the dynamic list is the same as the version node without scope and node name. See *Note VERSION:: for more information. `--dynamic-list-data' Include all global data symbols to the dynamic list. `--dynamic-list-cpp-new' Provide the builtin dynamic list for C++ operator new and delete. It is mainly useful for building shared libstdc++. `--dynamic-list-cpp-typeinfo' Provide the builtin dynamic list for C++ runtime type identification. `--check-sections' `--no-check-sections' Asks the linker _not_ to check section addresses after they have been assigned to see if there are any overlaps. Normally the linker will perform this check, and if it finds any overlaps it will produce suitable error messages. The linker does know about, and does make allowances for sections in overlays. The default behaviour can be restored by using the command line switch `--check-sections'. Section overlap is not usually checked for relocatable links. You can force checking in that case by using the `--check-sections' option. `--copy-dt-needed-entries' `--no-copy-dt-needed-entries' This option affects the treatment of dynamic libraries referred to by DT_NEEDED tags _inside_ ELF dynamic libraries mentioned on the command line. Normally the linker will add a DT_NEEDED tag to the output binary for each library mentioned in a DT_NEEDED tag in an input dynamic library. With `--no-copy-dt-needed-entries' specified on the command line however any dynamic libraries that follow it will have their DT_NEEDED entries ignored. The default behaviour can be restored with `--copy-dt-needed-entries'. This option also has an effect on the resolution of symbols in dynamic libraries. With the default setting dynamic libraries mentioned on the command line will be recursively searched, following their DT_NEEDED tags to other libraries, in order to resolve symbols required by the output binary. With `--no-copy-dt-needed-entries' specified however the searching of dynamic libraries that follow it will stop with the dynamic library itself. No DT_NEEDED links will be traversed to resolve symbols. `--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. `--no-define-common' This option inhibits the assignment of addresses to common symbols. The script command `INHIBIT_COMMON_ALLOCATION' has the same effect. *Note Miscellaneous Commands::. The `--no-define-common' option allows decoupling the decision to assign addresses to Common symbols from the choice of the output file type; otherwise a non-Relocatable output type forces assigning addresses to Common symbols. Using `--no-define-common' allows Common symbols that are referenced from a shared library to be assigned addresses only in the main program. This eliminates the unused duplicate space in the shared library, and also prevents any possible confusion over resolving to the wrong duplicate when there are many dynamic modules with specialized search paths for runtime symbol resolution. `--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: Assignments.). _Note:_ there should be no white space between SYMBOL, the equals sign ("<=>"), and EXPRESSION. `--demangle[=STYLE]' `--no-demangle' These options control whether to demangle symbol names in error messages and other output. When the linker is told to demangle, it tries to present symbol names in a readable fashion: it strips leading underscores if they are used by the object file format, and converts C++ mangled symbol names into user readable names. Different compilers have different mangling styles. The optional demangling style argument can be used to choose an appropriate demangling style for your compiler. The linker will demangle by default unless the environment variable `COLLECT_NO_DEMANGLE' is set. These options may be used to override the default. `-IFILE' `--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. `--fatal-warnings' `--no-fatal-warnings' Treat all warnings as errors. The default behaviour can be restored with the option `--no-fatal-warnings'. `--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. `--gc-sections' `--no-gc-sections' Enable garbage collection of unused input sections. It is ignored on targets that do not support this option. The default behaviour (of not performing this garbage collection) can be restored by specifying `--no-gc-sections' on the command line. `--gc-sections' decides which input sections are used by examining symbols and relocations. The section containing the entry symbol and all sections containing symbols undefined on the command-line will be kept, as will sections containing symbols referenced by dynamic objects. Note that when building shared libraries, the linker must assume that any visible symbol is referenced. Once this initial set of sections has been determined, the linker recursively marks as used any section referenced by their relocations. See `--entry' and `--undefined'. This option can be set when doing a partial link (enabled with option `-r'). In this case the root of symbols kept must be explicitly specified either by an `--entry' or `--undefined' option or by a `ENTRY' command in the linker script. `--print-gc-sections' `--no-print-gc-sections' List all sections removed by garbage collection. The listing is printed on stderr. This option is only effective if garbage collection has been enabled via the `--gc-sections') option. The default behaviour (of not listing the sections that are removed) can be restored by specifying `--no-print-gc-sections' on the command line. `--help' Print a summary of the command-line options on the standard output and exit. `--target-help' Print a summary of all target specific options