindex --no-add-needed @item --add-needed @itemx --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 @option{--no-add-needed} option. Normally, the linker will add a DT_NEEDED tag for each dynamic library from DT_NEEDED tags. @option{--no-add-needed} causes DT_NEEDED tags will never be emitted for those libraries from DT_NEEDED tags. @option{--add-needed} restores the default behaviour. @kindex -assert @var{keyword} @item -assert @var{keyword} This option is ignored for SunOS compatibility. @kindex -Bdynamic @kindex -dy @kindex -call_shared @item -Bdynamic @itemx -dy @itemx -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 @option{-l} options which follow it. @kindex -Bgroup @item -Bgroup Set the @code{DF_1_GROUP} flag in the @code{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. @option{--unresolved-symbols=report-all} is implied. This option is only meaningful on ELF platforms which support shared libraries. @kindex -Bstatic @kindex -dn @kindex -non_shared @kindex -static @item -Bstatic @itemx -dn @itemx -non_shared @itemx -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 @option{-l} options which follow it. This option also implies @option{--unresolved-symbols=report-all}. This option can be used with @option{-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. @kindex -Bsymbolic @item -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. @kindex --dynamic-list=@var{dynamic-list-file} @item --dynamic-list=@var{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 @ref{VERSION} for more information. @kindex --dynamic-list-cpp-typeinfo @item --dynamic-list-cpp-typeinfo Provide the builtin dynamic list for C++ runtime type identification. @kindex --check-sections @kindex --no-check-sections @item --check-sections @itemx --no-check-sections Asks the linker @emph{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 @option{--check-sections}. @cindex cross reference table @kindex --cref @item --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. @cindex common allocation @kindex --no-define-common @item --no-define-common This option inhibits the assignment of addresses to common symbols. The script command @code{INHIBIT_COMMON_ALLOCATION} has the same effect. @xref{Miscellaneous Commands}. The @samp{--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 @samp{--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. @cindex symbols, from command line @kindex --defsym @var{symbol}=@var{exp} @item --defsym @var{symbol}=@var{expression} Create a global symbol in the output file, containing the absolute address given by @var{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 @var{expression} in this context: you may give a hexadecimal constant or the name of an existing symbol, or use @code{+} and @code{-} to add or subtract hexadecimal constants or symbols. If you need more elaborate expressions, consider using the linker command language from a script (@pxref{Assignments,, Assignment: Symbol Definitions}). @emph{Note:} there should be no white space between @var{symbol}, the equals sign (``@key{=}''), and @var{expression}. @cindex demangling, from command line @kindex --demangle[=@var{style}] @kindex --no-demangle @item --demangle[=@var{style}] @itemx --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 @samp{COLLECT_NO_DEMANGLE} is set. These options may be used to override the default. @cindex dynamic linker, from command line @kindex -I@var{file} @kindex --dynamic-linker @var{file} @item --dynamic-linker @var{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. @kindex --fatal-warnings @item --fatal-warnings Treat all warnings as errors. @kindex --force-exe-suffix @item --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 @code{.exe} or @code{.dll} suffix, this option forces the linker to copy the output file to one of the same name with a @code{.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 @code{.exe} suffix. @kindex --gc-sections @kindex --no-gc-sections @cindex garbage collection @item --gc-sections @itemx --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 @samp{-r} or @samp{--emit-relocs}. The default behaviour (of not performing this garbage collection) can be restored by specifying @samp{--no-gc-sections} on the command line. @kindex --print-gc-sections @kindex --no-print-gc-sections @cindex garbage collection @item --print-gc-sections @itemx --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 @samp{--gc-sections}) option. The default behaviour (of not listing the sections that are removed) can be restored by specifying @samp{--no-print-gc-sections} on the command line. @cindex help @cindex usage @kindex --help @item --help Print a summary of the command-line options on the standard output and exit. @kindex --target-help @item --target-help Print a summary of all target specific options on the standard output and exit. @kindex -Map @item -Map @var{mapfile} Print a link map to the file @var{mapfile}. See the description of the @option{-M} option, above. @cindex memory usage @kindex --no-keep-memory @item --no-keep-memory @command{ld} normally optimizes for speed over memory usage by caching the symbol tables of input files in memory. This option tells @command{ld} to instead optimize for memory usage, by rereading the symbol tables as necessary. This may be required if @command{ld} runs out of memory space while linking a large executable. @kindex --no-undefined @kindex -z defs @item --no-undefined @itemx -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 @option{--[no-]allow-shlib-undefined} controls the behaviour for reporting unresolved references found in shared libraries being linked in. @kindex --allow-multiple-definition @kindex -z muldefs @item --allow-multiple-definition @itemx -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. @kindex --allow-shlib-undefined @kindex --no-allow-shlib-undefined @item --allow-shlib-undefined @itemx --no-allow-shlib-undefined Allows (the default) or disallows undefined symbols in shared libraries. This switch is similar to @option{--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 @option{--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. @kindex --no-undefined-version @item --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. @kindex --default-symver @item --default-symver Create and use a default symbol version (the soname) for unversioned exported symbols. @kindex --default-imported-symver @item --default-imported-symver Create and use a default symbol version (the soname) for unversioned imported symbols. @kindex --no-warn-mismatch @item --no-warn-mismatch Normally @command{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 @command{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. @kindex --no-whole-archive @item --no-whole-archive Turn off the effect of the @option{--whole-archive} option for subsequent archive files. @cindex output file after errors @kindex --noinhibit-exec @item --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. @kindex -nostdlib @item -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. @ifclear SingleFormat @kindex --oformat @item --oformat @var{output-format} @command{ld} may be configured to support more than one kind of object file. If your @command{ld} is configured this way, you can use the @samp{--oformat} option to specify the binary format for the output object file. Even when @command{ld} is configured to support alternative object formats, you don't usually need to specify this, as @command{ld} should be configured to produce as a default output format the most usual format on each machine. @var{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 @samp{objdump -i}.) The script command @code{OUTPUT_FORMAT} can also specify the output format, but this option overrides it. @xref{BFD}. @end ifclear @kindex -pie @kindex --pic-executable @item -pie @itemx --pic-executable @cindex position independent executables 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. @kindex -qmagic @item -qmagic This option is ignored for Linux compatibility. @kindex -Qy @item -Qy This option is ignored for SVR4 compatibility. @kindex --relax @cindex synthesizing linker @cindex relaxing addressing modes @item --relax An option with machine dependent effects. @ifset GENERIC This option is only supported on a few targets. @end ifset @ifset H8300 @xref{H8/300,,@command{ld} and the H8/300}. @end ifset @ifset I960 @xref{i960,, @command{ld} and the Intel 960 family}. @end ifset @ifset XTENSA @xref{Xtensa,, @command{ld} and Xtensa Processors}. @end ifset @ifset M68HC11 @xref{M68HC11/68HC12,,@command{ld} and the 68HC11 and 68HC12}. @end ifset @ifset POWERPC @xref{PowerPC ELF32,,@command{ld} and PowerPC 32-bit ELF Support}. @end ifset On some platforms, the @samp{--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. @ifset GENERIC This is known to be the case for the Matsushita MN10200 and MN10300 family of processors. @end ifset @ifset GENERIC On platforms where this is not supported, @samp{--relax} is accepted, but ignored. @end ifset @cindex retaining specified symbols @cindex stripping all but some symbols @cindex symbols, retaining selectively @item --retain-symbols-file @var{filename} Retain @emph{only} the symbols listed in the file @var{filename}, discarding all others. @var{filename} is simply a flat file, with one symbol name per line. This option is especially useful in environments @ifset GENERIC (such as VxWorks) @end ifset where a large global symbol table is accumulated gradually, to conserve run-time memory. @samp{--retain-symbols-file} does @emph{not} discard undefined symbols, or symbols needed for relocations. You may only specify @samp{--retain-symbols-file} once in the command line. It overrides @samp{-s} and @samp{-S}. @ifset GENERIC @item -rpath @var{dir} @cindex runtime library search path @kindex -rpath Add a directory to the runtime library search path. This is used when linking an ELF executable with shared objects. All @option{-rpath} arguments are concatenated and passed to the runtime linker, which uses them to locate shared objects at runtime. The @option{-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 @option{-rpath-link} option. If @option{-rpath} is not used when linking an ELF executable, the contents of the environment variable @code{LD_RUN_PATH} will be used if it is defined. The @option{-rpath} option may also be used on SunOS. By default, on SunOS, the linker will form a runtime search patch out of all the @option{-L} options it is given. If a @option{-rpath} option is used, the runtime search path will be formed exclusively using the @option{-rpath} options, ignoring the @option{-L} options. This can be useful when using gcc, which adds many @option{-L} options which may be on NFS mounted file systems. For compatibility with other ELF linkers, if the @option{-R} option is followed by a directory name, rather than a file name, it is treated as the @option{-rpath} option. @end ifset @ifset GENERIC @cindex link-time runtime library search path @kindex -rpath-link @item -rpath-link @var{DIR} When using ELF or SunOS, one shared library may require another. This happens when an @code{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 @option{-rpath-link} option specifies the first set of directories to search. The @option{-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: @enumerate @item Any directories specified by @option{-rpath-link} options. @item Any directories specified by @option{-rpath} options. The difference between @option{-rpath} and @option{-rpath-link} is that directories specified by @option{-rpath} options are included in the executable and used at runtime, whereas the @option{-rpath-link} option is only effective at link time. Searching @option{-rpath} in this way is only supported by native linkers and cross linkers which have been configured with the @option{--with-sysroot} option. @item On an ELF system, if the @option{-rpath} and @code{rpath-link} options were not used, search the contents of the environment variable @code{LD_RUN_PATH}. It is for the native linker only. @item On SunOS, if the @option{-rpath} option was not used, search any directories specified using @option{-L} options. @item For a native linker, the contents of the environment variable @code{LD_LIBRARY_PATH}. @item For a native ELF linker, the directories in @code{DT_RUNPATH} or @code{DT_RPATH} of a shared library are searched for shared libraries needed by it. The @code{DT_RPATH} entries are ignored if @code{DT_RUNPATH} entries exist. @item The default directories, normally @file{/lib} and @file{/usr/lib}. @item For a native linker on an ELF system, if the file @file{/etc/ld.so.conf} exists, the list of directories found in that file. @end enumerate If the required shared library is not found, the linker will issue a warning and continue with the link. @end ifset @kindex -shared @kindex -Bshareable @item -shared @itemx -Bshareable @cindex shared libraries 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 @option{-e} option is not used and there are undefined symbols in the link. @item --sort-common @kindex --sort-common This option tells @command{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. @kindex --sort-section name @item --sort-section name This option will apply @code{SORT_BY_NAME} to all wildcard section patterns in the linker script. @kindex --sort-section alignment @item --sort-section alignment This option will apply @code{SORT_BY_ALIGNMENT} to all wildcard section patterns in the linker script. @kindex --split-by-file @item --split-by-file [@var{size}] Similar to @option{--split-by-reloc} but creates a new output section for each input file when @var{size} is reached. @var{size} defaults to a size of 1 if not given. @kindex --split-by-reloc @item --split-by-reloc [@var{count}] Tries to creates extra sections in the output file so that no single output section in the file contains more than @var{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 @var{count} relocations one output section will contain that many relocations. @var{count} defaults to a value of 32768. @kindex --stats @item --stats Compute and display statistics about the operation of the linker, such as execution time and memory usage. @kindex --sysroot @item --sysroot=@var{directory} Use @var{directory} as the location of the sysroot, overriding the configure-time default. This option is only supported by linkers that were configured using @option{--with-sysroot}. @kindex --traditional-format @cindex traditional format @item --traditional-format For some targets, the output of @command{ld} is different in some ways from the output of some existing linker. This switch requests @command{ld} to use the traditional format instead. @cindex dbx For example, on SunOS, @command{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 @code{dbx} program can not read the resulting program (@code{gdb} has no trouble). The @samp{--traditional-format} switch tells @command{ld} to not combine duplicate entries. @kindex --section-start @var{sectionname}=@var{org} @item --section-start @var{sectionname}=@var{org} Locate a section in the output file at the absolute address given by @var{org}. You may use this option as many times as necessary to locate multiple sections in the command line. @var{org} must be a single hexadecimal integer; for compatibility with other linkers, you may omit the leading @samp{0x} usually associated with hexadecimal values. @emph{Note:} there should be no white space between @var{sectionname}, the equals sign (``@key{=}''), and @var{org}. @kindex -Tbss @var{org} @kindex -Tdata @var{org} @kindex -Ttext @var{org} @cindex segment origins, cmd line @item -Tbss @var{org} @itemx -Tdata @var{org} @itemx -Ttext @var{org} Same as --section-start, with @code{.bss}, @code{.data} or @code{.text} as the @var{sectionname}. @kindex --unresolved-symbols @item --unresolved-symbols=@var{method} Determine how to handle unresolved symbols. There are four possible values for @samp{method}: @table @samp @item ignore-all Do not report any unresolved symbols. @item report-all Report all unresolved symbols. This is the default. @item ignore-in-object-files Report unresolved symbols that are contained in shared libraries, but ignore them if they come from regular object files. @item 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. @end table The behaviour for shared libraries on their own can also be controlled by the @option{--[no-]allow-shlib-undefined} option. Normally the linker will generate an error message for each reported unresolved symbol but the option @option{--warn-unresolved-symbols} can change this to a warning. @kindex --verbose @cindex verbose @item --dll-verbose @itemx --verbose Display the version number for @command{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. @kindex --version-script=@var{version-scriptfile} @cindex version script, symbol versions @itemx --version-script=@var{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. @xref{VERSION}. @kindex --warn-common @cindex warnings, on combining symbols @cindex combining symbols, warnings on @item --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: @table @samp @item int i = 1; A definition, which goes in the initialized data section of the output file. @item 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. @item 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. @end table The @samp{--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. @enumerate @item Turning a common symbol into a reference, because there is already a definition for the symbol. @smallexample @var{file}(@var{section}): warning: common of `@var{symbol}' overridden by definition @var{file}(@var{section}): warning: defined here @end smallexample @item 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. @smallexample @var{file}(@var{section}): warning: definition of `@var{symbol}' overriding common @var{file}(@var{section}): warning: common is here @end smallexample @item Merging a common symbol with a previous same-sized common symbol. @smallexample @var{file}(@var{section}): warning: multiple common of `@var{symbol}' @var{file}(@var{section}): warning: previous common is here @end smallexample @item Merging a common symbol with a previous larger common symbol. @smallexample @var{file}(@var{section}): warning: common of `@var{symbol}' overridden by larger common @var{file}(@var{section}): warning: larger common is here @end smallexample @item 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. @smallexample @var{file}(@var{section}): warning: common of `@var{symbol}' overriding smaller common @var{file}(@var{section}): warning: smaller common is here @end smallexample @end enumerate @kindex --warn-constructors @item --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. @kindex --warn-multiple-gp @item --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. @kindex --warn-once @cindex warnings, on undefined symbols @cindex undefined symbols, warnings on @item --warn-once Only warn once for each undefined symbol, rather than once per module which refers to it. @kindex --warn-section-align @cindex warnings, on section alignment @cindex section alignment, warnings on @item --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 @code{SECTIONS} command does not specify a start address for the section (@pxref{SECTIONS}). @kindex --warn-shared-textrel @item --warn-shared-textrel Warn if the linker adds a DT_TEXTREL to a shared object. @kindex --warn-unresolved-symbols @item --warn-unresolved-symbols If the linker is going to report an unresolved symbol (see the option @option{--unresolved-symbols}) it will normally generate an error. This option makes it generate a warning instead. @kindex --error-unresolved-symbols @item --error-unresolved-symbols This restores the linker's default behaviour of generating errors when it is reporting unresolved symbols. @kindex --whole-archive @cindex including an entire archive @item --whole-archive For each archive mentioned on the command line after the @option{--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 @option{-Wl,-whole-archive}. Second, don't forget to use @option{-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. @kindex --wrap @item --wrap @var{symbol} Use a wrapper function for @var{symbol}. Any undefined reference to @var{symbol} will be resolved to @code{__wrap_@var{symbol}}. Any undefined reference to @code{__real_@var{symbol}} will be resolved to @var{symbol}. This can be used to provide a wrapper for a system function. The wrapper function should be called @code{__wrap_@var{symbol}}. If it wishes to call the system function, it should call @code{__real_@var{symbol}}. Here is a trivial example: @smallexample void * __wrap_malloc (size_t c) @{ printf ("malloc called with %zu\n", c); return __real_malloc (c); @} @end smallexample If you link other code with this file using @option{--wrap malloc}, then all calls to @code{malloc} will call the function @code{__wrap_malloc} instead. The call to @code{__real_malloc} in @code{__wrap_malloc} will call the real @code{malloc} function. You may wish to provide a @code{__real_malloc} function as well, so that links without the @option{--wrap} option will succeed. If you do this, you should not put the definition of @code{__real_malloc} in the same file as @code{__wrap_malloc}; if you do, the assembler may resolve the call before the linker has a chance to wrap it to @code{malloc}. @kindex --eh-frame-hdr @item --eh-frame-hdr Request creation of @code{.eh_frame_hdr} section and ELF @code{PT_GNU_EH_FRAME} segment header. @kindex --enable-new-dtags @kindex --disable-new-dtags @item --enable-new-dtags @itemx --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 @option{--enable-new-dtags}, the dynamic tags will be created as needed. If you specify @option{--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. @kindex --hash-size=@var{number} @item --hash-size=@var{number} Set the default size of the linker's hash tables to a prime number close to @var{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. @kindex --hash-style=@var{style} @item --hash-style=@var{style} Set the type of linker's hash table(s). @var{style} can be either @code{sysv} for classic ELF @code{.hash} section, @code{gnu} for new style GNU @code{.gnu.hash} section or @code{both} for both the classic ELF @code{.hash} and new style GNU @code{.gnu.hash} hash tables. The default is @code{sysv}. @kindex --reduce-memory-overheads @item --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 @option{--hash-size} switch has been used. The @option{--reduce-memory-overheads} switch may be also be used to enable other tradeoffs in future versions of the linker. @end table @c man end @subsection Options Specific to i386 PE Targets @c man begin OPTIONS The i386 PE linker supports the @option{-shared} option, which causes the output to be a dynamically linked library (DLL) instead of a normal executable. You should name the output @code{*.dll} when you use this option. In addition, the linker fully supports the standard @code{*.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. @table @gcctabopt @kindex --add-stdcall-alias @item --add-stdcall-alias If given, symbols with a stdcall suffix (@@@var{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] @kindex --base-file @item --base-file @var{file} Use @var{file} as the name of a file in which to save the base addresses of all the relocations needed for generating DLLs with @file{dlltool}. [This is an i386 PE specific option] @kindex --dll @item --dll Create a DLL instead of a regular executable. You may also use @option{-shared} or specify a @code{LIBRARY} in a given @code{.def} file. [This option is specific to the i386 PE targeted port of the linker] @kindex --enable-stdcall-fixup @kindex --disable-stdcall-fixup @item --enable-stdcall-fixup @itemx --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 @code{_foo} might be linked to the function @code{_foo@@12}, or the undefined symbol @code{_bar@@16} might be linked to the function @code{_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 @option{--enable-stdcall-fixup}, this feature is fully enabled and warnings are not printed. If you specify @option{--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] @cindex DLLs, creating @kindex --export-all-symbols @item --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 @code{DllMain@@12}, @code{DllEntryPoint@@0}, @code{DllMainCRTStartup@@12}, and @code{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 @code{_head_} or ending with @code{_iname}. In addition, no symbols from @code{libgcc}, @code{libstd++}, @code{libmingw32}, or @code{crtX.o} will be exported. Symbols whose names begin with @code{__rtti_} or @code{__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: @code{_cygwin_dll_entry@@12}, @code{_cygwin_crt0_common@@8}, @code{_cygwin_noncygwin_dll_entry@@12}, @code{_fmode}, @code{_impure_ptr}, @code{cygwin_attach_dll}, @code{cygwin_premain0}, @code{cygwin_premain1}, @code{cygwin_premain2}, @code{cygwin_premain3}, and @code{environ}. [This option is specific to the i386 PE targeted port of the linker] @kindex --exclude-symbols @item --exclude-symbols @var{symbol},@var{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] @kindex --file-alignment @item --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] @cindex heap size @kindex --heap @item --heap @var{reserve} @itemx --heap @var{reserve},@var{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] @cindex image base @kindex --image-base @item --image-base @var{value} Use @var{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] @kindex --kill-at @item --kill-at If given, the stdcall suffixes (@@@var{nn}) will be stripped from symbols before they are exported. [This option is specific to the i386 PE targeted port of the linker] @kindex --large-address-aware @item --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=@var{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] @kindex --major-image-version @item --major-image-version @var{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] @kindex --major-os-version @item --major-os-version @var{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] @kindex --major-subsystem-version @item --major-subsystem-version @var{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] @kindex --minor-image-version @item --minor-image-version @var{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] @kindex --minor-os-version @item --minor-os-version @var{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] @kindex --minor-subsystem-version @item --minor-subsystem-version @var{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] @cindex DEF files, creating @cindex DLLs, creating @kindex --output-def @item --output-def @var{file} The linker will create the file @var{file} which will contain a DEF file corresponding to the DLL the linker is generating. This DEF file (which should be called @code{*.def}) may be used to create an import library with @code{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] @cindex DLLs, creating @kindex --out-implib @item --out-implib @var{file} The linker will create the file @var{file} which will contain an import lib corresponding to the DLL the linker is generating. This import lib (which should be called @code{*.dll.a} or @code{*.a} may be used to link clients against the generated DLL; this behaviour makes it possible to skip a separate @code{dlltool} import library creation step. [This option is specific to the i386 PE targeted port of the linker] @kindex --enable-auto-image-base @item --enable-auto-image-base Automatically choose the image base for DLLs, unless one is specified using the @code{--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] @kindex --disable-auto-image-base @item --disable-auto-image-base Do not automatically generate a unique image base. If there is no user-specified image base (@code{--image-base}) then use the platform default. [This option is specific to the i386 PE targeted port of the linker] @cindex DLLs, linking to @kindex --dll-search-prefix @item --dll-search-prefix @var{string} When linking dynamically to a dll without an import library, search for @code{.dll} in preference to @code{lib.dll}. This behaviour allows easy distinction between DLLs built for the various "subplatforms": native, cygwin, uwin, pw, etc. For instance, cygwin DLLs typically use @code{--dll-search-prefix=cyg}. [This option is specific to the i386 PE targeted port of the linker] @kindex --enable-auto-import @item --enable-auto-import Do sophisticated linking of @code{_symbol} to @code{__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 '' can't be auto-imported. Please read the documentation for ld's @code{--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: @example extern type extern_array[]; extern_array[1] --> @{ volatile type *t=extern_array; t[1] @} @end example or @example extern type extern_array[]; extern_array[1] --> @{ volatile int t=1; extern_array[t] @} @end example For structs (and most other multiword data types) the only option is to make the struct itself (or the long long, or the ...) variable: @example extern struct s extern_struct; extern_struct.field --> @{ volatile struct s *t=&extern_struct; t->field @} @end example or @example extern long long extern_ll; extern_ll --> @{ volatile long long * local_ll=&extern_ll; *local_ll @} @end example A third method of dealing with this difficulty is to abandon 'auto-import' for the offending symbol and mark it with @code{__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: @example --foo.h extern int arr[]; --foo.c #include "foo.h" void main(int argc, char **argv)@{ printf("%d\n",arr[1]); @} @end example Solution 1: @example --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]); @} @end example Solution 2: @example --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]); @} @end example 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] @kindex --disable-auto-import @item --disable-auto-import Do not attempt to do sophisticated linking of @code{_symbol} to @code{__imp__symbol} for DATA imports from DLLs. [This option is specific to the i386 PE targeted port of the linker] @kindex --enable-runtime-pseudo-reloc @item --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] @kindex --disable-runtime-pseudo-reloc @item --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] @kindex --enable-extra-pe-debug @item --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] @kindex --section-alignment @item --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] @cindex stack size @kindex --stack @item --stack @var{reserve} @itemx --stack @var{reserve},@var{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] @kindex --subsystem @item --subsystem @var{which} @itemx --subsystem @var{which}:@var{major} @itemx --subsystem @var{which}:@var{major}.@var{minor} Specifies the subsystem under which your program will execute. The legal values for @var{which} are @code{native}, @code{windows}, @code{console}, @code{posix}, and @code{xbox}. You may optionally set the subsystem version also. Numeric values are also accepted for @var{which}. [This option is specific to the i386 PE targeted port of the linker] @end table @c man end @ifset M68HC11 @subsection Options specific to Motorola 68HC11 and 68HC12 targets @c man begin OPTIONS The 68HC11 and 68HC12 linkers support specific options to control the memory bank switching mapping and trampoline code generation. @table @gcctabopt @kindex --no-trampoline @item --no-trampoline This option disables the generation of trampoline. By default a trampoline is generated for each far function which is called using a @code{jsr} instruction (this happens when a pointer to a far function is taken). @kindex --bank-window @item --bank-window @var{name} This option indicates to the linker the name of the memory region in the @samp{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. @end table @c man end @end ifset @ifset UsesEnvVars @node Environment @section Environment Variables @c man begin ENVIRONMENT You can change the behaviour of @command{ld} with the environment variables @ifclear SingleFormat @code{GNUTARGET}, @end ifclear @code{LDEMULATION} and @code{COLLECT_NO_DEMANGLE}. @ifclear SingleFormat @kindex GNUTARGET @cindex default input format @code{GNUTARGET} determines the input-file object format if you don't use @samp{-b} (or its synonym @samp{--format}). Its value should be one of the BFD names for an input format (@pxref{BFD}). If there is no @code{GNUTARGET} in the environment, @command{ld} uses the natural format of the target. If @code{GNUTARGET} is set to @code{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. @end ifclear @kindex LDEMULATION @cindex default emulation @cindex emulation, default @code{LDEMULATION} determines the default emulation if you don't use the @samp{-m} option. The emulation can affect various aspects of linker behaviour, particularly the default linker script. You can list the available emulations with the @samp{--verbose} or @samp{-V} options. If the @samp{-m} option is not used, and the @code{LDEMULATION} environment variable is not defined, the default emulation depends upon how the linker was configured. @kindex COLLECT_NO_DEMANGLE @cindex demangling, default Normally, the linker will default to demangling symbols. However, if @code{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 @code{gcc} linker wrapper program. The default may be overridden by the @samp{--demangle} and @samp{--no-demangle} options. @c man end @end ifset @node Scripts @chapter Linker Scripts @cindex scripts @cindex linker scripts @cindex command files Every link is controlled by a @dfn{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 @samp{--verbose} command line option to display the default linker script. Certain command line options, such as @samp{-r} or @samp{-N}, will affect the default linker script. You may supply your own linker script by using the @samp{-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. @xref{Implicit Linker Scripts}. @menu * Basic Script Concepts:: Basic Linker Script Concepts * Script Format:: Linker Script Format * Simple Example:: Simple Linker Script Example * Simple Commands:: Simple Linker Script Commands * Assignments:: Assigning Values to Symbols * SECTIONS:: SECTIONS Command * MEMORY:: MEMORY Command * PHDRS:: PHDRS Command * VERSION:: VERSION Command * Expressions:: Expressions in Linker Scripts * Implicit Linker Scripts:: Implicit Linker Scripts @end menu @node Basic Script Concepts @section Basic Linker Script Concepts @cindex 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 @dfn{object file format}. Each file is called an @dfn{object file}. The output file is often called an @dfn{executable}, but for our purposes we will also call it an object file. Each object file has, among other things, a list of @dfn{sections}. We sometimes refer to a section in an input file as an @dfn{input section}; similarly, a section in the output file is an @dfn{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 @dfn{section contents}. A section may be marked as @dfn{loadable}, which mean that the contents should be loaded into memory when the output file is run. A section with no contents may be @dfn{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 @dfn{VMA}, or virtual memory address. This is the address the section will have when the output file is run. The second is the @dfn{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 @code{objdump} program with the @samp{-h} option. Every object file also has a list of @dfn{symbols}, known as the @dfn{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 @code{nm} program, or by using the @code{objdump} program with the @samp{-t} option. @node Script Format @section Linker Script Format @cindex 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 @samp{/*} and @samp{*/}. As in C, comments are syntactically equivalent to whitespace. @node Simple Example @section Simple Linker Script Example @cindex linker script example @cindex example of linker script Many linker scripts are fairly simple. The simplest possible linker script has just one command: @samp{SECTIONS}. You use the @samp{SECTIONS} command to describe the memory layout of the output file. The @samp{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 @samp{.text}, @samp{.data}, and @samp{.