diff options
Diffstat (limited to 'ld/ld.texinfo')
| -rw-r--r-- | ld/ld.texinfo | 4305 |
1 files changed, 4305 insertions, 0 deletions
diff --git a/ld/ld.texinfo b/ld/ld.texinfo new file mode 100644 index 00000000000..027f196d599 --- /dev/null +++ b/ld/ld.texinfo @@ -0,0 +1,4305 @@ +\input texinfo +@setfilename ld.info +@syncodeindex ky cp +@include configdoc.texi +@c (configdoc.texi is generated by the Makefile) +@include ldver.texi + +@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 version @value{VERSION}. + +Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 1999 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 Version @value{VERSION} +@author Steve Chamberlain +@author Ian Lance Taylor +@author Cygnus Solutions +@page + +@tex +{\parskip=0pt +\hfill Cygnus Solutions\par +\hfill ian\@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, 96, 97, 98, 1999 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 version @value{VERSION}. + +@menu +* Overview:: Overview +* Invocation:: Invocation +* Scripts:: Linker Scripts +@ifset GENERIC +* Machine Dependent:: Machine Dependent Features +@end ifset +@ifclear GENERIC +@ifset H8300 +* H8/300:: ld and the H8/300 +@end ifset +@ifset 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 + +* Reporting Bugs:: Reporting Bugs +* 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. Note that +specifying a script in this way should only be used to augment the main +linker script; if you want to use some command that logically can only +appear once, such as the @code{SECTIONS} or @code{MEMORY} command, you +must replace the default linker script using the @samp{-T} option. +@xref{Scripts}. + +For options whose names are a single letter, +option arguments must either follow the option letter without intervening +whitespace, or be given as separate arguments immediately following the +option that requires them. + +For options whose names are multiple letters, either one dash or two can +precede the option name; for example, @samp{--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{Format 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{Miscellaneous Commands}. + +@cindex entry point, from command line +@kindex -e @var{entry} +@kindex --entry=@var{entry} +@item -e @var{entry} +@itemx --entry=@var{entry} +Use @var{entry} as the explicit symbol for beginning execution of your +program, rather than the default entry point. If there is no symbol +named @var{entry}, the linker will try to parse @var{entry} as a number, +and use that as the entry address (the number will be interpreted in +base 10; you may use a leading @samp{0x} for base 16, or a leading +@samp{0} for base 8). @xref{Entry Point}, for a discussion of defaults +and other ways of specifying the entry point. + +@cindex dynamic symbol table +@kindex -E +@kindex --export-dynamic +@item -E +@itemx --export-dynamic +When creating a dynamically linked executable, add all symbols to the +dynamic symbol table. The dynamic symbol table is the set of symbols +which are visible from dynamic objects at run time. + +If you do not use this option, the dynamic symbol table will normally +contain only those symbols which are referenced by some dynamic object +mentioned in the link. + +If you use @code{dlopen} to load a dynamic object which needs to refer +back to the symbols defined by the program, rather than some other +dynamic object, then you will probably need to use this option when +linking the program itself. + +@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. + +@kindex -f +@kindex --auxiliary +@item -f +@itemx --auxiliary @var{name} +When creating an ELF shared object, set the internal DT_AUXILIARY field +to the specified name. This tells the dynamic linker that the symbol +table of the shared object should be used as an auxiliary filter on the +symbol table of the shared object @var{name}. + +If you later link a program against this filter object, then, when you +run the program, the dynamic linker will see the DT_AUXILIARY field. If +the dynamic linker resolves any symbols from the filter object, it will +first check whether there is a definition in the shared object +@var{name}. If there is one, it will be used instead of the definition +in the filter object. The shared object @var{name} need not exist. +Thus the shared object @var{name} may be used to provide an alternative +implementation of certain functions, perhaps for debugging or for +machine specific performance. + +This option may be specified more than once. The DT_AUXILIARY entries +will be created in the order in which they appear on the command line. + +@kindex -F +@kindex --filter +@item -F @var{name} +@itemx --filter @var{name} +When creating an ELF shared object, set the internal DT_FILTER field to +the specified name. This tells the dynamic linker that the symbol table +of the shared object which is being created should be used as a filter +on the symbol table of the shared object @var{name}. + +If you later link a program against this filter object, then, when you +run the program, the dynamic linker will see the DT_FILTER field. The +dynamic linker will resolve symbols according to the symbol table of the +filter object as usual, but it will actually link to the definitions +found in the shared object @var{name}. Thus the filter object can be +used to select a subset of the symbols provided by the object +@var{name}. + +Some older linkers used the @code{-F} option throughout a compilation +toolchain for specifying object-file format for both input and output +object files. The @sc{gnu} linker uses other mechanisms for this +purpose: the @code{-b}, @code{--format}, @code{--oformat} options, the +@code{TARGET} command in linker scripts, and the @code{GNUTARGET} +environment variable. The @sc{gnu} linker will ignore the @code{-F} +option when not creating an ELF shared object. + +@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. + +On systems which support shared libraries, @code{ld} may also search for +libraries with extensions other than @code{.a}. Specifically, on ELF +and SunOS systems, @code{ld} will search a directory for a library with +an extension of @code{.so} before searching for one with an extension of +@code{.a}. By convention, a @code{.so} extension indicates a shared +library. + +The linker will search an archive only once, at the location where it is +specified on the command line. If the archive defines a symbol which +was undefined in some object which appeared before the archive on the +command line, the linker will include the appropriate file(s) from the +archive. However, an undefined symbol in an object appearing later on +the command line will not cause the linker to search the archive again. + +See the @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. + +If the @samp{-m} option is not used, the emulation is taken from the +@code{LDEMULATION} environment variable, if that is defined. + +Otherwise, the default emulation depends upon how the linker was +configured. + +@cindex link map +@kindex -M +@kindex --print-map +@item -M +@itemx --print-map +Print a link map to the standard output. A link map provides +information about the link, including the following: + +@itemize @bullet +@item +Where object files and symbols are mapped into memory. +@item +How common symbols are allocated. +@item +All archive members included in the link, with a mention of the symbol +which caused the archive member to be brought in. +@end itemize + +@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. + +@kindex -O @var{level} +@cindex generating optimized output +@item -O @var{level} +If @var{level} is a numeric values greater than zero @code{ld} optimizes +the output. This might take significantly longer and therefore probably +should only be enabled for the final binary. + +@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{scriptfile} +@itemx --script=@var{scriptfile} +Use @var{scriptfile} as the linker script. This script replaces +@code{ld}'s default linker script (rather than adding to it), so +@var{commandfile} must specify everything necessary to describe the +output file. You must use this option if you want to use a command +which can only appear once in a linker script, such as the +@code{SECTIONS} or @code{MEMORY} command. @xref{Scripts}. If +@var{scriptfile} does not exist in the current directory, @code{ld} +looks for it in the directories specified by any preceding @samp{-L} +options. Multiple @samp{-T} options accumulate. + +@kindex -u @var{symbol} +@kindex --undefined=@var{symbol} +@cindex undefined symbol +@item -u @var{symbol} +@itemx --undefined=@var{symbol} +Force @var{symbol} to be entered in the output file as an undefined +symbol. Doing this may, for example, trigger linking of additional +modules from standard libraries. @samp{-u} may be repeated with +different option arguments to enter additional undefined symbols. This +option is equivalent to the @code{EXTERN} linker script command. + +@kindex -Ur +@cindex constructors +@item -Ur +For anything other than C++ programs, this option is equivalent to +@samp{-r}: it generates relocatable output---i.e., an output file that can in +turn serve as input to @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 -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. + +@kindex --check-sections +@kindex --no-check-sections +@item --check-sections +@item --no-check-sections +Asks the linker @emph{not} to check section addresses after they have +been assigned to see if there any overlaps. Normally the linker will +perform this check, and if it finds any overlaps it will produce +suitable error messages. The linker does know about, and does make +allowances for sections in overlays. The default behaviour can be +restored by using the command line switch @samp{--check-sections}. + +@cindex cross reference table +@kindex --cref +@item --cref +Output a cross reference table. If a linker map file is being +generated, the cross reference table is printed to the map file. +Otherwise, it is printed on the standard output. + +The format of the table is intentionally simple, so that it may be +easily processed by a script if necessary. The symbols are printed out, +sorted by name. For each symbol, a list of file names is given. If the +symbol is defined, the first file listed is the location of the +definition. The remaining files contain references to the symbol. + +@cindex symbols, from command line +@kindex --defsym @var{symbol}=@var{exp} +@item --defsym @var{symbol}=@var{expression} +Create a global symbol in the output file, containing the absolute +address given by @var{expression}. You may use this option as many +times as necessary to define multiple symbols in the command line. A +limited form of arithmetic is supported for the @var{expression} in this +context: you may give a hexadecimal constant or the name of an existing +symbol, or use @code{+} and @code{-} to add or subtract hexadecimal +constants or symbols. If you need more elaborate expressions, consider +using the linker command language from a script (@pxref{Assignments,, +Assignment: Symbol Definitions}). @emph{Note:} there should be no white +space between @var{symbol}, the equals sign (``@key{=}''), and +@var{expression}. + +@cindex demangling, from command line +@kindex --demangle +@kindex --no-demangle +@item --demangle +@itemx --no-demangle +These options control whether to demangle symbol names in error messages +and other output. When the linker is told to demangle, it tries to +present symbol names in a readable fashion: it strips leading +underscores if they are used by the object file format, and converts C++ +mangled symbol names into user readable names. The linker will demangle +by default unless the environment variable @samp{COLLECT_NO_DEMANGLE} is +set. These options may be used to override the default. + +@cindex dynamic linker, from command line +@kindex --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 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. + +@kindex --force-exe-suffix +@item --force-exe-suffix +Make sure that an output file has a .exe suffix. + +If a successfully built fully linked output file does not have a +@code{.exe} or @code{.dll} suffix, this option forces the linker to copy +the output file to one of the same name with a @code{.exe} suffix. This +option is useful when using unmodified Unix makefiles on a Microsoft +Windows host, since some versions of Windows won't run an image unless +it ends in a @code{.exe} suffix. + +@kindex --gc-sections +@kindex --no-gc-sections +@cindex garbage collection +@item --no-gc-sections +@itemx --gc-sections +Enable garbage collection of unused input sections. It is ignored on +targets that do not support this option. This option is not compatible +with @samp{-r}, nor should it be used with dynamic linking. The default +behaviour (of not performing this garbage collection) can be restored by +specifying @samp{--no-gc-sections} on the command line. + +@cindex help +@cindex usage +@kindex --help +@item --help +Print a summary of the command-line options on the standard output and exit. + +@kindex -Map +@item -Map @var{mapfile} +Print a link map to the file @var{mapfile}. See the description of the +@samp{-M} option, above. + +@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-undefined +@item --no-undefined +Normally when creating a non-symbolic shared library, undefined symbols +are allowed and left to be resolved by the runtime loader. This option +disallows such undefined symbols. + +@kindex --no-warn-mismatch +@item --no-warn-mismatch +Normally @code{ld} will give an error if you try to link together input +files that are mismatched for some reason, perhaps because they have +been compiled for different processors or for different endiannesses. +This option tells @code{ld} that it should silently permit such possible +errors. This option should only be used with care, in cases when you +have taken some special action that ensures that the linker errors are +inappropriate. + +@kindex --no-whole-archive +@item --no-whole-archive +Turn off the effect of the @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. + +On some platforms these link time global optimizations may make symbolic +debugging of the resulting executable impossible. +@ifset GENERIC +This is known to be +the case for the Matsushita MN10200 and MN10300 family of processors. +@end ifset + +@ifset GENERIC +On platforms where this is not supported, @samp{--relax} is accepted, +but ignored. +@end ifset + +@cindex retaining specified symbols +@cindex stripping all but some symbols +@cindex symbols, retaining selectively +@item --retain-symbols-file @var{filename} +Retain @emph{only} the symbols listed in the file @var{filename}, +discarding all others. @var{filename} is simply a flat file, with one +symbol name per line. This option is especially useful in environments +@ifset GENERIC +(such as