on the standard output and exit. `-Map=MAPFILE' Print a link map to the file MAPFILE. See the description of the `-M' option, above. `--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-undefined' `-z defs' Report unresolved symbol references from regular object files. This is done even if the linker is creating a non-symbolic shared library. The switch `--[no-]allow-shlib-undefined' controls the behaviour for reporting unresolved references found in shared libraries being linked in. `--allow-multiple-definition' `-z muldefs' Normally when a symbol is defined multiple times, the linker will report a fatal error. These options allow multiple definitions and the first definition will be used. `--allow-shlib-undefined' `--no-allow-shlib-undefined' Allows or disallows undefined symbols in shared libraries. This switch is similar to `--no-undefined' except that it determines the behaviour when the undefined symbols are in a shared library rather than a regular object file. It does not affect how undefined symbols in regular object files are handled. The default behaviour is to report errors for any undefined symbols referenced in shared libraries if the linker is being used to create an executable, but to allow them if the linker is being used to create a shared library. The reasons for allowing undefined symbol references in shared libraries specified at link time are that: * A shared library specified at link time may not be the same as the one that is available at load time, so the symbol might actually be resolvable at load time. * There are some operating systems, eg BeOS and HPPA, where undefined symbols in shared libraries are normal. The BeOS kernel for example patches shared libraries at load time to select whichever function is most appropriate for the current architecture. This is used, for example, to dynamically select an appropriate memset function. `--no-undefined-version' Normally when a symbol has an undefined version, the linker will ignore it. This option disallows symbols with undefined version and a fatal error will be issued instead. `--default-symver' Create and use a default symbol version (the soname) for unversioned exported symbols. `--default-imported-symver' Create and use a default symbol version (the soname) for unversioned imported symbols. `--no-warn-mismatch' Normally `ld' will give an error if you try to link together input files that are mismatched for some reason, perhaps because they have been compiled for different processors or for different endiannesses. This option tells `ld' that it should silently permit such possible errors. This option should only be used with care, in cases when you have taken some special action that ensures that the linker errors are inappropriate. `--no-warn-search-mismatch' Normally `ld' will give a warning if it finds an incompatible library during a library search. This option silences the warning. `--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. `-nostdlib' Only search library directories explicitly specified on the command line. Library directories specified in linker scripts (including linker scripts specified on the command line) are ignored. `--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::. `-pie' `--pic-executable' Create a position independent executable. This is currently only supported on ELF platforms. Position independent executables are similar to shared libraries in that they are relocated by the dynamic linker to the virtual address the OS chooses for them (which can vary between invocations). Like normal dynamically linked executables they can be executed and symbols defined in the executable cannot be overridden by shared libraries. `-qmagic' This option is ignored for Linux compatibility. `-Qy' This option is ignored for SVR4 compatibility. `--relax' `--no-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. *Note `ld' and Xtensa Processors: Xtensa. *Note `ld' and the 68HC11 and 68HC12: M68HC11/68HC12. *Note `ld' and PowerPC 32-bit ELF Support: PowerPC ELF32. On some platforms the `--relax' option performs target specific, global optimizations that become possible when the linker resolves addressing in the program, such as relaxing address modes, synthesizing new instructions, selecting shorter version of current instructions, and combinig constant values. On some platforms these link time global optimizations may make symbolic debugging of the resulting executable impossible. This is known to be the case for the Matsushita MN10200 and MN10300 family of processors. On platforms where this is not supported, `--relax' is accepted, but ignored. On platforms where `--relax' is accepted the option `--no-relax' can be used to disable the feature. `--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 file systems. 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-relocatable link, it will automatically try to locate the required shared library and include it in the link, if it is not included explicitly. In such a case, the `-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. This option should be used with caution as it overrides the search path that may have been hard compiled into a shared library. In such a case it is possible to use unintentionally a different search path than the runtime linker would do. The linker uses the following search paths to locate required shared libraries: 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. Searching `-rpath' in this way is only supported by native linkers and cross linkers which have been configured with the `--with-sysroot' option. 