\input texinfo @setfilename ld.info @syncodeindex ky cp @include configdoc.texi @c (configdoc.texi is generated by the Makefile) @c @smallbook @ifinfo @format START-INFO-DIR-ENTRY * Ld: (ld). The GNU linker. END-INFO-DIR-ENTRY @end format @end ifinfo @ifinfo This file documents the GNU linker LD. Copyright (C) 1991, 1992, 1993, 1994 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions. @ignore Permission is granted to process this file through Tex and print the results, provided the printed document carries copying permission notice identical to this one except for the removal of this paragraph (this paragraph not being relevant to the printed manual). @end ignore @end ifinfo @iftex @finalout @setchapternewpage odd @settitle Using LD, the GNU linker @titlepage @title Using ld @subtitle The GNU linker @sp 1 @subtitle @code{ld} version 2 @subtitle January 1994 @author Steve Chamberlain and Roland Pesch @author Cygnus Support @page @tex {\parskip=0pt \hfill Cygnus Support\par \hfill steve\@cygnus.com, pesch\@cygnus.com\par \hfill {\it Using LD, the GNU linker}\par \hfill Edited by Jeffrey Osier (jeffrey\@cygnus.com)\par \hfill and Roland Pesch (pesch\@cygnus.com)\par } \global\parindent=0pt % Steve likes it this way. @end tex @vskip 0pt plus 1filll Copyright @copyright{} 1991, 1992, 1993, 1994 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 GNU linker ld. @menu * Overview:: Overview * Invocation:: Invocation * Commands:: Command Language @ifset GENERIC * Machine Dependent:: Machine Dependent Features @end ifset @ifclear GENERIC @ifset H8300 * H8/300:: ld and the H8/300 @end ifset @ifset Hitachi * Hitachi:: ld and other Hitachi micros @end ifset @ifset I960 * i960:: ld and the Intel 960 family @end ifset @end ifclear @ifclear SingleFormat * BFD:: BFD @end ifclear @c Following blank line required for remaining bug in makeinfo conds/menus * MRI:: MRI Compatible Script Files * Index:: Index @end menu @end ifinfo @node Overview @chapter Overview @cindex 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 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 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 Here is a summary of the options you can use on the @code{ld} command line: @c FIXME! -relax only avail h8/300, i960. Conditionals screwed in examples. @smallexample ld [ -o @var{output} ] @var{objfile}@dots{} [ -A@var{architecture} ] [ -b @var{input-format} ] [ -Bstatic ] [ -c @var{MRI-commandfile} ] [ -d | -dc | -dp ] [ -defsym @var{symbol}=@var{expression} ] [ -e @var{entry} ] [ -F ] [ -F @var{format} ] [ -format @var{input-format} ] [ -g ] [ -G @var{size} ] [ -help ] [ -i ] [ -l@var{archive} ] [ -L@var{searchdir} ] [ -M ] [ -Map @var{mapfile} ] [ -m @var{emulation} ] [ -N | -n ] [ -noinhibit-exec ] [ -oformat @var{output-format} ] [ -R @var{filename} ] [ -relax ] [ -retain-symbols-file @var{filename} ] [ -r | -Ur ] [ -S ] [ -s ] [ -sort-common ] [ -stats ] [ -T @var{commandfile} ] [ -Ttext @var{org} ] [ -Tdata @var{org} ] [ -Tbss @var{org} ] [ -t ] [ -traditional-format ] [ -u @var{symbol}] [-V] [-v] [ -version ] [ -warn-common ] [ -y @var{symbol} ] [ -X ] [-x ] @end smallexample This plethora of command-line options may seem intimidating, 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}: @example ld -o @var{output} /lib/crt0.o hello.o -lc @end example 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. @ifclear SingleFormat The exceptions---which may meaningfully be used more than once---are @samp{-A}, @samp{-b} (or its synonym @samp{-format}), @samp{-defsym}, @samp{-L}, @samp{-l}, @samp{-R}, and @samp{-u}. @end ifclear @ifset SingleFormat The exceptions---which may meaningfully be used more than once---are @samp{-A}, @samp{-defsym}, @samp{-L}, @samp{-l}, @samp{-R}, and @samp{-u}. @end ifset @cindex object files The list of object files to be linked together, shown as @var{objfile}@dots{}, may follow, precede, or be mixed in with command-line options, except that an @var{objfile} 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}. 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 @ifset I960 @cindex architectures @kindex -A@var{arch} @item -A@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} @cindex input format @item -b @var{input-format} @cindex 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}.) @w{@samp{-format @var{input-format}}} has the same effect, as does the script command @code{TARGET}. @xref{BFD}. You may want to use this option if you are linking files with an unusual binary format. You can also use @samp{-b} to switch formats explicitly (when linking object files of different formats), by including @samp{-b @var{input-format}} before each group of object files in a particular format. The default format is taken from the environment variable @code{GNUTARGET}. @ifset UsesEnvVars @xref{Environment}. @end ifset You can also define the input format from a script, using the command @code{TARGET}; see @ref{Option Commands}. @end ifclear @kindex -Bstatic @item -Bstatic Ignored. This option is accepted for command-line compatibility with the SunOS linker. @kindex -c @var{MRI-cmdfile} @cindex compatibility, MRI @item -c @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 @item -d @kindex -dc @itemx -dc @kindex -dp @itemx -dp These three options are equivalent; multiple forms are supported for compatibility with other linkers. They assign space to common symbols even if a relocatable output file is specified (with @samp{-r}). The script command @code{FORCE_COMMON_ALLOCATION} has the same effect. @xref{Option Commands}. @cindex symbols, from command line @kindex -defsym @var{symbol}=@var{exp} @item -defsym @var{symbol}=@var{expression} Create a global symbol in the output file, containing the absolute address given by @var{expression}. You may use this option as many times as necessary to define multiple symbols in the command line. A limited form of arithmetic is supported for the @var{expression} in this context: you may give a hexadecimal constant or the name of an existing symbol, or use @code{+} and @code{-} to add or subtract hexadecimal constants or symbols. If you need more elaborate expressions, consider using the linker command language from a script (@pxref{Assignment, , Assignment: Symbol Definitions}). @emph{Note:} there should be no white space between @var{symbol}, the equals sign (``@key{=}''), and @var{expression}. @cindex entry point, from command line @kindex -e @var{entry} @item -e @var{entry} Use @var{entry} as the explicit symbol for beginning execution of your program, rather than the default entry point. @xref{Entry Point}, for a discussion of defaults and other ways of specifying the entry point. @ifclear SingleFormat @kindex -F @item -F @itemx -F@var{format} Ignored. Some older linkers used this option throughout a compilation toolchain for specifying object-file format for both input and output object files. The mechanisms @code{ld} uses for this purpose (the @samp{-b} or @samp{-format} options for input files, @samp{-oformat} option or the @code{TARGET} command in linker scripts for output files, the @code{GNUTARGET} environment variable) are more flexible, but @code{ld} accepts the @samp{-F} option for compatibility with scripts written to call the old linker. @kindex -format @item -format @var{input-format} Synonym for @samp{-b @var{input-format}}. @end ifclear @kindex -g @item -g Ignored. Provided for compatibility with other tools. @kindex -G @cindex object size @item -G@var{value} @itemx -G @var{value} Set