VxWorks) +@end ifset +where a large global symbol table is accumulated gradually, to conserve +run-time memory. + +@samp{--retain-symbols-file} does @emph{not} discard undefined symbols, +or symbols needed for relocations. + +You may only specify @samp{--retain-symbols-file} once in the command +line. It overrides @samp{-s} and @samp{-S}. + +@ifset GENERIC +@item -rpath @var{dir} +@cindex runtime library search path +@kindex -rpath +Add a directory to the runtime library search path. This is used when +linking an ELF executable with shared objects. All @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-relocatable link, it will automatically try to locate the required +shared library and include it in the link, if it is not included +explicitly. In such a case, the @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}. +@item +For a native linker on an ELF system, if the file @file{/etc/ld.so.conf} +exists, the list of directories found in that file. +@end enumerate + +If the required shared library is not found, the linker will issue a +warning and continue with the link. +@end ifset + +@kindex -shared +@kindex -Bshareable +@item -shared +@itemx -Bshareable +@cindex shared libraries +Create a shared library. This is currently only supported on ELF, XCOFF +and SunOS platforms. On SunOS, the linker will automatically create a +shared library if the @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 --verbose +@cindex verbose +@item --dll-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 --version-script=@var{version-scriptfile} +@cindex version script, symbol versions +@itemx --version-script=@var{version-scriptfile} +Specify the name of a version script to the linker. This is typically +used when creating shared libraries to specify additional information +about the version heirarchy for the library being created. This option +is only meaningful on ELF platforms which support shared libraries. +@xref{VERSION}. + +@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 --warn-section-align +@cindex warnings, on section alignment +@cindex section alignment, warnings on +@item --warn-section-align +Warn if the address of an output section is changed because of +alignment. Typically, the alignment will be set by an input section. +The address will only be changed if it not explicitly specified; that +is, if the @code{SECTIONS} command does not specify a start address for +the section (@pxref{SECTIONS}). + +@kindex --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 + +@subsection Options specific to i386 PE targets + +The i386 PE linker supports the @code{-shared} option, which causes +the output to be a dynamically linked library (DLL) instead of a +normal executable. You should name the output @code{*.dll} when you +use this option. In addition, the linker fully supports the standard +@code{*.def} files, which may be specified on the linker command line +like an object file (in fact, it should precede archives it exports +symbols from, to ensure that they get linked in, just like a normal +object file). + +In addition to the options common to all targets, the i386 PE linker +support additional command line options that are specific to the i386 +PE target. Options that take values may be separated from their +values by either a space or an equals sign. + +@table @code + +@kindex --add-stdcall-alias +@item --add-stdcall-alias +If given, symbols with a stdcall suffix (@@@var{nn}) will be exported +as-is and also with the suffix stripped. + +@kindex --base-file +@item --base-file @var{file} +Use @var{file} as the name of a file in which to save the base +addresses of all the relocations needed for generating DLLs with +@file{dlltool}. + +@kindex --dll +@item --dll +Create a DLL instead of a regular executable. You may also use +@code{-shared} or specify a @code{LIBRARY} in a given @code{.def} +file. + +@kindex --enable-stdcall-fixup +@kindex --disable-stdcall-fixup +@item --enable-stdcall-fixup +@itemx --disable-stdcall-fixup +If the link finds a symbol that it cannot resolve, it will attempt to +do "fuzzy linking" by looking for another defined symbol that differs +only in the format of the symbol name (cdecl vs stdcall) and will +resolve that symbol by linking to the match. For example, the +undefined symbol @code{_foo} might be linked to the function +@code{_foo@@12}, or the undefined symbol @code{_bar@@16} might be linked +to the function @code{_bar}. When the linker does this, it prints a +warning, since it normally should have failed to link, but sometimes +import libraries generated from third-party dlls may need this feature +to be usable. If you specify @code{--enable-stdcall-fixup}, this +feature is fully enabled and warnings are not printed. If you specify +@code{--disable-stdcall-fixup}, this feature is disabled and such +mismatches are considered to be errors. + +@cindex DLLs, creating +@kindex --export-all-symbols +@item --export-all-symbols +If given, all global symbols in the objects used to build a DLL will +be exported by the DLL. Note that this is the default if there +otherwise wouldn't be any exported symbols. When symbols are +explicitly exported via DEF files or implicitly exported via function +attributes, the default is to not export anything else unless this +option is given. Note that the symbols @code{DllMain@@12}, +@code{DllEntryPoint@@0}, and @code{impure_ptr} will not be automatically +exported. + +@kindex --exclude-symbols +@item --exclude-symbols @var{symbol,symbol,...} +Specifies a list of symbols which should not be automatically +exported. The symbol names may be delimited by commas or colons. + +@kindex --file-alignment +@item --file-alignment +Specify the file alignment. Sections in the file will always begin at +file offsets which are multiples of this number. This defaults to +512. + +@cindex heap size +@kindex --heap +@item --heap @var{reserve} +@itemx --heap @var{reserve},@var{commit} +Specify the amount of memory to reserve (and optionally commit) to be +used as heap for this program. The default is 1Mb reserved, 4K +committed. + +@cindex image base +@kindex --image-base +@item --image-base @var{value} +Use @var{value} as the base address of your program or dll. This is +the lowest memory location that will be used when your program or dll +is loaded. To reduce the need to relocate and improve performance of +your dlls, each should have a unique base address and not overlap any +other dlls. The default is 0x400000 for executables, and 0x10000000 +for dlls. + +@kindex --kill-at +@item --kill-at +If given, the stdcall suffixes (@@@var{nn}) will be stripped from +symbols before they are exported. + +@kindex --major-image-version +@item --major-image-version @var{value} +Sets the major number of the "image version". Defaults to 1. + +@kindex --major-os-version +@item --major-os-version @var{value} +Sets the major number of the "os version". Defaults to 4. + +@kindex --major-subsystem-version +@item --major-subsystem-version @var{value} +Sets the major number of the "subsystem version". Defaults to 4. + +@kindex --minor-image-version +@item --minor-image-version @var{value} +Sets the minor number of the "image version". Defaults to 0. + +@kindex --minor-os-version +@item --minor-os-version @var{value} +Sets the minor number of the "os version". Defaults to 0. + +@kindex --minor-subsystem-version +@item --minor-subsystem-version @var{value} +Sets the minor number of the "subsystem version". Defaults to 0. + +@cindex DEF files, creating +@cindex DLLs, creating +@kindex --output-def +@item --output-def @var{file} +The linker will create the file @var{file} which will contain a DEF +file corresponding to the DLL the linker is generating. This DEF file +(which should be called @code{*.def}) may be used to create an import +library with @code{dlltool} or may be used as a reference to +automatically or implicitly exported symbols. + +@kindex --section-alignment +@item --section-alignment +Sets the section alignment. Sections in memory will always begin at +addresses which are a multiple of this number. Defaults to 0x1000. + +@cindex stack size +@kindex --stack +@item --stack @var{reserve} +@itemx --stack @var{reserve},@var{commit} +Specify the amount of memory to reserve (and optionally commit) to be +used as stack for this program. The default is 32Mb reserved, 4K +committed. + +@kindex --subsystem +@item --subsystem @var{which} +@itemx --subsystem @var{which}:@var{major} +@itemx --subsystem @var{which}:@var{major}.@var{minor} +Specifies the subsystem under which your program will execute. The +legal values for @var{which} are @code{native}, @code{windows}, +@code{console}, and @code{posix}. You may optionally set the +subsystem version also. + +@end table + +@ifset UsesEnvVars +@node Environment +@section Environment Variables + +You can change the behavior of @code{ld} with the environment variables +@code{GNUTARGET}, @code{LDEMULATION}, and @code{COLLECT_NO_DEMANGLE}. + +@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. + +@kindex LDEMULATION +@cindex default emulation +@cindex emulation, default +@code{LDEMULATION} determines the default emulation if you don't use the +@samp{-m} option. The emulation can affect various aspects of linker +behaviour, particularly the default linker script. You can list the +available emulations with the @samp{--verbose} or @samp{-V} options. If +the @samp{-m} option is not used, and the @code{LDEMULATION} environment +variable is not defined, the default emulation depends upon how the +linker was configured. +@end ifset + +@kindex COLLECT_NO_DEMANGLE +@cindex demangling, default +Normally, the linker will default to demangling symbols. However, if +@code{COLLECT_NO_DEMANGLE} is set in the environment, then it will +default to not demangling symbols. This environment variable is used in +a similar fashion by the @code{gcc} linker wrapper program. The default +may be overridden by the @samp{--demangle} and @samp{--no-demangle} +options. + +@node Scripts +@chapter Linker Scripts + +@cindex scripts +@cindex linker scripts +@cindex command files +Every link is controlled by a @dfn{linker script}. This script is +written in the linker command language. + +The main purpose of the linker script is to describe how the sections in +the input files should be mapped into the output file, and to control +the memory layout of the output file. Most linker scripts do nothing +more than this. However, when necessary, the linker script can also +direct the linker to perform many other operations, using the commands +described below. + +The linker always uses a linker script. If you do not supply one +yourself, the linker will use a default script that is compiled into the +linker executable. You can use the @samp{--verbose} command line option +to display the default linker script. Certain command line options, +such as @samp{-r} or @samp{-N}, will affect the default linker script. + +You may supply your own linker script by using the @samp{-T} command +line option. When you do this, your linker script will replace the +default linker script. + +You may also use linker scripts implicitly by naming them as input files +to the linker, as though they were files to be linked. @xref{Implicit +Linker Scripts}. + +@menu +* Basic Script Concepts:: Basic Linker Script Concepts +* Script Format:: Linker Script Format +* Simple Example:: Simple Linker Script Example +* Simple Commands:: Simple Linker Script Commands +* Assignments:: Assigning Values to Symbols +* SECTIONS:: SECTIONS Command +* MEMORY:: MEMORY Command +* PHDRS:: PHDRS Command +* VERSION:: VERSION Command +* Expressions:: Expressions in Linker Scripts +* Implicit Linker Scripts:: Implicit Linker Scripts +@end menu + +@node Basic Script Concepts +@section Basic Linker Script Concepts +@cindex linker script concepts +We need to define some basic concepts and vocabulary in order to +describe the linker script language. + +The linker combines input files into a single output file. The output +file and each input file are in a special data format known as an +@dfn{object file format}. Each file is called an @dfn{object file}. +The output file is often called an @dfn{executable}, but for our +purposes we will also call it an object file. Each object file has, +among other things, a list of @dfn{sections}. We sometimes refer to a +section in an input file as an @dfn{input section}; similarly, a section +in the output file is an @dfn{output section}. + +Each section in an object file has a name and a size. Most sections +also have an associated block of data, known as the @dfn{section +contents}. A section may be marked as @dfn{loadable}, which mean that +the contents should be loaded into memory when the output file is run. +A section with no contents may be @dfn{allocatable}, which means that an +area in memory should be set aside, but nothing in particular should be +loaded there (in some cases this memory must be zeroed out). A section +which is neither loadable nor allocatable typically contains some sort +of debugging information. + +Every loadable or allocatable output section has two addresses. The +first is the @dfn{VMA}, or virtual memory address. This is the address +the section will have when the output file is run. The second is the +@dfn{LMA}, or load memory address. This is the address at which the +section will be loaded. In most cases the two addresses will be the +same. An example of when they might be different is when a data section +is loaded into ROM, and then copied into RAM when the program starts up +(this technique is often used to initialize global variables in a ROM +based system). In this case the ROM address would be the LMA, and the +RAM address would be the VMA. + +You can see the sections in an object file by using the @code{objdump} +program with the @samp{-h} option. + +Every object file also has a list of @dfn{symbols}, known as the +@dfn{symbol table}. A symbol may be defined or undefined. Each symbol +has a name, and each defined symbol has an address, among other +information. If you compile a C or C++ program into an object file, you +will get a defined symbol for every defined function and global or +static variable. Every undefined function or global variable which is +referenced in the input file will become an undefined symbol. + +You can see the symbols in an object file by using the @code{nm} +program, or by using the @code{objdump} program with the @samp{-t} +option. + +@node Script Format +@section Linker Script Format +@cindex linker script format +Linker scripts are text files. + +You write a linker script as a series of commands. Each command is +either a keyword, possibly followed by arguments, or an assignment to a +symbol. You may separate commands using semicolons. Whitespace is +generally ignored. + +Strings such as file or format names can normally be entered directly. +If the file name contains a character such as a comma which would +otherwise serve to separate file names, you may put the file name in +double quotes. There is no way to use a double quote character in a +file name. + +You may include comments in linker scripts just as in C, delimited by +@samp{/*} and @samp{*/}. As in C, comments are syntactically equivalent +to whitespace. + +@node Simple Example +@section Simple Linker Script Example +@cindex linker script example +@cindex example of linker script +Many linker scripts are fairly simple. + +The simplest possible linker script has just one command: +@samp{SECTIONS}. You use the @samp{SECTIONS} command to describe the +memory layout of the output file. + +The @samp{SECTIONS} command is a powerful command. Here we will +describe a simple use of it. Let's assume your program consists only of +code, initialized data, and uninitialized data. These will be in the +@samp{.text}, @samp{.data}, and @samp{.bss} sections, respectively. +Let's assume further that these are the only sections which appear in +your input files. + +For this example, let's say that the code should be loaded at address +0x10000, and that the data should start at address 0x8000000. Here is a +linker script which will do that: +@smallexample +SECTIONS +@{ + . = 0x10000; + .text : @{ *(.text) @} + . = 0x8000000; + .data : @{ *(.data) @} + .bss : @{ *(.bss) @} +@} +@end smallexample + +You write the @samp{SECTIONS} command as the keyword @samp{SECTIONS}, +followed by a series of symbol assignments and output section +descriptions enclosed in curly braces. + +The first line in the above example sets the special symbol @samp{.