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GNU Info File | 1996-07-15 | 46.4 KB | 1,150 lines

This is Info file ld.info, produced by Makeinfo-1.55 from the input file ./ld.texinfo. START-INFO-DIR-ENTRY * Ld: (ld). The GNU linker. END-INFO-DIR-ENTRY This file documents the GNU linker LD. Copyright (C) 1991, 92, 93, 94, 95, 1996 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions. File: ld.info, Node: Top, Next: Overview, Prev: (DIR), Up: (DIR) Using ld ******** This file documents the GNU linker ld. * Menu: * Overview:: Overview * Invocation:: Invocation * Commands:: Command Language * Machine Dependent:: Machine Dependent Features * BFD:: BFD * MRI:: MRI Compatible Script Files * Index:: Index File: ld.info, Node: Overview, Next: Invocation, Prev: Top, Up: Top Overview ******** `ld' combines a number of object and archive files, relocates their data and ties up symbol references. Usually the last step in compiling a program is to run `ld'. `ld' accepts Linker Command Language files written in a superset of AT&T's Link Editor Command Language syntax, to provide explicit and total control over the linking process. This version of `ld' uses the general purpose BFD libraries to operate on object files. This allows `ld' to read, combine, and write object files in many different formats--for example, COFF or `a.out'. Different formats may be linked together to produce any available kind of object file. *Note BFD::, for more information. Aside from its flexibility, the GNU linker is more helpful than other linkers in providing diagnostic information. Many linkers abandon execution immediately upon encountering an error; whenever possible, `ld' continues executing, allowing you to identify other errors (or, in some cases, to get an output file in spite of the error). File: ld.info, Node: Invocation, Next: Commands, Prev: Overview, Up: Top Invocation ********** The GNU linker `ld' is meant to cover a broad range of situations, and to be as compatible as possible with other linkers. As a result, you have many choices to control its behavior. * Menu: * Options:: Command Line Options * Environment:: Environment Variables File: ld.info, Node: Options, Next: Environment, Up: Invocation Command Line Options ==================== The linker supports a plethora of command-line options, but in actual practice few of them are used in any particular context. For instance, a frequent use of `ld' is to link standard Unix object files on a standard, supported Unix system. On such a system, to link a file `hello.o': ld -o OUTPUT /lib/crt0.o hello.o -lc This tells `ld' to produce a file called OUTPUT as the result of linking the file `/lib/crt0.o' with `hello.o' and the library `libc.a', which will come from the standard search directories. (See the discussion of the `-l' option below.) The command-line options to `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. 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 `-l', `-R', and the script command language. If *no* binary input files at all are specified, the linker does not produce any output, and issues the message `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 `-T'). This feature permits the linker to link against a file which appears to be an object or an archive, but actually merely defines some symbol values, or uses `INPUT' or `GROUP' to load other objects. *Note Commands::. For options whose names are a single letter, option arguments must either follow the option letter without intervening whitespace, or be given as separate arguments immediately following the option that requires them. For options whose names are multiple letters, either one dash or two can precede the option name; for example, `--oformat' and `-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, `--oformat srec' and `--oformat=srec' are equivalent. Unique abbreviations of the names of multiple-letter options are accepted. `-aKEYWORD' This option is supported for HP/UX compatibility. The KEYWORD argument must be one of the strings `archive', `shared', or `default'. `-aarchive' is functionally equivalent to `-Bstatic', and the other two keywords are functionally equivalent to `-Bdynamic'. This option may be used any number of times. `-AARCHITECTURE' `--architecture=ARCHITECTURE' In the current release of `ld', this option is useful only for the Intel 960 family of architectures. In that `ld' configuration, the ARCHITECTURE argument identifies the particular architecture in the 960 family, enabling some safeguards and modifying the archive-library search path. *Note `ld' and the Intel 960 family: i960, for details. Future releases of `ld' may support similar