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: @smallexample SECTIONS @{ . = 0x10000; .text : @{ *(.text) @} . = 0x8000000; .data : @{ *(.data) @} .bss : @{ *(.bss) @} @} @end smallexample You write the @samp{SECTIONS} command as the keyword @samp{SECTIONS}, followed by a series of symbol assignments and output section descriptions enclosed in curly braces. The first line inside the @samp{SECTIONS} command of the above example sets the value of the special symbol @samp{.}, 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 @samp{SECTIONS} command, the location counter has the value @samp{0}. The second line defines an output section, @samp{.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 @samp{*} is a wildcard which matches any file name. The expression @samp{*(.text)} means all @samp{.text} input sections in all input files. Since the location counter is @samp{0x10000} when the output section @samp{.text} is defined, the linker will set the address of the @samp{.text} section in the output file to be @samp{0x10000}. The remaining lines define the @samp{.data} and @samp{.bss} sections in the output file. The linker will place the @samp{.data} output section at address @samp{0x8000000}. After the linker places the @samp{.data} output section, the value of the location counter will be @samp{0x8000000} plus the size of the @samp{.data} output section. The effect is that the linker will place the @samp{.bss} output section immediately after the @samp{.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 @samp{.text} and @samp{.data} sections will probably satisfy any alignment constraints, but the linker may have to create a small gap between the @samp{.data} and @samp{.bss} sections. That's it! That's a simple and complete linker script. @node Simple Commands @section Simple Linker Script Commands @cindex linker script simple commands In this section we describe the simple linker script commands. @menu * Entry Point:: Setting the entry point * File Commands:: Commands dealing with files @ifclear SingleFormat * Format Commands:: Commands dealing with object file formats @end ifclear * Miscellaneous Commands:: Other linker script commands @end menu @node Entry Point @subsection Setting the Entry Point @kindex ENTRY(@var{symbol}) @cindex start of execution @cindex first instruction @cindex entry point The first instruction to execute in a program is called the @dfn{entry point}. You can use the @code{ENTRY} linker script command to set the entry point. The argument is a symbol name: @smallexample ENTRY(@var{symbol}) @end smallexample 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: @itemize @bullet @item the @samp{-e} @var{entry} command-line option; @item the @code{ENTRY(@var{symbol})} command in a linker script; @item the value of the symbol @code{start}, if defined; @item the address of the first byte of the @samp{.text} section, if present; @item The address @code{0}. @end itemize @node File Commands @subsection Commands Dealing with Files @cindex linker script file commands Several linker script commands deal with files. @table @code @item INCLUDE @var{filename} @kindex INCLUDE @var{filename} @cindex including a linker script Include the linker script @var{filename} at this point. The file will be searched for in the current directory, and in any directory specified with the @option{-L} option. You can nest calls to @code{INCLUDE} up to 10 levels deep. @item INPUT(@var{file}, @var{file}, @dots{}) @itemx INPUT(@var{file} @var{file} @dots{}) @kindex INPUT(@var{files}) @cindex input files in linker scripts @cindex input object files in linker scripts @cindex linker script input object files The @code{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 @file{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 @samp{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 @samp{-T} option. In case a @dfn{sysroot prefix} is configured, and the filename starts with the @samp{/} character, and the script being processed was located inside the @dfn{sysroot prefix}, the filename will be looked for in the @dfn{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 @samp{-L} in @ref{Options,,Command Line Options}. If you use @samp{INPUT (-l@var{file})}, @command{ld} will transform the name to @code{lib@var{file}.a}, as with the command line argument @samp{-l}. When you use the @code{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. @item GROUP(@var{file}, @var{file}, @dots{}) @itemx GROUP(@var{file} @var{file} @dots{}) @kindex GROUP(@var{files}) @cindex grouping input files The @code{GROUP} command is like @code{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 @samp{-(} in @ref{Options,,Command Line Options}. @item AS_NEEDED(@var{file}, @var{file}, @dots{}) @itemx AS_NEEDED(@var{file} @var{file} @dots{}) @kindex AS_NEEDED(@var{files}) This construct can appear only inside of the @code{INPUT} or @code{GROUP} commands, among other filenames. The files listed will be handled as if they appear directly in the @code{INPUT} or @code{GROUP} commands, with the exception of ELF shared libraries, that will be added only when they are actually needed. This construct essentially enables @option{--as-needed} option for all the files listed inside of it and restores previous @option{--as-needed} resp. @option{--no-as-needed} setting afterwards. @item OUTPUT(@var{filename}) @kindex OUTPUT(@var{filename}) @cindex output file name in linker script The @code{OUTPUT} command names the output file. Using @code{OUTPUT(@var{filename})} in the linker script is exactly like using @samp{-o @var{filename}} on the command line (@pxref{Options,,Command Line Options}). If both are used, the command line option takes precedence. You can use the @code{OUTPUT} command to define a default name for the output file other than the usual default of @file{a.out}. @item SEARCH_DIR(@var{path}) @kindex SEARCH_DIR(@var{path}) @cindex library search path in linker script @cindex archive search path in linker script @cindex search path in linker script The @code{SEARCH_DIR} command adds @var{path} to the list of paths where @command{ld} looks for archive libraries. Using @code{SEARCH_DIR(@var{path})} is exactly like using @samp{-L @var{path}} on the command line (@pxref{Options,,Command Line Options}). If both are used, then the linker will search both paths. Paths specified using the command line option are searched first. @item STARTUP(@var{filename}) @kindex STARTUP(@var{filename}) @cindex first input file The @code{STARTUP} command is just like the @code{INPUT} command, except that @var{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. @end table @ifclear SingleFormat @node Format Commands @subsection Commands Dealing with Object File Formats A couple of linker script commands deal with object file formats. @table @code @item OUTPUT_FORMAT(@var{bfdname}) @itemx OUTPUT_FORMAT(@var{default}, @var{big}, @var{little}) @kindex OUTPUT_FORMAT(@var{bfdname}) @cindex output file format in linker script The @code{OUTPUT_FORMAT} command names the BFD format to use for the output file (@pxref{BFD}). Using @code{OUTPUT_FORMAT(@var{bfdname})} is exactly like using @samp{--oformat @var{bfdname}} on the command line (@pxref{Options,,Command Line Options}). If both are used, the command line option takes precedence. You can use @code{OUTPUT_FORMAT} with three arguments to use different formats based on the @samp{-EB} and @samp{-EL} command line options. This permits the linker script to set the output format based on the desired endianness. If neither @samp{-EB} nor @samp{-EL} are used, then the output format will be the first argument, @var{default}. If @samp{-EB} is used, the output format will be the second argument, @var{big}. If @samp{-EL} is used, the output format will be the third argument, @var{little}. For example, the default linker script for the MIPS ELF target uses this command: @smallexample OUTPUT_FORMAT(elf32-bigmips, elf32-bigmips, elf32-littlemips) @end smallexample This says that the default format for the output file is @samp{elf32-bigmips}, but if the user uses the @samp{-EL} command line option, the output file will be created in the @samp{elf32-littlemips} format. @item TARGET(@var{bfdname}) @kindex TARGET(@var{bfdname}) @cindex input file format in linker script The @code{TARGET} command names the BFD format to use when reading input files. It affects subsequent @code{INPUT} and @code{GROUP} commands. This command is like using @samp{-b @var{bfdname}} on the command line (@pxref{Options,,Command Line Options}). If the @code{TARGET} command is used but @code{OUTPUT_FORMAT} is not, then the last @code{TARGET} command is also used to set the format for the output file. @xref{BFD}. @end table @end ifclear @node Miscellaneous Commands @subsection Other Linker Script Commands There are a few other linker scripts commands. @table @code @item ASSERT(@var{exp}, @var{message}) @kindex ASSERT @cindex assertion in linker script Ensure that @var{exp} is non-zero. If it is zero, then exit the linker with an error code, and print @var{message}. @item EXTERN(@var{symbol} @var{symbol} @dots{}) @kindex EXTERN @cindex undefined symbol in linker script Force @var{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 @var{symbol}s for each @code{EXTERN}, and you may use @code{EXTERN} multiple times. This command has the same effect as the @samp{-u} command-line option. @item FORCE_COMMON_ALLOCATION @kindex FORCE_COMMON_ALLOCATION @cindex common allocation in linker script This command has the same effect as the @samp{-d} command-line option: to make @command{ld} assign space to common symbols even if a relocatable output file is specified (@samp{-r}). @item INHIBIT_COMMON_ALLOCATION @kindex INHIBIT_COMMON_ALLOCATION @cindex common allocation in linker script This command has the same effect as the @samp{--no-define-common} command-line option: to make @code{ld} omit the assignment of addresses to common symbols even for a non-relocatable output file. @item NOCROSSREFS(@var{section} @var{section} @dots{}) @kindex NOCROSSREFS(@var{sections}) @cindex cross references This command may be used to tell @command{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 @code{NOCROSSREFS} command takes a list of output section names. If @command{ld} detects any cross references between the sections, it reports an error and returns a non-zero exit status. Note that the @code{NOCROSSREFS} command uses output section names, not input section names. @ifclear SingleFormat @item OUTPUT_ARCH(@var{bfdarch}) @kindex OUTPUT_ARCH(@var{bfdarch}) @cindex machine architecture @cindex architecture Specify a particular output machine architecture. The argument is one of the names used by the BFD library (@pxref{BFD}). You can see the architecture of an object file by using the @code{objdump} program with the @samp{-f} option. @end ifclear @end table @node Assignments @section Assigning Values to Symbols @cindex assignment in scripts @cindex symbol definition, scripts @cindex variables, defining You may assign a value to a symbol in a linker script. This will define the symbol and place it into the symbol table with a global scope. @menu * Simple Assignments:: Simple Assignments * PROVIDE:: PROVIDE * PROVIDE_HIDDEN:: PROVIDE_HIDDEN * Source Code Reference:: How to use a linker script defined symbol in source code @end menu @node Simple Assignments @subsection Simple Assignments You may assign to a symbol using any of the C assignment operators: @table @code @item @var{symbol} = @var{expression} ; @itemx @var{symbol} += @var{expression} ; @itemx @var{symbol} -= @var{expression} ; @itemx @var{symbol} *= @var{expression} ; @itemx @var{symbol} /= @var{expression} ; @itemx @var{symbol} <<= @var{expression} ; @itemx @var{symbol} >>= @var{expression} ; @itemx @var{symbol} &= @var{expression} ; @itemx @var{symbol} |= @var{expression} ; @end table The first case will define @var{symbol} to the value of @var{expression}. In the other cases, @var{symbol} must already be defined, and the value will be adjusted accordingly. The special symbol name @samp{.} indicates the location counter. You may only use this within a @code{SECTIONS} command. @xref{Location Counter}. The semicolon after @var{expression} is required. Expressions are defined below; see @ref{Expressions}. You may write symbol assignments as commands in their own right, or as statements within a @code{SECTIONS} command, or as part of an output section description in a @code{SECTIONS} command. The section of the symbol will be set from the section of the expression; for more information, see @ref{Expression Section}. Here is an example showing the three different places that symbol assignments may be used: @smallexample floating_point = 0; SECTIONS @{ .text : @{ *(.text) _etext = .; @} _bdata = (. + 3) & ~ 3; .data : @{ *(.data) @} @} @end smallexample @noindent In this example, the symbol @samp{floating_point} will be defined as zero. The symbol @samp{_etext} will be defined as the address following the last @samp{.text} input section. The symbol @samp{_bdata} will be defined as the address following the @samp{.text} output section aligned upward to a 4 byte boundary. @node PROVIDE @subsection PROVIDE @cindex 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 @samp{etext}. However, ANSI C requires that the user be able to use @samp{etext} as a function name without encountering an error. The @code{PROVIDE} keyword may be used to define a symbol, such as @samp{etext}, only if it is referenced but not defined. The syntax is @code{PROVIDE(@var{symbol} = @var{expression})}. Here is an example of using @code{PROVIDE} to define @samp{etext}: @smallexample SECTIONS @{ .text : @{ *(.text) _etext = .; PROVIDE(etext = .); @} @} @end smallexample In this example, if the program defines @samp{_etext} (with a leading underscore), the linker will give a multiple definition error. If, on the other hand, the program defines @samp{etext} (with no leading underscore), the linker will silently use the definition in the program. If the program references @samp{etext} but does not define it, the linker will use the definition in the linker script. @node PROVIDE_HIDDEN @subsection PROVIDE_HIDDEN @cindex PROVIDE_HIDDEN Similar to @code{PROVIDE}. For ELF targeted ports, the symbol will be hidden and won't be exported. @node Source Code Reference @subsection 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 @samp{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: @smallexample extern int foo; @end smallexample But in the linker script it might be defined as: @smallexample _foo = 1000; @end smallexample 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 @emph{value} of the symbol. The second is that the compiler creates an entry in the program's symbol table which holds the symbol's @emph{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: @smallexample int foo = 1000; @end smallexample creates a entry called @samp{foo} in the symbol table. This entry holds the address of an @samp{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: @smallexample foo = 1; @end smallexample looks up the symbol @samp{foo} in the symbol table, gets the address associated with this symbol and then writes the value 1 into that address. Whereas: @smallexample int * a = & foo; @end smallexample looks up the symbol @samp{foo} in the symbol table, gets it address and then copies this address into the block of memory associated with the variable @samp{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: @smallexample foo = 1000; @end smallexample creates an entry in the symbol table called @samp{foo} which holds the address of memory location 1000, but nothing special is stored at address 1000. This means that you cannot access the @emph{value} of a linker script defined symbol - it has no value - all you can do is access the @emph{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: @smallexample @group start_of_ROM = .ROM; end_of_ROM = .ROM + sizeof (.ROM) - 1; start_of_FLASH = .FLASH; @end group @end smallexample Then the C source code to perform the copy would be: @smallexample @group extern char start_of_ROM, end_of_ROM, start_of_FLASH; memcpy (& start_of_FLASH, & start_of_ROM, & end_of_ROM - & start_of_ROM); @end group @end smallexample Note the use of the @samp{&} operators. These are correct. @node SECTIONS @section SECTIONS Command @kindex SECTIONS The @code{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 @code{SECTIONS} command is: @smallexample SECTIONS @{ @var{sections-command} @var{sections-command} @dots{} @} @end smallexample Each @var{sections-command} may of be one of the following: @itemize @bullet @item an @code{ENTRY} command (@pxref{Entry Point,,Entry command}) @item a symbol assignment (@pxref{Assignments}) @item an output section description @item an overlay description @end itemize The @code{ENTRY} command and symbol assignments are permitted inside the @code{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 @code{SECTIONS} command in your linker script, the linker will place each input section into an identically named output section in the order that the sections are first encountered in the input files. If all input sections are present in the first file, for example, the order of sections in the output file will match the order in the first input file. The first section will be at address zero. @menu * Output Section Description:: Output section description * Output Section Name:: Output section name * Output Section Address:: Output section address * Input Section:: Input section description * Output Section Data:: Output section data * Output Section Keywords:: Output section keywords * Output Section Discarding:: Output section discarding * Output Section Attributes:: Output section attributes * Overlay Description:: Overlay description @end menu @node Output Section Description @subsection Output Section Description The full description of an output section looks like this: @smallexample @group @var{section} [@var{address}] [(@var{type})] : [AT(@var{lma})] [ALIGN(@var{section_align})] [SUBALIGN(@var{subsection_align})] @{ @var{output-section-command} @var{output-section-command} @dots{} @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}] @end group @end smallexample Most output sections do not use most of the optional section attributes. The whitespace around @var{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 @var{output-section-command} may be one of the following: @itemize @bullet @item a symbol assignment (@pxref{Assignments}) @item an input section description (@pxref{Input Section}) @item data values to include directly (@pxref{Output Section Data}) @item a special output section keyword (@pxref{Output Section Keywords}) @end itemize @node Output Section Name @subsection Output Section Name @cindex name, section @cindex section name The name of the output section is @var{section}. @var{section} must meet the constraints of your output format. In formats which only support a limited number of sections, such as @code{a.out}, the name must be one of the names supported by the format (@code{a.out}, for example, allows only @samp{.text}, @samp{.data} or @samp{.