}, +which is the location counter. If you do not specify the address of an +output section in some other way (other ways are described later), the +address is set from the current value of the location counter. The +location counter is then incremented by the size of the output section. + +The first line inside the @samp{SECTIONS} command of the above example +sets the value of the special symbol @samp{.}, which is the location +counter. If you do not specify the address of an output section in some +other way (other ways are described later), the address is set from the +current value of the location counter. The location counter is then +incremented by the size of the output section. At the start of the +@samp{SECTIONS} command, the location counter has the value @samp{0}. + +The second line defines an output section, @samp{.text}. The colon is +required syntax which may be ignored for now. Within the curly braces +after the output section name, you list the names of the input sections +which should be placed into this output section. The @samp{*} is a +wildcard which matches any file name. The expression @samp{*(.text)} +means all @samp{.text} input sections in all input files. + +Since the location counter is @samp{0x10000} when the output section +@samp{.text} is defined, the linker will set the address of the +@samp{.text} section in the output file to be @samp{0x10000}. + +The remaining lines define the @samp{.data} and @samp{.bss} sections in +the output file. The linker will place the @samp{.data} output section +at address @samp{0x8000000}. After the linker places the @samp{.data} +output section, the value of the location counter will be +@samp{0x8000000} plus the size of the @samp{.data} output section. The +effect is that the linker will place the @samp{.bss} output section +immediately after the @samp{.data} output section in memory + +The linker will ensure that each output section has the required +alignment, by increasing the location counter if necessary. In this +example, the specified addresses for the @samp{.text} and @samp{.data} +sections will probably satisfy any alignment constraints, but the linker +may have to create a small gap between the @samp{.data} and @samp{.bss} +sections. + +That's it! That's a simple and complete linker script. + +@node Simple Commands +@section Simple Linker Script Commands +@cindex linker script simple commands +In this section we describe the simple linker script commands. + +@menu +* Entry Point:: Setting the entry point +* File Commands:: Commands dealing with files +@ifclear SingleFormat +* Format Commands:: Commands dealing with object file formats +@end ifclear + +* Miscellaneous Commands:: Other linker script commands +@end menu + +@node Entry Point +@subsection Setting the entry point +@kindex ENTRY(@var{symbol}) +@cindex start of execution +@cindex first instruction +@cindex entry point +The first instruction to execute in a program is called the @dfn{entry +point}. You can use the @code{ENTRY} linker script command to set the +entry point. The argument is a symbol name: +@smallexample +ENTRY(@var{symbol}) +@end smallexample + +There are several ways to set the entry point. The linker will set the +entry point by trying each of the following methods in order, and +stopping when one of them succeeds: +@itemize @bullet +@item +the @samp{-e} @var{entry} command-line option; +@item +the @code{ENTRY(@var{symbol})} command in a linker script; +@item +the value of the symbol @code{start}, if defined; +@item +the address of the first byte of the @samp{.text} section, if present; +@item +The address @code{0}. +@end itemize + +@node File Commands +@subsection Commands dealing with files +@cindex linker script file commands +Several linker script commands deal with files. + +@table @code +@item INCLUDE @var{filename} +@kindex INCLUDE @var{filename} +@cindex including a linker script +Include the linker script @var{filename} at this point. The file will +be searched for in the current directory, and in any directory specified +with the @code{-L} option. You can nest calls to @code{INCLUDE} up to +10 levels deep. + +@item INPUT(@var{file}, @var{file}, @dots{}) +@itemx INPUT(@var{file} @var{file} @dots{}) +@kindex INPUT(@var{files}) +@cindex input files in linker scripts +@cindex input object files in linker scripts +@cindex linker script input object files +The @code{INPUT} command directs the linker to include the named files +in the link, as though they were named on the command line. + +For example, if you always want to include @file{subr.o} any time you do +a link, but you can't be bothered to put it on every link command line, +then you can put @samp{INPUT (subr.o)} in your linker script. + +In fact, if you like, you can list all of your input files in the linker +script, and then invoke the linker with nothing but a @samp{-T} option. + +The linker will first try to open the file in the current directory. If +it is not found, the linker will search through the archive library +search path. See the description of @samp{-L} in @ref{Options,,Command +Line Options}. + +If you use @samp{INPUT (-l@var{file})}, @code{ld} will transform the +name to @code{lib@var{file}.a}, as with the command line argument +@samp{-l}. + +When you use the @code{INPUT} command in an implicit linker script, the +files will be included in the link at the point at which the linker +script file is included. This can affect archive searching. + +@item GROUP(@var{file}, @var{file}, @dots{}) +@itemx GROUP(@var{file} @var{file} @dots{}) +@kindex GROUP(@var{files}) +@cindex grouping input files +The @code{GROUP} command is like @code{INPUT}, except that the named +files should all be archives, and they are searched repeatedly until no +new undefined references are created. See the description of @samp{-(} +in @ref{Options,,Command Line Options}. + +@item OUTPUT(@var{filename}) +@kindex OUTPUT(@var{filename}) +@cindex output file name in linker scripot +The @code{OUTPUT} command names the output file. Using +@code{OUTPUT(@var{filename})} in the linker script is exactly like using +@samp{-o @var{filename}} on the command line (@pxref{Options,,Command +Line Options}). If both are used, the command line option takes +precedence. + +You can use the @code{OUTPUT} command to define a default name for the +output file other than the usual default of @file{a.out}. + +@item SEARCH_DIR(@var{path}) +@kindex SEARCH_DIR(@var{path}) +@cindex library search path in linker script +@cindex archive search path in linker script +@cindex search path in linker script +The @code{SEARCH_DIR} command adds @var{path} to the list of paths where +@code{ld} looks for archive libraries. Using +@code{SEARCH_DIR(@var{path})} is exactly like using @samp{-L @var{path}} +on the command line (@pxref{Options,,Command Line Options}). If both +are used, then the linker will search both paths. Paths specified using +the command line option are searched first. + +@item STARTUP(@var{filename}) +@kindex STARTUP(@var{filename}) +@cindex first input file +The @code{STARTUP} command is just like the @code{INPUT} command, except +that @var{filename} will become the first input file to be linked, as +though it were specified first on the command line. This may be useful +when using a system in which the entry point is always the start of the +first file. +@end table + +@ifclear SingleFormat +@node Format Commands +@subsection Commands dealing with object file formats +A couple of linker script commands deal with object file formats. + +@table @code +@item OUTPUT_FORMAT(@var{bfdname}) +@itemx OUTPUT_FORMAT(@var{default}, @var{big}, @var{little}) +@kindex OUTPUT_FORMAT(@var{bfdname}) +@cindex output file format in linker script +The @code{OUTPUT_FORMAT} command names the BFD format to use for the +output file (@pxref{BFD}). Using @code{OUTPUT_FORMAT(@var{bfdname})} is +exactly like using @samp{-oformat @var{bfdname}} on the command line +(@pxref{Options,,Command Line Options}). If both are used, the command +line option takes precedence. + +You can use @code{OUTPUT_FORMAT} with three arguments to use different +formats based on the @samp{-EB} and @samp{-EL} command line options. +This permits the linker script to set the output format based on the +desired endianness. + +If neither @samp{-EB} nor @samp{-EL} are used, then the output format +will be the first argument, @var{default}. If @samp{-EB} is used, the +output format will be the second argument, @var{big}. If @samp{-EL} is +used, the output format will be the third argument, @var{little}. + +For example, the default linker script for the MIPS ELF target uses this +command: +@smallexample +OUTPUT_FORMAT(elf32-bigmips, elf32-bigmips, elf32-littlemips) +@end smallexample +This says that the default format for the output file is +@samp{elf32-bigmips}, but if the user uses the @samp{-EL} command line +option, the output file will be created in the @samp{elf32-littlemips} +format. + +@item TARGET(@var{bfdname}) +@kindex TARGET(@var{bfdname}) +@cindex input file format in linker script +The @code{TARGET} command names the BFD format to use when reading input +files. It affects subsequent @code{INPUT} and @code{GROUP} commands. +This command is like using @samp{-b @var{bfdname}} on the command line +(@pxref{Options,,Command Line Options}). If the @code{TARGET} command +is used but @code{OUTPUT_FORMAT} is not, then the last @code{TARGET} +command is also used to set the format for the output file. @xref{BFD}. +@end table +@end ifclear + +@node Miscellaneous Commands +@subsection Other linker script commands +There are a few other linker scripts commands. + +@table @code +@item ASSERT(@var{exp}, @var{message}) +@kindex ASSERT +@cindex assertion in linker script +Ensure that @var{exp} is non-zero. If it is zero, then exit the linker +with an error code, and print @var{message}. + +@item EXTERN(@var{symbol} @var{symbol} @dots{}) +@kindex EXTERN +@cindex undefined symbol in linker script +Force @var{symbol} to be entered in the output file as an undefined +symbol. Doing this may, for example, trigger linking of additional +modules from standard libraries. You may list several @var{symbol}s for +each @code{EXTERN}, and you may use @code{EXTERN} multiple times. This +command has the same effect as the @samp{-u} command-line option. + +@item FORCE_COMMON_ALLOCATION +@kindex FORCE_COMMON_ALLOCATION +@cindex common allocation in linker script +This command has the same effect as the @samp{-d} command-line option: +to make @code{ld} assign space to common symbols even if a relocatable +output file is specified (@samp{-r}). + +@item NOCROSSREFS(@var{section} @var{section} @dots{}) +@kindex NOCROSSREFS(@var{sections}) +@cindex cross references +This command may be used to tell @code{ld} to issue an error about any +references among certain output sections. + +In certain types of programs, particularly on embedded systems when +using overlays, when one section is loaded into memory, another section +will not be. Any direct references between the two sections would be +errors. For example, it would be an error if code in one section called +a function defined in the other section. + +The @code{NOCROSSREFS} command takes a list of output section names. If +@code{ld} detects any cross references between the sections, it reports +an error and returns a non-zero exit status. Note that the +@code{NOCROSSREFS} command uses output section names, not input section +names. + +@ifclear SingleFormat +@item OUTPUT_ARCH(@var{bfdarch}) +@kindex OUTPUT_ARCH(@var{bfdarch}) +@cindex machine architecture +@cindex architecture +Specify a particular output machine architecture. The argument is one +of the names used by the BFD library (@pxref{BFD}). You can see the +architecture of an object file by using the @code{objdump} program with +the @samp{-f} option. +@end ifclear +@end table + +@node Assignments +@section Assigning Values to Symbols +@cindex assignment in scripts +@cindex symbol definition, scripts +@cindex variables, defining +You may assign a value to a symbol in a linker script. This will define +the symbol as a global symbol. + +@menu +* Simple Assignments:: Simple Assignments +* PROVIDE:: PROVIDE +@end menu + +@node Simple Assignments +@subsection Simple Assignments + +You may assign to a symbol using any of the C assignment operators: + +@table @code +@item @var{symbol} = @var{expression} ; +@itemx @var{symbol} += @var{expression} ; +@itemx @var{symbol} -= @var{expression} ; +@itemx @var{symbol} *= @var{expression} ; +@itemx @var{symbol} /= @var{expression} ; +@itemx @var{symbol} <<= @var{expression} ; +@itemx @var{symbol} >>= @var{expression} ; +@itemx @var{symbol} &= @var{expression} ; +@itemx @var{symbol} |= @var{expression} ; +@end table + +The first case will define @var{symbol} to the value of +@var{expression}. In the other cases, @var{symbol} must already be +defined, and the value will be adjusted accordingly. + +The special symbol name @samp{.