\input texinfo @setfilename ld.info @syncodeindex ky cp @include configdoc.texi @c (configdoc.texi is generated by the Makefile) @c @smallbook @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. Copyright (C) 1991, 92, 93, 94, 95, 1996 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions. @ignore 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 Using LD, the GNU linker @titlepage @title Using ld @subtitle The GNU linker @sp 1 @subtitle @code{ld} version 2 @subtitle January 1994 @author Steve Chamberlain @author Cygnus Support @page @tex {\parskip=0pt \hfill Cygnus Support\par \hfill steve\@cygnus.com, doc\@cygnus.com\par \hfill {\it Using LD, 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 Copyright @copyright{} 1991, 92, 93, 94, 95, 1996 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions. @end titlepage @end iftex @c FIXME: Talk about importance of *order* of args, cmds to linker! @ifinfo @node Top @top Using ld This file documents the @sc{gnu} linker ld. @menu * Overview:: Overview * Invocation:: Invocation * Commands:: Command Language @ifset GENERIC * Machine Dependent:: Machine Dependent Features @end ifset @ifclear GENERIC @ifset H8300 * H8/300:: ld and the H8/300 @end ifset @ifset Hitachi * Hitachi:: ld and other Hitachi micros @end ifset @ifset I960 * i960:: ld and the Intel 960 family @end ifset @end ifclear @ifclear SingleFormat * BFD:: BFD @end ifclear @c Following blank line required for remaining bug in makeinfo conds/menus * MRI:: MRI Compatible Script Files * Index:: Index @end menu @end ifinfo @node Overview @chapter Overview @cindex @sc{gnu} linker @cindex what is this? @code{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 @code{ld}. @code{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. @ifclear SingleFormat This version of @code{ld} uses the general purpose BFD libraries to operate on object files. This allows @code{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, @code{ld} continues executing, allowing you to identify other errors (or, in some cases, to get an output file in spite of the error). @node Invocation @chapter Invocation The @sc{gnu} linker @code{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. @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 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 @code{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 @code{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.) The command-line options to @code{ld} may be specified in any order, and may be repeated at will. Repeating most 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 objects files 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 can not recognize the format of an object file, it will assume that it is a linker script. A script specified in this way augments the main linker script used for the link (either the default linker script or the one specified by using @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. @xref{Commands}. 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{--oformat} and @samp{-oformat} are equivalent. 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{--oformat srec} and @samp{--oformat=srec} are equivalent. Unique abbreviations of the names of multiple-letter options are accepted. @table @code @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 @code{ld}, this option is useful only for the Intel 960 family of architectures. In that @code{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,,@code{ld} and the Intel 960 family}, for details. Future releases of @code{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} @code{ld} may be configured to support more than one kind of object file. If your @code{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 @code{ld} is configured to support alternative object formats, you don't usually need to specify this, as @code{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}; see @ref{Option Commands}. @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, @code{ld} accepts script files written in an alternate, restricted command language, described in @ref{MRI,,MRI Compatible Script Files}. Introduce MRI script files with the option @samp{-c}; use the @samp{-T} option to run linker scripts written in the general-purpose @code{ld} scripting language. If @var{MRI-cmdfile} does not exist, @code{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{Option 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. @xref{Entry Point}, for a discussion of defaults and other ways of specifying the entry point. @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. Normally, the dynamic symbol table contains only symbols which are used by a dynamic object. This option is needed for some uses of @code{dlopen}. @ifclear SingleFormat @kindex -F @item -F @itemx -F@var{format} Ignored. Some older linkers used this option throughout a compilation toolchain for specifying object-file format for both input and output object files. The mechanisms @code{ld} uses for this purpose (the @samp{-b} or @samp{-format} options for input files, @samp{-oformat} option or the @code{TARGET} command in linker scripts for output files, the @code{GNUTARGET} environment variable) are more flexible, but @code{ld} accepts the @samp{-F} option for compatibility with scripts written to call the old linker. @end ifclear @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 -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 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. @code{ld} will search its path-list for occurrences of @code{lib@var{archive}.a} for every @var{archive} specified. File extensions other than @code{.a} may be used on certain systems. 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 @code{-(} 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 @code{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 @code{ld} will search for archive libraries and @code{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 @code{-L} options apply to all @code{-l} options, regardless of the order in which the options appear. @ifset UsesEnvVars The default set of paths searched (without being specified with @samp{-L}) depends on which emulation mode @code{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. The default depends on how your @code{ld} was configured. @cindex link map @kindex -M @kindex --print-map @item -M @itemx --print-map Print (to the standard output) a link map---diagnostic information about where symbols are mapped by @code{ld}, and information on global common storage allocation. @kindex -n @cindex read-only text @cindex NMAGIC @kindex --nmagic @item -n @itemx --nmagic Set the text segment to be read only, 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. If the output format supports Unix style magic numbers, mark the output as @code{OMAGIC}. @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 @code{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. @cindex partial link @cindex relocatable output @kindex -r @kindex --relocateable @item -r @itemx --relocateable Generate relocatable output---i.e., generate an output file that can in turn serve as input to @code{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 @code{-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}. 