the maximum size of objects to be optimized using the GP register to @var{size} under MIPS ECOFF. Ignored for other object file formats. @cindex help @cindex usage @kindex -help @item -help Print a summary of the command-line options on the standard output and exit. @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} @item -l@var{ar} 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{ar}.a} for every @var{archive} specified. @cindex search directory, from cmd line @kindex -L@var{dir} @item -L@var{searchdir} @itemx -L @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. @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. @cindex link map @kindex -M @item -M Print (to the standard output) a link map---diagnostic information about where symbols are mapped by @code{ld}, and information on global common storage allocation. @cindex link map @kindex -Map @item -Map @var{mapfile} Print to the file @var{mapfile} a link map---diagnostic information about where symbols are mapped by @code{ld}, and information on global common storage allocation. @cindex emulation @kindex -m @var{emulation} @item -m@var{emulation} @itemx -m @var{emulation} Emulate the @var{emulation} linker. You can list the available emulations with the @samp{-V} option. The default depends on how your @code{ld} was configured. @kindex -N @cindex read/write from cmd line @kindex OMAGIC @item -N 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 -n @cindex read-only text @kindex NMAGIC @item -n Set the text segment to be read only, and mark the output as @code{NMAGIC} if possible. @item -noinhibit-exec @cindex output file after errors @kindex -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. @item -o @var{output} @kindex -o @var{output} @cindex naming the output file 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. @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 @item -R @var{filename} @kindex -R @var{file} @cindex symbol-only input 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. @kindex -relax @cindex synthesizing linker @cindex relaxing addressing modes @item -relax An option with machine dependent effects. @ifset GENERIC Currently this option is only supported on the H8/300 and the Intel 960. @end ifset @ifset H8300 @xref{H8/300,,@code{ld} and the H8/300}. @end ifset @ifset I960 @xref{i960,, @code{ld} and the Intel 960 family}. @end ifset On some platforms, the @samp{-relax} option performs global optimizations that become possible when the linker resolves addressing in the program, such as relaxing address modes and synthesizing new instructions in the output object file. @ifset GENERIC On platforms where this is not supported, @samp{-relax} is accepted, but ignored. @end ifset @item -retain-symbols-file @var{filename} @cindex retaining specified symbols @cindex stripping all but some symbols @cindex symbols, retaining selectively 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}. @cindex partial link @cindex relocatable output @kindex -r @item -r 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 -S @cindex strip debugger symbols @item -S Omit debugger symbol information (but not all symbols) from the output file. @kindex -s @cindex strip all symbols @item -s Omit all symbol information from the output file. @item -sort-common Normally, when @code{ld} places the global common symbols in the appropriate output sections, it sorts them by size. 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. This option disables that sorting. @item -stats Compute and display statistics about the operation of the linker, such as execution time and memory usage. @item -Tbss @var{org} @kindex -Tbss @var{org} @itemx -Tdata @var{org} @kindex -Tdata @var{org} @itemx -Ttext @var{org} @kindex -Ttext @var{org} @cindex segment origins, cmd line 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. @item -T @var{commandfile} @itemx -T@var{commandfile} @kindex -T @var{script} @cindex script files Read link commands from the file @var{commandfile}. These commands replace @code{ld}'s default link script (rather than adding to it), so @var{commandfile} must specify everything necessary to describe the target format. @xref{Commands}. If @var{commandfile} does not exist, @code{ld} looks for it in the directories specified by any preceding @samp{-L} options. Multiple @samp{-T} options accumulate. @kindex -t @cindex verbose @cindex input files, displaying @item -t Print the names of the input files as @code{ld} processes them. @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. @item -u @var{symbol} @kindex -u @var{symbol} @cindex undefined symbol Force @var{symbol} to be entered in the output file as an undefined symbol. Doing this may, for example, trigger linking of additional modules from standard libraries. @samp{-u} may be repeated with different option arguments to enter additional undefined symbols. @c Nice idea, but no such command: This option is equivalent @c to the @code{EXTERN} linker command. @kindex -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 @cindex version @item -V Display the version number for @code{ld} and list the linker emulations supported. Display which input files can and cannot be opened. @kindex -v @cindex version @item -v Display the version number for @code{ld}. @item -version @kindex -version Display the version number for @code{ld} and exit. @item -warn-common @kindex -warn-comon @cindex warnings, on combining symbols @cindex combining symbols, warnings on 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 -X @cindex local symbols, deleting @cindex L, deleting symbols beginning @item -X If @samp{-s} or @samp{-S} is also specified, delete only local symbols beginning with @samp{L}. @kindex -x @cindex deleting local symbols @item -x If @samp{-s} or @samp{-S} is also specified, delete all local symbols, not just those beginning with @samp{L}. @item -y @var{symbol} @kindex -y @var{symbol} @cindex symbol tracing 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. @end table @ifset UsesEnvVars @node Environment @section Environment Variables You can change the behavior of @code{ld} with the environment variable @code{GNUTARGET}. @kindex GNUTARGET @cindex default input format @code{GNUTARGET} determines the input-file object format if you don't use @samp{-b} (or its synonym @samp{-format}). Its value should be one of the BFD names for an input format (@pxref{BFD}). If there is no @code{GNUTARGET} in the environment, @code{ld} uses the natural format of the target. If @code{GNUTARGET} is set to @code{default} then BFD attempts to discover the input format by examining binary input files; this method often succeeds, but there are potential ambiguities, since there is no method of ensuring that the magic number used to specify object-file formats is unique. However, the configuration procedure for BFD on each system places the conventional format for that system first in the search-list, so ambiguities are resolved in favor of convention. @end ifset @node Commands @chapter Command Language @cindex command files The command language provides explicit control over the link process, allowing complete specification of the mapping between the linker's input files and its output. It controls: @itemize @bullet @item input files @item file formats @item output file layout @item addresses of sections @item placement of common blocks @end itemize You may supply a command file (also known as a link script) to the linker either explicitly through the @samp{-T} option, or