} indicates the location counter. You +may only use this within a @code{SECTIONS} command. + +The semicolon after @var{expression} is required. + +Expressions are defined below; see @ref{Expressions}. + +You may write symbol assignments as commands in their own right, or as +statements within a @code{SECTIONS} command, or as part of an output +section description in a @code{SECTIONS} command. + +The section of the symbol will be set from the section of the +expression; for more information, see @ref{Expression Section}. + +Here is an example showing the three different places that symbol +assignments may be used: + +@smallexample +floating_point = 0; +SECTIONS +@{ + .text : + @{ + *(.text) + _etext = .; + @} + _bdata = (. + 3) & ~ 4; + .data : @{ *(.data) @} +@} +@end smallexample +@noindent +In this example, the symbol @samp{floating_point} will be defined as +zero. The symbol @samp{_etext} will be defined as the address following +the last @samp{.text} input section. The symbol @samp{_bdata} will be +defined as the address following the @samp{.text} output section aligned +upward to a 4 byte boundary. + +@node PROVIDE +@subsection PROVIDE +@cindex PROVIDE +In some cases, it is desirable for a linker script to define a symbol +only if it is referenced and is not defined by any object included in +the link. For example, traditional linkers defined the symbol +@samp{etext}. However, ANSI C requires that the user be able to use +@samp{etext} as a function name without encountering an error. The +@code{PROVIDE} keyword may be used to define a symbol, such as +@samp{etext}, only if it is referenced but not defined. The syntax is +@code{PROVIDE(@var{symbol} = @var{expression})}. + +Here is an example of using @code{PROVIDE} to define @samp{etext}: +@smallexample +SECTIONS +@{ + .text : + @{ + *(.text) + _etext = .; + PROVIDE(etext = .); + @} +@} +@end smallexample + +In this example, if the program defines @samp{_etext} (with a leading +underscore), the linker will give a multiple definition error. If, on +the other hand, the program defines @samp{etext} (with no leading +underscore), the linker will silently use the definition in the program. +If the program references @samp{etext} but does not define it, the +linker will use the definition in the linker script. + +@node SECTIONS +@section SECTIONS command +@kindex SECTIONS +The @code{SECTIONS} command tells the linker how to map input sections +into output sections, and how to place the output sections in memory. + +The format of the @code{SECTIONS} command is: +@smallexample +SECTIONS +@{ + @var{sections-command} + @var{sections-command} + @dots{} +@} +@end smallexample + +Each @var{sections-command} may of be one of the following: + +@itemize @bullet +@item +an @code{ENTRY} command (@pxref{Entry Point,,Entry command}) +@item +a symbol assignment (@pxref{Assignments}) +@item +an output section description +@item +an overlay description +@end itemize + +The @code{ENTRY} command and symbol assignments are permitted inside the +@code{SECTIONS} command for convenience in using the location counter in +those commands. This can also make the linker script easier to +understand because you can use those commands at meaningful points in +the layout of the output file. + +Output section descriptions and overlay descriptions are described +below. + +If you do not use a @code{SECTIONS} command in your linker script, the +linker will place each input section into an identically named output +section in the order that the sections are first encountered in the +input files. If all input sections are present in the first file, for +example, the order of sections in the output file will match the order +in the first input file. The first section will be at address zero. + +@menu +* Output Section Description:: Output section description +* Output Section Name:: Output section name +* Output Section Address:: Output section address +* Input Section:: Input section description +* Output Section Data:: Output section data +* Output Section Keywords:: Output section keywords +* Output Section Discarding:: Output section discarding +* Output Section Attributes:: Output section attributes +* Overlay Description:: Overlay description +@end menu + +@node Output Section Description +@subsection Output section description +The full description of an output section looks like this: +@smallexample +@group +@var{section} [@var{address}] [(@var{type})] : [AT(@var{lma})] + @{ + @var{output-section-command} + @var{output-section-command} + @dots{} + @} [>@var{region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}] +@end group +@end smallexample + +Most output sections do not use most of the optional section attributes. + +The whitespace around @var{section} is required, so that the section +name is unambiguous. The colon and the curly braces are also required. +The line breaks and other white space are optional. + +Each @var{output-section-command} may be one of the following: + +@itemize @bullet +@item +a symbol assignment (@pxref{Assignments}) +@item +an input section description (@pxref{Input Section}) +@item +data values to include directly (@pxref{Output Section Data}) +@item +a special output section keyword (@pxref{Output Section Keywords}) +@end itemize + +@node Output Section Name +@subsection Output section name +@cindex name, section +@cindex section name +The name of the output section is @var{section}. @var{section} must +meet the constraints of your output format. In formats which only +support a limited number of sections, such as @code{a.out}, the name +must be one of the names supported by the format (@code{a.out}, for +example, allows only @samp{.text}, @samp{.data} or @samp{.bss}). If the +output format supports any number of sections, but with numbers and not +names (as is the case for Oasys), the name should be supplied as a +quoted numeric string. A section name may consist of any sequence of +characters, but a name which contains any unusual characters such as +commas must be quoted. + +The output section name @samp{/DISCARD/} is special; @ref{Output Section +Discarding}. + +@node Output Section Address +@subsection Output section address +@cindex address, section +@cindex section address +The @var{address} is an expression for the VMA (the virtual memory +address) of the output section. If you do not provide @var{address}, +the linker will set it based on @var{region} if present, or otherwise +based on the current value of the location counter. + +If you provide @var{address}, the address of the output section will be +set to precisely that. If you provide neither @var{address} nor +@var{region}, then the address of the output section will be set to the +current value of the location counter aligned to the alignment +requirements of the output section. The alignment requirement of the +output section is the strictest alignment of any input section contained +within the output section. + +For example, +@smallexample +.text . : @{ *(.text) @} +@end smallexample +@noindent +and +@smallexample +.text : @{ *(.text) @} +@end smallexample +@noindent +are subtly different. The first will set the address of the +@samp{.text} output section to the current value of the location +counter. The second will set it to the current value of the location +counter aligned to the strictest alignment of a @samp{.text} input +section. + +The @var{address} may be an arbitrary expression; @ref{Expressions}. +For example, if you want to align the section on a 0x10 byte boundary, +so that the lowest four bits of the section address are zero, you could +do something like this: +@smallexample +.text ALIGN(0x10) : @{ *(.text) @} +@end smallexample +@noindent +This works because @code{ALIGN} returns the current location counter +aligned upward to the specified value. + +Specifying @var{address} for a section will change the value of the +location counter. + +@node Input Section +@subsection Input section description +@cindex input sections +@cindex mapping input sections to output sections +The most common output section command is an input section description. + +The input section description is the most basic linker script operation. +You use output sections to tell the linker how to lay out your program +in memory. You use input section descriptions to tell the linker how to +map the input files into your memory layout. + +@menu +* Input Section Basics:: Input section basics +* Input Section Wildcards:: Input section wildcard patterns +* Input Section Common:: Input section for common symbols +* Input Section Keep:: Input section and garbage collection +* Input Section Example:: Input section example +@end menu + +@node Input Section Basics +@subsubsection Input section basics +@cindex input section basics +An input section description consists of a file name optionally followed +by a list of section names in parentheses. + +The file name and the section name may be wildcard patterns, which we +describe further below (@pxref{Input Section Wildcards}). + +The most common input section description is to include all input +sections with a particular name in the output section. For example, to +include all input @samp{.text} sections, you would write: +@smallexample +*(.text) +@end smallexample +@noindent +Here the @samp{*} is a wildcard which matches any file name. To exclude a file +from matching the file name wildcard, EXCLUDE_FILE may be used to match all files +except the one specified by EXCLUDE_FILE. For example: +@smallexample +(*(EXCLUDE_FILE (*crtend.o) .ctors)) +@end smallexample +will cause all .ctors sections from all files except crtend.o to be included. + +There are two ways to include more than one section: +@smallexample +*(.text .rdata) +*(.text) *(.rdata) +@end smallexample +@noindent +The difference between these is the order in which the @samp{.text} and +@samp{.rdata} input sections will appear in the output section. In the +first example, they will be intermingled. In the second example, all +@samp{.text} input sections will appear first, followed by all +@samp{.rdata} input sections. + +You can specify a file name to include sections from a particular file. +You would do this if one or more of your files contain special data that +needs to be at a particular location in memory. For example: +@smallexample +data.o(.data) +@end smallexample + +If you use a file name without a list of sections, then all sections in +the input file will be included in the output section. This is not +commonly done, but it may by useful on occasion. For example: +@smallexample +data.o +@end smallexample + +When you use a file name which does not contain any wild card +characters, the linker will first see if you also specified the file +name on the linker command line or in an @code{INPUT} command. If you +did not, the linker will attempt to open the file as an input file, as +though it appeared on the command line. Note that this differs from an +@code{INPUT} command, because the linker will not search for the file in +the archive search path. + +@node Input Section Wildcards +@subsubsection Input section wildcard patterns +@cindex input section wildcards +@cindex wildcard file name patterns +@cindex file name wildcard patterns +@cindex section name wildcard patterns +In an input section description, either the file name or the section +name or both may be wildcard patterns. + +The file name of @samp{*} seen in many examples is a simple wildcard +pattern for the file name. + +The wildcard patterns are like those used by the Unix shell. + +@table @samp +@item * +matches any number of characters +@item ? +matches any single character +@item [@var{chars}] +matches a single instance of any of the @var{chars}; the @samp{-} +character may be used to specify a range of characters, as in +@samp{[a-z]} to match any lower case letter +@item \ +quotes the following character +@end table + +When a file name is matched with a wildcard, the wildcard characters +will not match a @samp{/} character (used to separate directory names on +Unix). A pattern consisting of a single @samp{*} character is an +exception; it will always match any file name, whether it contains a +@samp{/} or not. In a section name, the wildcard characters will match +a @samp{/} character. + +File name wildcard patterns only match files which are explicitly +specified on the command line or in an @code{INPUT} command. The linker +does not search directories to expand wildcards. + +If a file name matches more than one wildcard pattern, or if a file name +appears explicitly and is also matched by a wildcard pattern, the linker +will use the first match in the linker script. For example, this +sequence of input section descriptions is probably in error, because the +@file{data.o} rule will not be used: +@smallexample +.data : @{ *(.data) @} +.data1 : @{ data.o(.data) @} +@end smallexample + +@cindex SORT +Normally, the linker will place files and sections matched by wildcards +in the order in which they are seen during the link. You can change +this by using the @code{SORT} keyword, which appears before a wildcard +pattern in parentheses (e.g., @code{SORT(.text*)}). When the +@code{SORT} keyword is used, the linker will sort the files or sections +into ascending order by name before placing them in the output file. + +If you ever get confused about where input sections are going, use the +@samp{-M} linker option to generate a map file. The map file shows +precisely how input sections are mapped to output sections. + +This example shows how wildcard patterns might be used to partition +files. This linker script directs the linker to place all @samp{.text} +sections in @samp{.text} and all @samp{.bss} sections in @samp{.bss}. +The linker will place the @samp{.data} section from all files beginning +with an upper case character in @samp{.DATA}; for all other files, the +linker will place the @samp{.data} section in @samp{.data}. +@smallexample +@group +SECTIONS @{ + .text : @{ *(.text) @} + .DATA : @{ [A-Z]*(.data) @} + .data : @{ *(.data) @} + .bss : @{ *(.bss) @} +@} +@end group +@end smallexample + +@node Input Section Common +@subsubsection Input section for common symbols +@cindex common symbol placement +@cindex uninitialized data placement +A special notation is needed for common symbols, because in many object +file formats common symbols do not have a particular input section. The +linker treats common symbols as though they are in an input section +named @samp{COMMON}. + +You may use file names with the @samp{COMMON} section just as with any +other input sections. You can use this to place common symbols from a +particular input file in one section while common symbols from other +input files are placed in another section. + +In most cases, common symbols in input files will be placed in the +@samp{.bss} section in the output file. For example: +@smallexample +.bss @{ *(.bss) *(COMMON) @} +@end smallexample + +@cindex scommon section +@cindex small common symbols +Some object file formats have more than one type of common symbol. For +example, the MIPS ELF object file format distinguishes standard common +symbols and small common symbols. In this case, the linker will use a +different special section name for other types of common symbols. In +the case