functionality for other architecture families. `-b INPUT-FORMAT' `--format=INPUT-FORMAT' `ld' may be configured to support more than one kind of object file. If your `ld' is configured this way, you can use the `-b' option to specify the binary format for input object files that follow this option on the command line. Even when `ld' is configured to support alternative object formats, you don't usually need to specify this, as `ld' should be configured to expect as a default input format the most usual format on each machine. INPUT-FORMAT is a text string, the name of a particular format supported by the BFD libraries. (You can list the available binary formats with `objdump -i'.) *Note BFD::. You may want to use this option if you are linking files with an unusual binary format. You can also use `-b' to switch formats explicitly (when linking object files of different formats), by including `-b INPUT-FORMAT' before each group of object files in a particular format. The default format is taken from the environment variable `GNUTARGET'. *Note Environment::. You can also define the input format from a script, using the command `TARGET'; see *Note Option Commands::. `-c MRI-COMMANDFILE' `--mri-script=MRI-COMMANDFILE' For compatibility with linkers produced by MRI, `ld' accepts script files written in an alternate, restricted command language, described in *Note MRI Compatible Script Files: MRI. Introduce MRI script files with the option `-c'; use the `-T' option to run linker scripts written in the general-purpose `ld' scripting language. If MRI-CMDFILE does not exist, `ld' looks for it in the directories specified by any `-L' options. `-d' `-dc' `-dp' These three options are equivalent; multiple forms are supported for compatibility with other linkers. They assign space to common symbols even if a relocatable output file is specified (with `-r'). The script command `FORCE_COMMON_ALLOCATION' has the same effect. *Note Option Commands::. `-e ENTRY' `--entry=ENTRY' Use ENTRY as the explicit symbol for beginning execution of your program, rather than the default entry point. *Note Entry Point::, for a discussion of defaults and other ways of specifying the entry point. `-E' `-export-dynamic' When creating a dynamically linked executable, add all symbols to the dynamic symbol table. Normally, the dynamic symbol table contains only symbols which are used by a dynamic object. This option is needed for some uses of `dlopen'. `-F' `-FFORMAT' 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 `ld' uses for this purpose (the `-b' or `-format' options for input files, `-oformat' option or the `TARGET' command in linker scripts for output files, the `GNUTARGET' environment variable) are more flexible, but `ld' accepts the `-F' option for compatibility with scripts written to call the old linker. `--force-exe-suffix' Make sure that an output file has a .exe suffix. If a successfully built fully linked output file does not have a `.exe' or `.dll' suffix, this option forces the linker to copy the output file to one of the same name with a `.exe' suffix. This option is useful when using unmodified Unix makefiles on a Microsoft Windows host, since some versions of Windows won't run an image unless it ends in a `.exe' suffix. `-g' Ignored. Provided for compatibility with other tools. `-GVALUE' `--gpsize=VALUE' Set the maximum size of objects to be optimized using the GP register to SIZE. This is only meaningful for object file formats such as MIPS ECOFF which supports putting large and small objects into different sections. This is ignored for other object file formats. `-hNAME' `-soname=NAME' When creating an ELF shared object, set the internal DT_SONAME field to the specified name. When an executable is linked with a shared object which has a DT_SONAME field, then when the executable is run the dynamic linker will attempt to load the shared object specified by the DT_SONAME field rather than the using the file name given to the linker. `-i' Perform an incremental link (same as option `-r'). `-lARCHIVE' `--library=ARCHIVE' Add archive file ARCHIVE to the list of files to link. This option may be used any number of times. `ld' will search its path-list for occurrences of `libARCHIVE.a' for every ARCHIVE specified. File extensions other than `.a' may be used on certain systems. `-LSEARCHDIR' `--library-path=SEARCHDIR' Add path SEARCHDIR to the list of paths that `ld' will search for archive libraries and `ld' control scripts. You may use this option any number of times. The directories are searched in the order in which they are specified on the command line. Directories specified on the command line are searched