implicitly as an ordinary file. If the linker opens a file which it cannot recognize as a supported object or archive format, it reports an error. @menu * Scripts:: Linker Scripts * Expressions:: Expressions * MEMORY:: MEMORY Command * SECTIONS:: SECTIONS Command * Entry Point:: The Entry Point * Option Commands:: Option Commands @end menu @node Scripts @section Linker Scripts The @code{ld} command language is a collection of statements; some are simple keywords setting a particular option, some are used to select and group input files or name output files; and two statement types have a fundamental and pervasive impact on the linking process. @cindex fundamental script commands @cindex commands, fundamental @cindex output file layout @cindex layout of output file The most fundamental command of the @code{ld} command language is the @code{SECTIONS} command (@pxref{SECTIONS}). Every meaningful command script must have a @code{SECTIONS} command: it specifies a ``picture'' of the output file's layout, in varying degrees of detail. No other command is required in all cases. The @code{MEMORY} command complements @code{SECTIONS} by describing the available memory in the target architecture. This command is optional; if you don't use a @code{MEMORY} command, @code{ld} assumes sufficient memory is available in a contiguous block for all output. @xref{MEMORY}. @cindex comments You may include comments in linker scripts just as in C: delimited by @samp{/*} and @samp{*/}. As in C, comments are syntactically equivalent to whitespace. @node Expressions @section Expressions @cindex expression syntax @cindex arithmetic Many useful commands involve arithmetic expressions. The syntax for expressions in the command language is identical to that of C expressions, with the following features: @itemize @bullet @item All expressions evaluated as integers and are of ``long'' or ``unsigned long'' type. @item All constants are integers. @item All of the C arithmetic operators are provided. @item You may reference, define, and create global variables. @item You may call special purpose built-in functions. @end itemize @menu * Integers:: Integers * Symbols:: Symbol Names * Location Counter:: The Location Counter * Operators:: Operators * Evaluation:: Evaluation * Assignment:: Assignment: Defining Symbols * Arithmetic Functions:: Built-In Functions @end menu @node Integers @subsection Integers @cindex integer notation @cindex octal integers An octal integer is @samp{0} followed by zero or more of the octal digits (@samp{01234567}). @example _as_octal = 0157255; @end example @cindex decimal integers A decimal integer starts with a non-zero digit followed by zero or more digits (@samp{0123456789}). @example _as_decimal = 57005; @end example @cindex hexadecimal integers @kindex 0x A hexadecimal integer is @samp{0x} or @samp{0X} followed by one or more hexadecimal digits chosen from @samp{0123456789abcdefABCDEF}. @example _as_hex = 0xdead; @end example @cindex negative integers To write a negative integer, use the prefix operator @samp{-}; @pxref{Operators}. @example _as_neg = -57005; @end example @cindex scaled integers @cindex K and M integer suffixes @cindex M and K integer suffixes @cindex suffixes for integers @cindex integer suffixes Additionally the suffixes @code{K} and @code{M} may be used to scale a constant by @c TEXI2ROFF-KILL @ifinfo @c END TEXI2ROFF-KILL @code{1024} or @code{1024*1024} @c TEXI2ROFF-KILL @end ifinfo @tex ${\rm 1024}$ or ${\rm 1024}^2$ @end tex @c END TEXI2ROFF-KILL respectively. For example, the following all refer to the same quantity: @example _fourk_1 = 4K; _fourk_2 = 4096; _fourk_3 = 0x1000; @end example @node Symbols @subsection Symbol Names @cindex symbol names @cindex names @cindex quoted symbol names @kindex " Unless quoted, symbol names start with a letter, underscore, or point and may include any letters, underscores, digits, points, and hyphens. Unquoted symbol names must not conflict with any keywords. You can specify a symbol which contains odd characters or has the same name as a keyword, by surrounding the symbol name in double quotes: @example "SECTION" = 9; "with a space" = "also with a space" + 10; @end example Since symbols can contain many non-alphabetic characters, it is safest to delimit symbols with spaces. For example, @samp{A-B} is one symbol, whereas @samp{A - B} is an expression involving subtraction. @node Location Counter @subsection The Location Counter @kindex . @cindex dot @cindex location counter @cindex current output location The special linker variable @dfn{dot} @samp{.} always contains the current output location counter. Since the @code{.} always refers to a location in an output section, it must always appear in an expression within a @code{SECTIONS} command. The @code{.} symbol may appear anywhere that an ordinary symbol is allowed in an expression, but its assignments have a side effect. Assigning a value to the @code{.} symbol will cause the location counter to be moved. @cindex holes This may be used to create holes in the output section. The location counter may never be moved backwards. @example SECTIONS @{ output : @{ file1(.text) . = . + 1000; file2(.text) . += 1000; file3(.text) @} = 0x1234; @} @end example @noindent In the previous example, @code{file1} is located at the beginning of the output section, then there is a 1000 byte gap. Then @code{file2} appears, also with a 1000 byte gap following before @code{file3} is loaded. The notation @samp{= 0x1234} specifies what data to write in the gaps (@pxref{Section Options}). @iftex @vfill @end iftex @need 5000 @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 @example 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 example Notes: (1) Prefix operators (2) @xref{Assignment} @c TEXI2ROFF-KILL @end ifinfo @tex \vskip \baselineskip %"lispnarrowing" is the extra indent used generally for @example \hskip\lispnarrowing\vbox{\offinterlineskip \hrule \halign {\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ {\tt #}\ \hfil&\vrule#\cr height2pt&\omit&&\omit&&\omit&\cr &Precedence&& Associativity &&{\rm Operators}&\cr height2pt&\omit&&\omit&&\omit&\cr \noalign{\hrule} height2pt&\omit&&\omit&&\omit&\cr &highest&&&&&\cr % '176 is tilde, '~' in tt font &1&&left&&\qquad- \char'176\ !\qquad\dag&\cr &2&&left&&* / \%&\cr &3&&left&&+ -&\cr &4&&left&&>> <<&\cr &5&&left&&== != > < <= >=&\cr &6&&left&&\&&\cr &7&&left&&|&\cr &8&&left&&{\&\&}&\cr &9&&left&&||&\cr &10&&right&&? :&\cr &11&&right&&\qquad\&= += -= *= /=\qquad\ddag&\cr &lowest&&&&&\cr height2pt&\omit&&\omit&&\omit&\cr} \hrule} @end tex @iftex { @obeylines@parskip=0pt@parindent=0pt @dag@quad Prefix operators. @ddag@quad @xref{Assignment}. } @end iftex @c END TEXI2ROFF-KILL @node Evaluation @subsection Evaluation @cindex lazy evaluation @cindex expression evaluation order The linker uses ``lazy evaluation'' for expressions; it only calculates an expression when absolutely necessary. The linker needs the value of the start address, and the lengths of memory regions, in order to do any linking at all; these values are computed as soon as possible when the linker reads in the command file. However, other values (such as symbol values) are not known or needed until after storage allocation. Such values are evaluated later, when other information (such as the sizes of output sections) is available for use in the symbol assignment expression. @node Assignment @subsection Assignment: Defining Symbols @cindex assignment in scripts @cindex symbol