of MIPS ELF, the linker uses @samp{COMMON} for standard common +symbols and @samp{.scommon} for small common symbols. This permits you +to map the different types of common symbols into memory at different +locations. + +@cindex [COMMON] +You will sometimes see @samp{[COMMON]} in old linker scripts. This +notation is now considered obsolete. It is equivalent to +@samp{*(COMMON)}. + +@node Input Section Keep +@subsubsection Input section and garbage collection +@cindex KEEP +@cindex garbage collection +When link-time garbage collection is in use (@samp{--gc-sections}), +it is often useful to mark sections that should not be eliminated. +This is accomplished by surrounding an input section's wildcard entry +with @code{KEEP()}, as in @code{KEEP(*(.init))} or +@code{KEEP(SORT(*)(.ctors))}. + +@node Input Section Example +@subsubsection Input section example +The following example is a complete linker script. It tells the linker +to read all of the sections from file @file{all.o} and place them at the +start of output section @samp{outputa} which starts at location +@samp{0x10000}. All of section @samp{.input1} from file @file{foo.o} +follows immediately, in the same output section. All of section +@samp{.input2} from @file{foo.o} goes into output section +@samp{outputb}, followed by section @samp{.input1} from @file{foo1.o}. +All of the remaining @samp{.input1} and @samp{.input2} sections from any +files are written to output section @samp{outputc}. + +@smallexample +@group +SECTIONS @{ + outputa 0x10000 : + @{ + all.o + foo.o (.input1) + @} + outputb : + @{ + foo.o (.input2) + foo1.o (.input1) + @} + outputc : + @{ + *(.input1) + *(.input2) + @} +@} +@end group +@end smallexample + +@node Output Section Data +@subsection Output section data +@cindex data +@cindex section data +@cindex output section data +@kindex BYTE(@var{expression}) +@kindex SHORT(@var{expression}) +@kindex LONG(@var{expression}) +@kindex QUAD(@var{expression}) +@kindex SQUAD(@var{expression}) +You can include explicit bytes of data in an output section by using +@code{BYTE}, @code{SHORT}, @code{LONG}, @code{QUAD}, or @code{SQUAD} as +an output section command. Each keyword is followed by an expression in +parentheses providing the value to store (@pxref{Expressions}). The +value of the expression is stored at the current value of the location +counter. + +The @code{BYTE}, @code{SHORT}, @code{LONG}, and @code{QUAD} commands +store one, two, four, and eight bytes (respectively). After storing the +bytes, the location counter is incremented by the number of bytes +stored. + +For example, this will store the byte 1 followed by the four byte value +of the symbol @samp{addr}: +@smallexample +BYTE(1) +LONG(addr) +@end smallexample + +When using a 64 bit host or target, @code{QUAD} and @code{SQUAD} are the +same; they both store an 8 byte, or 64 bit, value. When both host and +target are 32 bits, an expression is computed as 32 bits. In this case +@code{QUAD} stores a 32 bit value zero extended to 64 bits, and +@code{SQUAD} stores a 32 bit value sign extended to 64 bits. + +If the object file format of the output file has an explicit endianness, +which is the normal case, the value will be stored in that endianness. +When the object file format does not have an explicit endianness, as is +true of, for example, S-records, the value will be stored in the +endianness of the first input object file. + +@kindex FILL(@var{expression}) +@cindex holes, filling +@cindex unspecified memory +You may use the @code{FILL} command to set the fill pattern for the +current section. It is followed by an expression in parentheses. Any +otherwise unspecified regions of memory within the section (for example, +gaps left due to the required alignment of input sections) are filled +with the two least significant bytes of the expression, repeated as +necessary. A @code{FILL} statement covers memory locations after the +point at which it occurs in the section definition; by including more +than one @code{FILL} statement, you can have different fill patterns in +different parts of an output section. + +This example shows how to fill unspecified regions of memory with the +value @samp{0x9090}: +@smallexample +FILL(0x9090) +@end smallexample + +The @code{FILL} command is similar to the @samp{=@var{fillexp}} output +section attribute (@pxref{Output Section Fill}), but it only affects the +part of the section following the @code{FILL} command, rather than the +entire section. If both are used, the @code{FILL} command takes +precedence. + +@node Output Section Keywords +@subsection Output section keywords +There are a couple of keywords which can appear as output section +commands. + +@table @code +@kindex CREATE_OBJECT_SYMBOLS +@cindex input filename symbols +@cindex filename symbols +@item CREATE_OBJECT_SYMBOLS +The command tells the linker to create a symbol for each input file. +The name of each symbol will be the name of the corresponding input +file. The section of each symbol will be the output section in which +the @code{CREATE_OBJECT_SYMBOLS} command appears. + +This is conventional for the a.out object file format. It is not +normally used for any other object file format. + +@kindex CONSTRUCTORS +@cindex C++ constructors, arranging in link +@cindex constructors, arranging in link +@item CONSTRUCTORS +When linking using the a.out object file format, the linker uses an +unusual set construct to support C++ global constructors and +destructors. When linking object file formats which do not support +arbitrary sections, such as ECOFF and XCOFF, the linker will +automatically recognize C++ global constructors and destructors by name. +For these object file formats, the @code{CONSTRUCTORS} command tells the +linker to place constructor information in the output section where the +@code{CONSTRUCTORS} command appears. The @code{CONSTRUCTORS} command is +ignored for other object file formats. + +The symbol @w{@code{__CTOR_LIST__}} marks the start of the global +constructors, and the symbol @w{@code{__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++ normally calls constructors from a subroutine +@code{__main}; a call to @code{__main} is automatically inserted into +the startup code for @code{main}. @sc{gnu} C++ normally runs +destructors either by using @code{atexit}, or directly from the function +@code{exit}. + +For object file formats such as @code{COFF} or @code{ELF} which support +arbitrary section names, @sc{gnu} C++ will normally arrange to put the +addresses of global constructors and destructors into the @code{.ctors} +and @code{.dtors} sections. Placing the following sequence into your +linker script will build the sort of table which the @sc{gnu} C++ +runtime code expects to see. + +@smallexample + __CTOR_LIST__ = .; + LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2) + *(.ctors) + LONG(0) + __CTOR_END__ = .; + __DTOR_LIST__ = .; + LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2) + *(.dtors) + LONG(0) + __DTOR_END__ = .; +@end smallexample + +If you are using the @sc{gnu} C++ support for initialization priority, +which provides some control over the order in which global constructors +are run, you must sort the constructors at link time to ensure that they +are executed in the correct order. When using the @code{CONSTRUCTORS} +command, use @samp{SORT(CONSTRUCTORS)} instead. When using the +@code{.ctors} and @code{.dtors} sections, use @samp{*(SORT(.ctors))} and +@samp{*(SORT(.dtors))} instead of just @samp{*(.ctors)} and +@samp{*(.dtors)}. + +Normally the compiler and linker will handle these issues automatically, +and you will not need to concern yourself with them. However, you may +need to consider this if you are using C++ and writing your own linker +scripts. + +@end table + +@node Output Section Discarding +@subsection Output section discarding +@cindex discarding sections +@cindex sections, discarding +@cindex removing sections +The linker will not create output section which do not have any +contents. This is for convenience when referring to input sections that +may or may not be present in any of the input files. For example: +@smallexample +.foo @{ *(.foo) @} +@end smallexample +@noindent +will only create a @samp{.foo} section in the output file if there is a +@samp{.foo} section in at least one input file. + +If you use anything other than an input section description as an output +section command, such as a symbol assignment, then the output section +will always be created, even if there are no matching input sections. + +@cindex /DISCARD/ +The special output section name @samp{/DISCARD/} may be used to discard +input sections. Any input sections which are assigned to an output +section named @samp{/DISCARD/} are not included in the output file. + +@node Output Section Attributes +@subsection Output section attributes +@cindex output section attributes +We showed above that the full description of an output section looked +like this: +@smallexample +@group +@var{section} [@var{address}] [(@var{type})] : [AT(@var{lma})] + @{ + @var{output-section-command} + @var{output-section-command} + @dots{} + @} [>@var{region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}] +@end group +@end smallexample +We've already described @var{section}, @var{address}, and +@var{output-section-command}. In this section we will describe the +remaining section attributes. + +@menu +* Output Section Type:: Output section type +* Output Section LMA:: Output section LMA +* Output Section Region:: Output section region +* Output Section Phdr:: Output section phdr +* Output Section Fill:: Output section fill +@end menu + +@node Output Section Type +@subsubsection Output section type +Each output section may have a type. The type is a keyword in +parentheses. The following types are defined: + +@table @code +@item NOLOAD +The section should be marked as not loadable, so that it will not be +loaded into memory when the program is run. +@item DSECT +@itemx COPY +@itemx INFO +@itemx OVERLAY +These type names are supported for backward compatibility, and are +rarely used. They all have the same effect: the section should be +marked as not allocatable, so that no memory is allocated for the +section when the program is run. +@end table + +@kindex NOLOAD +@cindex prevent unnecessary loading +@cindex loading, preventing +The linker normally sets the attributes of an output section based on +the input sections which map into it. You can override this by using +the section type. For example, in the script sample below, the +@samp{ROM} section is addressed at memory location @samp{0} and does not +need to be loaded when the program is run. The contents of the +@samp{ROM} section will appear in the linker output file as usual. +@smallexample +@group +SECTIONS @{ + ROM 0 (NOLOAD) : @{ @dots{} @} + @dots{} +@} +@end group +@end smallexample + +@node Output Section LMA +@subsubsection Output section LMA +@kindex AT(@var{lma}) +@cindex load address +@cindex section load address +Every section has a virtual address (VMA) and a load address (LMA); see +@ref{Basic Script Concepts}. The address expression which may appear in +an output section description sets the VMA (@pxref{Output Section +Address}). + +The linker will normally set the LMA equal to the VMA. You can change +that by using the @code{AT} keyword. The expression @var{lma} that +follows the @code{AT} keyword specifies the load address of the section. + +@cindex ROM initialized data +@cindex initialized data in ROM +This feature is designed to make it easy to build a ROM image. For +example, the following linker script creates three output sections: one +called @samp{.text}, which starts at @code{0x1000}, one called +@samp{.mdata}, which is loaded at the end of the @samp{.text} section +even though its VMA is @code{0x2000}, and one called @samp{.bss} to hold +uninitialized data at address @code{0x3000}. The symbol @code{_data} is +defined with the value @code{0x2000}, which shows that the location +counter holds the VMA value, not the LMA value. + +@smallexample +@group +SECTIONS + @{ + .text 0x1000 : @{ *(.text) _etext = . ; @} + .mdata 0x2000 : + AT ( ADDR (.text) + SIZEOF (.text) ) + @{ _data = . ; *(.data); _edata = . ; @} + .bss 0x3000 : + @{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;@} +@} +@end group +@end smallexample + +The run-time initialization code for use with a program generated with +this linker script would include something like the following, to copy +the initialized data from the ROM image to its runtime address. Notice +how this code takes advantage of the symbols defined by the linker +script. + +@smallexample +@group +extern char _etext, _data, _edata, _bstart, _bend; +char *src = &_etext; +char *dst = &_data; + +/* ROM has data at end of text; copy it. */ +while (dst < &_edata) @{ + *dst++ = *src++; +@} + +/* Zero bss */ +for (dst = &_bstart; dst< &_bend; dst++) + *dst = 0; +@end group +@end smallexample + +@node Output Section Region +@subsubsection Output section region +@kindex >@var{region} +@cindex section, assigning to memory region +@cindex memory regions and sections +You can assign a section to a previously defined region of memory by +using @samp{>@var{region}}. @xref{MEMORY}. + +Here is a simple example: +@smallexample +@group +MEMORY @{ rom : ORIGIN = 0x1000, LENGTH = 0x1000 @} +SECTIONS @{ ROM : @{ *(.text) @} >rom @} +@end group +@end smallexample + +@node Output Section Phdr +@subsubsection Output section phdr +@kindex :@var{phdr} +@cindex section, assigning to program header +@cindex program headers and sections +You can assign a section to a previously defined program segment by +using @samp{:@var{phdr}}. @xref{PHDRS}. If a section is assigned to +one or more segments, then all subsequent allocated sections will be +assigned to those segments as well, unless they use an explicitly +@code{:@var{phdr}} modifier. You can use @code{:NONE} to tell the +linker to not put the section in any segment at all. + +Here is a simple example: +@smallexample +@group +PHDRS @{ text PT_LOAD ; @} +SECTIONS @{ .text : @{ *(.text) @} :text @} +@end group +@end smallexample + +@node Output Section Fill +@subsubsection Output section fill +@kindex =@var{fillexp} +@cindex section fill pattern +@cindex fill pattern, entire section +You can set the fill pattern for an entire section by using +@samp{=@var{fillexp}}. @var{fillexp} is an expression +(@pxref{Expressions}). Any otherwise unspecified regions of memory +within the output section (for example, gaps left due to the required +alignment of input sections) will be filled with the two least +significant bytes of the value, repeated as necessary. + +You can also change the fill value with a @code{FILL} command in the +output section commands; see @ref{Output Section Data}. + +Here is a simple example: +@smallexample +@group +SECTIONS @{ .text : @{ *(.text) @} =0x9090 @} +@end group +@end smallexample + +@node