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 @samp{/DISCARD/} is special; @ref{Output Section Discarding}. @node Output Section Address @subsection Output Section Address @cindex address, section @cindex section address The @var{address} is an expression for the VMA (the virtual memory address) of the output section. If you do not provide @var{address}, the linker will set it based on @var{region} if present, or otherwise based on the current value of the location counter. If you provide @var{address}, the address of the output section will be set to precisely that. If you provide neither @var{address} nor @var{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, @smallexample .text . : @{ *(.text) @} @end smallexample @noindent and @smallexample .text : @{ *(.text) @} @end smallexample @noindent are subtly different. The first will set the address of the @samp{.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 @samp{.text} input section. The @var{address} may be an arbitrary expression; @ref{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: @smallexample .text ALIGN(0x10) : @{ *(.text) @} @end smallexample @noindent This works because @code{ALIGN} returns the current location counter aligned upward to the specified value. Specifying @var{address} for a section will change the value of the location counter. @node Input Section @subsection Input Section Description @cindex input sections @cindex mapping input sections to output sections The most common output section command is an input section description. The input section description is the most basic linker script operation. You use output sections to tell the linker how to lay out your program in memory. You use input section descriptions to tell the linker how to map the input files into your memory layout. @menu * Input Section Basics:: Input section basics * Input Section Wildcards:: Input section wildcard patterns * Input Section Common:: Input section for common symbols * Input Section Keep:: Input section and garbage collection * Input Section Example:: Input section example @end menu @node Input Section Basics @subsubsection Input Section Basics @cindex 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 (@pxref{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 @samp{.text} sections, you would write: @smallexample *(.text) @end smallexample @noindent Here the @samp{*} 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: @smallexample (*(EXCLUDE_FILE (*crtend.o *otherfile.o) .ctors)) @end smallexample will cause all .ctors sections from all files except @file{crtend.o} and @file{otherfile.o} to be included. There are two ways to include more than one section: @smallexample *(.text .rdata) *(.text) *(.rdata) @end smallexample @noindent The difference between these is the order in which the @samp{.text} and @samp{.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 @samp{.text} input sections will appear first, followed by all @samp{.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: @smallexample data.o(.data) @end smallexample 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: @smallexample data.o @end smallexample 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 @code{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 @code{INPUT} command, because the linker will not search for the file in the archive search path. @node Input Section Wildcards @subsubsection Input Section Wildcard Patterns @cindex input section wildcards @cindex wildcard file name patterns @cindex file name wildcard patterns @cindex section name 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 @samp{*} seen in many examples is a simple wildcard pattern for the file name. The wildcard patterns are like those used by the Unix shell. @table @samp @item * matches any number of characters @item ? matches any single character @item [@var{chars}] matches a single instance of any of the @var{chars}; the @samp{-} character may be used to specify a range of characters, as in @samp{[a-z]} to match any lower case letter @item \ quotes the following character @end table When a file name is matched with a wildcard, the wildcard characters will not match a @samp{/} character (used to separate directory names on Unix). A pattern consisting of a single @samp{*} character is an exception; it will always match any file name, whether it contains a @samp{/} or not. In a section name, the wildcard characters will match a @samp{/} character. File name wildcard patterns only match files which are explicitly specified on the command line or in an @code{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 @file{data.o} rule will not be used: @smallexample .data : @{ *(.data) @} .data1 : @{ data.o(.data) @} @end smallexample @cindex SORT_BY_NAME 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 @code{SORT_BY_NAME} keyword, which appears before a wildcard pattern in parentheses (e.g., @code{SORT_BY_NAME(.text*)}). When the @code{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. @cindex SORT_BY_ALIGNMENT @code{SORT_BY_ALIGNMENT} is very similar to @code{SORT_BY_NAME}. The difference is @code{SORT_BY_ALIGNMENT} will sort sections into ascending order by alignment before placing them in the output file. @cindex SORT @code{SORT} is an alias for @code{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. @enumerate @item @code{SORT_BY_NAME} (@code{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. @item @code{SORT_BY_ALIGNMENT} (@code{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. @item @code{SORT_BY_NAME} (@code{SORT_BY_NAME} (wildcard section pattern)) is treated the same as @code{SORT_BY_NAME} (wildcard section pattern). @item @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)) is treated the same as @code{SORT_BY_ALIGNMENT} (wildcard section pattern). @item All other nested section sorting commands are invalid. @end enumerate 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. @enumerate @item @code{SORT_BY_NAME} (wildcard section pattern ) with @option{--sort-sections alignment} is equivalent to @code{SORT_BY_NAME} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)). @item @code{SORT_BY_ALIGNMENT} (wildcard section pattern) with @option{--sort-section name} is equivalent to @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_NAME} (wildcard section pattern)). @end enumerate 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 @samp{-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 @samp{.text} sections in @samp{.text} and all @samp{.bss} sections in @samp{.bss}. The linker will place the @samp{.data} section from all files beginning with an upper case character in @samp{.DATA}; for all other files, the linker will place the @samp{.data} section in @samp{.data}. @smallexample @group SECTIONS @{ .text : @{ *(.text) @} .DATA : @{ [A-Z]*(.data) @} .data : @{ *(.data) @} .bss : @{ *(.bss) @} @} @end group @end smallexample @node Input Section Common @subsubsection Input Section for Common Symbols @cindex common symbol placement @cindex uninitialized data placement 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 @samp{COMMON}. You may use file names with the @samp{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 @samp{.bss} section in the output file. For example: @smallexample .bss @{ *(.bss) *(COMMON) @} @end smallexample @cindex scommon section @cindex small common symbols 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 @samp{COMMON} for standard common symbols and @samp{.scommon} for small common symbols. This permits you to map the different types of common symbols into memory at different locations. @cindex [COMMON] You will sometimes see @samp{[COMMON]} in old linker scripts. This notation is now considered obsolete. It is equivalent to @samp{*(COMMON)}. @node Input Section Keep @subsubsection Input Section and Garbage Collection @cindex KEEP @cindex garbage collection When link-time garbage collection is in use (@samp{--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 @code{KEEP()}, as in @code{KEEP(*(.init))} or @code{KEEP(SORT_BY_NAME(*)(.ctors))}. @node Input Section Example @subsubsection Input Section Example The following example is a complete linker script. It tells the linker to read all of the sections from file @file{all.o} and place them at the start of output section @samp{outputa} which starts at location @samp{0x10000}. All of section @samp{.input1} from file @file{foo.o} follows immediately, in the same output section. All of section @samp{.input2} from @file{foo.o} goes into output section @samp{outputb}, followed by section @samp{.input1} from @file{foo1.o}. All of the remaining @samp{.input1} and @samp{.input2} sections from any files are written to output section @samp{outputc}. @smallexample @group SECTIONS @{ outputa 0x10000 : @{ all.o foo.o (.input1) @} @end group @group outputb : @{ foo.o (.input2) foo1.o (.input1) @} @end group @group outputc : @{ *(.input1) *(.input2) @} @} @end group @end smallexample @node Output Section Data @subsection Output Section Data @cindex data @cindex section data @cindex output section data @kindex BYTE(@var{expression}) @kindex SHORT(@var{expression}) @kindex LONG(@var{expression}) @kindex QUAD(@var{expression}) @kindex SQUAD(@var{expression}) You can include explicit bytes of data in an output section by using @code{BYTE}, @code{SHORT}, @code{LONG}, @code{QUAD}, or @code{SQUAD} as an output section command. Each keyword is followed by an expression in parentheses providing the value to store (@pxref{Expressions}). The value of the expression is stored at the current value of the location counter. The @code{BYTE}, @code{SHORT}, @code{LONG}, and @code{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 @samp{addr}: @smallexample BYTE(1) LONG(addr) @end smallexample When using a 64 bit host or target, @code{QUAD} and @code{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 @code{QUAD} stores a 32 bit value zero extended to 64 bits, and @code{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: @smallexample SECTIONS @{@ .text : @{@ *(.text) @}@ LONG(1) .data : @{@ *(.data) @}@ @}@ @end smallexample whereas this will work: @smallexample SECTIONS @{@ .text : @{@ *(.text) ; LONG(1) @}@ .data : @{@ *(.data) @}@ @}@ @end smallexample @kindex FILL(@var{expression}) @cindex holes, filling @cindex unspecified memory You may use the @code{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 @code{FILL} statement covers memory locations after the point at which it occurs in the section definition; by including more than one @code{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 @samp{0x90}: @smallexample FILL(0x90909090) @end smallexample The @code{FILL} command is similar to the @samp{=@var{fillexp}} output section attribute, but it only affects the part of the section following the @code{FILL} command, rather than the entire section. If both are used, the @code{FILL} command takes precedence. @xref{Output Section Fill}, for details on the fill expression. @node Output Section Keywords @subsection Output Section Keywords There are a couple of keywords which can appear as output section commands. @table @code @kindex CREATE_OBJECT_SYMBOLS @cindex input filename symbols @cindex filename symbols @item 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 @code{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. @kindex CONSTRUCTORS @cindex C++ constructors, arranging in link @cindex constructors, arranging in link @item 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 @code{CONSTRUCTORS} command tells the linker to place constructor information in the output section where the @code{CONSTRUCTORS} command appears. The @code{CONSTRUCTORS} command is ignored for other object file formats. The symbol @w{@code{__CTOR_LIST__}} marks the start of the global constructors, and the symbol @w{@code{__CTOR_END__}} marks the end. Similarly, @w{@code{__DTOR_LIST__}} and @w{@code{__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 @sc{gnu} C++ normally calls constructors from a subroutine @code{__main}; a call to @code{__main} is automatically inserted into the startup code for @code{main}. @sc{gnu} C++ normally runs destructors either by using @code{atexit}, or directly from the function @code{exit}. For object file formats such as @code{COFF} or @code{ELF} which support arbitrary section names, @sc{gnu} C++ will normally arrange to put the addresses of global constructors and destructors into the @code{.ctors} and @code{.dtors} sections. Placing the following sequence into your linker script will build the sort of table which the @sc{gnu} C++ runtime code expects to see. @smallexample __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__ = .; @end smallexample If you are using the @sc{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 @code{CONSTRUCTORS} command, use @samp{SORT_BY_NAME(CONSTRUCTORS)} instead. When using the @code{.ctors} and @code{.dtors} sections, use @samp{*(SORT_BY_NAME(.ctors))} and @samp{*(SORT_BY_NAME(.dtors))} instead of just @samp{*(.ctors)} and @samp{*(.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. @end table @node Output Section Discarding @subsection Output Section Discarding @cindex discarding sections @cindex sections, discarding @cindex removing sections 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: @smallexample .foo : @{ *(.foo) @} @end smallexample @noindent will only create a @samp{.foo} section in the output file if there is a @samp{.