definition, scripts @cindex variables, defining You may create global symbols, and assign values (addresses) to global symbols, using any of the C assignment operators: @table @code @item @var{symbol} = @var{expression} ; @itemx @var{symbol} &= @var{expression} ; @itemx @var{symbol} += @var{expression} ; @itemx @var{symbol} -= @var{expression} ; @itemx @var{symbol} *= @var{expression} ; @itemx @var{symbol} /= @var{expression} ; @end table Two things distinguish assignment from other operators in @code{ld} expressions. @itemize @bullet @item Assignment may only be used at the root of an expression; @samp{a=b+3;} is allowed, but @samp{a+b=3;} is an error. @kindex ; @cindex semicolon @item You must place a trailing semicolon (``@key{;}'') at the end of an assignment statement. @end itemize Assignment statements may appear: @itemize @bullet @item as commands in their own right in an @code{ld} script; or @item as independent statements within a @code{SECTIONS} command; or @item as part of the contents of a section definition in a @code{SECTIONS} command. @end itemize The first two cases are equivalent in effect---both define a symbol with an absolute address. The last case defines a symbol whose address is relative to a particular section (@pxref{SECTIONS}). @cindex absolute and relocatable symbols @cindex relocatable and absolute symbols @cindex symbols, relocatable and absolute When a linker expression is evaluated and assigned to a variable, it is given either an absolute or a relocatable type. An absolute expression type is one in which the symbol contains the value that it will have in the output file; a relocatable expression type is one in which the value is expressed as a fixed offset from the base of a section. The type of the expression is controlled by its position in the script file. A symbol assigned within a section definition is created relative to the base of the section; a symbol assigned in any other place is created as an absolute symbol. Since a symbol created within a section definition is relative to the base of the section, it will remain relocatable if relocatable output is requested. A symbol may be created with an absolute value even when assigned to within a section definition by using the absolute assignment function @code{ABSOLUTE}. For example, to create an absolute symbol whose address is the last byte of an output section named @code{.data}: @example SECTIONS@{ @dots{} .data : @{ *(.data) _edata = ABSOLUTE(.) ; @} @dots{} @} @end example The linker tries to put off the evaluation of an assignment until all the terms in the source expression are known (@pxref{Evaluation}). For instance, the sizes of sections cannot be known until after allocation, so assignments dependent upon these are not performed until after allocation. Some expressions, such as those depending upon the location counter @dfn{dot}, @samp{.} must be evaluated during allocation. If the result of an expression is required, but the value is not available, then an error results. For example, a script like the following @example SECTIONS @{ @dots{} text 9+this_isnt_constant : @{ @dots{} @} @dots{} @} @end example @kindex Non constant expression @noindent will cause the error message ``@code{Non constant expression for initial address}''. @node Arithmetic Functions @subsection Arithmetic Functions @cindex functions in expression language The command language includes a number of built-in functions for use in link script expressions. @table @code @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. @item ADDR(@var{section}) @kindex ADDR(@var{section}) @cindex section address Return the absolute address of the named @var{section}. Your script must previously have defined the location of that section. In the following example, @code{symbol_1} and @code{symbol_2} are assigned identical values: @example SECTIONS@{ @dots{} .output1 : @{ start_of_output_1 = ABSOLUTE(.); @dots{} @} .output : @{ symbol_1 = ADDR(.output1); symbol_2 = start_of_output_1; @} @dots{} @} @end example @item ALIGN(@var{exp}) @kindex ALIGN(@var{exp}) @cindex rounding up location counter Return the result of the current location counter (@code{.}) aligned to the next @var{exp} boundary. @var{exp} must be an expression whose value is a power of two. This is equivalent to @example (. + @var{exp} - 1) & ~(@var{exp} - 1) @end example @code{ALIGN} doesn't change the value of the location counter---it just does arithmetic on it. As an example, to align the output @code{.data} section to the next @code{0x2000} byte boundary after the preceding section and to set a variable within the section to the next @code{0x8000} boundary after the input sections: @example SECTIONS@{ @dots{} .data ALIGN(0x2000): @{ *(.data) variable = ALIGN(0x8000); @} @dots{} @} @end example @noindent The first use of @code{ALIGN} in this example specifies the location of a section because it is used as the optional @var{start} attribute of a section definition (@pxref{Section Options}). The second use simply defines the value of a variable. The built-in @code{NEXT} is closely related to @code{ALIGN}. @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 command-file fragment shows how to set a global symbol @code{begin} to the first location in the @code{.text} section---but if a symbol called @code{begin} already existed, its value is preserved: @smallexample SECTIONS@{ @dots{} .text : @{ begin = DEFINED(begin) ? begin : . ; @dots{} @} @dots{} @} @end smallexample @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. In the following example, @code{symbol_1} and @code{symbol_2} are assigned identical values: @c What does it return if the section hasn't been allocated? 0? @example SECTIONS@{ @dots{} .output @{ .start = . ; @dots{} .end = . ; @} symbol_1 = .end - .start ; symbol_2 = SIZEOF(.output); @dots{} @} @end example @item SIZEOF_HEADERS @kindex SIZEOF_HEADERS @cindex header size @itemx sizeof_headers @kindex sizeof_headers Return the size in bytes of the output file's headers. You can use this number as the start address of the first section, if you choose, to facilitate paging. @end table @node MEMORY @section Memory Layout @kindex MEMORY @cindex regions of memory @cindex discontinuous memory @cindex allocating memory The linker's default configuration permits allocation of all available memory. You can override this configuration by using the @code{MEMORY} command. The @code{MEMORY} command describes the location and size of blocks of memory in the target. By using it carefully, you can describe which memory regions may be used by the linker, and which memory regions it must avoid. The linker does not shuffle sections to fit into the available regions, but does move the requested sections into the correct regions and issue errors when the regions become too full. A command file may contain at most one use of the @code{MEMORY} command; however, you can define as many blocks of memory within it as you wish. The syntax is: @example MEMORY @{ @var{name} (@var{attr}) : ORIGIN = @var{origin}, LENGTH = @var{len} @dots{} @} @end example @table @code @cindex naming memory regions @item @var{name} is a name used internally by the linker to refer to the region. Any symbol name may be used. The region names are stored in a separate name space, and will not conflict with symbols, file names or section names. Use distinct names to specify multiple regions. @cindex memory region attributes @item (@var{attr}) is an optional list of attributes, permitted for