Overlay Description +@subsection Overlay description +@kindex OVERLAY +@cindex overlays +An overlay description provides an easy way to describe sections which +are to be loaded as part of a single memory image but are to be run at +the same memory address. At run time, some sort of overlay manager will +copy the overlaid sections in and out of the runtime memory address as +required, perhaps by simply manipulating addressing bits. This approach +can be useful, for example, when a certain region of memory is faster +than another. + +Overlays are described using the @code{OVERLAY} command. The +@code{OVERLAY} command is used within a @code{SECTIONS} command, like an +output section description. The full syntax of the @code{OVERLAY} +command is as follows: +@smallexample +@group +OVERLAY [@var{start}] : [NOCROSSREFS] [AT ( @var{ldaddr} )] + @{ + @var{secname1} + @{ + @var{output-section-command} + @var{output-section-command} + @dots{} + @} [:@var{phdr}@dots{}] [=@var{fill}] + @var{secname2} + @{ + @var{output-section-command} + @var{output-section-command} + @dots{} + @} [:@var{phdr}@dots{}] [=@var{fill}] + @dots{} + @} [>@var{region}] [:@var{phdr}@dots{}] [=@var{fill}] +@end group +@end smallexample + +Everything is optional except @code{OVERLAY} (a keyword), and each +section must have a name (@var{secname1} and @var{secname2} above). The +section definitions within the @code{OVERLAY} construct are identical to +those within the general @code{SECTIONS} contruct (@pxref{SECTIONS}), +except that no addresses and no memory regions may be defined for +sections within an @code{OVERLAY}. + +The sections are all defined with the same starting address. The load +addresses of the sections are arranged such that they are consecutive in +memory starting at the load address used for the @code{OVERLAY} as a +whole (as with normal section definitions, the load address is optional, +and defaults to the start address; the start address is also optional, +and defaults to the current value of the location counter). + +If the @code{NOCROSSREFS} keyword is used, and there any references +among the sections, the linker will report an error. Since the sections +all run at the same address, it normally does not make sense for one +section to refer directly to another. @xref{Miscellaneous Commands, +NOCROSSREFS}. + +For each section within the @code{OVERLAY}, the linker automatically +defines two symbols. The symbol @code{__load_start_@var{secname}} is +defined as the starting load address of the section. The symbol +@code{__load_stop_@var{secname}} is defined as the final load address of +the section. Any characters within @var{secname} which are not legal +within C identifiers are removed. C (or assembler) code may use these +symbols to move the overlaid sections around as necessary. + +At the end of the overlay, the value of the location counter is set to +the start address of the overlay plus the size of the largest section. + +Here is an example. Remember that this would appear inside a +@code{SECTIONS} construct. +@smallexample +@group + OVERLAY 0x1000 : AT (0x4000) + @{ + .text0 @{ o1/*.o(.text) @} + .text1 @{ o2/*.o(.text) @} + @} +@end group +@end smallexample +@noindent +This will define both @samp{.text0} and @samp{.text1} to start at +address 0x1000. @samp{.text0} will be loaded at address 0x4000, and +@samp{.text1} will be loaded immediately after @samp{.text0}. The +following symbols will be defined: @code{__load_start_text0}, +@code{__load_stop_text0}, @code{__load_start_text1}, +@code{__load_stop_text1}. + +C code to copy overlay @code{.text1} into the overlay area might look +like the following. + +@smallexample +@group + extern char __load_start_text1, __load_stop_text1; + memcpy ((char *) 0x1000, &__load_start_text1, + &__load_stop_text1 - &__load_start_text1); +@end group +@end smallexample + +Note that the @code{OVERLAY} command is just syntactic sugar, since +everything it does can be done using the more basic commands. The above +example could have been written identically as follows. + +@smallexample +@group + .text0 0x1000 : AT (0x4000) @{ o1/*.o(.text) @} + __load_start_text0 = LOADADDR (.text0); + __load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0); + .text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) @{ o2/*.o(.text) @} + __load_start_text1 = LOADADDR (.text1); + __load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1); + . = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1)); +@end group +@end smallexample + +@node MEMORY +@section MEMORY command +@kindex MEMORY +@cindex memory regions +@cindex regions of memory +@cindex allocating memory +@cindex discontinuous memory +The linker's default configuration permits allocation of all available +memory. You can override this by using the @code{MEMORY} command. + +The @code{MEMORY} command describes the location and size of blocks of +memory in the target. You can use it to describe which memory regions +may be used by the linker, and which memory regions it must avoid. You +can then assign sections to particular memory regions. The linker will +set section addresses based on the memory regions, and will warn about +regions that become too full. The linker will not shuffle sections +around to fit into the available regions. + +A linker script may contain at most one use of the @code{MEMORY} +command. However, you can define as many blocks of memory within it as +you wish. The syntax is: +@smallexample +@group +MEMORY + @{ + @var{name} [(@var{attr})] : ORIGIN = @var{origin}, LENGTH = @var{len} + @dots{} + @} +@end group +@end smallexample + +The @var{name} is a name used in the linker script to refer to the +region. The region name has no meaning outside of the linker script. +Region names are stored in a separate name space, and will not conflict +with symbol names, file names, or section names. Each memory region +must have a distinct name. + +@cindex memory region attributes +The @var{attr} string is an optional list of attributes that specify +whether to use a particular memory region for an input section which is +not explicitly mapped in the linker script. As described in +@ref{SECTIONS}, if you do not specify an output section for some input +section, the linker will create an output section with the same name as +the input section. If you define region attributes, the linker will use +them to select the memory region for the output section that it creates. + +The @var{attr} string must consist only of the following characters: +@table @samp +@item R +Read-only section +@item W +Read/write section +@item X +Executable section +@item A +Allocatable section +@item I +Initialized section +@item L +Same as @samp{I} +@item ! +Invert the sense of any of the preceding attributes +@end table + +If a unmapped section matches any of the listed attributes other than +@samp{!}, it will be placed in the memory region. The @samp{!} +attribute reverses this test, so that an unmapped section will be placed +in the memory region only if it does not match any of the listed +attributes. + +@kindex ORIGIN = +@kindex o = +@kindex org = +The @var{origin} is an expression for the start address of the memory +region. The expression must evaluate to a constant before memory +allocation is performed, which means that you may not use any section +relative symbols. The keyword @code{ORIGIN} may be abbreviated to +@code{org} or @code{o} (but not, for example, @code{ORG}). + +@kindex LENGTH = +@kindex len = +@kindex l = +The @var{len} is an expression for the size in bytes of the memory +region. As with the @var{origin} expression, the expression must +evaluate to a constant before memory allocation is performed. The +keyword @code{LENGTH} may be abbreviated to @code{len} or @code{l}. + +In the following example, we specify that there are two memory regions +available for allocation: one starting at @samp{0} for 256 kilobytes, +and the other starting at @samp{0x40000000} for four megabytes. The +linker will place into the @samp{rom} memory region every section which +is not explicitly mapped into a memory region, and is either read-only +or executable. The linker will place other sections which are not +explicitly mapped into a memory region into the @samp{ram} memory +region. + +@smallexample +@group +MEMORY + @{ + rom (rx) : ORIGIN = 0, LENGTH = 256K + ram (!rx) : org = 0x40000000, l = 4M + @} +@end group +@end smallexample + +Once you define a memory region, you can direct the linker to place +specific output sections into that memory region by using the +@samp{>@var{region}} output section attribute. For example, if you have +a memory region named @samp{mem}, you would use @samp{>mem} in the +output section definition. @xref{Output Section Region}. If no address +was specified for the output section, the linker will set the address to +the next available address within the memory region. If the combined +output sections directed to a memory region are too large for the +region, the linker will issue an error message. + +@node PHDRS +@section PHDRS Command +@kindex PHDRS +@cindex program headers +@cindex ELF program headers +@cindex program segments +@cindex segments, ELF +The ELF object file format uses @dfn{program headers}, also knows as +@dfn{segments}. The program headers describe how the program should be +loaded into memory. You can print them out by using the @code{objdump} +program with the @samp{-p} option. + +When you run an ELF program on a native ELF system, the system loader +reads the program headers in order to figure out how to load the +program. This will only work if the program headers are set correctly. +This manual does not describe the details of how the system loader +interprets program headers; for more information, see the ELF ABI. + +The linker will create reasonable program headers by default. However, +in some cases, you may need to specify the program headers more +precisely. You may use the @code{PHDRS} command for this purpose. When +the linker sees the @code{PHDRS} command in the linker script, it will +not create any program headers other than the ones specified. + +The linker only pays attention to the @code{PHDRS} command when +generating an ELF output file. In other cases, the linker will simply +ignore @code{PHDRS}. + +This is the syntax of the @code{PHDRS} command. The words @code{PHDRS}, +@code{FILEHDR}, @code{AT}, and @code{FLAGS} are keywords. + +@smallexample +@group +PHDRS +@{ + @var{name} @var{type} [ FILEHDR ] [ PHDRS ] [ AT ( @var{address} ) ] + [ FLAGS ( @var{flags} ) ] ; +@} +@end group +@end smallexample + +The @var{name} is used only for reference in the @code{SECTIONS} command +of the linker script. It is not put into the output file. Program +header names are stored in a separate name space, and will not conflict +with symbol names, file names, or section names. Each program header +must have a distinct name. + +Certain program header types describe segments of memory which the +system loader will load from the file. In the linker script, you +specify the contents of these segments by placing allocatable output +sections in the segments. You use the @samp{:@var{phdr}} output section +attribute to place a section in a particular segment. @xref{Output +Section Phdr}. + +It is normal to put certain sections in more than one segment. This +merely implies that one segment of memory contains another. You may +repeat @samp{:@var{phdr}}, using it once for each segment which should +contain the section. + +If you place a section in one or more segments using @samp{:@var{phdr}}, +then the linker will place all subsequent allocatable sections which do +not specify @samp{:@var{phdr}} in the same segments. This is for +convenience, since generally a whole set of contiguous sections will be +placed in a single segment. You can use @code{:NONE} to override the +default segment and tell the linker to not put the section in any +segment at all. + +@kindex FILEHDR +@kindex PHDRS +You may use the @code{FILEHDR} and @code{PHDRS} keywords appear after +the program header type to further describe the contents of the segment. +The @code{FILEHDR} keyword means that the segment should include the ELF +file header. The @code{PHDRS} keyword means that the segment should +include the ELF program headers themselves. + +The @var{type} may be one of the following. The numbers indicate the +value of the keyword. + +@table @asis +@item @code{PT_NULL} (0) +Indicates an unused program header. + +@item @code{PT_LOAD} (1) +Indicates that this program header describes a segment to be loaded from +the file. + +@item @code{PT_DYNAMIC} (2) +Indicates a segment where dynamic linking information can be found. + +@item @code{PT_INTERP} (3) +Indicates a segment where the name of the program interpreter may be +found. + +@item @code{PT_NOTE} (4) +Indicates a segment holding note information. + +@item @code{PT_SHLIB} (5) +A reserved program header type, defined but not specified by the ELF +ABI. + +@item @code{PT_PHDR} (6) +Indicates a segment where the program headers may be found. + +@item @var{expression} +An expression giving the numeric type of the program header. This may +be used for types not defined above. +@end table + +You can specify that a segment should be loaded at a particular address +in memory by using an @code{AT} expression. This is identical to the +@code{AT} command used as an output section attribute (@pxref{Output +Section LMA}). The @code{AT} command for a program header overrides the +output section attribute. + +The linker will normally set the segment flags based on the sections +which comprise the segment. You may use the @code{FLAGS} keyword to +explicitly specify the segment flags. The value of @var{flags} must be +an integer. It is used to set the @code{p_flags} field of the program +header. + +Here is an example of @code{PHDRS}. This shows a typical set of program +headers used on a native ELF system. + +@example +@group +PHDRS +@{ + headers PT_PHDR PHDRS ; + interp PT_INTERP ; + text PT_LOAD FILEHDR PHDRS ; + data PT_LOAD ; + dynamic PT_DYNAMIC ; +@} + +SECTIONS +@{ + . = SIZEOF_HEADERS; + .interp : @{ *(.interp) @} :text :interp + .text : @{ *(.text) @} :text + .rodata : @{ *(.rodata) @} /* defaults to :text */ + @dots{} + . = . + 0x1000; /* move to a new page in memory */ + .data : @{ *(.data) @} :data + .dynamic : @{ *(.dynamic) @} :data :dynamic + @dots{} +@} +@end group +@end example + +@node VERSION +@section VERSION Command +@kindex VERSION @{script text@} +@cindex symbol versions +@cindex version script +@cindex versions of symbols +The linker supports symbol versions when using ELF. Symbol versions are +only useful when using shared libraries. The dynamic linker can use +symbol versions to select a specific version of a function when it runs +a program that may have been linked against an earlier version of the +shared library. + +You can include a version script directly in the