before the default directories. All `-L' options apply to all `-l' options, regardless of the order in which the options appear. The default set of paths searched (without being specified with `-L') depends on which emulation mode `ld' is using, and in some cases also on how it was configured. *Note Environment::. The paths can also be specified in a link script with the `SEARCH_DIR' command. Directories specified this way are searched at the point in which the linker script appears in the command line. `-mEMULATION' Emulate the EMULATION linker. You can list the available emulations with the `--verbose' or `-V' options. The default depends on how your `ld' was configured. `-M' `--print-map' Print (to the standard output) a link map--diagnostic information about where symbols are mapped by `ld', and information on global common storage allocation. `-n' `--nmagic' Set the text segment to be read only, and mark the output as `NMAGIC' if possible. `-N' `--omagic' Set the text and data sections to be readable and writable. Also, do not page-align the data segment. If the output format supports Unix style magic numbers, mark the output as `OMAGIC'. `-o OUTPUT' `--output=OUTPUT' Use OUTPUT as the name for the program produced by `ld'; if this option is not specified, the name `a.out' is used by default. The script command `OUTPUT' can also specify the output file name. `-r' `--relocateable' Generate relocatable output--i.e., generate an output file that can in turn serve as input to `ld'. This is often called "partial linking". As a side effect, in environments that support standard Unix magic numbers, this option also sets the output file's magic number to `OMAGIC'. If this option is not specified, an absolute file is produced. When linking C++ programs, this option *will not* resolve references to constructors; to do that, use `-Ur'. This option does the same thing as `-i'. `-R FILENAME' `--just-symbols=FILENAME' Read symbol names and their addresses from FILENAME, but do not relocate it or include it in the output. This allows your output file to refer symbolically to absolute locations of memory defined in other programs. You may use this option more than once. For compatibility with other ELF linkers, if the `-R' option is followed by a directory name, rather than a file name, it is treated as the `-rpath' option. `-s' `--strip-all' Omit all symbol information from the output file. `-S' `--strip-debug' Omit debugger symbol information (but not all symbols) from the output file. `-t' `--trace' Print the names of the input files as `ld' processes them. `-T COMMANDFILE' `--script=COMMANDFILE' Read link commands from the file COMMANDFILE. These commands replace `ld''s default link script (rather than adding to it), so COMMANDFILE must specify everything necessary to describe the target format. *Note Commands::. If COMMANDFILE does not exist, `ld' looks for it in the directories specified by any preceding `-L' options. Multiple `-T' options accumulate. `-u SYMBOL' `--undefined=SYMBOL' Force SYMBOL to be entered in the output file as an undefined symbol. Doing this may, for example, trigger linking of additional modules from standard libraries. `-u' may be repeated with different option arguments to enter additional undefined symbols. `-v' `--version' `-V' Display the version number for `ld'. The `-V' option also lists the supported emulations. `-x' `--discard-all' Delete all local symbols. `-X' `--discard-locals' Delete all temporary local symbols. For most targets, this is all local symbols whose names begin with `L'. `-y SYMBOL' `--trace-symbol=SYMBOL' Print the name of each linked file in which SYMBOL appears. This option may be given any number of times. On many systems it is necessary to prepend an underscore. This option is useful when you have an undefined symbol in your link but don't know where the reference is coming from. `-Y PATH' Add PATH to the default library search path. This option exists for Solaris compatibility. `-z KEYWORD' This option is ignored for Solaris compatibility. `-( ARCHIVES -)' `--start-group ARCHIVES --end-group' The ARCHIVES should be a list of archive files. They may be either explicit file names, or `-l' options. The specified archives are searched repeatedly until no new undefined references are created. Normally, an archive is searched only once in the order that it is specified on the command line. If a symbol in that archive is needed to resolve an undefined symbol referred to by an object in an archive that appears later on the command line, the linker would not be able to resolve that reference. By grouping the archives, they all be searched repeatedly