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 (@pxref{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. @cindex /DISCARD/ The special output section name @samp{/DISCARD/} may be used to discard input sections. Any input sections which are assigned to an output section named @samp{/DISCARD/} are not included in the output file. @node Output Section Attributes @subsection Output Section Attributes @cindex output section attributes We showed above that the full description of an output section looked like this: @smallexample @group @var{section} [@var{address}] [(@var{type})] : [AT(@var{lma})] [ALIGN(@var{section_align})] [SUBALIGN(@var{subsection_align})] @{ @var{output-section-command} @var{output-section-command} @dots{} @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}] @end group @end smallexample We've already described @var{section}, @var{address}, and @var{output-section-command}. In this section we will describe the remaining section attributes. @menu * Output Section Type:: Output section type * Output Section LMA:: Output section LMA * Forced Output Alignment:: Forced Output Alignment * Forced Input Alignment:: Forced Input Alignment * Output Section Region:: Output section region * Output Section Phdr:: Output section phdr * Output Section Fill:: Output section fill @end menu @node Output Section Type @subsubsection Output Section Type Each output section may have a type. The type is a keyword in parentheses. The following types are defined: @table @code @item NOLOAD The section should be marked as not loadable, so that it will not be loaded into memory when the program is run. @item DSECT @itemx COPY @itemx INFO @itemx 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. @end table @kindex NOLOAD @cindex prevent unnecessary loading @cindex loading, preventing 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 @samp{ROM} section is addressed at memory location @samp{0} and does not need to be loaded when the program is run. The contents of the @samp{ROM} section will appear in the linker output file as usual. @smallexample @group SECTIONS @{ ROM 0 (NOLOAD) : @{ @dots{} @} @dots{} @} @end group @end smallexample @node Output Section LMA @subsubsection Output Section LMA @kindex AT>@var{lma_region} @kindex AT(@var{lma}) @cindex load address @cindex section load address Every section has a virtual address (VMA) and a load address (LMA); see @ref{Basic Script Concepts}. The address expression which may appear in an output section description sets the VMA (@pxref{Output Section Address}). The expression @var{lma} that follows the @code{AT} keyword specifies the load address of the section. Alternatively, with @samp{AT>@var{lma_region}} expression, you may specify a memory region for the section's load address. @xref{MEMORY}. Note that if the section has not had a VMA assigned to it then the linker will use the @var{lma_region} as the VMA region as well. If neither @code{AT} nor @code{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. @xref{Output Section Region}. @cindex ROM initialized data @cindex initialized data in ROM 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 @samp{.text}, which starts at @code{0x1000}, one called @samp{.mdata}, which is loaded at the end of the @samp{.text} section even though its VMA is @code{0x2000}, and one called @samp{.bss} to hold uninitialized data at address @code{0x3000}. The symbol @code{_data} is defined with the value @code{0x2000}, which shows that the location counter holds the VMA value, not the LMA value. @smallexample @group SECTIONS @{ .text 0x1000 : @{ *(.text) _etext = . ; @} .mdata 0x2000 : AT ( ADDR (.text) + SIZEOF (.text) ) @{ _data = . ; *(.data); _edata = . ; @} .bss 0x3000 : @{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;@} @} @end group @end smallexample 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. @smallexample @group 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; @end group @end smallexample @node Forced Output Alignment @subsubsection Forced Output Alignment @kindex ALIGN(@var{section_align}) @cindex forcing output section alignment @cindex output section alignment You can increase an output section's alignment by using ALIGN. @node Forced Input Alignment @subsubsection Forced Input Alignment @kindex SUBALIGN(@var{subsection_align}) @cindex forcing input section alignment @cindex input section 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. @node Output Section Region @subsubsection Output Section Region @kindex >@var{region} @cindex section, assigning to memory region @cindex memory regions and sections You can assign a section to a previously defined region of memory by using @samp{>@var{region}}. @xref{MEMORY}. Here is a simple example: @smallexample @group MEMORY @{ rom : ORIGIN = 0x1000, LENGTH = 0x1000 @} SECTIONS @{ ROM : @{ *(.text) @} >rom @} @end group @end smallexample @node Output Section Phdr @subsubsection Output Section Phdr @kindex :@var{phdr} @cindex section, assigning to program header @cindex program headers and sections You can assign a section to a previously defined program segment by using @samp{:@var{phdr}}. @xref{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 @code{:@var{phdr}} modifier. You can use @code{:NONE} to tell the linker to not put the section in any segment at all. Here is a simple example: @smallexample @group PHDRS @{ text PT_LOAD ; @} SECTIONS @{ .text : @{ *(.text) @} :text @} @end group @end smallexample @node Output Section Fill @subsubsection Output Section Fill @kindex =@var{fillexp} @cindex section fill pattern @cindex fill pattern, entire section You can set the fill pattern for an entire section by using @samp{=@var{fillexp}}. @var{fillexp} is an expression (@pxref{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 @samp{0x} and without a trailing @samp{k} or @samp{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 @code{+}, 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 @code{FILL} command in the output section commands; (@pxref{Output Section Data}). Here is a simple example: @smallexample @group SECTIONS @{ .text : @{ *(.text) @} =0x90909090 @} @end group @end smallexample @node Overlay Description @subsection Overlay Description @kindex OVERLAY @cindex overlays 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 @code{OVERLAY} command. The @code{OVERLAY} command is used within a @code{SECTIONS} command, like an output section description. The full syntax of the @code{OVERLAY} command is as follows: @smallexample @group OVERLAY [@var{start}] : [NOCROSSREFS] [AT ( @var{ldaddr} )] @{ @var{secname1} @{ @var{output-section-command} @var{output-section-command} @dots{} @} [:@var{phdr}@dots{}] [=@var{fill}] @var{secname2} @{ @var{output-section-command} @var{output-section-command} @dots{} @} [:@var{phdr}@dots{}] [=@var{fill}] @dots{} @} [>@var{region}] [:@var{phdr}@dots{}] [=@var{fill}] @end group @end smallexample Everything is optional except @code{OVERLAY} (a keyword), and each section must have a name (@var{secname1} and @var{secname2} above). The section definitions within the @code{OVERLAY} construct are identical to those within the general @code{SECTIONS} contruct (@pxref{SECTIONS}), except that no addresses and no memory regions may be defined for sections within an @code{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 @code{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 @code{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. @xref{Miscellaneous Commands, NOCROSSREFS}. For each section within the @code{OVERLAY}, the linker automatically defines two symbols. The symbol @code{__load_start_@var{secname}} is defined as the starting load address of the section. The symbol @code{__load_stop_@var{secname}} is defined as the final load address of the section. Any characters within @var{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 @code{SECTIONS} construct. @smallexample @group OVERLAY 0x1000 : AT (0x4000) @{ .text0 @{ o1/*.o(.text) @} .text1 @{ o2/*.o(.text) @} @} @end group @end smallexample @noindent This will define both @samp{.text0} and @samp{.text1} to start at address 0x1000. @samp{.text0} will be loaded at address 0x4000, and @samp{.text1} will be loaded immediately after @samp{.text0}. The following symbols will be defined: @code{__load_start_text0}, @code{__load_stop_text0}, @code{__load_start_text1}, @code{__load_stop_text1}. C code to copy overlay @code{.text1} into the overlay area might look like the following. @smallexample @group extern char __load_start_text1, __load_stop_text1; memcpy ((char *) 0x1000, &__load_start_text1, &__load_stop_text1 - &__load_start_text1); @end group @end smallexample Note that the @code{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. @smallexample @group .text0 0x1000 : AT (0x4000) @{ o1/*.o(.text) @} __load_start_text0 = LOADADDR (.text0); __load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0); .text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) @{ o2/*.o(.text) @} __load_start_text1 = LOADADDR (.text1); __load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1); . = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1)); @end group @end smallexample @node MEMORY @section MEMORY Command @kindex MEMORY @cindex memory regions @cindex regions of memory @cindex allocating memory @cindex discontinuous memory The linker's default configuration permits allocation of all available memory. You can override this by using the @code{MEMORY} command. The @code{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 @code{MEMORY} command. However, you can define as many blocks of memory within it as you wish. The syntax is: @smallexample @group MEMORY @{ @var{name} [(@var{attr})] : ORIGIN = @var{origin}, LENGTH = @var{len} @dots{} @} @end group @end smallexample The @var{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. @cindex memory region attributes The @var{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 @ref{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 @var{attr} string must consist only of the following characters: @table @samp @item R Read-only section @item W Read/write section @item X Executable section @item A Allocatable section @item I Initialized section @item L Same as @samp{I} @item ! Invert the sense of any of the preceding attributes @end table If a unmapped section matches any of the listed attributes other than @samp{!}, it will be placed in the memory region. The @samp{!} 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. @kindex ORIGIN = @kindex o = @kindex org = The @var{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 @code{ORIGIN} may be abbreviated to @code{org} or @code{o} (but not, for example, @code{ORG}). @kindex LENGTH = @kindex len = @kindex l = The @var{len} is an expression for the size in bytes of the memory region. As with the @var{origin} expression, the expression must be numerical only and must evaluate to a constant. The keyword @code{LENGTH} may be abbreviated to @code{len} or @code{l}. In the following example, we specify that there are two memory regions available for allocation: one starting at @samp{0} for 256 kilobytes, and the other starting at @samp{0x40000000} for four megabytes. The linker will place into the @samp{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 @samp{ram} memory region. @smallexample @group MEMORY @{ rom (rx) : ORIGIN = 0, LENGTH = 256K ram (!rx) : org = 0x40000000, l = 4M @} @end group @end smallexample Once you define a memory region, you can direct the linker to place specific output sections into that memory region by using the @samp{>@var{region}} output section attribute. For example, if you have a memory region named @samp{mem}, you would use @samp{>mem} in the output section definition. @xref{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 @code{ORIGIN(@var{memory})} and @code{LENGTH(@var{memory})} functions: @smallexample @group _fstack = ORIGIN(ram) + LENGTH(ram) - 4; @end group @end smallexample @node PHDRS @section PHDRS Command @kindex PHDRS @cindex program headers @cindex ELF program headers @cindex program segments @cindex segments, ELF The ELF object file format uses @dfn{program headers}, also knows as @dfn{segments}. The program headers describe how the program should be loaded into memory. You can print them out by using the @code{objdump} program with the @samp{-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 @code{PHDRS} command for this purpose. When the linker sees the @code{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 @code{PHDRS} command when generating an ELF output file. In other cases, the linker will simply ignore @code{PHDRS}. This is the syntax of the @code{PHDRS} command. The words @code{PHDRS}, @code{FILEHDR}, @code{AT}, and @code{FLAGS} are keywords. @smallexample @group PHDRS @{ @var{name} @var{type} [ FILEHDR ] [ PHDRS ] [ AT ( @var{address} ) ] [ FLAGS ( @var{flags} ) ] ; @} @end group @end smallexample The @var{name} is used only for reference in the @code{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 @samp{:@var{phdr}} output section attribute to place a section in a particular segment. @xref{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 @samp{:@var{phdr}}, using it once for each segment which should contain the section. If you place a section in one or more segments using @samp{:@var{phdr}}, then the linker will place all subsequent allocatable sections which do not specify @samp{:@var{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 @code{:NONE} to override the default segment and tell the linker to not put the section in any segment at all. @kindex FILEHDR @kindex PHDRS You may use the @code{FILEHDR} and @code{PHDRS} keywords appear after the program header type to further describe the contents of the segment. The @code{FILEHDR} keyword means that the segment should include the ELF file header. The @code{PHDRS} keyword means that the segment should include the ELF program headers themselves. The @var{type} may be one of the following. The numbers indicate the value of the keyword. @table @asis @item @code{PT_NULL} (0) Indicates an unused program header. @item @code{PT_LOAD} (1) Indicates that this program header describes a segment to be loaded from the file. @item @code{PT_DYNAMIC} (2) Indicates a segment where dynamic linking information can be found. @item @code{PT_INTERP} (3) Indicates a segment where the name of the program interpreter may be found. @item @code{PT_NOTE} (4) Indicates a segment holding note information. @item @code{PT_SHLIB} (5) A reserved program header type, defined but not specified by the ELF ABI. @item @code{PT_PHDR} (6) Indicates a segment where the program headers may be found. @item @var{expression} An expression giving the numeric type of the program header. This may be used for types not defined above. @end table You can specify that a segment should be loaded at a particular address in memory by using an @code{AT} expression. This is identical to the @code{AT} command used as an output section attribute (@pxref{Output Section LMA}). The @code{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 @code{FLAGS} keyword to explicitly specify the segment flags. The value of @var{flags} must be an integer. It is used to set the @code{p_flags} field of the program header. Here is an example of @code{PHDRS}. This shows a typical set of program headers used on a native ELF system. @example @group 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 */ @dots{} . = . + 0x1000; /* move to a new page in memory */ .data : @{ *(.data) @} :data .dynamic : @{ *(.dynamic) @} :data :dynamic @dots{} @} @end group @end example @node VERSION @section VERSION Command @kindex VERSION @{script text@} @cindex symbol versions @cindex version script @cindex versions of symbols 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 @samp{--version-script} linker option. The syntax of the @code{VERSION} command is simply @smallexample VERSION @{ version-script-commands @} @end smallexample 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. @smallexample 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; @end smallexample This example version script defines three version nodes. The first version node defined is @samp{VERS_1.1}; it has no other dependencies. The script binds the symbol @samp{foo1} to @samp{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 @samp{old}, @samp{original}, or @samp{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 @samp{VERS_1.2}. This node depends upon @samp{VERS_1.1}. The script binds the symbol @samp{foo2} to the version node @samp{VERS_1.2}. Finally, the version script defines node @samp{VERS_2.0}. This node depends upon @samp{VERS_1.2}. The scripts binds the symbols @samp{bar1} and @samp{bar2} are bound to the version node @samp{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 @samp{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 @samp{2.0} version could just as well have appeared in between @samp{1.1} and @samp{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. @smallexample @{ global: foo; bar; local: *; @}; @end smallexample 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: @smallexample __asm__(".symver original_foo,foo@@VERS_1.1"); @end smallexample @noindent in the C source file. This renames the function @samp{original_foo} to be an alias for @samp{foo} bound to the version node @samp{VERS_1.1}. The @samp{local:} directive can be used to prevent the symbol @samp{original_foo} from being exported. A @samp{.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 @samp{.symver} directives in the source file. Here is an example: @smallexample __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"); @end smallexample In this example, @samp{foo@@} represents the symbol @samp{foo} bound to the unspecified base version of the symbol. The source file that contains this example would define 4 C functions: @samp{original_foo}, @samp{old_foo}, @samp{old_foo1}, and @samp{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 @samp{foo@@@@VERS_2.0} type of @samp{.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., @samp{old_foo}), or you can use the @samp{.symver} directive to specifically bind to an external version of the function in question. You can also specify the language in the version script: @smallexample VERSION extern "lang" @{ version-script-commands @} @end smallexample The supported @samp{lang}s are @samp{C}, @samp{C++}, and @samp{Java}. The linker will iterate over the list of symbols at the link time and demangle them according to @samp{lang} before matching them to the patterns specified in @samp{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. @node Expressions @section Expressions in Linker Scripts @cindex expressions @cindex arithmetic The syntax for expressions in the linker script language is identical to that of C expressions. All expressions are evaluated as integers. All expressions are evaluated in the same size, which is 32 bits if both the host and target are 32 bits, and is otherwise 64 bits. You can use and set symbol values in expressions. The linker defines several special purpose builtin functions for use in expressions. @menu * Constants:: Constants * Symbols:: Symbol Names * Orphan Sections:: Orphan Sections * Location Counter:: The Location Counter * Operators:: Operators * Evaluation:: Evaluation * Expression Section:: The Section of an Expression * Builtin Functions:: Builtin Functions @end menu @node Constants @subsection Constants @cindex integer notation @cindex constants in linker scripts All constants are integers. As in C, the linker considers an integer beginning with @samp{0} to be octal, and an integer beginning with @samp{0x} or @samp{0X} to be hexadecimal. The linker considers other integers to be decimal. @cindex scaled integers @cindex K and M integer suffixes @cindex M and K integer suffixes @cindex suffixes for integers @cindex integer suffixes In addition, you can use the suffixes @code{K} and @code{M} to scale a constant by @c TEXI2ROFF-KILL @ifnottex @c END TEXI2ROFF-KILL @code{1024} or @code{1024*1024} @c TEXI2ROFF-KILL @end ifnottex @tex ${\rm 1024}$ or ${\rm 1024}^2$ @end tex @c END TEXI2ROFF-KILL respectively. For example, the following all refer to the same quantity: @smallexample _fourk_1 = 4K; _fourk_2 = 4096; _fourk_3 = 0x1000; @end smallexample @node Symbols @subsection Symbol Names @cindex symbol names @cindex names @cindex quoted symbol names @kindex " 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: @smallexample "SECTION" = 9; "with a space" = "also with a space" + 10; @end smallexample Since symbols can contain many non-alphabetic characters, it is safest to delimit symbols with spaces. For example, @samp{A-B} is one symbol, whereas @samp{A - B} is an expression involving subtraction. @node Orphan Sections @subsection Orphan Sections @cindex orphan 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. @node Location Counter @subsection The Location Counter @kindex . @cindex dot @cindex location counter @cindex current output location The special linker variable @dfn{dot} @samp{.