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 @code{-R} option is followed by a directory name, rather than a file name, it is treated as the @code{-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 @code{ld} processes them. @kindex -T @var{script} @kindex --script=@var{script} @cindex script files @item -T @var{commandfile} @itemx --script=@var{commandfile} Read link commands from the file @var{commandfile}. These commands replace @code{ld}'s default link script (rather than adding to it), so @var{commandfile} must specify everything necessary to describe the target format. @xref{Commands}. If @var{commandfile} does not exist, @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. @c Nice idea, but no such command: This option is equivalent @c to the @code{EXTERN} linker command. @kindex -v @kindex -V @kindex --version @cindex version @item -v @itemx --version @itemx -V Display the version number for @code{ld}. The @code{-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 @cindex L, deleting symbols beginning @item -X @itemx --discard-locals Delete all temporary local symbols. For most targets, this is all local symbols whose names begin with @samp{L}. @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} This option is 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 -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 @code{-l} options which follow it. @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 @code{-l} options which follow it. @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. @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 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{Assignment, , Assignment: Symbol Definitions}). @emph{Note:} there should be no white space between @var{symbol}, the equals sign (``@key{=}''), and @var{expression}. @cindex dynamic linker, from command line @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. @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. @cindex MIPS embedded PIC code @kindex -embedded-relocs @item -embedded-relocs This option is only meaningful when linking MIPS embedded PIC code, generated by the -membedded-pic option to the @sc{gnu} compiler and assembler. It causes the linker to create a table which may be used at runtime to relocate any data which was statically initialized to pointer values. See the code in testsuite/ld-empic for details. @cindex help @cindex usage @kindex --help @item --help Print a summary of the command-line options on the standard output and exit. @cindex link map @kindex -Map @item -Map @var{mapfile} Print to the file @var{mapfile} a link map---diagnostic information about where symbols are mapped by @code{ld}, and information on global common storage allocation. @cindex memory usage @kindex --no-keep-memory @item --no-keep-memory @code{ld} normally optimizes for speed over memory usage by caching the symbol tables of input files in memory. This option tells @code{ld} to instead optimize for memory usage, by rereading the symbol tables as necessary. This may be required if @code{ld} runs out of memory space while linking a large executable. @kindex --no-whole-archive @item --no-whole-archive Turn off the effect of the @code{--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. @ifclear SingleFormat @kindex -oformat @item -oformat @var{output-format} @code{ld} may be configured to support more than one kind of object file. If your @code{ld} is configured this way, you can use the @samp{-oformat} option to specify the binary format for the output object file. Even when @code{ld} is configured to support alternative object formats, you don't usually need to specify this, as @code{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 -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,,@code{ld} and the H8/300}. @end ifset @ifset I960 @xref{i960,, @code{ld} and the Intel 960 family}. @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. @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 @code{-rpath} arguments are concatenated and passed to the runtime linker, which uses them to locate shared objects at runtime. The @code{-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 @code{-rpath-link} option. If @code{-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 @code{-rpath} option may also be used on SunOS. By default, on SunOS, the linker will form a runtime search patch out of all the @code{-L} options it is given. If a @code{-rpath} option is used, the runtime search path will be formed exclusively using the @code{-rpath} options, ignoring the @code{-L} options. This can be useful when using gcc, which adds many @code{-L} options which may be on NFS mounted filesystems. For compatibility with other ELF linkers, if the @code{-R} option is followed by a directory name, rather than a file name, it is treated as the @code{-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-relocateable 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 @code{-rpath-link} option specifies the first set of directories to search. The @code{-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. The linker uses the following search paths to locate required shared libraries. @enumerate @item Any directories specified by @code{-rpath-link} options. @item Any directories specified by @code{-rpath} options. The difference between @code{-rpath} and @code{-rpath-link} is that directories specified by @code{-rpath} options are included in the executable and used at runtime, whereas the @code{-rpath-link} option is only effective at link time. @item On an ELF system, if the @code{-rpath} and @code{rpath-link} options were not used, search the contents of the environment variable @code{LD_RUN_PATH}. @item On SunOS, if the @code{-rpath} option was not used, search any directories specified using @code{-L} options. @item For a native linker, the contents of the environment variable @code{LD_LIBRARY_PATH}. @item The default directories, normally @file{/lib} and @file{/usr/lib}. @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 @code{-e} option is not used and there are undefined symbols in the link. @item --sort-common @kindex --sort-common This option tells @code{ld} to sort the common symbols by size when it places them in the appropriate output sections. First come all the one byte symbols, then all the two bytes, then all the four bytes, and then everything else. This is to prevent gaps between symbols due to alignment constraints. @kindex --split-by-file @item --split-by-file Similar to @code{--split-by-reloc} but creates a new output section for each input file. @kindex --split-by-reloc @item --split-by-reloc @var{count} Trys 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 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. @kindex --stats @item --stats Compute and display statistics about the operation of the linker, such as execution time and memory usage. @kindex -traditional-format @cindex traditional format @item -traditional-format For some targets, the output of @code{ld} is different in some ways from the output of some existing linker. This switch requests @code{ld} to use the traditional format instead. @cindex dbx For example, on SunOS, @code{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 @code{ld} to not combine duplicate entries. @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} Use @var{org} as the starting address for---respectively---the @code{bss}, @code{data}, or the @code{text} segment of the output file. @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. @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 @code{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 --verbose @cindex verbose @item --verbose Display the version number for @code{ld} and list the linker emulations supported. Display which input files can and cannot be opened. Display the linker script if using a default builtin script. @kindex -warn-comon @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 practice, but linkers on some other operating systems do not. This option allows you to find potential problems from combining global symbols. Unfortunately, some C libraries use this practice, so you may get some warnings about symbols in the libraries as well as in your programs. There are three kinds of global symbols, illustrated here by C examples: @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 --whole-archive @cindex including an entire archive @item --whole-archive For each archive mentioned on the command line after the @code{--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. @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 (int c) @{ printf ("malloc called with %ld\n", c); return __real_malloc (c); @} @end smallexample If you link other code with this file using @code{--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 @code{--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}. @end table @ifset UsesEnvVars @node Environment @section Environment Variables You can change the behavior of @code{ld} with the environment variable @code{GNUTARGET}. @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, @code{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 ifset @node Commands @chapter Command Language @cindex command files The command language provides explicit control over the link process, allowing complete specification of the mapping between the linker's input files and its output. It controls: @itemize @bullet @item input files @item file formats @item output file layout @item addresses of sections @item placement of common blocks @end itemize You may supply a command file (also known as a link script) to the linker either explicitly through the @samp{-T} option, or implicitly as an ordinary file. If the linker opens a file which it cannot recognize as a supported object or archive format, it reports an error. @menu * Scripts:: Linker Scripts * Expressions:: Expressions * MEMORY:: MEMORY Command * SECTIONS:: SECTIONS Command * PHDRS:: PHDRS Command * Entry Point:: The Entry Point * Option Commands:: Option Commands @end menu @node Scripts @section Linker Scripts The @code{ld} command language is a collection of statements; some are simple keywords setting a particular option, some are used to select and group input files or name output files; and two statement types have a fundamental and pervasive impact on the linking process. @cindex fundamental script commands @cindex commands, fundamental @cindex output file layout @cindex layout of output file The most fundamental command of the @code{ld} command language is the @code{SECTIONS} command (@pxref{SECTIONS}). Every meaningful command script must have a @code{SECTIONS} command: it specifies a ``picture'' of the output file's layout, in varying degrees of detail. No other command is required in all cases. The @code{MEMORY} command complements @code{SECTIONS} by describing the available memory in the target architecture. This command is optional; if you don't use a @code{MEMORY} command, @code{ld} assumes sufficient memory is available in a contiguous block for all output. @xref{MEMORY}. @cindex comments 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 Expressions @section Expressions @cindex expression syntax @cindex arithmetic Many useful commands involve arithmetic expressions. The syntax for expressions in the command language is identical to that of C expressions, with the following features: @itemize @bullet @item All expressions evaluated as integers and are of ``long'' or ``unsigned long'' type. @item All constants are integers. @item All of the C arithmetic operators are provided. @item You may reference, define, and create global variables. @item You may call special purpose built-in functions. @end itemize @menu * Integers:: Integers * Symbols:: Symbol Names * Location Counter:: The Location Counter * Operators:: Operators * Evaluation:: Evaluation * Assignment:: Assignment: Defining Symbols * Arithmetic Functions:: Built-In Functions * Semicolons:: Semicolon Usage @end menu @node Integers @subsection Integers @cindex integer notation @cindex octal integers An octal integer is @samp{0} followed by zero or more of the octal digits (@samp{01234567}). @smallexample _as_octal = 0157255; @end smallexample @cindex decimal integers A decimal integer starts with a non-zero digit followed by zero or more digits (@samp{0123456789}). @smallexample _as_decimal = 57005; @end smallexample @cindex hexadecimal integers @kindex 0x A hexadecimal integer is @samp{0x} or @samp{0X} followed by one or more hexadecimal digits chosen from @samp{0123456789abcdefABCDEF}. @smallexample _as_hex = 0xdead; @end smallexample @cindex negative integers To write a negative integer, use the prefix operator @samp{-} (@pxref{Operators}). @smallexample _as_neg = -57005; @end smallexample @cindex scaled integers @cindex K and M integer suffixes @cindex M and K integer suffixes @cindex suffixes for integers @cindex integer suffixes Additionally the suffixes @code{K} and @code{M} may be used to scale a constant by @c TEXI2ROFF-KILL @ifinfo @c END TEXI2ROFF-KILL @code{1024} or @code{1024*1024} @c TEXI2ROFF-KILL @end ifinfo @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 point and may include any letters, underscores, digits, points, 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 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 must always appear in an expression within a @code{SECTIONS} command. The @code{.} symbol may appear anywhere that an ordinary symbol is allowed in an expression, but its assignments have a side effect. Assigning a value to the @code{.} symbol will cause the location counter to be moved. @cindex holes This may be used to create holes in the output section. The location counter may never be moved backwards. @smallexample SECTIONS @{ output : @{ file1(.text) . = . + 1000; file2(.text) . += 1000; file3(.text) @} = 0x1234; @} @end smallexample @noindent In the previous example, @code{file1} is located at the beginning of the output section, then there is a 1000 byte gap. Then @code{file2} appears, also with a 1000 byte gap following before @code{file3} is loaded. The notation @samp{= 0x1234} specifies what data to write in the gaps (@pxref{Section Options}). @iftex @vfill @end iftex @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 @ifinfo @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{Assignment}. @c TEXI2ROFF-KILL @end ifinfo @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{Assignment}. } @end iftex @c END TEXI2ROFF-KILL @node Evaluation @subsection Evaluation @cindex lazy evaluation @cindex expression evaluation order The linker uses ``lazy evaluation'' for expressions; it only calculates an expression when absolutely necessary. The linker needs the value of the start address, and the 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 command file. 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. @node Assignment @subsection Assignment: Defining Symbols @cindex assignment in scripts @cindex symbol definition, scripts @cindex variables, defining You may create global symbols, and assign values (addresses) to global symbols, 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} ; @end table Two things distinguish assignment from other operators in @code{ld} expressions. @itemize @bullet @item Assignment may only be used at the root of an expression; @samp{a=b+3;} is allowed, but @samp{a+b=3;} is an error. @kindex ; @cindex semicolon @item You must place a trailing semicolon (``@key{;}'') at the end of an assignment statement. @end itemize Assignment statements may appear: @itemize @bullet @item as commands in their own right in an @code{ld} script; or @item as independent statements within a @code{SECTIONS} command; or @item as part of the contents of a section definition in a @code{SECTIONS} command. @end itemize The first two cases are equivalent in effect---both define a symbol with an absolute address. The last case defines a symbol whose address is relative to a particular section (@pxref{SECTIONS}). @cindex absolute and relocatable symbols @cindex relocatable and absolute symbols @cindex symbols, relocatable and absolute When a linker expression is evaluated and assigned to a variable, it is given either an absolute or a relocatable type. An absolute expression type is one in which the symbol contains the value that it will have in the output file; a relocatable expression type is one in which the value is expressed as a fixed offset from the base of a section. The type of the expression is controlled by its position in the script file. A symbol assigned within a section definition is created relative to the base of the section; a symbol assigned in any other place is created as an absolute symbol. Since a symbol created within a section definition is relative to the base of the section, it will remain relocatable if relocatable output is requested. A symbol may be created with an absolute value even when assigned to within a section definition by using the absolute assignment function @code{ABSOLUTE}. For example, to create an absolute symbol whose address is the last byte of an output section named @code{.data}: @smallexample SECTIONS@{ @dots{} .data : @{ *(.data) _edata = ABSOLUTE(.) ; @} @dots{} @} @end smallexample The linker tries to put off the evaluation of an assignment until all the terms in the source expression are known (@pxref{Evaluation}). For instance, 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 @dfn{dot}, @samp{.