until all possible references are resolved. Using this option has a significant performance cost. It is best to use it only when there are unavoidable circular references between two or more archives. `-assert KEYWORD' This option is ignored for SunOS compatibility. `-Bdynamic' `-dy' `-call_shared' Link against dynamic libraries. This is only meaningful on platforms for which shared libraries are supported. This option is normally the default on such platforms. The different variants of this option are for compatibility with various systems. You may use this option multiple times on the command line: it affects library searching for `-l' options which follow it. `-Bstatic' `-dn' `-non_shared' `-static' Do not link against shared libraries. This is only meaningful on platforms for which shared libraries are supported. The different variants of this option are for compatibility with various systems. You may use this option multiple times on the command line: it affects library searching for `-l' options which follow it. `-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. `--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. `--defsym SYMBOL=EXPRESSION' Create a global symbol in the output file, containing the absolute address given by EXPRESSION. You may use this option as many times as necessary to define multiple symbols in the command line. A limited form of arithmetic is supported for the EXPRESSION in this context: you may give a hexadecimal constant or the name of an existing symbol, or use `+' and `-' to add or subtract hexadecimal constants or symbols. If you need more elaborate expressions, consider using the linker command language from a script (*note Assignment: Symbol Definitions: Assignment.). *Note:* there should be no white space between SYMBOL, the equals sign ("="), and EXPRESSION. `--dynamic-linker FILE' Set the name of the dynamic linker. This is only meaningful when generating dynamically linked ELF executables. The default dynamic linker is normally correct; don't use this unless you know what you are doing. `-EB' Link big-endian objects. This affects the default output format. `-EL' Link little-endian objects. This affects the default output format. `-embedded-relocs' This option is only meaningful when linking MIPS embedded PIC code, generated by the -membedded-pic option to the 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. `--help' Print a summary of the command-line options on the standard output and exit. `-Map MAPFILE' Print to the file MAPFILE a link map--diagnostic information about where symbols are mapped by `ld', and information on global common storage allocation. `--no-keep-memory' `ld' normally optimizes for speed over memory usage by caching the symbol tables of input files in memory. This option tells `ld' to instead optimize for memory usage, by rereading the symbol tables as necessary. This may be required if `ld' runs out of memory space while linking a large executable. `--no-whole-archive' Turn off the effect of the `--whole-archive' option for subsequent archive files. `--noinhibit-exec' Retain the executable output file whenever it is still usable. Normally, the linker will not produce an output file if it encounters errors during the link process; it exits without writing an output file when it issues any error whatsoever. `-oformat OUTPUT-FORMAT' `ld' may be configured to support more than one kind of object file. If your `ld' is configured this way, you can use the `-oformat' option to specify the binary format for the output object file. Even when `ld' is configured to support alternative object formats, you don't usually need to specify this, as `ld' should be configured to produce as a default output format the most usual format on each machine. OUTPUT-FORMAT is a text string, the name of a particular format supported by the BFD libraries. (You can list the available binary formats with `objdump -i'.) The script command `OUTPUT_FORMAT' can also specify the output format, but this option overrides it. *Note BFD::. `-qmagic' This option is ignored for Linux compatibility. `-Qy' This option is ignored for SVR4 compatibility. `--relax' An option with machine dependent effects. This option is only supported on a few targets. *Note `ld' and the H8/300: H8/300. *Note `ld' and the Intel 960 family: i960. On some platforms, the `--relax' option performs global optimizations that become possible when the linker resolves addressing in the program, such as relaxing address modes and synthesizing new instructions in the output object file. On platforms where this is not supported, `-relax' is accepted, but ignored. `--retain-symbols-file