3. On an ELF system, for native linkers, 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 search the contents of the environment variable `LD_LIBRARY_PATH'. 6. For a native ELF linker, the directories in `DT_RUNPATH' or `DT_RPATH' of a shared library are searched for shared libraries needed by it. The `DT_RPATH' entries are ignored if `DT_RUNPATH' entries exist. 7. The default directories, normally `/lib' and `/usr/lib'. 8. For a native linker on an ELF system, if the file `/etc/ld.so.conf' exists, the list of directories found in that file. 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' `--sort-common=ascending' `--sort-common=descending' This option tells `ld' to sort the common symbols by alignment in ascending or descending order when it places them in the appropriate output sections. The symbol alignments considered are sixteen-byte or larger, eight-byte, four-byte, two-byte, and one-byte. This is to prevent gaps between symbols due to alignment constraints. If no sorting order is specified, then descending order is assumed. `--sort-section=name' This option will apply `SORT_BY_NAME' to all wildcard section patterns in the linker script. `--sort-section=alignment' This option will apply `SORT_BY_ALIGNMENT' to all wildcard section patterns in the linker script. `--split-by-file[=SIZE]' Similar to `--split-by-reloc' but creates a new output section for each input file when SIZE is reached. SIZE defaults to a size of 1 if not given. `--split-by-reloc[=COUNT]' Tries 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 files for downloading into certain real time kernels with the COFF object file format; since COFF cannot represent more than 65535 relocations in a single section. Note that this will fail to work with object file formats which do not support arbitrary sections. The linker will not split up individual input sections for redistribution, so if a single input section contains more than COUNT relocations one output section will contain that many relocations. COUNT defaults to a value of 32768. `--stats' Compute and display statistics about the operation of the linker, such as execution time and memory usage. `--sysroot=DIRECTORY' Use DIRECTORY as the location of the sysroot, overriding the configure-time default. This option is only supported by linkers that were configured using `--with-sysroot'. `--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. `--section-start=SECTIONNAME=ORG' Locate a section in the output file at the absolute address given by ORG. You may use this option as many times as necessary to locate multiple sections in the command line. ORG must be a single hexadecimal integer; for compatibility with other linkers, you may omit the leading `0x' usually associated with hexadecimal values. _Note:_ there should be no white space between SECTIONNAME, the equals sign ("<=>"), and ORG. `-Tbss=ORG' `-Tdata=ORG' `-Ttext=ORG' Same as `--section-start', with `.bss', `.data' or `.text' as the SECTIONNAME. `-Ttext-segment=ORG' When creating an ELF executable or shared object, it will set the address of the first byte of the text segment. `--unresolved-symbols=METHOD' Determine how to handle unresolved symbols. There are four possible values for `method': `ignore-all' Do not report any unresolved symbols. `report-all' Report all unresolved symbols. This is the default. `ignore-in-object-files' Report unresolved symbols that are contained in shared libraries, but ignore them if they come from regular object files. `ignore-in-shared-libs' Report unresolved symbols that come from regular object files, but ignore them if they come from shared libraries. This can be useful when creating a dynamic binary and it is known that all the shared libraries that it should be referencing are included on the linker's command line. The behaviour for shared libraries on their own can also be controlled by the `--[no-]allow-shlib-undefined' option. Normally the linker will generate an error message for each reported unresolved symbol but the option `--warn-unresolved-symbols' can change this to a warning. `--dll-verbose' `--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 being used by the linker. `--version-script=VERSION-SCRIPTFILE' Specify the name of a version script to the linker. This is typically used when creating shared libraries to specify additional information about the version hierarchy for the library being created. This option is only fully supported on ELF platforms which support shared libraries; see *Note VERSION::. It is partially supported on PE platforms, which can use version scripts to filter symbol visibility in auto-export mode: any symbols marked `local' in the version script will not be exported. *Note WIN32::. `--warn-common' Warn when a common symbol is combined with another common symbol or with a symbol definition. Unix linkers allow this somewhat sloppy practise, but linkers on some other operating systems do not. This option allows you to find potential problems from combining global symbols. Unfortunately, some C libraries use this practise, so you may get some warnings about symbols in the libraries as well as in your programs. There are three kinds of global symbols, illustrated here by C examples: `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. `--warn-section-align' Warn if the address of an output section is changed because of alignment. Typically, the alignment will be set by an input section. The address will only be changed if it not explicitly specified; that is, if the `SECTIONS' command does not specify a start address for the section (*note SECTIONS::). `--warn-shared-textrel' Warn if the linker adds a DT_TEXTREL