} must be evaluated during 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 SECTIONS @{ @dots{} text 9+this_isnt_constant : @{ @dots{} @} @dots{} @} @end smallexample @kindex Non constant expression @noindent will cause the error message ``@code{Non constant expression for initial address}''. @cindex provide In some cases, it is desirable for a linker script to define a symbol only if it is referenced, and only if it 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})}. @node Arithmetic Functions @subsection Arithmetic Functions @cindex functions in expression language The command language includes a number of built-in functions for use in link script expressions. @table @code @kindex ABSOLUTE(@var{exp}) @cindex expression, absolute @item ABSOLUTE(@var{exp}) 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. @kindex ADDR(@var{section}) @cindex section address @item ADDR(@var{section}) Return the absolute address 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 @kindex LOADADDR(@var{section}) @cindex section load address @item LOADADDR(@var{section}) Return the absolute load address of the named @var{section}. This is normally the same as @code{ADDR}, but it may be different if the @code{AT} keyword is used in the section definition (@pxref{Section Options}). @kindex ALIGN(@var{exp}) @cindex rounding up location counter @item ALIGN(@var{exp}) Return the result of the current location counter (@code{.}) aligned to the next @var{exp} boundary. @var{exp} must be an expression whose value is a power of two. This is equivalent to @smallexample (. + @var{exp} - 1) & ~(@var{exp} - 1) @end smallexample @code{ALIGN} doesn't change the value of the location counter---it just does arithmetic on it. As an example, to align the output @code{.data} section to the next @code{0x2000} byte boundary after the preceding section and to set 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{start} attribute of a section definition (@pxref{Section Options}). The second use simply defines the value of a variable. The built-in @code{NEXT} is closely related to @code{ALIGN}. @kindex DEFINED(@var{symbol}) @cindex symbol defaults @item DEFINED(@var{symbol}) Return 1 if @var{symbol} is in the linker global symbol table and is defined, otherwise return 0. You can use this function to provide default values for symbols. For example, the following command-file fragment shows how to set a global symbol @code{begin} to the first location in the @code{.text} section---but if a symbol called @code{begin} already existed, its value is preserved: @smallexample @group SECTIONS@{ @dots{} .text : @{ begin = DEFINED(begin) ? begin : . ; @dots{} @} @dots{} @} @end group @end smallexample @kindex NEXT(@var{exp}) @cindex unallocated address, next @item NEXT(@var{exp}) 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. @kindex SIZEOF(@var{section}) @cindex section size @item SIZEOF(@var{section}) Return the size in bytes of the named @var{section}, if that section has been allocated. In the following example, @code{symbol_1} and @code{symbol_2} are assigned identical values: @c What does it return if the section hasn't been allocated? 0? @smallexample @group SECTIONS@{ @dots{} .output @{ .start = . ; @dots{} .end = . ; @} symbol_1 = .end - .start ; symbol_2 = SIZEOF(.output); @dots{} @} @end group @end smallexample @kindex SIZEOF_HEADERS @cindex header size @kindex sizeof_headers @item SIZEOF_HEADERS @itemx sizeof_headers Return the size in bytes of the output file's headers. You can use this number as the start address of the first section, if you choose, to facilitate paging. @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}. @end table @node Semicolons @subsection Semicolons Semicolons (``@key{;}'') are required in the following places. In all other places they can appear for aesthetic reasons but are otherwise ignored. @table @code @item Assignment Semicolons must appear at the end of assignment expressions. @xref{Assignment} @item PHDRS Semicolons must appear at the end of a @code{PHDRS} statement. @xref{PHDRS} @end table @node MEMORY @section Memory Layout @kindex MEMORY @cindex regions of memory @cindex discontinuous memory @cindex allocating memory The linker's default configuration permits allocation of all available memory. You can override this configuration by using the @code{MEMORY} command. The @code{MEMORY} command describes the location and size of blocks of memory in the target. By using it carefully, you can describe which memory regions may be used by the linker, and which memory regions it must avoid. The linker does not shuffle sections to fit into the available regions, but does move the requested sections into the correct regions and issue errors when the regions become too full. A command file 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 @table @code @cindex naming memory regions @item @var{name} is a name used internally by the linker to refer to the region. Any symbol name may be used. The region names are stored in a separate name space, and will not conflict with symbols, file names or section names. Use distinct names to specify multiple regions. @cindex memory region attributes @item (@var{attr}) is an optional list of attributes, permitted for compatibility with the AT&T linker but not used by @code{ld} beyond checking that the attribute list is valid. Valid attribute lists must be made up of the characters ``@code{LIRWX}''. If you omit the attribute list, you may omit the parentheses around it as well. @kindex ORIGIN = @kindex o = @kindex org = @item @var{origin} is the start address of the region in physical memory. It is an expression that must evaluate to a constant before memory allocation is performed. The keyword @code{ORIGIN} may be abbreviated to @code{org} or @code{o} (but not, for example, @samp{ORG}). @kindex LENGTH = @kindex len = @kindex l = @item @var{len} is the size in bytes of the region (an expression). The keyword @code{LENGTH} may be abbreviated to @code{len} or @code{l}. @end table For example, to specify that memory has two regions available for allocation---one starting at 0 for 256 kilobytes, and the other starting at @code{0x40000000} for four megabytes: @smallexample @group MEMORY @{ rom : ORIGIN = 0, LENGTH = 256K ram : org = 0x40000000, l = 4M @} @end group @end smallexample Once you have defined a region of memory named @var{mem}, you can direct specific output sections there by using a command ending in @samp{>@var{mem}} within the @code{SECTIONS} command (@pxref{Section Options}). If the combined output sections directed to a region are too big for the region, the linker will issue an error message. @node SECTIONS @section Specifying Output Sections @kindex SECTIONS The @code{SECTIONS} command controls exactly where input sections are placed into output sections, their order in the output file, and to which output sections they are allocated. You may use at