} always contains the current output location counter. Since the @code{.} always refers to a location in an output section, it may only appear in an expression within a @code{SECTIONS} command. The @code{.} symbol may appear anywhere that an ordinary symbol is allowed in an expression. @cindex holes Assigning a value to @code{.} 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. @smallexample SECTIONS @{ output : @{ file1(.text) . = . + 1000; file2(.text) . += 1000; file3(.text) @} = 0x12345678; @} @end smallexample @noindent In the previous example, the @samp{.text} section from @file{file1} is located at the beginning of the output section @samp{output}. It is followed by a 1000 byte gap. Then the @samp{.text} section from @file{file2} appears, also with a 1000 byte gap following before the @samp{.text} section from @file{file3}. The notation @samp{= 0x12345678} specifies what data to write in the gaps (@pxref{Output Section Fill}). @cindex dot inside sections Note: @code{.} actually refers to the byte offset from the start of the current containing object. Normally this is the @code{SECTIONS} statement, whose start address is 0, hence @code{.} can be used as an absolute address. If @code{.} 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: @smallexample SECTIONS @{ . = 0x100 .text: @{ *(.text) . = 0x200 @} . = 0x500 .data: @{ *(.data) . += 0x600 @} @} @end smallexample The @samp{.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 @samp{.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 @code{.} backwards). The @samp{.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 @samp{.data} input sections and before the end of the @samp{.data} output section itself. @cindex dot outside sections 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: @smallexample SECTIONS @{ start_of_text = . ; .text: @{ *(.text) @} end_of_text = . ; start_of_data = . ; .data: @{ *(.data) @} end_of_data = . ; @} @end smallexample If the linker needs to place some input section, e.g. @code{.rodata}, not mentioned in the script, it might choose to place that section between @code{.text} and @code{.data}. You might think the linker should place @code{.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 @code{.}. I.e., the linker will place the orphan @code{.rodata} section as if the script was written as follows: @smallexample SECTIONS @{ start_of_text = . ; .text: @{ *(.text) @} end_of_text = . ; start_of_data = . ; .rodata: @{ *(.rodata) @} .data: @{ *(.data) @} end_of_data = . ; @} @end smallexample This may or may not be the script author's intention for the value of @code{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 @code{.} is setting the start address of a following output section and thus should be grouped with that section. So you could write: @smallexample SECTIONS @{ start_of_text = . ; .text: @{ *(.text) @} end_of_text = . ; . = . ; start_of_data = . ; .data: @{ *(.data) @} end_of_data = . ; @} @end smallexample Now, the orphan @code{.rodata} section will be placed between @code{end_of_text} and @code{start_of_data}. @need 2000 @node Operators @subsection Operators @cindex operators for arithmetic @cindex arithmetic operators @cindex precedence in expressions The linker recognizes the standard C set of arithmetic operators, with the standard bindings and precedence levels: @c TEXI2ROFF-KILL @ifnottex @c END TEXI2ROFF-KILL @smallexample 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) @end smallexample Notes: (1) Prefix operators (2) @xref{Assignments}. @c TEXI2ROFF-KILL @end ifnottex @tex \vskip \baselineskip %"lispnarrowing" is the extra indent used generally for smallexample \hskip\lispnarrowing\vbox{\offinterlineskip \hrule \halign {\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ {\tt #}\ \hfil&\vrule#\cr height2pt&\omit&&\omit&&\omit&\cr &Precedence&& Associativity &&{\rm Operators}&\cr height2pt&\omit&&\omit&&\omit&\cr \noalign{\hrule} height2pt&\omit&&\omit&&\omit&\cr &highest&&&&&\cr % '176 is tilde, '~' in tt font &1&&left&&\qquad- \char'176\ !\qquad\dag&\cr &2&&left&&* / \%&\cr &3&&left&&+ -&\cr &4&&left&&>> <<&\cr &5&&left&&== != > < <= >=&\cr &6&&left&&\&&\cr &7&&left&&|&\cr &8&&left&&{\&\&}&\cr &9&&left&&||&\cr &10&&right&&? :&\cr &11&&right&&\qquad\&= += -= *= /=\qquad\ddag&\cr &lowest&&&&&\cr height2pt&\omit&&\omit&&\omit&\cr} \hrule} @end tex @iftex { @obeylines@parskip=0pt@parindent=0pt @dag@quad Prefix operators. @ddag@quad @xref{Assignments}. } @end iftex @c END TEXI2ROFF-KILL @node Evaluation @subsection Evaluation @cindex lazy evaluation @cindex expression evaluation order 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 @samp{.}, 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 @smallexample @group SECTIONS @{ .text 9+this_isnt_constant : @{ *(.text) @} @} @end group @end smallexample @noindent will cause the error message @samp{non constant expression for initial address}. @node Expression Section @subsection The Section of an Expression @cindex expression sections @cindex absolute expressions @cindex relative expressions @cindex absolute and relocatable symbols @cindex relocatable and absolute symbols @cindex symbols, relocatable and absolute 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 @samp{-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 @code{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 @samp{.data}: @smallexample SECTIONS @{ .data : @{ *(.data) _edata = ABSOLUTE(.); @} @} @end smallexample @noindent If @samp{ABSOLUTE} were not used, @samp{_edata} would be relative to the @samp{.data} section. @node Builtin Functions @subsection Builtin Functions @cindex functions in expressions The linker script language includes a number of builtin functions for use in linker script expressions. @table @code @item ABSOLUTE(@var{exp}) @kindex ABSOLUTE(@var{exp}) @cindex expression, absolute Return the absolute (non-relocatable, as opposed to non-negative) value of the expression @var{exp}. Primarily useful to assign an absolute value to a symbol within a section definition, where symbol values are normally section relative. @xref{Expression Section}. @item ADDR(@var{section}) @kindex ADDR(@var{section}) @cindex section address in expression Return the absolute address (the VMA) of the named @var{section}. Your script must previously have defined the location of that section. In the following example, @code{symbol_1} and @code{symbol_2} are assigned identical values: @smallexample @group SECTIONS @{ @dots{} .output1 : @{ start_of_output_1 = ABSOLUTE(.); @dots{} @} .output : @{ symbol_1 = ADDR(.output1); symbol_2 = start_of_output_1; @} @dots{} @} @end group @end smallexample @item ALIGN(@var{align}) @itemx ALIGN(@var{exp},@var{align}) @kindex ALIGN(@var{align}) @kindex ALIGN(@var{exp},@var{align}) @cindex round up location counter @cindex align location counter @cindex round up expression @cindex align expression Return the location counter (@code{.}) or arbitrary expression aligned to the next @var{align} boundary. The single operand @code{ALIGN} doesn't change the value of the location counter---it just does arithmetic on it. The two operand @code{ALIGN} allows an arbitrary expression to be aligned upwards (@code{ALIGN(@var{align})} is equivalent to @code{ALIGN(., @var{align})}). Here is an example which aligns the output @code{.data} section to the next @code{0x2000} byte boundary after the preceding section and sets a variable within the section to the next @code{0x8000} boundary after the input sections: @smallexample @group SECTIONS @{ @dots{} .data ALIGN(0x2000): @{ *(.data) variable = ALIGN(0x8000); @} @dots{} @} @end group @end smallexample @noindent The first use of @code{ALIGN} in this example specifies the location of a section because it is used as the optional @var{address} attribute of a section definition (@pxref{Output Section Address}). The second use of @code{ALIGN} is used to defines the value of a symbol. The builtin function @code{NEXT} is closely related to @code{ALIGN}. @item BLOCK(@var{exp}) @kindex BLOCK(@var{exp}) This is a synonym for @code{ALIGN}, for compatibility with older linker scripts. It is most often seen when setting the address of an output section. @item DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize}) @kindex DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize}) This is equivalent to either @smallexample (ALIGN(@var{maxpagesize}) + (. & (@var{maxpagesize} - 1))) @end smallexample or @smallexample (ALIGN(@var{maxpagesize}) + (. & (@var{maxpagesize} - @var{commonpagesize}))) @end smallexample @noindent depending on whether the latter uses fewer @var{commonpagesize} sized pages for the data segment (area between the result of this expression and @code{DATA_SEGMENT_END}) than the former or not. If the latter form is used, it means @var{commonpagesize} bytes of runtime memory will be saved at the expense of up to @var{commonpagesize} wasted bytes in the on-disk file. This expression can only be used directly in @code{SECTIONS} commands, not in any output section descriptions and only once in the linker script. @var{commonpagesize} should be less or equal to @var{maxpagesize} and should be the system page size the object wants to be optimized for (while still working on system page sizes up to @var{maxpagesize}). @noindent Example: @smallexample . = DATA_SEGMENT_ALIGN(0x10000, 0x2000); @end smallexample @item DATA_SEGMENT_END(@var{exp}) @kindex DATA_SEGMENT_END(@var{exp}) This defines the end of data segment for @code{DATA_SEGMENT_ALIGN} evaluation purposes. @smallexample . = DATA_SEGMENT_END(.); @end smallexample @item DATA_SEGMENT_RELRO_END(@var{offset}, @var{exp}) @kindex DATA_SEGMENT_RELRO_END(@var{offset}, @var{exp}) This defines the end of the @code{PT_GNU_RELRO} segment when @samp{-z relro} option is used. Second argument is returned. When @samp{-z relro} option is not present, @code{DATA_SEGMENT_RELRO_END} does nothing, otherwise @code{DATA_SEGMENT_ALIGN} is padded so that @var{exp} + @var{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 @code{DATA_SEGMENT_ALIGN} and @code{DATA_SEGMENT_END}. @smallexample . = DATA_SEGMENT_RELRO_END(24, .); @end smallexample @item DEFINED(@var{symbol}) @kindex DEFINED(@var{symbol}) @cindex symbol defaults Return 1 if @var{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 @samp{begin} to the first location in the @samp{.text} section---but if a symbol called @samp{begin} already existed, its value is preserved: @smallexample @group SECTIONS @{ @dots{} .text : @{ begin = DEFINED(begin) ? begin : . ; @dots{} @} @dots{} @} @end group @end smallexample @item LENGTH(@var{memory}) @kindex LENGTH(@var{memory}) Return the length of the memory region named @var{memory}. @item LOADADDR(@var{section}) @kindex LOADADDR(@var{section}) @cindex section load address in expression Return the absolute LMA of the named @var{section}. This is normally the same as @code{ADDR}, but it may be different if the @code{AT} attribute is used in the output section definition (@pxref{Output Section LMA}). @kindex MAX @item MAX(@var{exp1}, @var{exp2}) Returns the maximum of @var{exp1} and @var{exp2}. @kindex MIN @item MIN(@var{exp1}, @var{exp2}) Returns the minimum of @var{exp1} and @var{exp2}. @item NEXT(@var{exp}) @kindex NEXT(@var{exp}) @cindex unallocated address, next Return the next unallocated address that is a multiple of @var{exp}. This function is closely related to @code{ALIGN(@var{exp})}; unless you use the @code{MEMORY} command to define discontinuous memory for the output file, the two functions are equivalent. @item ORIGIN(@var{memory}) @kindex ORIGIN(@var{memory}) Return the origin of the memory region named @var{memory}. @item SEGMENT_START(@var{segment}, @var{default}) @kindex SEGMENT_START(@var{segment}, @var{default}) Return the base address of the named @var{segment}. If an explicit value has been given for this segment (with a command-line @samp{-T} option) that value will be returned; otherwise the value will be @var{default}. At present, the @samp{-T} command-line option can only be used to set the base address for the ``text'', ``data'', and ``bss'' sections, but you use @code{SEGMENT_START} with any segment name. @item SIZEOF(@var{section}) @kindex SIZEOF(@var{section}) @cindex section size Return the size in bytes of the named @var{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, @code{symbol_1} and @code{symbol_2} are assigned identical values: @smallexample @group SECTIONS@{ @dots{} .output @{ .start = . ; @dots{} .end = . ; @} symbol_1 = .end - .start ; symbol_2 = SIZEOF(.output); @dots{} @} @end group @end smallexample @item SIZEOF_HEADERS @itemx sizeof_headers @kindex SIZEOF_HEADERS @cindex header size 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. @cindex not enough room for program headers @cindex program headers, not enough room When producing an ELF output file, if the linker script uses the @code{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 @samp{not enough room for program headers}. To avoid this error, you must avoid using the @code{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 @code{PHDRS} command (@pxref{PHDRS}). @end table @node Implicit Linker Scripts @section Implicit Linker Scripts @cindex 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 @code{INPUT}, @code{GROUP}, or @code{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. @ifset GENERIC @node Machine Dependent @chapter Machine Dependent Features @cindex machine dependencies @command{ld} has additional features on some platforms; the following sections describe them. Machines where @command{ld} has no additional functionality are not listed. @menu @ifset H8300 * H8/300:: @command{ld} and the H8/300 @end ifset @ifset I960 * i960:: @command{ld} and the Intel 960 family @end ifset @ifset ARM * ARM:: @command{ld} and the ARM family @end ifset @ifset HPPA * HPPA ELF32:: @command{ld} and HPPA 32-bit ELF @end ifset @ifset MMIX * MMIX:: @command{ld} and MMIX @end ifset @ifset MSP430 * MSP430:: @command{ld} and MSP430 @end ifset @ifset M68HC11 * M68HC11/68HC12:: @code{ld} and the Motorola 68HC11 and 68HC12 families @end ifset @ifset POWERPC * PowerPC ELF32:: @command{ld} and PowerPC 32-bit ELF Support @end ifset @ifset POWERPC64 * PowerPC64 ELF64:: @command{ld} and PowerPC64 64-bit ELF Support @end ifset @ifset TICOFF * TI COFF:: @command{ld} and TI COFF @end ifset @ifset WIN32 * WIN32:: @command{ld} and WIN32 (cygwin/mingw) @end ifset @ifset XTENSA * Xtensa:: @command{ld} and Xtensa Processors @end ifset @end menu @end ifset @ifset H8300 @ifclear GENERIC @raisesections @end ifclear @node H8/300 @section @command{ld} and the H8/300 @cindex H8/300 support For the H8/300, @command{ld} can perform these global optimizations when you specify the @samp{--relax} command-line option. @table @emph @cindex relaxing on H8/300 @item relaxing address modes @command{ld} finds all @code{jsr} and @code{jmp} instructions whose targets are within eight bits, and turns them into eight-bit program-counter relative @code{bsr} and @code{bra} instructions, respectively. @cindex synthesizing on H8/300 @item synthesizing instructions @c FIXME: specifically mov.b, or any mov instructions really? @command{ld} finds all @code{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 @samp{mov.b @code{@@}@var{aa}:16} into @samp{mov.b @code{@@}@var{aa}:8} whenever the address @var{aa} is in the top page of memory). @item bit manipulation instructions @command{ld} finds all bit manipulation instructions like @code{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 @samp{bset #xx:3,@code{@@}@var{aa}:32} into @samp{bset #xx:3,@code{@@}@var{aa}:8} whenever the address @var{aa} is in the top page of memory). @item system control instructions @command{ld} finds all @code{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 @samp{ldc.w @code{@@}@var{aa}:32,ccr} into @samp{ldc.w @code{@@}@var{aa}:16,ccr} whenever the address @var{aa} is in the top page of memory). @end table @ifclear GENERIC @lowersections @end ifclear @end ifset @ifclear GENERIC @ifset Renesas @c This stuff is pointless to say unless you're especially concerned @c with Renesas chips; don't enable it for generic case, please. @node Renesas @chapter @command{ld} and Other Renesas Chips @command{ld} also supports the Renesas (formerly Hitachi) H8/300H, H8/500, and