compatibility with the AT&T linker but not used by @code{ld} beyond checking that the attribute list is valid. Valid attribute lists must be made up of the characters ``@code{LIRWX}''. If you omit the attribute list, you may omit the parentheses around it as well. @kindex ORIGIN = @kindex o = @kindex org = @item @var{origin} is the start address of the region in physical memory. It is an expression that must evaluate to a constant before memory allocation is performed. The keyword @code{ORIGIN} may be abbreviated to @code{org} or @code{o} (but not, for example, @samp{ORG}). @kindex LENGTH = @kindex len = @kindex l = @item @var{len} is the size in bytes of the region (an expression). The keyword @code{LENGTH} may be abbreviated to @code{len} or @code{l}. @end table For example, to specify that memory has two regions available for allocation---one starting at 0 for 256 kilobytes, and the other starting at @code{0x40000000} for four megabytes: @example MEMORY @{ rom : ORIGIN = 0, LENGTH = 256K ram : org = 0x40000000, l = 4M @} @end example Once you have defined a region of memory named @var{mem}, you can direct specific output sections there by using a command ending in @samp{>@var{mem}} within the @code{SECTIONS} command (@pxref{Section Options}). If the combined output sections directed to a region are too big for the region, the linker will issue an error message. @node SECTIONS @section Specifying Output Sections @kindex SECTIONS The @code{SECTIONS} command controls exactly where input sections are placed into output sections, their order in the output file, and to which output sections they are allocated. You may use at most one @code{SECTIONS} command in a script file, but you can have as many statements within it as you wish. Statements within the @code{SECTIONS} command can do one of three things: @itemize @bullet @item define the entry point; @item assign a value to a symbol; @item describe the placement of a named output section, and which input sections go into it. @end itemize You can also use the first two operations---defining the entry point and defining symbols---outside the @code{SECTIONS} command: @pxref{Entry Point}, and @pxref{Assignment}. They are permitted here as well for your convenience in reading the script, so that symbols and the entry point can be defined at meaningful points in your output-file layout. If you do not use a @code{SECTIONS} command, the linker places each input section into an identically named output section in the order that the sections are first encountered in the input files. If all input sections are present in the first file, for example, the order of sections in the output file will match the order in the first input file. @menu * Section Definition:: Section Definitions * Section Placement:: Section Placement * Section Data Expressions:: Section Data Expressions * Section Options:: Optional Section Attributes @end menu @node Section Definition @subsection Section Definitions @cindex section definition The most frequently used statement in the @code{SECTIONS} command is the @dfn{section definition}, which specifies the properties of an output section: its location, alignment, contents, fill pattern, and target memory region. Most of these specifications are optional; the simplest form of a section definition is @example SECTIONS @{ @dots{} @var{secname} : @{ @var{contents} @} @dots{} @} @end example @cindex naming output sections @noindent @var{secname} is the name of the output section, and @var{contents} a specification of what goes there---for example, a list of input files or sections of input files (@pxref{Section Placement}). As you might assume, the whitespace shown is optional. You do need the colon @samp{:} and the braces @samp{@{@}}, however. @var{secname} must meet the constraints of your output format. In formats which only support a limited number of sections, such as @code{a.out}, the name must be one of the names supported by the format (@code{a.out}, for example, allows only @code{.text}, @code{.data} or @code{.bss}). If the output format supports any number of sections, but with numbers and not names (as is the case for Oasys), the name should be supplied as a quoted numeric string. A section name may consist of any sequence of characters, but any name which does not conform to the standard @code{ld} symbol name syntax must be quoted. @xref{Symbols, , Symbol Names}. The linker will not create output sections which do not have any contents. This is for convenience when referring to input sections that may or may not exist. For example, @example .foo @{ *(.foo @} @end example will only create a @samp{.foo} section in the output file if there is a @samp{.foo} section in at least one input file. @node Section Placement @subsection Section Placement @cindex contents of a section In a section definition, you can specify the contents of an output section by listing particular input files, by listing particular input-file sections, or by a combination of the two. You can also place arbitrary data in the section, and define symbols relative to the beginning of the section. The @var{contents} of a section definition may include any of the following kinds of statement. You can include as many of these as you like in a single section definition, separated from one another by whitespace. @table @code @kindex @var{filename} @cindex input files, section defn @cindex files, including in output sections @item @var{filename} You may simply name a particular input file to be placed in the current output section; @emph{all} sections from that file are placed in the current section definition. If the file name has already been mentioned in another section definition, with an explicit section name list, then only those sections which have not yet been allocated are used. To specify a list of particular files by name: @example .data : @{ afile.o bfile.o cfile.o @} @end example @noindent The example also illustrates that multiple statements can be included in the contents of a section definition, since each file name is a separate statement. @kindex @var{filename}(@var{section}) @cindex files and sections, section defn @item @var{filename}( @var{section} ) @itemx @var{filename}( @var{section}, @var{section}, @dots{} ) @itemx @var{filename}( @var{section} @var{section} @dots{} ) You can name one or more sections from your input files, for insertion in the current output section. If you wish to specify a list of input-file sections inside the parentheses, you may separate the section names by either commas or whitespace. @cindex input sections to output section @kindex *(@var{section}) @item * (@var{section}) @itemx * (@var{section}, @var{section}, @dots{}) @itemx * (@var{section} @var{section} @dots{}) Instead of explicitly naming particular input files in a link control script, you can refer to @emph{all} files from the @code{ld} command line: use @samp{*} instead of a particular file name before the parenthesized input-file section list. If you have already explicitly included some files by name, @samp{*} refers to all @emph{remaining} files---those whose places in the output file have not yet been defined. For example, to copy sections @code{1} through @code{4} from an Oasys file into the @code{.text} section of an @code{a.out} file, and sections @code{13} and @code{14} into the @code{.data} section: @example SECTIONS @{ .text :@{ *("1" "2" "3" "4") @} .data :@{ *("13" "14") @} @} @end example @cindex @code{[@var{section}@dots{}]}, not supported @samp{[ @var{section} @dots{} ]} used to be accepted as an alternate way to specify named sections from all unallocated input files. Because some operating systems (VMS) allow brackets in file names, that notation is no longer supported. @cindex uninitialized data @cindex commons in output @kindex *( COMMON ) @item @var{filename}@code{( COMMON )} @itemx *( COMMON ) Specify where in your output file to place uninitialized data with this notation. @code{*(COMMON)} by itself refers to all uninitialized data from all input files (so far as it is not yet allocated); @var{filename}@code{(COMMON)} refers to uninitialized data from a particular file. Both are special cases of the general mechanisms for specifying where to place input-file sections: @code{ld} permits you to refer to uninitialized data as if it were in an input-file section named @code{COMMON}, regardless of the input file's format. @end table For example, the following command script arranges the output file into three consecutive sections, named @code{.text}, @code{.data}, and @code{.bss}, taking the input for each from the correspondingly named sections of all the input files: @example SECTIONS @{ .text : @{ *(.text) @} .data : @{ *(.data) @} .bss : @{ *(.bss) *(COMMON) @} @} @end example The following example reads all of the sections from file @code{all.o} and places them at the start of output section @code{outputa} which starts at location @code{0x10000}. All of section @code{.input1} from file @code{foo.o} follows immediately, in the same output section. All of section @code{.input2} from @code{foo.o} goes into output section @code{outputb}, followed by section @code{.input1} from @code{foo1.o}. All of the remaining @code{.input1} and @code{.input2} sections from any files are written to output section @code{outputc}. @example SECTIONS @{ outputa 0x10000 : @{ all.o foo.o (.input1) @} outputb : @{ foo.o (.input2) foo1.o (.input1) @} outputc : @{ *(.input1) *(.input2) @} @} @end example @node Section Data Expressions @subsection Section Data Expressions @cindex expressions in a section The foregoing statements arrange, in your output file, data originating from your input files. You can also place data directly in an output section from the link command script. Most of these additional statements involve expressions; @pxref{Expressions}. Although these statements are shown separately here for ease of presentation, no such segregation is needed within a section definition in the @code{SECTIONS} command; you can intermix them freely with any of the statements we've just described. @table @code @cindex input filename symbols @cindex filename symbols @kindex CREATE_OBJECT_SYMBOLS @item CREATE_OBJECT_SYMBOLS Create a symbol for each input file in the current section, set to the address of the first byte of data written from that input file. For instance, with @code{a.out} files it is conventional to have a symbol for each input file. You can accomplish this by defining the output @code{.text} section as follows: @example SECTIONS @{ .text 0x2020 : @{ CREATE_OBJECT_SYMBOLS *(.text) _etext = ALIGN(0x2000); @} @dots{} @} @end example If @code{sample.ld} is a file containing this script, and @code{a.o}, @code{b.o}, @code{c.o}, and @code{d.o} are four input files with contents like the following--- @example /* a.c */ afunction() @{ @} int adata=1; int abss; @end example @noindent @samp{ld -M -T sample.ld a.o b.o c.o d.o} would create a map like this, containing symbols matching the object file names: @example 00000000 A __DYNAMIC 00004020 B _abss 00004000 D _adata 00002020 T _afunction 00004024 B _bbss 00004008 D _bdata 00002038 T _bfunction 00004028 B _cbss 00004010 D _cdata 00002050 T _cfunction 0000402c B _dbss 00004018 D _ddata 00002068 T _dfunction 00004020 D _edata 00004030 B _end 00004000 T _etext 00002020 t a.o 00002038 t b.o 00002050 t c.o 00002068 t d.o @end example @kindex @var{symbol} = @var{expression} ; @kindex @var{symbol} @var{f}= @var{expression} ; @item @var{symbol} = @var{expression} ; @itemx @var{symbol} @var{f}= @var{expression} ; @var{symbol} is any symbol name (@pxref{Symbols}). ``@var{f}='' refers to any of the operators @code{&= += -= *= /=} which combine arithmetic and assignment. @cindex assignment, in section defn When you assign a value to a symbol within a particular section definition, the value is relative to the beginning of the section (@pxref{Assignment}). If you write @example SECTIONS @{ abs = 14 ; @dots{} .data : @{ @dots{} rel = 14 ; @dots{} @} abs2 = 14 + ADDR(.data); @dots{} @} @end example @c FIXME: Try above example! @noindent @code{abs} and @code{rel} do not have the same value; @code{rel} has the same value as @code{abs2}. @kindex BYTE(@var{expression}) @kindex SHORT(@var{expression}) @kindex LONG(@var{expression}) @kindex QUAD(@var{expression}) @cindex direct output @item BYTE(@var{expression}) @itemx SHORT(@var{expression}) @itemx LONG(@var{expression}) @itemx QUAD(@var{expression}) By including one of these four statements in a section definition, you can explicitly place one, two, four, or eight bytes (respectively) at the current address of that section. @code{QUAD} is only supported when using a 64 bit host or target. @ifclear SingleFormat Multiple-byte quantities are represented in whatever byte order is appropriate for the output file format (@pxref{BFD}). @end ifclear @item FILL(@var{expression}) @kindex FILL(@var{expression}) @cindex holes, filling @cindex unspecified memory Specify the ``fill pattern'' for the current section. Any otherwise unspecified regions of memory within the section (for example, regions you skip over by assigning a new value to the location counter @samp{.}) are filled with the two least significant bytes from the @var{expression} argument. A @code{FILL} statement covers memory locations @emph{after} the point it occurs in the section definition; by including more than one @code{FILL} statement, you can have different fill patterns in different parts of an output section. @end table @node Section Options @subsection Optional Section Attributes @cindex section defn, full syntax Here is the full syntax of a section definition, including all the optional portions: @smallexample SECTIONS @{ @dots{} @var{secname} @var{start} BLOCK(@var{align}) (NOLOAD) : AT ( @var{ldadr} ) @{ @var{contents} @} >@var{region} =@var{fill} @dots{} @} @end smallexample @var{secname} and @var{contents} are required. @xref{Section Definition}, and @pxref{Section Placement} for details on @var{contents}. The remaining elements---@var{start}, @code{BLOCK(@var{align)}}, @code{(NOLOAD)}, @code{AT ( @var{ldadr} )}, @code{>@var{region}}, and @code{=@var{fill}}---are all optional. @table @code @cindex start address, section @cindex section start @cindex section address @item @var{start} You can force the output section to be loaded at a specified address by specifying @var{start} immediately following the section name. @var{start} can be represented as any expression. The following example generates section @var{output} at location @code{0x40000000}: @example SECTIONS @{ @dots{} output 0x40000000: @{ @dots{} @} @dots{} @} @end example @kindex BLOCK(@var{align}) @cindex section alignment @cindex aligning sections @item BLOCK(@var{align}) You can include @code{BLOCK()} specification to advance the location counter @code{.