most one @code{SECTIONS} command in a script file, but you can have as many statements within it as you wish. Statements within the @code{SECTIONS} command can do one of three things: @itemize @bullet @item define the entry point; @item assign a value to a symbol; @item describe the placement of a named output section, and which input sections go into it. @end itemize You can also use the first two operations---defining the entry point and defining symbols---outside the @code{SECTIONS} command: @pxref{Entry Point}, and @ref{Assignment}. They are permitted here as well for your convenience in reading the script, so that symbols and the entry point can be defined at meaningful points in your output-file layout. If you do not use a @code{SECTIONS} command, the linker places 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. @menu * Section Definition:: Section Definitions * Section Placement:: Section Placement * Section Data Expressions:: Section Data Expressions * Section Options:: Optional Section Attributes * Overlays:: Overlays @end menu @node Section Definition @subsection Section Definitions @cindex section definition The most frequently used statement in the @code{SECTIONS} command is the @dfn{section definition}, which specifies the properties of an output section: its location, alignment, contents, fill pattern, and target memory region. Most of these specifications are optional; the simplest form of a section definition is @smallexample SECTIONS @{ @dots{} @var{secname} : @{ @var{contents} @} @dots{} @} @end smallexample @cindex naming output sections @noindent @var{secname} is the name of the output section, and @var{contents} a specification of what goes there---for example, a list of input files or sections of input files (@pxref{Section Placement}). As you might assume, the whitespace shown is optional. You do need the colon @samp{:} and the braces @samp{@{@}}, however. @var{secname} 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 @code{.text}, @code{.data} or @code{.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 any name which does not conform to the standard @code{ld} symbol name syntax must be quoted. @xref{Symbols, , Symbol Names}. The special @var{secname} @samp{/DISCARD/} may be used to discard input sections. Any sections which are assigned to an output section named @samp{/DISCARD/} are not included in the final link output. The linker will not create output sections which do not have any contents. This is for convenience when referring to input sections that may or may not exist. For example, @smallexample .foo @{ *(.foo) @} @end smallexample will only create a @samp{.foo} section in the output file if there is a @samp{.foo} section in at least one input file. @node Section Placement @subsection Section Placement @cindex contents of a section In a section definition, you can specify the contents of an output section by listing particular input files, by listing particular input-file sections, or by a combination of the two. You can also place arbitrary data in the section, and define symbols relative to the beginning of the section. The @var{contents} of a section definition may include any of the following kinds of statement. You can include as many of these as you like in a single section definition, separated from one another by whitespace. @table @code @kindex @var{filename} @cindex input files, section defn @cindex files, including in output sections @item @var{filename} You may simply name a particular input file to be placed in the current output section; @emph{all} sections from that file are placed in the current section definition. If the file name has already been mentioned in another section definition, with an explicit section name list, then only those sections which have not yet been allocated are used. To specify a list of particular files by name: @smallexample .data : @{ afile.o bfile.o cfile.o @} @end smallexample @noindent The example also illustrates that multiple statements can be included in the contents of a section definition, since each file name is a separate statement. @kindex @var{filename}(@var{section}) @cindex files and sections, section defn @item @var{filename}( @var{section} ) @itemx @var{filename}( @var{section} , @var{section}, @dots{} ) @itemx @var{filename}( @var{section} @var{section} @dots{} ) You can name one or more sections from your input files, for insertion in the current output section. If you wish to specify a list of input-file sections inside the parentheses, you may separate the section names by either commas or whitespace. @cindex input sections to output section @kindex *(@var{section}) @item * (@var{section}) @itemx * (@var{section}, @var{section}, @dots{}) @itemx * (@var{section} @var{section} @dots{}) Instead of explicitly naming particular input files in a link control script, you can refer to @emph{all} files from the @code{ld} command line: use @samp{*} instead of a particular file name before the parenthesized input-file section list. If you have already explicitly included some files by name, @samp{*} refers to all @emph{remaining} files---those whose places in the output file have not yet been defined. For example, to copy sections @code{1} through @code{4} from an Oasys file into the @code{.text} section of an @code{a.out} file, and sections @code{13} and @code{14} into the @code{.data} section: @smallexample @group SECTIONS @{ .text :@{ *("1" "2" "3" "4") @} .data :@{ *("13" "14") @} @} @end group @end smallexample @cindex @code{[@var{section}@dots{}]}, not supported @samp{[ @var{section} @dots{} ]} used to be accepted as an alternate way to specify named sections from all unallocated input files. Because some operating systems (VMS) allow brackets in file names, that notation is no longer supported. @cindex uninitialized data @cindex commons in output @kindex *( COMMON ) @item @var{filename}@code{( COMMON )} @itemx *( COMMON ) Specify where in your output file to place uninitialized data with this notation. @code{*(COMMON)} by itself refers to all uninitialized data from all input files (so far as it is not yet allocated); @var{filename}@code{(COMMON)} refers to uninitialized data from a particular file. Both are special cases of the general mechanisms for specifying where to place input-file sections: @code{ld} permits you to refer to uninitialized data as if it were in an input-file section named @code{COMMON}, regardless of the input file's format. @end table In any place where you may use a specific file or section name, you may also use a wildcard pattern. The linker handles wildcards much as the Unix shell does. A @samp{*} character matches any number of characters. A @samp{?} character matches any single character. The sequence @samp{[@var{chars}]} will match 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. A @samp{\} character may be used to quote the following character. 