SH chips. No special features, commands, or command-line options are required for these chips. @end ifset @end ifclear @ifset I960 @ifclear GENERIC @raisesections @end ifclear @node i960 @section @command{ld} and the Intel 960 Family @cindex i960 support You can use the @samp{-A@var{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 @command{ld} command line included @w{@samp{-ACA}} as well as @w{@samp{-ltry}}, the linker would look (in its built-in search paths, and in any paths you specify with @samp{-L}) for a library with the names @smallexample @group try libtry.a tryca libtryca.a @end group @end smallexample @noindent The first two possibilities would be considered in any event; the last two are due to the use of @w{@samp{-ACA}}. You can meaningfully use @samp{-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 @w{@samp{-l}} specifies a library. @cindex @option{--relax} on i960 @cindex relaxing on i960 @command{ld} supports the @samp{--relax} option for the i960 family. If you specify @samp{--relax}, @command{ld} finds all @code{balx} and @code{calx} instructions whose targets are within 24 bits, and turns them into 24-bit program-counter relative @code{bal} and @code{cal} instructions, respectively. @command{ld} also turns @code{cal} instructions into @code{bal} instructions when it determines that the target subroutine is a leaf routine (that is, the target subroutine does not itself call any subroutines). @ifclear GENERIC @lowersections @end ifclear @end ifset @ifset ARM @ifclear GENERIC @raisesections @end ifclear @ifset M68HC11 @ifclear GENERIC @raisesections @end ifclear @node M68HC11/68HC12 @section @command{ld} and the Motorola 68HC11 and 68HC12 families @cindex M68HC11 and 68HC12 support @subsection Linker Relaxation For the Motorola 68HC11, @command{ld} can perform these global optimizations when you specify the @samp{--relax} command-line option. @table @emph @cindex relaxing on M68HC11 @item relaxing address modes @command{ld} finds all @code{jsr} and @code{jmp} instructions whose targets are within eight bits, and turns them into eight-bit program-counter relative @code{bsr} and @code{bra} instructions, respectively. @command{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). @item relaxing gcc instruction group When @command{gcc} is called with @option{-mrelax}, it can emit group of instructions that the linker can optimize to use a 68HC11 direct addressing mode. These instructions consists of @code{bclr} or @code{bset} instructions. @end table @subsection Trampoline Generation @cindex trampoline generation on M68HC11 @cindex trampoline generation on M68HC12 For 68HC11 and 68HC12, @command{ld} can generate trampoline code to call a far function using a normal @code{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. @ifclear GENERIC @lowersections @end ifclear @end ifset @node ARM @section @command{ld} and the ARM family @cindex ARM interworking support @kindex --support-old-code For the ARM, @command{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 @samp{-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 @samp{--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. @cindex thumb entry point @cindex entry point, thumb @kindex --thumb-entry=@var{entry} The @samp{--thumb-entry} switch is a duplicate of the generic @samp{--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. @cindex BE8 @kindex --be8 The @samp{--be8} switch instructs @command{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. @cindex TARGET1 @kindex --target1-rel @kindex --target1-abs The @samp{R_ARM_TARGET1} relocation is typically used for entries in the @samp{.init_array} section. It is interpreted as either @samp{R_ARM_REL32} or @samp{R_ARM_ABS32}, depending on the target. The @samp{--target1-rel} and @samp{--target1-abs} switches override the default. @cindex TARGET2 @kindex --target2=@var{type} The @samp{--target2=type} switch overrides the default definition of the @samp{R_ARM_TARGET2} relocation. Valid values for @samp{type}, their meanings, and target defaults are as follows: @table @samp @item rel @samp{R_ARM_REL32} (arm*-*-elf, arm*-*-eabi) @item abs @samp{R_ARM_ABS32} (arm*-*-symbianelf) @item got-rel @samp{R_ARM_GOT_PREL} (arm*-*-linux, arm*-*-*bsd) @end table @cindex FIX_V4BX @kindex --fix-v4bx The @samp{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 @option{--fix-v4bx} must be passed to the linker, which causes v4t @code{BX rM} instructions to be rewritten as @code{MOV PC,rM}, since v4 processors do not have a @code{BX} instruction. In the former case, the switch should not be used, and @samp{R_ARM_V4BX} relocations are ignored. @cindex USE_BLX @kindex --use-blx The @samp{--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. @ifclear GENERIC @lowersections @end ifclear @end ifset @ifset HPPA @ifclear GENERIC @raisesections @end ifclear @node HPPA ELF32 @section @command{ld} and HPPA 32-bit ELF Support @cindex HPPA multiple sub-space stubs @kindex --multi-subspace When generating a shared library, @command{ld} will by default generate import stubs suitable for use with a single sub-space application. The @samp{--multi-subspace} switch causes @command{ld} to generate export stubs, and different (larger) import stubs suitable for use with multiple sub-spaces. @cindex HPPA stub grouping @kindex --stub-group-size=@var{N} Long branch stubs and import/export stubs are placed by @command{ld} in stub sections located between groups of input sections. @samp{--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 @samp{N} chooses this scheme, ensuring that branches to stubs always use a negative offset. Two special values of @samp{N} are recognized, @samp{1} and @samp{-1}. These both instruct @command{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 @samp{N} respectively. Note that @samp{--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. @ifclear GENERIC @lowersections @end ifclear @end ifset @ifset MMIX @ifclear GENERIC @raisesections @end ifclear @node MMIX @section @code{ld} and MMIX For MMIX, there is a choice of generating @code{ELF} object files or @code{mmo} object files when linking. The simulator @code{mmix} understands the @code{mmo} format. The binutils @code{objcopy} utility can translate between the two formats. There is one special section, the @samp{.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 @samp{.MMIX.reg_contents} section corresponds to the first allocated global register multiplied by 8. Register @code{$255} is not included in this section; it is always set to the program entry, which is at the symbol @code{Main} for @code{mmo} files. Symbols with the prefix @code{__.MMIX.start.}, for example @code{__.MMIX.start..text} and @code{__.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. @ifclear GENERIC @lowersections @end ifclear @end ifset @ifset MSP430 @ifclear GENERIC @raisesections @end ifclear @node MSP430 @section @code{ld} and MSP430 For the MSP430 it is possible to select the MPU architecture. The flag @samp{-m [mpu type]} will select an appropriate linker script for selected MPU type. (To get a list of known MPUs just pass @samp{-m help} option to the linker). @cindex MSP430 extra sections The linker will recognize some extra sections which are MSP430 specific: @table @code @item @samp{.vectors} Defines a portion of ROM where interrupt vectors located. @item @samp{.bootloader} Defines the bootloader portion of the ROM (if applicable). Any code in this section will be uploaded to the MPU. @item @samp{.infomem} Defines an information memory section (if applicable). Any code in this section will be uploaded to the MPU. @item @samp{.infomemnobits} This is the same as the @samp{.infomem} section except that any code in this section will not be uploaded to the MPU. @item @samp{.noinit} Denotes a portion of RAM located above @samp{.bss} section. The last two sections are used by gcc. @end table @ifclear GENERIC @lowersections @end ifclear @end ifset @ifset POWERPC @ifclear GENERIC @raisesections @end ifclear @node PowerPC ELF32 @section @command{ld} and PowerPC 32-bit ELF Support @cindex PowerPC long branches @kindex --relax on PowerPC Branches on PowerPC processors are limited to a signed 26-bit displacement, which may result in @command{ld} giving @samp{relocation truncated to fit} errors with very large programs. @samp{--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. @cindex PowerPC ELF32 options @table @option @cindex PowerPC PLT @kindex --bss-plt @item --bss-plt Current PowerPC GCC accepts a @samp{-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 @command{ld} will generate this layout, including stubs to access the PLT, if all input files (including startup and static libraries) were compiled with @samp{-msecure-plt}. @samp{--bss-plt} forces the old BSS PLT (and GOT layout) which can give slightly better performance. @cindex PowerPC GOT @kindex --sdata-got @item --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 @code{.plt} must change because the new secure PLT is an initialized section while the old PLT is uninitialized. The reason for the @code{.got} change is more subtle: The new placement allows @code{.got} to be read-only in applications linked with @samp{-z relro -z now}. However, this placement means that @code{.sdata} cannot always be used in shared libraries, because the PowerPC ABI accesses @code{.sdata} in shared libraries from the GOT pointer. @samp{--sdata-got} forces the old GOT placement. PowerPC GCC doesn't use @code{.sdata} in shared libraries, so this option is really only useful for other compilers that may do so. @cindex PowerPC stub symbols @kindex --emit-stub-syms @item --emit-stub-syms This option causes @command{ld} to label linker stubs with a local symbol that encodes the stub type and destination. @cindex PowerPC TLS optimization @kindex --no-tls-optimize @item --no-tls-optimize PowerPC @command{ld} normally performs some optimization of code sequences used to access Thread-Local Storage. Use this option to disable the optimization. @end table @ifclear GENERIC @lowersections @end ifclear @end ifset @ifset POWERPC64 @ifclear GENERIC @raisesections @end ifclear @node PowerPC64 ELF64 @section @command{ld} and PowerPC64 64-bit ELF Support @cindex PowerPC64 ELF64 options @table @option @cindex PowerPC64 stub grouping @kindex --stub-group-size @item --stub-group-size Long branch stubs, PLT call stubs and TOC adjusting stubs are placed by @command{ld} in stub sections located between groups of input sections. @samp{--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 @samp{N} chooses this scheme, ensuring that branches to stubs always use a negative offset. Two special values of @samp{N} are recognized, @samp{1} and @samp{-1}. These both instruct @command{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 @samp{N} respectively. Note that @samp{--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. @cindex PowerPC64 stub symbols @kindex --emit-stub-syms @item --emit-stub-syms This option causes @command{ld} to label linker stubs with a local symbol that encodes the stub type and destination. @cindex PowerPC64 dot symbols @kindex --dotsyms @kindex --no-dotsyms @item --dotsyms, --no-dotsyms These two options control how @command{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 (@samp{.}). To properly version a function @samp{foo}, the version script thus needs to control both @samp{foo} and @samp{.foo}. The option @samp{--dotsyms}, on by default, automatically adds the required dot-prefixed patterns. Use @samp{--no-dotsyms} to disable this feature. @cindex PowerPC64 TLS optimization @kindex --no-tls-optimize @item --no-tls-optimize PowerPC64 @command{ld} normally performs some optimization of code sequences used to access Thread-Local Storage. Use this option to disable the optimization. @cindex PowerPC64 OPD optimization @kindex --no-opd-optimize @item --no-opd-optimize PowerPC64 @command{ld} normally removes @code{.opd} section entries corresponding to deleted link-once functions, or functions removed by the action of @samp{--gc-sections} or linker scrip @code{/DISCARD/}. Use this option to disable @code{.opd} optimization. @cindex PowerPC64 OPD spacing @kindex --non-overlapping-opd @item --non-overlapping-opd Some PowerPC64 compilers have an option to generate compressed @code{.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. @cindex PowerPC64 TOC optimization @kindex --no-toc-optimize @item --no-toc-optimize PowerPC64 @command{ld} normally removes unused @code{.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. @cindex PowerPC64 multi-TOC @kindex --no-multi-toc @item --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 @command{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. @command{ld} does not split apart input sections, so cannot help if a single input file has a @code{.toc} section that exceeds 64K, most likely from linking multiple files with @command{ld -r}. Use this option to turn off this feature. @end table @ifclear GENERIC @lowersections @end ifclear @end ifset @ifset TICOFF @ifclear GENERIC @raisesections @end ifclear @node TI COFF @section @command{ld}'s Support for Various TI COFF Versions @cindex TI COFF versions @kindex --format=@var{version} The @samp{--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; @command{ld} will read any version or byte order, but the output header format depends on the default specified by the specific target. @ifclear GENERIC @lowersections @end ifclear @end ifset @ifset WIN32 @ifclear GENERIC @raisesections @end ifclear @node WIN32 @section @command{ld} and WIN32 (cygwin/mingw) This section describes some of the win32 specific @command{ld} issues. See @ref{Options,,Command Line Options} for detailed description of the command line options mentioned here. @table @emph @cindex import libraries @item 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 @command{ld} have specific support for creating such libraries provided with the @samp{--out-implib} command line option. @item exporting DLL symbols @cindex exporting DLL symbols The cygwin/mingw @command{ld} has several ways to export symbols for dll's. @table @emph @item using auto-export functionality @cindex using auto-export functionality By default @command{ld} exports symbols with the auto-export functionality, which is controlled by the following command line options: @itemize @item --export-all-symbols [This is the default] @item --exclude-symbols @item --exclude-libs @end itemize If, however, @samp{--export-all-symbols} is not given explicitly on the command line, then the default auto-export behavior will be @emph{disabled} if either of the following are true: @itemize @item A DEF file is used. @item Any symbol in any object file was marked with the __declspec(dllexport) attribute. @end itemize @item using a DEF file @cindex 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 @samp{.def} and is added as any other object file to the linker's command line. The file's name must end in @samp{.def} or @samp{.DEF}. @example gcc -o .def @end example Using a DEF file turns off the normal auto-export behavior, unless the @samp{--export-all-symbols} option is also used. Here is an example of a DEF file for a shared library called @samp{xyz.dll}: @example LIBRARY "xyz.dll" BASE=0x20000000 EXPORTS foo bar _bar = bar another_foo = abc.dll.afoo var1 DATA @end example This example defines a DLL with a non-default base address and five symbols in the export table. The third exported symbol @code{_bar} is an alias for the second. The fourth symbol, @code{another_foo} is resolved by "forwarding" to another module and treating it as an alias for @code{afoo} exported from the DLL @samp{abc.dll}. The final symbol @code{var1} is declared to be a data object. The optional @code{LIBRARY } command indicates the @emph{internal} name of the output DLL. If @samp{} does not include a suffix, the default library suffix, @samp{.DLL} is appended. When the .DEF file is used to build an application, rather than a library, the @code{NAME } command should be used instead of @code{LIBRARY}. If @samp{} does not include a suffix, the default executable suffix, @samp{.EXE} is appended. With either @code{LIBRARY } or @code{NAME } the optional specification @code{BASE = } may be used to specify a non-default base address for the image. If neither @code{LIBRARY } nor @code{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: @example EXPORTS ( ( ( [ = ] ) | ( = . )) [ @@ ] [NONAME] [DATA] [CONSTANT] [PRIVATE] ) * @end example Declares @samp{} as an exported symbol from the DLL, or declares @samp{} as an exported alias for @samp{}; or declares @samp{} as a "forward" alias for the symbol @samp{} in the DLL @samp{}. Optionally, the symbol may be exported by the specified ordinal @samp{} alias. The optional keywords that follow the declaration indicate: @code{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 @code{PRIVATE} is also specified. @code{DATA}: The symbol is a variable or object, rather than a function. The import lib will export only an indirect reference to @code{foo} as the symbol @code{_imp__foo} (ie, @code{foo} must be resolved as @code{*_imp__foo}). @code{CONSTANT}: Like @code{DATA}, but put the undecorated @code{foo} as well as @code{_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 @code{dllimport} attribute and also fails to explicitly add the extra indirection that the use of the attribute enforces, the application will behave unexpectedly. @code{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 @code{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 @cindex creating a DEF file While linking a shared dll, @command{ld} is able to create a DEF file with the @samp{--output-def } command line option. @item Using decorations @cindex 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: @example __declspec(dllexport) int a_variable __declspec(dllexport) void a_function(int with_args) @end example 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 @samp{--export-all-symbols} option is also used. Note that object files that wish to access these symbols must @emph{not} decorate them with dllexport. Instead, they should use dllimport, instead: @example __declspec(dllimport) int a_variable __declspec(dllimport) void a_function(int with_args) @end example 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 @samp{--enable-auto-import} and @samp{automatic data imports} for more information. @end table @cindex automatic data imports @item 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 @samp{--enable-auto-import} command-line option, although it is enabled by default on cygwin/mingw. The @samp{--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 '' can't be auto-imported. Please read the documentation for ld's @code{--enable-auto-import} for details." The @samp{--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 @emph{runtime pseudo-relocs} feature, described below. @cindex runtime pseudo-relocation For complex variables imported from DLLs (such as structs or classes), object files typically contain a base address for the variable and an offset (@emph{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 @samp{--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. @samp{--enable-runtime-pseudo-relocs} is not the default; it must be explicitly enabled as needed. @cindex direct linking to a dll @item direct linking to a dll The cygwin/mingw ports of @command{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 @command{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 @samp{-L} and @samp{-l}, because @command{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 @samp{-lxxx} it will attempt to find, in the first directory of its search path, @example libxxx.dll.a xxx.dll.a libxxx.a xxx.lib cygxxx.dll (*) libxxx.dll xxx.dll @end example before moving on to the next directory in the search path. (*) Actually, this is not @samp{cygxxx.dll} but in fact is @samp{xxx.dll}, where @samp{} is set by the @command{ld} option @samp{--dll-search-prefix=}. In the case of cygwin, the standard gcc spec file includes @samp{--dll-search-prefix=cyg}, so in effect we actually search for @samp{cygxxx.dll}. Other win32-based unix environments, such as mingw or pw32, may use other @samp{}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 @samp{bin} directory for applications and dll's and a @samp{lib} directory for the import libraries (using cygwin nomenclature): @example bin/ cygxxx.dll lib/ libxxx.dll.a (in case of dll's) libxxx.a (in case of static archive) @end example Linking directly to a dll without using the import library can be done two ways: 1. Use the dll directly by adding the @samp{bin} path to the link line @example gcc -Wl,-verbose -o a.exe -L../bin/ -lxxx @end example However, as the dll's often have version numbers appended to their names (@samp{cygncurses-5.dll}) this will often fail, unless one specifies @samp{-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 @samp{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. @example ln -s bin/cygxxx.dll lib/[cyg|lib|]xxx.dll[.a] @end example Then you can link without any make environment changes. @example gcc -Wl,-verbose -o a.exe -L../lib/ -lxxx @end example This technique also avoids the version number problems, because the following is perfectly legal @example bin/ cygxxx-5.dll lib/ libxxx.dll.a -> ../bin/cygxxx-5.dll @end example Linking directly to a dll without using an import lib will work even when auto-import features are exercised, and even when @samp{--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 @emph{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. @item symbol aliasing @table @emph @item adding additional names Sometimes, it is useful to export symbols with additional names. A symbol @samp{foo} will be exported as @samp{foo}, but it can also be exported as @samp{_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: @example LIBRARY "xyz.dll" BASE=0x61000000 EXPORTS foo _foo = foo @end example The line @samp{_foo = foo} maps the symbol @samp{foo} to @samp{_foo}. Another method for creating a symbol alias is to create it in the source code using the "weak" attribute: @example void foo () @{ /* Do something. */; @} void _foo () __attribute__ ((weak, alias ("foo"))); @end example See the gcc manual for more information about attributes and weak symbols. @item renaming symbols Sometimes it is useful to rename exports. For instance, the cygwin kernel does this regularly. A symbol @samp{_foo} can be exported as @samp{foo} but not as @samp{_foo} by using special directives in the DEF file. (This will also affect the import library, if it is created). In the following example: @example LIBRARY "xyz.dll" BASE=0x61000000 EXPORTS _foo = foo @end example The line @samp{_foo = foo} maps the exported symbol @samp{foo} to @samp{_foo}. @end table Note: using a DEF file disables the default auto-export behavior, unless the @samp{--export-all-symbols} command line option is used. If, however, you are trying to rename symbols, then you should list @emph{all} desired exports in the DEF file, including the symbols that are not being renamed, and do @emph{not} use the @samp{--export-all-symbols} option. If you list only the renamed symbols in the DEF file, and use @samp{--export-all-symbols} to handle the other symbols, then the both the new names @emph{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. @cindex weak externals @item 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: @itemize @item Definition is searched for in objects and libraries, historically called lazy externals. @item Definition is searched for only in other objects, not in libraries. This form is not presently implemented. @item No search; the symbol is an alias. This form is not presently implemented. @end itemize 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. @end table @ifclear GENERIC @lowersections @end ifclear @end ifset @ifset XTENSA @ifclear GENERIC @raisesections @end ifclear @node Xtensa @section @code{ld} and Xtensa Processors @cindex Xtensa processors The default @command{ld} behavior for Xtensa processors is to interpret @code{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: @smallexample SECTIONS @{ .text : @{ *(.literal .text) @} @} @end smallexample @noindent @command{ld} may interleave some of the @code{.literal} and @code{.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 @code{.literal} sections from an initial group of files followed by the @code{.text} sections of that group of files. Then, the @code{.literal} sections from the rest of the files and the @code{.text} sections from the rest of the files would follow. @cindex @option{--relax} on Xtensa @cindex relaxing on Xtensa Relaxation is enabled by default for the Xtensa version of @command{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 @code{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 @code{L32R} instructions. The second optimization is to remove unnecessary overhead from assembler-generated ``longcall'' sequences of @code{L32R}/@code{CALLX@var{n}} when the target functions are within range of direct @code{CALL@var{n}} instructions. For each of these cases where an indirect call sequence can be optimized to a direct call, the linker will change the @code{CALLX@var{n}} instruction to a @code{CALL@var{n}} instruction, remove the @code{L32R} instruction, and remove the literal referenced by the @code{L32R} instruction if it is not used for anything else. Removing the @code{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 @code{L32R} instruction that was removed. If code size is more important than performance, the @option{--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: @cindex Xtensa options @table @option @kindex --no-relax @item --no-relax Since the Xtensa version of @code{ld} enables the @option{--relax} option by default, the @option{--no-relax} option is provided to disable relaxation. @item --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. @end table @ifclear GENERIC @lowersections @end ifclear @end ifset @ifclear SingleFormat @node BFD @chapter BFD @cindex back end @cindex object file management @cindex object formats available @kindex objdump -i 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 @code{objdump -i} (@pxref{objdump,,objdump,binutils.info,The GNU Binary Utilities}) to list all the formats available for your configuration. @cindex BFD requirements @cindex requirements for BFD 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. @xref{BFD information loss}. @menu * BFD outline:: How it works: an outline of BFD @end menu @node BFD outline @section How It Works: An Outline of BFD @cindex opening object files @include bfdsumm.texi @end ifclear @node Reporting Bugs @chapter Reporting Bugs @cindex bugs in @command{ld} @cindex reporting bugs in @command{ld} Your bug reports play an essential role in making @command{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 @command{ld} work better. Bug reports are your contribution to the maintenance of @command{ld}. In order for a bug report to serve its purpose, you must include the information that enables us to fix the bug. @menu * Bug Criteria:: Have you found a bug? * Bug Reporting:: How to report bugs @end menu @node Bug Criteria @section Have You Found a Bug? @cindex bug criteria If you are not sure whether you have found a bug, here are some guidelines: @itemize @bullet @cindex fatal signal @cindex linker crash @cindex crash of linker @item If the linker gets a fatal signal, for any input whatever, that is a @command{ld} bug. Reliable linkers never crash. @cindex error on valid input @item If @command{ld} produces an error message for valid input, that is a bug. @cindex invalid input @item If @command{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. @item If you are an experienced user of linkers, your suggestions for improvement of @command{ld} are welcome in any case. @end itemize @node Bug Reporting @section How to Report Bugs @cindex bug reports @cindex @command{ld} bugs, reporting A number of companies and individuals offer support for @sc{gnu} products. If you obtained @command{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 @file{etc/SERVICE} in the @sc{gnu} Emacs distribution. Otherwise, send bug reports for @command{ld} to @samp{bug-binutils@@gnu.org}. The fundamental principle of reporting bugs usefully is this: @strong{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: @itemize @bullet @item The version of @command{ld}. @command{ld} announces it if you start it with the @samp{--version} argument. Without this, we will not know whether there is any point in looking for the bug in the current version of @command{ld}. @item Any patches you may have applied to the @command{ld} source, including any patches made to the @code{BFD} library. @item The type of machine you are using, and the operating system name and version number. @item What compiler (and its version) was used to compile @command{ld}---e.g. ``@code{gcc-2.7}''. @item 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. @item 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 @code{gas} or compiled using @code{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 @code{gas} or @code{gcc} was used to produce the object files. Also say how @code{gas} or @code{gcc} were configured. @item 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 @command{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 @command{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. @item If you wish to suggest changes to the @command{ld} source, send us context diffs, as generated by @code{diff} with the @samp{-u}, @samp{-c}, or @samp{-p} option. Always send diffs from the old file to the new file. If you even discuss something in the @command{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. @end itemize Here are some things that are not necessary: @itemize @bullet @item 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 @emph{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. @item 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 @command{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. @item 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. @end itemize @node MRI @appendix MRI Compatible Script Files @cindex MRI compatibility To aid users making the transition to @sc{gnu} @command{ld} from the MRI linker, @command{ld} can use MRI compatible linker scripts as an alternative to the more general-purpose linker scripting language described in @ref{Scripts}. MRI compatible linker scripts have a much simpler command set than the scripting language otherwise used with @command{ld}. @sc{gnu} @command{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 @code{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 @samp{-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, @command{ld} issues a warning message, but continues processing the script. Lines beginning with @samp{*} are comments. You can write these commands using all upper-case letters, or all lower case; for example, @samp{chip} is the same as @samp{CHIP}. The following list shows only the upper-case form of each command. @table @code @cindex @code{ABSOLUTE} (MRI) @item ABSOLUTE @var{secname} @itemx ABSOLUTE @var{secname}, @var{secname}, @dots{} @var{secname} Normally, @command{ld} includes in the output file all sections from all the input files. However, in an MRI-compatible script, you can use the @code{ABSOLUTE} command to restrict the sections that will be present in your output program. If the @code{ABSOLUTE} command is used at all in a script, then only the sections named explicitly in @code{ABSOLUTE} commands will appear in the linker output. You can still use other input sections (whatever you select on the command line, or using @code{LOAD}) to resolve addresses in the output file. @cindex @code{ALIAS} (MRI) @item ALIAS @var{out-secname}, @var{in-secname} Use this command to place the data from input section @var{in-secname} in a section called @var{out-secname} in the linker output file. @var{in-secname} may be an integer. @cindex @code{ALIGN} (MRI) @item ALIGN @var{secname} = @var{expression} Align the section called @var{secname} to @var{expression}. The @var{expression} should be a power of two. @cindex @code{BASE} (MRI) @item BASE @var{expression} Use the value of @var{expression} as the lowest address (other than absolute addresses) in the output file. @cindex @code{CHIP} (MRI) @item CHIP @var{expression} @itemx CHIP @var{expression}, @var{expression} This command does nothing; it is accepted only for compatibility. @cindex @code{END} (MRI) @item END This command does nothing whatever; it's only accepted for compatibility. @cindex @code{FORMAT} (MRI) @item FORMAT @var{output-format} Similar to the @code{OUTPUT_FORMAT} command in the more general linker language, but restricted to one of these output formats: @enumerate @item S-records, if @var{output-format} is @samp{S} @item IEEE, if @var{output-format} is @samp{IEEE} @item COFF (the @samp{coff-m68k} variant in BFD), if @var{output-format} is @samp{COFF} @end enumerate @cindex @code{LIST} (MRI) @item LIST @var{anything}@dots{} Print (to the standard output file) a link map, as produced by the @command{ld} command-line option @samp{-M}. The keyword @code{LIST} may be followed by anything on the same line, with no change in its effect. @cindex @code{LOAD} (MRI) @item LOAD @var{filename} @itemx LOAD @var{filename}, @var{filename}, @dots{} @var{filename} Include one or more object file @var{filename} in the link; this has the same effect as specifying @var{filename} directly on the @command{ld} command line. @cindex @code{NAME} (MRI) @item NAME @var{output-name} @var{output-name} is the name for the program produced by @command{ld}; the MRI-compatible command @code{NAME} is equivalent to the command-line option @samp{-o} or the general script language command @code{OUTPUT}. @cindex @code{ORDER} (MRI) @item ORDER @var{secname}, @var{secname}, @dots{} @var{secname} @itemx ORDER @var{secname} @var{secname} @var{secname} Normally, @command{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 @code{ORDER} command. The sections you list with @code{ORDER} will appear first in your output file, in the order specified. @cindex @code{PUBLIC} (MRI) @item PUBLIC @var{name}=@var{expression} @itemx PUBLIC @var{name},@var{expression} @itemx PUBLIC @var{name} @var{expression} Supply a value (@var{expression}) for external symbol @var{name} used in the linker input files. @cindex @code{SECT} (MRI) @item SECT @var{secname}, @var{expression} @itemx SECT @var{secname}=@var{expression} @itemx SECT @var{secname} @var{expression} You can use any of these three forms of the @code{SECT} command to specify the start address (@var{expression}) for section @var{secname}. If you have more than one @code{SECT} statement for the same @var{secname}, only the @emph{first} sets the start address. @end table @include fdl.texi @node LD Index @unnumbered LD Index @printindex cp @tex % I think something like @colophon should be in texinfo. In the % meantime: \long\def\colophon{\hbox to0pt{}\vfill \centerline{The body of this manual is set in} \centerline{\fontname\tenrm,} \centerline{with headings in {\bf\fontname\tenbf}} \centerline{and examples in {\tt\fontname\tentt}.} \centerline{{\it\fontname\tenit\/} and} \centerline{{\sl\fontname\tensl\/}} \centerline{are used for emphasis.}\vfill} \page\colophon % Blame: doc@cygnus.com, 28mar91. @end tex @contents @bye