FILENAME' Retain *only* the symbols listed in the file FILENAME, discarding all others. FILENAME is simply a flat file, with one symbol name per line. This option is especially useful in environments (such as VxWorks) where a large global symbol table is accumulated gradually, to conserve run-time memory. `-retain-symbols-file' does *not* discard undefined symbols, or symbols needed for relocations. You may only specify `-retain-symbols-file' once in the command line. It overrides `-s' and `-S'. `-rpath DIR' Add a directory to the runtime library search path. This is used when linking an ELF executable with shared objects. All `-rpath' arguments are concatenated and passed to the runtime linker, which uses them to locate shared objects at runtime. The `-rpath' option is also used when locating shared objects which are needed by shared objects explicitly included in the link; see the description of the `-rpath-link' option. If `-rpath' is not used when linking an ELF executable, the contents of the environment variable `LD_RUN_PATH' will be used if it is defined. The `-rpath' option may also be used on SunOS. By default, on SunOS, the linker will form a runtime search patch out of all the `-L' options it is given. If a `-rpath' option is used, the runtime search path will be formed exclusively using the `-rpath' options, ignoring the `-L' options. This can be useful when using gcc, which adds many `-L' options which may be on NFS mounted filesystems. For compatibility with other ELF linkers, if the `-R' option is followed by a directory name, rather than a file name, it is treated as the `-rpath' option. `-rpath-link DIR' When using ELF or SunOS, one shared library may require another. This happens when an `ld -shared' link includes a shared library as one of the input files. When the linker encounters such a dependency when doing a non-shared, non-relocateable link, it will automatically try to locate the required shared library and include it in the link, if it is not included explicitly. In such a case, the `-rpath-link' option specifies the first set of directories to search. The `-rpath-link' option may specify a sequence of directory names either by specifying a list of names separated by colons, or by appearing multiple times. The linker uses the following search paths to locate required shared libraries. 1. Any directories specified by `-rpath-link' options. 2. Any directories specified by `-rpath' options. The difference between `-rpath' and `-rpath-link' is that directories specified by `-rpath' options are included in the executable and used at runtime, whereas the `-rpath-link' option is only effective at link time. 3. On an ELF system, if the `-rpath' and `rpath-link' options were not used, search the contents of the environment variable `LD_RUN_PATH'. 4. On SunOS, if the `-rpath' option was not used, search any directories specified using `-L' options. 5. For a native linker, the contents of the environment variable `LD_LIBRARY_PATH'. 6. The default directories, normally `/lib' and `/usr/lib'. If the required shared library is not found, the linker will issue a warning and continue with the link. `-shared' `-Bshareable' Create a shared library. This is currently only supported on ELF, XCOFF and SunOS platforms. On SunOS, the linker will automatically create a shared library if the `-e' option is not used and there are undefined symbols in the link. `--sort-common' This option tells `ld' to sort the common symbols by size when it places them in the appropriate output sections. First come all the one byte symbols, then all the two bytes, then all the four bytes, and then everything else. This is to prevent gaps between symbols due to alignment constraints. `--split-by-file' Similar to `--split-by-reloc' but creates a new output section for each input file. `--split-by-reloc COUNT' Trys to creates extra sections in the output file so that no single output section in the file contains more than COUNT relocations. This is useful when generating huge relocatable for downloading into certain real time kernels with the COFF object file format; since COFF cannot represent more than 65535 relocations in a single section. Note that this will fail to work with object file formats which do not support arbitrary sections. The linker will not split up individual input sections for redistribution, so if a single input section contains more than COUNT relocations one output section will contain that many relocations. `--stats' Compute and display statistics about the operation of the linker, such as execution time and memory usage. `-traditional-format' For some targets, the output of `ld' is