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. In a section name, the wildcard characters will match a @samp{/} character. Wildcards only match files which are explicitly specified on the command line. The linker does not search directories to expand wildcards. However, if you specify a simple file name---a name with no wildcard characters---in a linker script, and the file name is not also specified on the command line, the linker will attempt to open the file as though it appeared on the command line. In the following example, the command script arranges the output file into three consecutive sections, named @code{.text}, @code{.data}, and @code{.bss}, taking the input for each from the correspondingly named sections of all the input files: @smallexample @group SECTIONS @{ .text : @{ *(.text) @} .data : @{ *(.data) @} .bss : @{ *(.bss) *(COMMON) @} @} @end group @end smallexample The following example reads all of the sections from file @code{all.o} and places them at the start of output section @code{outputa} which starts at location @code{0x10000}. All of section @code{.input1} from file @code{foo.o} follows immediately, in the same output section. All of section @code{.input2} from @code{foo.o} goes into output section @code{outputb}, followed by section @code{.input1} from @code{foo1.o}. All of the remaining @code{.input1} and @code{.input2} sections from any files are written to output section @code{outputc}. @smallexample @group SECTIONS @{ outputa 0x10000 : @{ all.o foo.o (.input1) @} outputb : @{ foo.o (.input2) foo1.o (.input1) @} outputc : @{ *(.input1) *(.input2) @} @} @end group @end smallexample This example shows how wildcard patterns might be used to partition files. All @code{.text} sections are placed in @code{.text}, and all @code{.bss} sections are placed in @code{.bss}. For all files beginning with an upper case character, the @code{.data} section is placed into @code{.DATA}; for all other files, the @code{.data} section is placed into @code{.data}. @smallexample @group SECTIONS @{ .text : @{ *(.text) @} .DATA : @{ [A-Z]*(.data) @} .data : @{ *(.data) @} .bss : @{ *(.bss) @} @} @end group @end smallexample @node Section Data Expressions @subsection Section Data Expressions @cindex expressions in a section The foregoing statements arrange, in your output file, data originating from your input files. You can also place data directly in an output section from the link command script. Most of these additional statements involve expressions (@pxref{Expressions}). Although these statements are shown separately here for ease of presentation, no such segregation is needed within a section definition in the @code{SECTIONS} command; you can intermix them freely with any of the statements we've just described. @table @code @cindex input filename symbols @cindex filename symbols @kindex CREATE_OBJECT_SYMBOLS @item CREATE_OBJECT_SYMBOLS Create a symbol for each input file in the current section, set to the address of the first byte of data written from that input file. For instance, with @code{a.out} files it is conventional to have a symbol for each input file. You can accomplish this by defining the output @code{.text} section as follows: @smallexample @group SECTIONS @{ .text 0x2020 : @{ CREATE_OBJECT_SYMBOLS *(.text) _etext = ALIGN(0x2000); @} @dots{} @} @end group @end smallexample If @code{sample.ld} is a file containing this script, and @code{a.o}, @code{b.o}, @code{c.o}, and @code{d.o} are four input files with contents like the following--- @smallexample @group /* a.c */ afunction() @{ @} int adata=1; int abss; @end group @end smallexample @noindent @samp{ld -M -T sample.ld a.o b.o c.o d.o} would create a map like this, containing symbols matching the object file names: @smallexample 00000000 A __DYNAMIC 00004020 B _abss 00004000 D _adata 00002020 T _afunction 00004024 B _bbss 00004008 D _bdata 00002038 T _bfunction 00004028 B _cbss 00004010 D _cdata 00002050 T _cfunction 0000402c B _dbss 00004018 D _ddata 00002068 T _dfunction 00004020 D _edata 00004030 B _end 00004000 T _etext 00002020 t a.o 00002038 t b.o 00002050 t c.o 00002068 t d.o @end smallexample @kindex @var{symbol} = @var{expression} ; @kindex @var{symbol} @var{f}= @var{expression} ; @item @var{symbol} = @var{expression} ; @itemx @var{symbol} @var{f}= @var{expression} ; @var{symbol} is any symbol name (@pxref{Symbols}). ``@var{f}='' refers to any of the operators @code{&= += -= *= /=} which combine arithmetic and assignment. @cindex assignment, in section defn When you assign a value to a symbol within a particular section definition, the value is relative to the beginning of the section (@pxref{Assignment}). If you write @smallexample @group SECTIONS @{ abs = 14 ; @dots{} .data : @{ @dots{} rel = 14 ; @dots{} @} abs2 = 14 + ADDR(.data); @dots{} @} @end group @end smallexample @c FIXME: Try above example! @noindent @code{abs} and @code{rel} do not have the same value; @code{rel} has the same value as @code{abs2}. @kindex BYTE(@var{expression}) @kindex SHORT(@var{expression}) @kindex LONG(@var{expression}) @kindex QUAD(@var{expression}) @cindex direct output @item BYTE(@var{expression}) @itemx SHORT(@var{expression}) @itemx LONG(@var{expression}) @itemx QUAD(@var{expression}) By including one of these four statements in a section definition, you can explicitly place one, two, four, or eight bytes (respectively) at the current address of that section. @code{QUAD} is only supported when using a 64 bit host or target. @ifclear SingleFormat Multiple-byte quantities are represented in whatever byte order is appropriate for the output file format (@pxref{BFD}). @end ifclear @kindex FILL(@var{expression}) @cindex holes, filling @cindex unspecified memory @item FILL(@var{expression}) Specify the ``fill pattern'' for the current section. Any otherwise unspecified regions of memory within the section (for example, regions you skip over by assigning a new value to the location counter @samp{.}) are filled with the two least significant bytes from the @var{expression} argument. A @code{FILL} statement covers memory locations @emph{after} the point 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. @end table @node Section Options @subsection Optional Section Attributes @cindex section defn, full syntax Here is the full syntax of a section definition, including all the optional portions: @smallexample @group SECTIONS @{ @dots{} @var{secname} @var{start} BLOCK(@var{align}) (NOLOAD) : AT ( @var{ldadr} ) @{ @var{contents} @} >@var{region} :@var{phdr} =@var{fill} @dots{} @} @end group @end smallexample @var{secname} and @var{contents} are required. @xref{Section Definition}, and @ref{Section Placement}, for details on @var{contents}. The remaining elements---@var{start}, @code{BLOCK(@var{align)}}, @code{(NOLOAD)}, @code{AT ( @var{ldadr} )}, @code{>@var{region}}, @code{:@var{phdr}}, and @code{=@var{fill}}---are all optional. @table @code @cindex start address, section @cindex section start @cindex section address @item @var{start} You can force the output section to be loaded at a specified address by specifying @var{start} immediately following the section name. @var{start} can be represented as any expression. The following example generates section @var{output} at location @code{0x40000000}: @smallexample @group SECTIONS @{ @dots{} output 0x40000000: @{ @dots{} @} @dots{} @} @end group @end smallexample @kindex BLOCK(@var{align}) @cindex section alignment @cindex aligning sections @item BLOCK(@var{align}) You can include @code{BLOCK()} specification to advance the location counter @code{.} prior to the beginning of the section, so that the section will begin at the specified alignment. @var{align} is an expression. @kindex NOLOAD @cindex prevent unnecessary loading @cindex loading, preventing @item (NOLOAD) Use @samp{(NOLOAD)} to prevent a section from being loaded into memory each time it is accessed. For example, in the script sample below, the @code{ROM} segment is addressed at memory location @samp{0} and does not need to be loaded into each object file: @smallexample @group SECTIONS @{ ROM 0 (NOLOAD) : @{ @dots{} @} @dots{} @} @end group @end smallexample @kindex AT ( @var{ldadr} ) @cindex specify load address @cindex load address, specifying @item AT ( @var{ldadr} ) The expression @var{ldadr} that follows the @code{AT} keyword specifies the load address of the section. The default (if you do not use the @code{AT} keyword) is to make the load address the same as the relocation address. This feature is designed to make it easy to build a ROM image. For example, this @code{SECTIONS} definition creates two output sections: one called @samp{.text}, which starts at @code{0x1000}, and one called @samp{.mdata}, which is loaded at the end of the @samp{.text} section even though its relocation address is @code{0x2000}. The symbol @code{_data} is defined with the value @code{0x2000}: @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 C programs, usually @code{crt0}) for use with a ROM generated this way has to include something like the following, to copy the initialized data from the ROM image to its runtime address: @smallexample @group 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 @kindex >@var{region} @cindex section, assigning to memory region @cindex memory regions and sections @item >@var{region} Assign this section to a previously defined region of memory. @xref{MEMORY}. @kindex :@var{phdr} @cindex section, assigning to program header @cindex program headers and sections @item :@var{phdr} Assign this section to a segment described by a program header. @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. To prevent a section from being assigned to a segment when it would normally default to one, use @code{:NONE}. @kindex =@var{fill} @cindex section fill pattern @cindex fill pattern, entire section @item =@var{fill} Including @code{=@var{fill}} in a section definition specifies the initial fill value for that section. You may use any expression to specify @var{fill}. Any unallocated holes in the current output section when written to the output file will be filled with the two least significant bytes of the value, repeated as necessary. You can also change the fill value with a @code{FILL} statement in the @var{contents} of a section definition. @end table @node Overlays @subsection Overlays @kindex OVERLAY @cindex overlays The @code{OVERLAY} command 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. The @code{OVERLAY} command is used within a @code{SECTIONS} command. It appears as follows: @smallexample @group OVERLAY @var{start} : [ NOCROSSREFS ] AT ( @var{ldaddr} ) @{ @var{secname1} @{ @var{contents} @} :@var{phdr} =@var{fill} @var{secname2} @{ @var{contents} @} :@var{phdr} =@var{fill} @dots{} @} >@var{region} :@var{phdr} =@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 @code{.}). 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{Option 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 @code{.} 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 This will define both @code{.text0} and @code{.text1} to start at address 0x1000. @code{.text0} will be loaded at address 0x4000, and @code{.text1} will be loaded immediately after @code{.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 PHDRS @section ELF Program Headers @kindex PHDRS @cindex program headers @cindex ELF program headers The ELF object file format uses @dfn{program headers}, which are read by the system loader and describe how the program should be loaded into memory. These program headers must be set correctly in order to run the program on a native ELF system. The linker will create reasonable program headers by default. However, in some cases, it is desirable to specify the program headers more precisely; the @code{PHDRS} command may be used for this purpose. When the @code{PHDRS} command is used, the linker will not generate any program headers itself. The @code{PHDRS} command is only meaningful when generating an ELF output file. It is ignored in other cases. This manual does not describe the details of how the system loader interprets program headers; for more information, see the ELF ABI. The program headers of an ELF file may be displayed using the @samp{-p} option of the @code{objdump} command. 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 does not get put into the output file. Certain program header types describe segments of memory which are loaded from the file by the system loader. In the linker script, the contents of these segments are specified by directing allocated output sections to be placed in the segment. To do this, the command describing the output section in the @code{SECTIONS} command should use @samp{:@var{name}}, where @var{name} is the name of the program header as it appears in the @code{PHDRS} command. @xref{Section Options}. It is normal for certain sections to appear in more than one segment. This merely implies that one segment of memory contains another. This is specified by repeating @samp{:@var{name}}, using it once for each program header in which the section is to appear. If a section is placed in one or more segments using @samp{:@var{name}}, then all subsequent allocated sections which do not specify @samp{:@var{name}} are placed in the same segments. This is for convenience, since generally a whole set of contiguous sections will be placed in a single segment. To prevent a section from being assigned to a segment when it would normally default to one, use @code{:NONE}. The @code{FILEHDR} and @code{PHDRS} keywords which may appear after the program header type also indicate contents of the segment of memory. 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 It is possible to specify that a segment should be loaded at a particular address in memory. This is done using an @code{AT} expression. This is identical to the @code{AT} command used in the @code{SECTIONS} command (@pxref{Section Options}). Using the @code{AT} command for a program header overrides any information in the @code{SECTIONS} command. Normally the segment flags are set based on the sections. The @code{FLAGS} keyword may be used 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 the use 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 Entry Point @section The Entry Point @kindex ENTRY(@var{symbol}) @cindex start of execution @cindex first instruction The linker command language includes a command specifically for defining the first executable instruction in an output file (its @dfn{entry point}). Its argument is a symbol name: @smallexample ENTRY(@var{symbol}) @end smallexample Like symbol assignments, the @code{ENTRY} command may be placed either as an independent command in the command file, or among the section definitions within the @code{SECTIONS} command---whatever makes the most sense for your layout. @cindex entry point, defaults @code{ENTRY} is only one of several ways of choosing the entry point. You may indicate it in any of the following ways (shown in descending order of priority: methods higher in the list override methods lower down). @itemize @bullet @item the @samp{-e} @var{entry} command-line option; @item the @code{ENTRY(@var{symbol})} command in a linker control script; @item the value of the symbol @code{start}, if present; @item the address of the first byte of the @code{.text} section, if present; @item The address @code{0}. @end itemize For example, you can use these rules to generate an entry point with an assignment statement: if no symbol @code{start} is defined within your input files, you can simply define it, assigning it an appropriate value--- @smallexample start = 0x2020; @end smallexample @noindent The example shows an absolute address, but you can use any expression. For example, if your input object files use some other symbol-name convention for the entry point, you can just assign the value of whatever symbol contains the start address to @code{start}: @smallexample start = other_symbol ; @end smallexample @node Option Commands @section Option Commands The command language includes a number of other commands that you can use for specialized purposes. They are similar in purpose to command-line options. @table @code @kindex CONSTRUCTORS @cindex C++ constructors, arranging in link @cindex constructors, arranging in link @item CONSTRUCTORS When linking using the @code{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 @code{ECOFF} and @code{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 where this information should be placed. 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{__DTOR_LIST}} marks the end. 