} prior to the beginning of the section, so that the section will begin at the specified alignment. @var{align} is an expression. @kindex NOLOAD @cindex prevent unnecessary loading @cindex loading, preventing @item (NOLOAD) Use @samp{(NOLOAD)} to prevent a section from being loaded into memory each time it is accessed. For example, in the script sample below, the @code{ROM} segment is addressed at memory location @samp{0} and does not need to be loaded into each object file: @example SECTIONS @{ ROM 0 (NOLOAD) : @{ @dots{} @} @dots{} @} @end example @kindex AT ( @var{ldadr} ) @cindex specify load address @cindex load address, specifying @item AT ( @var{ldadr} ) The expression @var{ldadr} that follows the @code{AT} keyword specifies the load address of the section. The default (if you do not use the @code{AT} keyword) is to make the load address the same as the relocation address. This feature is designed to make it easy to build a ROM image. For example, this @code{SECTIONS} definition creates two output sections: one called @samp{.text}, which starts at @code{0x1000}, and one called @samp{.mdata}, which is loaded at the end of the @samp{.text} section even though its relocation address is @code{0x2000}. The symbol @code{_data} is defined with the value @code{0x2000}: @smallexample SECTIONS @{ .text 0x1000 : @{ *(.text) _etext = . ; @} .mdata 0x2000 : AT ( ADDR(.text) + SIZEOF ( .text ) ) @{ _data = . ; *(.data); _edata = . ; @} .bss 0x3000 : @{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;@} @} @end smallexample The run-time initialization code (for C programs, usually @code{crt0}) for use with a ROM generated this way has to include something like the following, to copy the initialized data from the ROM image to its runtime address: @smallexample 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 smallexample @kindex >@var{region} @cindex section, assigning to memory region @cindex memory regions and sections @item >@var{region} Assign this section to a previously defined region of memory. @xref{MEMORY}. @kindex =@var{fill} @cindex section fill pattern @cindex fill pattern, entire section @item =@var{fill} Including @code{=@var{fill}} in a section definition specifies the initial fill value for that section. You may use any expression to specify @var{fill}. Any unallocated holes in the current output section when written to the output file will be filled with the two least significant bytes of the value, repeated as necessary. You can also change the fill value with a @code{FILL} statement in the @var{contents} of a section definition. @end table @node Entry Point @section The Entry Point @kindex ENTRY(@var{symbol}) @cindex start of execution @cindex first instruction The linker command language includes a command specifically for defining the first executable instruction in an output file (its @dfn{entry point}). Its argument is a symbol name: @example ENTRY(@var{symbol}) @end example Like symbol assignments, the @code{ENTRY} command may be placed either as an independent command in the command file, or among the section definitions within the @code{SECTIONS} command---whatever makes the most sense for your layout. @cindex entry point, defaults @code{ENTRY} is only one of several ways of choosing the entry point. You may indicate it in any of the following ways (shown in descending order of priority: methods higher in the list override methods lower down). @itemize @bullet @item the @samp{-e} @var{entry} command-line option; @item the @code{ENTRY(@var{symbol})} command in a linker control script; @item the value of the symbol @code{start}, if present; @item the value of the symbol @code{_main}, if present; @item the address of the first byte of the @code{.text} section, if present; @item The address @code{0}. @end itemize For example, you can use these rules to generate an entry point with an assignment statement: if no symbol @code{start} is defined within your input files, you can simply define it, assigning it an appropriate value--- @example start = 0x2020; @end example @noindent The example shows an absolute address, but you can use any expression. For example, if your input object files use some other symbol-name convention for the entry point, you can just assign the value of whatever symbol contains the start address to @code{start}: @example start = other_symbol ; @end example @node Option Commands @section Option Commands The command language includes a number of other commands that you can use for specialized purposes. They are similar in purpose to command-line options. @table @code @kindex CONSTRUCTORS @cindex C++ constructors, arranging in link @cindex constructors, arranging in link @item CONSTRUCTORS This command ties up C++ style constructor and destructor records. The details of the constructor representation vary from one object format to another, but usually lists of constructors and destructors appear as special sections. The @code{CONSTRUCTORS} command specifies where the linker is to place the data from these sections, relative to the rest of the linked output. Constructor data is marked by the symbol @w{@code{__CTOR_LIST__}} at the start, and @w{@code{__CTOR_LIST_END}} at the end; destructor data is bracketed similarly, between @w{@code{__DTOR_LIST__}} and @w{@code{__DTOR_LIST_END}}. (The compiler must arrange to actually run this code; GNU C++ calls constructors from a subroutine @code{__main}, which it inserts automatically into the startup code for @code{main}, and destructors from @code{_exit}.) @need 1000 @kindex FLOAT @kindex NOFLOAT @item FLOAT @itemx NOFLOAT These keywords were used in some older linkers to request a particular math subroutine library. @code{ld} doesn't use the keywords, assuming instead that any necessary subroutines are in libraries specified using the general mechanisms for linking to archives; but to permit the use of scripts that were written for the older linkers, the keywords @code{FLOAT} and @code{NOFLOAT} are accepted and ignored. @kindex FORCE_COMMON_ALLOCATION @cindex common allocation @item FORCE_COMMON_ALLOCATION This command has the same effect as the @samp{-d} command-line option: to make @code{ld} assign space to common symbols even if a relocatable output file is specified (@samp{-r}). @kindex INPUT ( @var{files} ) @cindex binary input files @item INPUT ( @var{file}, @var{file}, @dots{} ) @itemx INPUT ( @var{file} @var{file} @dots{} ) Use this command to include binary input files in the link, without including them in a particular section definition. Specify the full name for each @var{file}, including @samp{.a} if required. @code{ld} searches for each @var{file} through the archive-library search path, just as for files you specify on the command line. See the description of @samp{-L} in @ref{Options,,Command Line Options}. @ignore @item MAP ( @var{name} ) @kindex MAP ( @var{name} ) @c MAP(...) appears to look for an F in the arg, ignoring all other @c chars; if it finds one, it sets "map_option_f" to true. But nothing @c checks map_option_f. Apparently a stub for the future... @end ignore @item OUTPUT ( @var{filename} ) @kindex OUTPUT ( @var{filename} ) @cindex naming the output file Use this command to name the link output file @var{filename}. The effect of @code{OUTPUT(@var{filename})} is identical to the effect of @w{@samp{-o @var{filename}}}, which overrides it. You can use this command to supply a default output-file name other than @code{a.out}. @ifclear SingleFormat @item OUTPUT_ARCH ( @var{bfdname} ) @kindex OUTPUT_ARCH ( @var{bfdname} ) @cindex machine architecture, output Specify a particular output machine architecture, with one of the names used by the