different in some ways from the output of some existing linker. This switch requests `ld' to use the traditional format instead. For example, on SunOS, `ld' combines duplicate entries in the symbol string table. This can reduce the size of an output file with full debugging information by over 30 percent. Unfortunately, the SunOS `dbx' program can not read the resulting program (`gdb' has no trouble). The `-traditional-format' switch tells `ld' to not combine duplicate entries. `-Tbss ORG' `-Tdata ORG' `-Ttext ORG' Use ORG as the starting address for--respectively--the `bss', `data', or the `text' segment of the output file. ORG must be a single hexadecimal integer; for compatibility with other linkers, you may omit the leading `0x' usually associated with hexadecimal values. `-Ur' For anything other than C++ programs, this option is equivalent to `-r': it generates relocatable output--i.e., an output file that can in turn serve as input to `ld'. When linking C++ programs, `-Ur' *does* resolve references to constructors, unlike `-r'. It does not work to use `-Ur' on files that were themselves linked with `-Ur'; once the constructor table has been built, it cannot be added to. Use `-Ur' only for the last partial link, and `-r' for the others. `--verbose' Display the version number for `ld' and list the linker emulations supported. Display which input files can and cannot be opened. Display the linker script if using a default builtin script. `-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: `int i = 1;' A definition, which goes in the initialized data section of the output file. `extern int i;' An undefined reference, which does not allocate space. There must be either a definition or a common symbol for the variable somewhere. `int i;' A common symbol. If there are only (one or more) common symbols for a variable, it goes in the uninitialized data area of the output file. The linker merges multiple common symbols for the same variable into a single symbol. If they are of different sizes, it picks the largest size. The linker turns a common symbol into a declaration, if there is a definition of the same variable. The `-warn-common' option can produce five kinds of warnings. Each warning consists of a pair of lines: the first describes the symbol just encountered, and the second describes the previous symbol encountered with the same name. One or both of the two symbols will be a common symbol. 1. Turning a common symbol into a reference, because there is already a definition for the symbol. FILE(SECTION): warning: common of `SYMBOL' overridden by definition FILE(SECTION): warning: defined here 2. Turning a common symbol into a reference, because a later definition for the symbol is encountered. This is the same as the previous case, except that the symbols are encountered in a different order. FILE(SECTION): warning: definition of `SYMBOL' overriding common FILE(SECTION): warning: common is here 3. Merging a common symbol with a previous same-sized common symbol. FILE(SECTION): warning: multiple common of `SYMBOL' FILE(SECTION): warning: previous common is here 4. Merging a common symbol with a previous larger common symbol. FILE(SECTION): warning: common of `SYMBOL' overridden by larger common FILE(SECTION): warning: larger common is here 5. Merging a common symbol with a previous smaller common symbol. This is the same as the previous case, except that the symbols are encountered in a different order. FILE(SECTION): warning: common of `SYMBOL' overriding smaller common FILE(SECTION): warning: smaller common is here `-warn-constructors' Warn if any global constructors are used. This is only useful for a few object file formats. For formats like COFF or ELF, the linker can not detect the use of global constructors. `-warn-multiple-gp' Warn if multiple global pointer values are required in the output file. This is only meaningful for certain processors, such as the Alpha. Specifically, some processors put large-valued constants in a special section. A special register (the global pointer) points into the middle of this section, so that constants can be loaded efficiently via a base-register relative addressing mode. Since the offset in base-register relative mode is fixed and relatively small (e.g., 16 bits), this limits the maximum size of the constant pool. Thus, in large programs, it is often necessary to use multiple global pointer values in order to be able to address all possible constants. This option causes a warning to be issued whenever this case occurs. `-warn-once' Only warn once for each undefined symbol, rather than once per module which refers to it. `--whole-archive' For each