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++ 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++ 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 multiple sections, @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 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. @need 1000 @kindex FLOAT @kindex NOFLOAT @item FLOAT @itemx NOFLOAT These keywords were used in some older linkers to request a particular math subroutine library. @code{ld} doesn't use the keywords, assuming instead that any necessary subroutines are in libraries specified using the general mechanisms for linking to archives; but to permit the use of scripts that were written for the older linkers, the keywords @code{FLOAT} and @code{NOFLOAT} are accepted and ignored. @kindex FORCE_COMMON_ALLOCATION @cindex common allocation @item FORCE_COMMON_ALLOCATION This command has the same effect as the @samp{-d} command-line option: to make @code{ld} assign space to common symbols even if a relocatable output file is specified (@samp{-r}). @kindex INPUT ( @var{files} ) @cindex binary input files @item INPUT ( @var{file}, @var{file}, @dots{} ) @itemx INPUT ( @var{file} @var{file} @dots{} ) Use this command to include binary input files in the link, without including them in a particular section definition. Specify the full name for each @var{file}, including @samp{.a} if required. @code{ld} searches for each @var{file} through the archive-library search path, just as for files you specify on the command line. See the description of @samp{-L} in @ref{Options,,Command Line Options}. If you use @samp{-l@var{file}}, @code{ld} will transform the name to @code{lib@var{file}.a} as with the command line argument @samp{-l}. @kindex GROUP ( @var{files} ) @cindex grouping input files @item GROUP ( @var{file}, @var{file}, @dots{} ) @itemx GROUP ( @var{file} @var{file} @dots{} ) This 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}. @ignore @kindex MAP ( @var{name} ) @item MAP ( @var{name} ) @c MAP(...) appears to look for an F in the arg, ignoring all other @c chars; if it finds one, it sets "map_option_f" to true. But nothing @c checks map_option_f. Apparently a stub for the future... @end ignore @kindex OUTPUT ( @var{filename} ) @cindex naming the output file @item OUTPUT ( @var{filename} ) Use this command to name the link output file @var{filename}. The effect of @code{OUTPUT(@var{filename})} is identical to the effect of @w{@samp{-o @var{filename}}}, which overrides it. You can use this command to supply a default output-file name other than @code{a.out}. @ifclear SingleFormat @kindex OUTPUT_ARCH ( @var{bfdname} ) @cindex machine architecture, output @item OUTPUT_ARCH ( @var{bfdname} ) Specify a particular output machine architecture, with one of the names used by the BFD back-end routines (@pxref{BFD}). This command is often unnecessary; the architecture is most often set implicitly by either the system BFD configuration or as a side effect of the @code{OUTPUT_FORMAT} command. @kindex OUTPUT_FORMAT ( @var{bfdname} ) @cindex format, output file @item OUTPUT_FORMAT ( @var{bfdname} ) When @code{ld} is configured to support multiple object code formats, you can use this command to specify a particular output format. @var{bfdname} is one of the names used by the BFD back-end routines (@pxref{BFD}). The effect is identical to the effect of the @samp{-oformat} command-line option. This selection affects only the output file; the related command @code{TARGET} affects primarily input files. @end ifclear @kindex SEARCH_DIR ( @var{path} ) @cindex path for libraries @cindex search path, libraries @item SEARCH_DIR ( @var{path} ) Add @var{path} to the list of paths where @code{ld} looks for archive libraries. @code{SEARCH_DIR(@var{path})} has the same effect as @samp{-L@var{path}} on the command line. @kindex STARTUP ( @var{filename} ) @cindex first input file @item STARTUP ( @var{filename} ) Ensure that @var{filename} is the first input file used in the link process. @ifclear SingleFormat @cindex input file format @kindex TARGET ( @var{format} ) @item TARGET ( @var{format} ) When @code{ld} is configured to support multiple object code formats, you can use this command to change the input-file object code format (like the command-line option @samp{-b} or its synonym @samp{-format}). The argument @var{format} is one of the strings used by BFD to name binary formats. If @code{TARGET} is specified but @code{OUTPUT_FORMAT} is not, the last @code{TARGET} argument is also used as the default format for the @code{ld} output file. @xref{BFD}. @kindex GNUTARGET If you don't use the @code{TARGET} command, @code{ld} uses the value of the environment variable @code{GNUTARGET}, if available, to select the output file format. If that variable is also absent, @code{ld} uses the default format configured for your machine in the BFD libraries. @end ifclear @cindex cross references @kindex NOCROSSREFS ( @var{sections} ) @item NOCROSSREFS ( @var{section} @var{section} @dots{} ) This command may be used to tell @code{ld} to issue an error about any references among certain sections. In certain types of programs, particularly on embedded systems, 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 section names. If @code{ld} detects any cross references between the sections, it reports an error and returns a non-zero exit status. The @code{NOCROSSREFS} command uses output section names, defined in the @code{SECTIONS} command. It does not use the names of input sections. @end table @ifset GENERIC @node Machine Dependent @chapter Machine Dependent Features @cindex machine dependencies @code{ld} has additional features on some platforms; the following sections describe them. Machines where @code{ld} has no additional functionality are not listed. @menu * H8/300:: @code{ld} and the H8/300 * i960:: @code{ld} and the Intel 960 family @end menu @end ifset @c FIXME! This could use @raisesections/@lowersections, but there seems to be a conflict @c between those and node-defaulting. @ifset H8300 @ifclear GENERIC @raisesections @end ifclear @node H8/300 @section @code{ld} and the H8/300 @cindex H8/300 support For the H8/300, @code{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 @code{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? @code{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). @end table @ifclear GENERIC @lowersections @end ifclear @end ifset @ifclear GENERIC @ifset Hitachi @c This stuff is pointless to say unless you're especially concerned @c with Hitachi chips; don't enable it for generic case, please. @node Hitachi @chapter @code{ld} and other Hitachi chips @code{ld} also supports the H8/300H, the H8/500, and the Hitachi SH. 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 @code{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 @code{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 @code{-relax} on i960 @cindex relaxing on i960 @code{ld} supports the @samp{-relax} option for the i960 family. If you specify @samp{-relax}, @code{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. @code{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 @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 MRI @appendix MRI Compatible Script Files @cindex MRI compatibility To aid users making the transition to @sc{gnu} @code{ld} from the MRI linker, @code{ld} can use MRI compatible linker scripts as an alternative to the more general-purpose linker scripting language described in @ref{Commands,,Command Language}. MRI compatible linker scripts have a much simpler command set than the scripting language otherwise used with @code{ld}. @sc{gnu} @code{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, @code{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, @code{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 @code{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 @code{ld} command line. @cindex @code{NAME} (MRI) @item NAME @var{output-name} @var{output-name} is the name for the program produced by @code{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, @code{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 @node Index @unnumbered 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