main linker script, or +you can supply the version script as an implicit linker script. You can +also use the @samp{--version-script} linker option. + +The syntax of the @code{VERSION} command is simply +@smallexample +VERSION @{ version-script-commands @} +@end smallexample + +The format of the version script commands is identical to that used by +Sun's linker in Solaris 2.5. The version script defines a tree of +version nodes. You specify the node names and interdependencies in the +version script. You can specify which symbols are bound to which +version nodes, and you can reduce a specified set of symbols to local +scope so that they are not globally visible outside of the shared +library. + +The easiest way to demonstrate the version script language is with a few +examples. + +@smallexample +VERS_1.1 @{ + global: + foo1; + local: + old*; + original*; + new*; +@}; + +VERS_1.2 @{ + foo2; +@} VERS_1.1; + +VERS_2.0 @{ + bar1; bar2; +@} VERS_1.2; +@end smallexample + +This example version script defines three version nodes. The first +version node defined is @samp{VERS_1.1}; it has no other dependencies. +The script binds the symbol @samp{foo1} to @samp{VERS_1.1}. It reduces +a number of symbols to local scope so that they are not visible outside +of the shared library. + +Next, the version script defines node @samp{VERS_1.2}. This node +depends upon @samp{VERS_1.1}. The script binds the symbol @samp{foo2} +to the version node @samp{VERS_1.2}. + +Finally, the version script defines node @samp{VERS_2.0}. This node +depends upon @samp{VERS_1.2}. The scripts binds the symbols @samp{bar1} +and @samp{bar2} are bound to the version node @samp{VERS_2.0}. + +When the linker finds a symbol defined in a library which is not +specifically bound to a version node, it will effectively bind it to an +unspecified base version of the library. You can bind all otherwise +unspecified symbols to a given version node by using @samp{global: *} +somewhere in the version script. + +The names of the version nodes have no specific meaning other than what +they might suggest to the person reading them. The @samp{2.0} version +could just as well have appeared in between @samp{1.1} and @samp{1.2}. +However, this would be a confusing way to write a version script. + +When you link an application against a shared library that has versioned +symbols, the application itself knows which version of each symbol it +requires, and it also knows which version nodes it needs from each +shared library it is linked against. Thus at runtime, the dynamic +loader can make a quick check to make sure that the libraries you have +linked against do in fact supply all of the version nodes that the +application will need to resolve all of the dynamic symbols. In this +way it is possible for the dynamic linker to know with certainty that +all external symbols that it needs will be resolvable without having to +search for each symbol reference. + +The symbol versioning is in effect a much more sophisticated way of +doing minor version checking that SunOS does. The fundamental problem +that is being addressed here is that typically references to external +functions are bound on an as-needed basis, and are not all bound when +the application starts up. If a shared library is out of date, a +required interface may be missing; when the application tries to use +that interface, it may suddenly and unexpectedly fail. With symbol +versioning, the user will get a warning when they start their program if +the libraries being used with the application are too old. + +There are several GNU extensions to Sun's versioning approach. The +first of these is the ability to bind a symbol to a version node in the +source file where the symbol is defined instead of in the versioning +script. This was done mainly to reduce the burden on the library +maintainer. You can do this by putting something like: +@smallexample +__asm__(".symver original_foo,foo@@VERS_1.1"); +@end smallexample +@noindent +in the C source file. This renames the function @samp{original_foo} to +be an alias for @samp{foo} bound to the version node @samp{VERS_1.1}. +The @samp{local:} directive can be used to prevent the symbol +@samp{original_foo} from being exported. + +The second GNU extension is to allow multiple versions of the same +function to appear in a given shared library. In this way you can make +an incompatible change to an interface without increasing the major +version number of the shared library, while still allowing applications +linked against the old interface to continue to function. + +To do this, you must use multiple @samp{.symver} directives in the +source file. Here is an example: + +@smallexample +__asm__(".symver original_foo,foo@@"); +__asm__(".symver old_foo,foo@@VERS_1.1"); +__asm__(".symver old_foo1,foo@@VERS_1.2"); +__asm__(".symver new_foo,foo@@@@VERS_2.0"); +@end smallexample + +In this example, @samp{foo@@} represents the symbol @samp{foo} bound to the +unspecified base version of the symbol. The source file that contains this +example would define 4 C functions: @samp{original_foo}, @samp{old_foo}, +@samp{old_foo1}, and @samp{new_foo}. + +When you have multiple definitions of a given symbol, there needs to be +some way to specify a default version to which external references to +this symbol will be bound. You can do this with the +@samp{foo@@@@VERS_2.0} type of @samp{.symver} directive. You can only +declare one version of a symbol as the default in this manner; otherwise +you would effectively have multiple definitions of the same symbol. + +If you wish to bind a reference to a specific version of the symbol +within the shared library, you can use the aliases of convenience +(i.e. @samp{old_foo}), or you can use the @samp{.symver} directive to +specifically bind to an external version of the function in question. + +@node Expressions +@section Expressions in Linker Scripts +@cindex expressions +@cindex arithmetic +The syntax for expressions in the linker script language is identical to +that of C expressions. All expressions are evaluated as integers. All +expressions are evaluated in the same size, which is 32 bits if both the +host and target are 32 bits, and is otherwise 64 bits. + +You can use and set symbol values in expressions. + +The linker defines several special purpose builtin functions for use in +expressions. + +@menu +* Constants:: Constants +* Symbols:: Symbol Names +* Location Counter:: The Location Counter +* Operators:: Operators +* Evaluation:: Evaluation +* Expression Section:: The Section of an Expression +* Builtin Functions:: Builtin Functions +@end menu + +@node Constants +@subsection Constants +@cindex integer notation +@cindex constants in linker scripts +All constants are integers. + +As in C, the linker considers an integer beginning with @samp{0} to be +octal, and an integer beginning with @samp{0x} or @samp{0X} to be +hexadecimal. The linker considers other integers to be decimal. + +@cindex scaled integers +@cindex K and M integer suffixes +@cindex M and K integer suffixes +@cindex suffixes for integers +@cindex integer suffixes +In addition, you can use the suffixes @code{K} and @code{M} to scale a +constant by +@c TEXI2ROFF-KILL +@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 period +and may include letters, digits, underscores, periods, and hyphens. +Unquoted symbol names must not conflict with any keywords. You can +specify a symbol which contains odd characters or has the same name as a +keyword by surrounding the symbol name in double quotes: +@smallexample + "SECTION" = 9; + "with a space" = "also with a space" + 10; +@end smallexample + +Since symbols can contain many non-alphabetic characters, it is safest +to delimit symbols with spaces. For example, @samp{A-B} is one symbol, +whereas @samp{A - B} is an expression involving subtraction. + +@node Location Counter +@subsection The Location Counter +@kindex . +@cindex dot +@cindex location counter +@cindex current output location +The special linker variable @dfn{dot} @samp{.} always contains the +current output location counter. Since the @code{.} always refers to a +location in an output section, it may only appear in an expression +within a @code{SECTIONS} command. The @code{.} symbol may appear +anywhere that an ordinary symbol is allowed in an expression. + +@cindex holes +Assigning a value to @code{.} will cause the location counter to be +moved. This may be used to create holes in the output section. The +location counter may never be moved backwards. + +@smallexample +SECTIONS +@{ + output : + @{ + file1(.text) + . = . + 1000; + file2(.text) + . += 1000; + file3(.text) + @} = 0x1234; +@} +@end smallexample +@noindent +In the previous example, the @samp{.text} section from @file{file1} is +located at the beginning of the output section @samp{output}. It is +followed by a 1000 byte gap. Then the @samp{.text} section from +@file{file2} appears, also with a 1000 byte gap following before the +@samp{.text} section from @file{file3}. The notation @samp{= 0x1234} +specifies what data to write in the gaps (@pxref{Output Section Fill}). + +@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{Assignments}. +@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{Assignments}. +} +@end iftex +@c END TEXI2ROFF-KILL + +@node Evaluation +@subsection Evaluation +@cindex lazy evaluation +@cindex expression evaluation order +The linker evaluates expressions lazily. It only computes the value of +an expression when absolutely necessary. + +The linker needs some information, such as the value of the start +address of the first section, and the origins and lengths of memory +regions, in order to do any linking at all. These values are computed +as soon as possible when the linker reads in the linker script. + +However, other values (such as symbol values) are not known or needed +until after storage allocation. Such values are evaluated later, when +other information (such as the sizes of output sections) is available +for use in the symbol assignment expression. + +The sizes of sections cannot be known until after allocation, so +assignments dependent upon these are not performed until after +allocation. + +Some expressions, such as those depending upon the location counter +@samp{.}, must be evaluated during section allocation. + +If the result of an expression is required, but the value is not +available, then an error results. For example, a script like the +following +@smallexample +@group +SECTIONS + @{ + .text 9+this_isnt_constant : + @{ *(.text) @} + @} +@end group +@end smallexample +@noindent +will cause the error message @samp{non constant expression for initial +address}. + +@node Expression Section +@subsection The Section of an Expression +@cindex expression sections +@cindex absolute expressions +@cindex relative expressions +@cindex absolute and relocatable symbols +@cindex relocatable and absolute symbols +@cindex symbols, relocatable and absolute +When the linker evaluates an expression, the result is either absolute +or relative to some section. A relative expression is expressed as a +fixed offset from the base of a section. + +The position of the expression within the linker script determines +whether it is absolute or relative. An expression which appears within +an output section definition is relative to the base of the output +section. An expression which appears elsewhere will be absolute. + +A symbol set to a relative expression will be relocatable if you request +relocatable output using the @samp{-r} option. That means that a +further link operation may change the value of the symbol. The symbol's +section will be the section of the relative expression. + +A symbol set to an absolute expression will retain the same value +through any further link operation. The symbol will be absolute, and +will not have any particular associated section. + +You can use the builtin function @code{ABSOLUTE} to force an expression +to be absolute when it would otherwise be relative. For example, to +create an absolute symbol set to the address of the end of the output +section @samp{.data}: +@smallexample +SECTIONS + @{ + .data : @{ *(.data) _edata = ABSOLUTE(.); @} + @} +@end smallexample +@noindent +If @samp{ABSOLUTE} were not used, @samp{_edata} would be relative to the +@samp{.data} section. + +@node Builtin Functions +@subsection Builtin Functions +@cindex functions in expressions +The linker script language includes a number of builtin functions for +use in linker script expressions. + +@table @code +@item ABSOLUTE(@var{exp}) +@kindex ABSOLUTE(@var{exp}) +@cindex expression, absolute +Return the absolute (non-relocatable, as opposed to non-negative) value +of the expression @var{exp}. Primarily useful to assign an absolute +value to a symbol within a section definition, where symbol values are +normally section relative. @xref{Expression Section}. + +@item ADDR(@var{section}) +@kindex ADDR(@var{section}) +@cindex section address in expression +Return the absolute address (the VMA) of the named @var{section}. Your +script must previously have defined the location of that section. In +the following example, @code{symbol_1} and @code{symbol_2} are assigned +identical values: +@smallexample +@group +SECTIONS @{ @dots{} + .output1 : + @{ + start_of_output_1 = ABSOLUTE(.); + @dots{} + @} + .output : + @{ + symbol_1 = ADDR(.output1); + symbol_2 = start_of_output_1; + @} +@dots{} @} +@end group +@end smallexample + +@item ALIGN(@var{exp}) +@kindex ALIGN(@var{exp}) +@cindex round up location counter +@cindex align location counter +Return the 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. Here is an example which aligns the output +@code{.data} section to the next @code{0x2000} byte boundary after the +preceding section and sets a variable within the section to the next +@code{0x8000} boundary after the input sections: +@smallexample +@group +SECTIONS @{ @dots{} + .data ALIGN(0x2000): @{ + *(.data) + variable = ALIGN(0x8000); + @} +@dots{} @} +@end group +@end smallexample +@noindent +The first use of @code{ALIGN} in this example specifies the location of +a section because it is used as the optional @var{address} attribute of +a section definition (@pxref{Output Section Address}). The second use +of @code{ALIGN} is used to defines the value of a symbol. + +The builtin function @code{NEXT} is closely related to @code{ALIGN}. + +@item BLOCK(@var{exp}) +@kindex BLOCK(@var{exp}) +This is a synonym for @code{ALIGN}, for compatibility with older linker +scripts. It is most often seen when setting the address of an output +section. + +@item DEFINED(@var{symbol}) +@kindex DEFINED(@var{symbol}) +@cindex symbol defaults +Return 1 if @var{symbol} is in the linker global symbol table and is +defined, otherwise return 0. You can use this function to provide +default values for symbols. For example, the following script fragment +shows how to set a global symbol @samp{begin} to the first location in +the @samp{.text} section---but if a symbol called @samp{begin} already +existed, its value is preserved: + +@smallexample +@group +SECTIONS @{ @dots{} + .text : @{ + begin = DEFINED(begin) ? begin : . ; + @dots{} + @} + @dots{} +@} +@end group +@end smallexample + +@item LOADADDR(@var{section}) +@kindex LOADADDR(@var{section}) +@cindex section load address in expression +Return the absolute LMA of the named @var{section}. This is normally +the same as @code{ADDR}, but it may be different if the @code{AT} +attribute is used in the output section definition (@pxref{Output +Section LMA}). + +@kindex MAX +@item MAX(@var{exp1}, @var{exp2}) +Returns the maximum of @var{exp1} and @var{exp2}. + +@kindex MIN +@item MIN(@var{exp1}, @var{exp2}) +Returns the minimum of @var{exp1} and @var{exp2}. + +@item NEXT(@var{exp}) +@kindex NEXT(@var{exp}) +@cindex unallocated address, next +Return the next unallocated address that is a multiple of @var{exp}. +This function is closely related to @code{ALIGN(@var{exp})}; unless you +use the @code{MEMORY} command to define discontinuous memory for the +output file, the two functions are equivalent. + +@item SIZEOF(@var{section}) +@kindex SIZEOF(@var{section}) +@cindex section size +Return the size in bytes of the named @var{section}, if that section has +been allocated. If the section has not been allocated when this is +evaluated, the linker will report an error. In the following example, +@code{symbol_1} and @code{symbol_2} are assigned identical values: +@smallexample +@group +SECTIONS@{ @dots{} + .output @{ + .start = . ; + @dots{} + .end = . ; + @} + symbol_1 = .end - .start ; + symbol_2 = SIZEOF(.output); +@dots{} @} +@end group +@end smallexample + +@item SIZEOF_HEADERS +@itemx sizeof_headers +@kindex SIZEOF_HEADERS +@cindex header size +Return the size in bytes of the output file's headers. This is +information which appears at the start of the output file. You can use +this number when setting the start address of the first section, if you +choose, to facilitate paging. + +@cindex not enough room for program headers +@cindex program headers, not enough room +When producing an ELF output file, if the linker script uses the +@code{SIZEOF_HEADERS} builtin function, the linker must compute the +number of program headers before it has determined all the section +addresses and sizes. If the linker later discovers that it needs +additional program headers, it will report an error @samp{not enough +room for program headers}. To avoid this error, you must avoid using +the @code{SIZEOF_HEADERS} function, or you must rework your linker +script to avoid forcing the linker to use additional program headers, or +you must define the program headers yourself using the @code{PHDRS} +command (@pxref{PHDRS}). +@end table + +@node Implicit Linker Scripts +@section Implicit Linker Scripts +@cindex implicit linker scripts +If you specify a linker input file which the linker can not recognize as +an object file or an archive file, it will try to read the file as a +linker script. If the file can not be parsed as a linker script, the +linker will report an error. + +An implicit linker script will not replace the default linker script. + +Typically an implicit linker script would contain only symbol +assignments, or the @code{INPUT}, @code{GROUP}, or @code{VERSION} +commands. + +Any input files read because of an implicit linker script will be read +at the position in the command line where the implicit linker script was +read. This can affect archive searching. + +@ifset GENERIC +@node Machine Dependent +@chapter Machine Dependent Features + +@cindex machine dependencies +@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 +* ARM:: @code{ld} and the ARM 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 GENERIC +@raisesections +@end ifclear + +@node ARM +@section @code{ld}'s support for interworking between ARM and Thumb code + +@cindex ARM interworking support +@cindex --support-old-code +For the ARM, @code{ld} will generate code stubs to allow functions calls +betweem ARM and Thumb code. These stubs only work with code that has +been compiled and assembled with the @samp{-mthumb-interwork} command +line option. If it is necessary to link with old ARM object files or +libraries, which have not been compiled with the -mthumb-interwork +option then the @samp{--support-old-code} command line switch should be +given to the linker. This will make it generate larger stub functions +which will work with non-interworking aware ARM code. Note, however, +the linker does not support generating stubs for function calls to +non-interworking aware Thumb code. + +@ifclear GENERIC +@lowersections +@end ifclear + +@ifclear SingleFormat +@node BFD +@chapter BFD + +@cindex back end +@cindex object file management +@cindex object formats available +@kindex objdump -i +The linker accesses object and archive files using the BFD libraries. +These libraries allow the linker to use the same routines to operate on +object files whatever the object file format. A different object file +format can be supported simply by creating a new BFD back end and adding +it to the library. To conserve runtime memory, however, the linker and +associated tools are usually configured to support only a subset of the +object file formats available. You can use @code{objdump -i} +(@pxref{objdump,,objdump,binutils.info,The GNU Binary Utilities}) to +list all the formats available for your configuration. + +@cindex BFD requirements +@cindex requirements for BFD +As with most implementations, BFD is a compromise between +several conflicting requirements. The major factor influencing +BFD design was efficiency: any time used converting between +formats is time which would not have been spent had BFD not +been involved. This is partly offset by abstraction payback; since +BFD simplifies applications and back ends, more time and care +may be spent optimizing algorithms for a greater speed. + +One minor artifact of the BFD solution which you should bear in +mind is the potential for information loss. There are two places where +useful information can be lost using the BFD mechanism: during +conversion and during output. @xref{BFD information loss}. + +@menu +* BFD outline:: How it works: an outline of BFD +@end menu + +@node BFD outline +@section How it works: an outline of BFD +@cindex opening object files +@include bfdsumm.texi +@end ifclear + +@node Reporting Bugs +@chapter Reporting Bugs +@cindex bugs in @code{ld} +@cindex reporting bugs in @code{ld} + +Your bug reports play an essential role in making @code{ld} reliable. + +Reporting a bug may help you by bringing a solution to your problem, or +it may not. But in any case the principal function of a bug report is +to help the entire community by making the next version of @code{ld} +work better. Bug reports are your contribution to the maintenance of +@code{ld}. + +In order for a bug report to serve its purpose, you must include the +information that enables us to fix the bug. + +@menu +* Bug Criteria:: Have you found a bug? +* Bug Reporting:: How to report bugs +@end menu + +@node Bug Criteria +@section Have you found a bug? +@cindex bug criteria + +If you are not sure whether you have found a bug, here are some guidelines: + +@itemize @bullet +@cindex fatal signal +@cindex linker crash +@cindex crash of linker +@item +If the linker gets a fatal signal, for any input whatever, that is a +@code{ld} bug. Reliable linkers never crash. + +@cindex error on valid input +@item +If @code{ld} produces an error message for valid input, that is a bug. + +@cindex invalid input +@item +If @code{ld} does not produce an error message for invalid input, that +may be a bug. In the general case, the linker can not verify that +object files are correct. + +@item +If you are an experienced user of linkers, your suggestions for +improvement of @code{ld} are welcome in any case. +@end itemize + +@node Bug Reporting +@section How to report bugs +@cindex bug reports +@cindex @code{ld} bugs, reporting + +A number of companies and individuals offer support for @sc{gnu} +products. If you obtained @code{ld} from a support organization, we +recommend you contact that organization first. + +You can find contact information for many support companies and +individuals in the file @file{etc/SERVICE} in the @sc{gnu} Emacs +distribution. + +Otherwise, send bug reports for @code{ld} to +@samp{bug-gnu-utils@@gnu.org}. + +The fundamental principle of reporting bugs usefully is this: +@strong{report all the facts}. If you are not sure whether to state a +fact or leave it out, state it! + +Often people omit facts because they think they know what causes the +problem and assume that some details do not matter. Thus, you might +assume that the name of a symbol you use in an example does not matter. +Well, probably it does not, but one cannot be sure. Perhaps the bug is +a stray memory reference which happens to fetch from the location where +that name is stored in memory; perhaps, if the name were different, the +contents of that location would fool the linker into doing the right +thing despite the bug. Play it safe and give a specific, complete +example. That is the easiest thing for you to do, and the most helpful. + +Keep in mind that the purpose of a bug report is to enable us to fix the bug if +it is new to us. Therefore, always write your bug reports on the assumption +that the bug has not been reported previously. + +Sometimes people give a few sketchy facts and ask, ``Does this ring a +bell?'' Those bug reports are useless, and we urge everyone to +@emph{refuse to respond to them} except to chide the sender to report +bugs properly. + +To enable us to fix the bug, you should include all these things: + +@itemize @bullet +@item +The version of @code{ld}. @code{ld} announces it if you start it with +the @samp{--version} argument. + +Without this, we will not know whether there is any point in looking for +the bug in the current version of @code{ld}. + +@item +Any patches you may have applied to the @code{ld} source, including any +patches made to the @code{BFD} library. + +@item +The type of machine you are using, and the operating system name and +version number. + +@item +What compiler (and its version) was used to compile @code{ld}---e.g. +``@code{gcc-2.7}''. + +@item +The command arguments you gave the linker to link your example and +observe the bug. To guarantee you will not omit something important, +list them all. A copy of the Makefile (or the output from make) is +sufficient. + +If we were to try to guess the arguments, we would probably guess wrong +and then we might not encounter the bug. + +@item +A complete input file, or set of input files, that will reproduce the +bug. It is generally most helpful to send the actual object files, +uuencoded if necessary to get them through the mail system. Making them +available for anonymous FTP is not as good, but may be the only +reasonable choice for large object files. + +If the source files were assembled using @code{gas} or compiled using +@code{gcc}, then it may be OK to send the source files rather than the +object files. In this case, be sure to say exactly what version of +@code{gas} or @code{gcc} was used to produce the object files. Also say +how @code{gas} or @code{gcc} were configured. + +@item +A description of what behavior you observe that you believe is +incorrect. For example, ``It gets a fatal signal.'' + +Of course, if the bug is that @code{ld} gets a fatal signal, then we +will certainly notice it. But if the bug is incorrect output, we might +not notice unless it is glaringly wrong. You might as well not give us +a chance to make a mistake. + +Even if the problem you experience is a fatal signal, you should still +say so explicitly. Suppose something strange is going on, such as, your +copy of @code{ld} is out of synch, or you have encountered a bug in the +C library on your system. (This has happened!) Your copy might crash +and ours would not. If you told us to expect a crash, then when ours +fails to crash, we would know that the bug was not happening for us. If +you had not told us to expect a crash, then we would not be able to draw +any conclusion from our observations. + +@item +If you wish to suggest changes to the @code{ld} source, send us context +diffs, as generated by @code{diff} with the @samp{-u}, @samp{-c}, or +@samp{-p} option. Always send diffs from the old file to the new file. +If you even discuss something in the @code{ld} source, refer to it by +context, not by line number. + +The line numbers in our development sources will not match those in your +sources. Your line numbers would convey no useful information to us. +@end itemize + +Here are some things that are not necessary: + +@itemize @bullet +@item +A description of the envelope of the bug. + +Often people who encounter a bug spend a lot of time investigating +which changes to the input file will make the bug go away and which +changes will not affect it. + +This is often time consuming and not very useful, because the way we +will find the bug is by running a single example under the debugger +with breakpoints, not by pure deduction from a series of examples. +We recommend that you save your time for something else. + +Of course, if you can find a simpler example to report @emph{instead} +of the original one, that is a convenience for us. Errors in the +output will be easier to spot, running under the debugger will take +less time, and so on. + +However, simplification is not vital; if you do not want to do this, +report the bug anyway and send us the entire test case you used. + +@item +A patch for the bug. + +A patch for the bug does help us if it is a good one. But do not omit +the necessary information, such as the test case, on the assumption that +a patch is all we need. We might see problems with your patch and decide +to fix the problem another way, or we might not understand it at all. + +Sometimes with a program as complicated as @code{ld} it is very hard to +construct an example that will make the program follow a certain path +through the code. If you do not send us the example, we will not be +able to construct one, so we will not be able to verify that the bug is +fixed. + +And if we cannot understand what bug you are trying to fix, or why your +patch should be an improvement, we will not install it. A test case will +help us to understand. + +@item +A guess about what the bug is or what it depends on. + +Such guesses are usually wrong. Even we cannot guess right about such +things without first using the debugger to find the facts. +@end itemize + +@node MRI +@appendix MRI Compatible Script Files +@cindex MRI compatibility +To aid users making the transition to @sc{gnu} @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{Scripts}. 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 + + |