archive mentioned on the command line after the `--whole-archive' option, include every object file in the archive in the link, rather than searching the archive for the required object files. This is normally used to turn an archive file into a shared library, forcing every object to be included in the resulting shared library. This option may be used more than once. `--wrap SYMBOL' Use a wrapper function for SYMBOL. Any undefined reference to SYMBOL will be resolved to `__wrap_SYMBOL'. Any undefined reference to `__real_SYMBOL' will be resolved to SYMBOL. This can be used to provide a wrapper for a system function. The wrapper function should be called `__wrap_SYMBOL'. If it wishes to call the system function, it should call `__real_SYMBOL'. Here is a trivial example: void * __wrap_malloc (int c) { printf ("malloc called with %ld\n", c); return __real_malloc (c); } If you link other code with this file using `--wrap malloc', then all calls to `malloc' will call the function `__wrap_malloc' instead. The call to `__real_malloc' in `__wrap_malloc' will call the real `malloc' function. You may wish to provide a `__real_malloc' function as well, so that links without the `--wrap' option will succeed. If you do this, you should not put the definition of `__real_malloc' in the same file as `__wrap_malloc'; if you do, the assembler may resolve the call before the linker has a chance to wrap it to `malloc'. File: ld.info, Node: Environment, Prev: Options, Up: Invocation Environment Variables ===================== You can change the behavior of `ld' with the environment variable `GNUTARGET'. `GNUTARGET' determines the input-file object format if you don't use `-b' (or its synonym `-format'). Its value should be one of the BFD names for an input format (*note BFD::.). If there is no `GNUTARGET' in the environment, `ld' uses the natural format of the target. If `GNUTARGET' is set to `default' then BFD attempts to discover the input format by examining binary input files; this method often succeeds, but there are potential ambiguities, since there is no method of ensuring that the magic number used to specify object-file formats is unique. However, the configuration procedure for BFD on each system places the conventional format for that system first in the search-list, so ambiguities are resolved in favor of convention. File: ld.info, Node: Commands, Next: Machine Dependent, Prev: Invocation, Up: Top Command Language **************** 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: * input files * file formats * output file layout * addresses of sections * placement of common blocks You may supply a command file (also known as a link script) to the linker either explicitly through the `-T' option, or implicitly as an ordinary file. If the linker opens a file which it cannot recognize as a supported object or archive format, it reports an error. * Menu: * Scripts:: Linker Scripts * Expressions:: Expressions * MEMORY:: MEMORY Command * SECTIONS:: SECTIONS Command * PHDRS:: PHDRS Command * Entry Point:: The Entry Point * Option Commands:: Option Commands File: ld.info, Node: Scripts, Next: Expressions, Up: Commands Linker Scripts ============== The `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. The most fundamental command of the `ld' command language is the `SECTIONS' command (*note SECTIONS::.). Every meaningful command script must have a `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 `MEMORY' command complements `SECTIONS' by describing the available memory in the target architecture. This command is optional; if you don't use a `MEMORY' command, `ld' assumes sufficient memory is available in a contiguous block for all output. *Note MEMORY::. You may include comments in linker scripts just as in C: delimited by `/*' and `*/'. As in C, comments are syntactically equivalent to whitespace. File: ld.info, Node: Expressions, Next: MEMORY, Prev: Scripts, Up: Commands Expressions =========== 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: * All expressions evaluated as integers and are of "long" or "unsigned long" type. * All constants are integers. * All of the C arithmetic operators are provided. * You may reference, define, and create global variables. * You may call special purpose built-in functions. * Menu: * Integers:: Integers * Symbols:: Symbol Names * Location Counter:: The Location Counter * Operators:: Operators * Evaluation:: Evaluation * Assignment:: Assignment: Defining Symbols * Arithmetic Functions:: Built-In Functions * Semicolons:: Semicolon Usage File: ld.info, Node: Integers, Next: Symbols, Up: Expressions Integers -------- An octal integer is `0' followed by