BFD back-end routines (@pxref{BFD}). This command is often unnecessary; the architecture is most often set implicitly by either the system BFD configuration or as a side effect of the @code{OUTPUT_FORMAT} command. @item OUTPUT_FORMAT ( @var{bfdname} ) @kindex OUTPUT_FORMAT ( @var{bfdname} ) @cindex format, output file When @code{ld} is configured to support multiple object code formats, you can use this command to specify a particular output format. @var{bfdname} is one of the names used by the BFD back-end routines (@pxref{BFD}). The effect is identical to the effect of the @samp{-oformat} command-line option. This selection affects only the output file; the related command @code{TARGET} affects primarily input files. @end ifclear @item SEARCH_DIR ( @var{path} ) @kindex SEARCH_DIR ( @var{path} ) @cindex path for libraries @cindex search path, libraries Add @var{path} to the list of paths where @code{ld} looks for archive libraries. @code{SEARCH_DIR(@var{path})} has the same effect as @samp{-L@var{path}} on the command line. @item STARTUP ( @var{filename} ) @kindex STARTUP ( @var{filename} ) @cindex first input file Ensure that @var{filename} is the first input file used in the link process. @ifclear SingleFormat @item TARGET ( @var{format} ) @cindex input file format @kindex TARGET ( @var{format} ) When @code{ld} is configured to support multiple object code formats, you can use this command to change the input-file object code format (like the command-line option @samp{-b} or its synonym @samp{-format}). The argument @var{format} is one of the strings used by BFD to name binary formats. If @code{TARGET} is specified but @code{OUTPUT_FORMAT} is not, the last @code{TARGET} argument is also used as the default format for the @code{ld} output file. @xref{BFD}. @kindex GNUTARGET If you don't use the @code{TARGET} command, @code{ld} uses the value of the environment variable @code{GNUTARGET}, if available, to select the output file format. If that variable is also absent, @code{ld} uses the default format configured for your machine in the BFD libraries. @end ifclear @end table @ifset GENERIC @node Machine Dependent @chapter Machine Dependent Features @cindex machine dependencies @code{ld} has additional features on some platforms; the following sections describe them. Machines where @code{ld} has no additional functionality are not listed. @menu * H8/300:: @code{ld} and the H8/300 * i960:: @code{ld} and the Intel 960 family @end menu @end ifset @c FIXME! This could use @raisesections/@lowersections, but there seems to be a conflict @c between those and node-defaulting. @ifset H8300 @ifclear GENERIC @raisesections @end ifclear @node H8/300 @section @code{ld} and the H8/300 @cindex H8/300 support For the H8/300, @code{ld} can perform these global optimizations when you specify the @samp{-relax} command-line option. @table @emph @item relaxing address modes @cindex relaxing on H8/300 @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. @item synthesizing instructions @cindex synthesizing on H8/300 @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 @example try libtry.a tryca libtryca.a @end example @noindent The first two possibilities would be considered in any event; the last two are due to the use of @w{@samp{-ACA}}. You can meaningfully use @samp{-A} more than once on a command line, since the 960 architecture family allows combination of target architectures; each use will add another pair of name variants to search for when @w{@samp{-l}} specifies a library. @cindex @code{-relax} on i960 @cindex relaxing on i960 @code{ld} supports the @samp{-relax} option for the i960 family. If you specify @samp{-relax}, @code{ld} finds all @code{balx} and @code{calx} instructions whose targets are within 24 bits, and turns them into 24-bit program-counter relative @code{bal} and @code{cal} instructions, respectively. @code{ld} also turns @code{cal} instructions into @code{bal} instructions when it determines that the target subroutine is a leaf routine (that is, the target subroutine does not itself call any subroutines). @ifclear GENERIC @lowersections @end ifclear @end ifset @ifclear SingleFormat @node BFD @chapter BFD @cindex back end @cindex object file management @cindex object formats available @kindex objdump -i The linker accesses object and archive files using the BFD libraries. These libraries allow the linker to use the same routines to operate on object files whatever the object file format. A different object file format can be supported simply by creating a new BFD back end and adding it to the library. To conserve runtime memory, however, the linker and associated tools are usually configured to support only a subset of the object file formats available. You can use @code{objdump -i} (@pxref{objdump,,objdump,binutils.info,The GNU Binary Utilities}) to list all the formats available for your configuration. @cindex BFD requirements @cindex requirements for BFD As with most implementations, BFD is a compromise between several conflicting requirements. The major factor influencing BFD design was efficiency: any time used converting between formats is time which would not have been spent had BFD not been involved. This is partly offset by abstraction payback; since BFD simplifies applications and back ends, more time and care may be spent optimizing algorithms for a greater speed. One minor artifact of the BFD solution which you should bear in mind is the potential for information loss. There are two places where useful information can be lost using the BFD mechanism: during conversion and during output. @xref{BFD information loss}. @menu * BFD outline:: How it works: an outline of BFD @end menu @node BFD outline @section How it works: an outline of BFD @cindex opening object files @include bfdsumm.texi @end ifclear @node MRI @appendix MRI Compatible Script Files @cindex MRI compatibility To aid users making the transition to @sc{gnu} @code{ld} from the MRI linker, @code{ld} can use MRI compatible linker scripts as an alternative to the more general-purpose linker scripting language described in @ref{Commands,,Command Language}. MRI compatible linker scripts have a much simpler command set than the scripting language otherwise used with @code{ld}. @sc{gnu} @code{ld} supports the most commonly used MRI linker commands; these commands are described here. In general, MRI scripts aren't of much use with the @code{a.out} object file format, since it only has three sections and MRI scripts lack some features to make use of them. You can specify a file containing an MRI-compatible script using the @samp{-c} command-line option. Each command in an MRI-compatible script occupies its own line; each command line starts with the keyword that identifies the command (though blank lines are also allowed for punctuation). If a line of an MRI-compatible script begins with an unrecognized keyword, @code{ld} issues a warning message, but continues processing the script. Lines beginning with @samp{*} are comments. You can write these commands using all upper-case letters, or all lower case; for example, @samp{chip} is the same as @samp{CHIP}. The following list shows only the upper-case form of each command. @table @code @cindex @code{ABSOLUTE} (MRI) @item ABSOLUTE @var{secname} @item 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{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} @item 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: pesch@cygnus.com, 28mar91. @end tex @contents @bye