zero or more of the octal digits (`01234567'). _as_octal = 0157255; A decimal integer starts with a non-zero digit followed by zero or more digits (`0123456789'). _as_decimal = 57005; A hexadecimal integer is `0x' or `0X' followed by one or more hexadecimal digits chosen from `0123456789abcdefABCDEF'. _as_hex = 0xdead; To write a negative integer, use the prefix operator `-' (*note Operators::.). _as_neg = -57005; Additionally the suffixes `K' and `M' may be used to scale a constant by `1024' or `1024*1024' respectively. For example, the following all refer to the same quantity: _fourk_1 = 4K; _fourk_2 = 4096; _fourk_3 = 0x1000; File: ld.info, Node: Symbols, Next: Location Counter, Prev: Integers, Up: Expressions Symbol Names ------------ 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: "SECTION" = 9; "with a space" = "also with a space" + 10; Since symbols can contain many non-alphabetic characters, it is safest to delimit symbols with spaces. For example, `A-B' is one symbol, whereas `A - B' is an expression involving subtraction. File: ld.info, Node: Location Counter, Next: Operators, Prev: Symbols, Up: Expressions The Location Counter -------------------- The special linker variable "dot" `.' always contains the current output location counter. Since the `.' always refers to a location in an output section, it must always appear in an expression within a `SECTIONS' command. The `.' 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 `.' symbol 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. SECTIONS { output : { file1(.text) . = . + 1000; file2(.text) . += 1000; file3(.text) } = 0x1234; } In the previous example, `file1' is located at the beginning of the output section, then there is a 1000 byte gap. Then `file2' appears, also with a 1000 byte gap following before `file3' is loaded. The notation `= 0x1234' specifies what data to write in the gaps (*note Section Options::.). File: ld.info, Node: Operators, Next: Evaluation, Prev: Location Counter, Up: Expressions Operators --------- The linker recognizes the standard C set of arithmetic operators, with the standard bindings and precedence levels: precedence associativity Operators Notes (highest) 1 left ! - ~ (1) 2 left * / % 3 left + - 4 left >> << 5 left == != > < <= >= 6 left & 7 left | 8 left && 9 left || 10 right ? : 11 right &= += -= *= /= (2) (lowest) Notes: (1) Prefix operators (2) *Note Assignment::. File: ld.info, Node: Evaluation, Next: Assignment, Prev: Operators, Up: Expressions Evaluation ---------- 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. File: ld.info, Node: Assignment, Next: Arithmetic Functions, Prev: Evaluation, Up: Expressions Assignment: Defining Symbols ---------------------------- You may create global symbols, and assign values (addresses) to global symbols, using any of the C assignment operators: `SYMBOL = EXPRESSION ;' `SYMBOL &= EXPRESSION ;' `SYMBOL += EXPRESSION ;' `SYMBOL -= EXPRESSION ;' `SYMBOL *= EXPRESSION ;' `SYMBOL /= EXPRESSION ;' Two things distinguish assignment from other operators in `ld' expressions. * Assignment may only be used at the root of an expression; `a=b+3;' is allowed, but `a+b=3;' is an error. * You must place a trailing semicolon (";") at the end of an assignment statement. Assignment statements may appear: * as commands in their own right in an `ld' script; or * as independent statements within a `SECTIONS' command; or * as part of the contents of a section definition in a `SECTIONS' command. 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 (*note SECTIONS::.). 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 `ABSOLUTE'. For example, to create an absolute symbol whose address is the last byte of an output section named `.data': SECTIONS{ ... .data : { *(.data) _edata = ABSOLUTE(.) ; } ... } The linker tries to put off the evaluation of an assignment until all the terms in the source expression are known (*note 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 "dot", `.' 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 SECTIONS { ... text 9+this_isnt_constant : { ... } ... } will cause the error message "`Non constant expression for initial address'". In some cases, it is desirable for a linker script to define a symbol only if it is referenced, and only if it is not defined by any object included in the link. For example, traditional linkers defined the symbol `etext'. However, ANSI C requires that the user be able to use `etext' as a function name without encountering an error. The `PROVIDE' keyword may be used to define a symbol, such as `etext', only if it is referenced but not defined. The syntax is `PROVIDE(SYMBOL = EXPRESSION)'.