to a shared object. `--warn-alternate-em' Warn if an object has alternate ELF machine code. `--warn-unresolved-symbols' If the linker is going to report an unresolved symbol (see the option `--unresolved-symbols') it will normally generate an error. This option makes it generate a warning instead. `--error-unresolved-symbols' This restores the linker's default behaviour of generating errors when it is reporting unresolved symbols. `--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. Two notes when using this option from gcc: First, gcc doesn't know about this option, so you have to use `-Wl,-whole-archive'. Second, don't forget to use `-Wl,-no-whole-archive' after your list of archives, because gcc will add its own list of archives to your link and you may not want this flag to affect those as well. `--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 (size_t c) { printf ("malloc called with %zu\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'. `--eh-frame-hdr' Request creation of `.eh_frame_hdr' section and ELF `PT_GNU_EH_FRAME' segment header. `--enable-new-dtags' `--disable-new-dtags' This linker can create the new dynamic tags in ELF. But the older ELF systems may not understand them. If you specify `--enable-new-dtags', the dynamic tags will be created as needed. If you specify `--disable-new-dtags', no new dynamic tags will be created. By default, the new dynamic tags are not created. Note that those options are only available for ELF systems. `--hash-size=NUMBER' Set the default size of the linker's hash tables to a prime number close to NUMBER. Increasing this value can reduce the length of time it takes the linker to perform its tasks, at the expense of increasing the linker's memory requirements. Similarly reducing this value can reduce the memory requirements at the expense of speed. `--hash-style=STYLE' Set the type of linker's hash table(s). STYLE can be either `sysv' for classic ELF `.hash' section, `gnu' for new style GNU `.gnu.hash' section or `both' for both the classic ELF `.hash' and new style GNU `.gnu.hash' hash tables. The default is `sysv'. `--reduce-memory-overheads' This option reduces memory requirements at ld runtime, at the expense of linking speed. This was introduced to select the old O(n^2) algorithm for link map file generation, rather than the new O(n) algorithm which uses about 40% more memory for symbol storage. Another effect of the switch is to set the default hash table size to 1021, which again saves memory at the cost of lengthening the linker's run time. This is not done however if the `--hash-size' switch has been used. The `--reduce-memory-overheads' switch may be also be used to enable other tradeoffs in future versions of the linker. `--build-id' `--build-id=STYLE' Request creation of `.note.gnu.build-id' ELF note section. The contents of the note are unique bits identifying this linked file. STYLE can be `uuid' to use 128 random bits, `sha1' to use a 160-bit SHA1 hash on the normative parts of the output contents, `md5' to use a 128-bit MD5 hash on the normative parts of the output contents, or `0xHEXSTRING' to use a chosen bit string specified as an even number of hexadecimal digits (`-' and `:' characters between digit pairs are ignored). If STYLE is omitted, `sha1' is used. The `md5' and `sha1' styles produces an identifier that is always the same in an identical output file, but will be unique among all nonidentical output files. It is not intended to be compared as a checksum for the file's contents. A linked file may be changed later by other tools, but the build ID bit string identifying the original linked file does not change. Passing `none' for STYLE disables the setting from any `--build-id' options earlier on the command line. 2.1.1 Options Specific to i386 PE Targets ----------------------------------------- The i386 PE linker supports the `-shared' option, which causes the output to be a dynamically linked library (DLL) instead of a normal executable. You should name the output `*.dll' when you use this option. In addition, the linker fully supports the standard `*.def' files, which may be specified on the linker command line like an object file (in fact, it should precede archives it exports symbols from, to ensure that they get linked in, just like a normal object file). In addition to the options common to all targets, the i386 PE linker support additional command line options that are specific to the i386 PE target. Options that take values may be separated from their values by either a space or an equals sign. `--add-stdcall-alias' If given, symbols with a stdcall suffix (@NN) will be exported as-is and also with the suffix stripped. [This option is specific to the i386 PE targeted port of the linker] `--base-file FILE' Use FILE as the name of a file in which to save the base addresses of all the relocations needed for generating DLLs with `dlltool'. [This is an i386 PE specific option] `--dll' Create a DLL instead of a regular executable. You may also use `-shared' or specify a `LIBRARY' in a given `.def' file. [This option is specific to the i386 PE targeted port of the linker] `--enable-long-section-names' `--disable-long-section-names' The PE variants of the Coff object format add an extension that permits the use of section names longer than eight characters, the normal limit for Coff. By default, these names are only allowed in object files, as fully-linked executable images do not carry the Coff string table required to support the longer names. As a GNU extension, it is possible to allow their use in executable images as well, or to (probably pointlessly!) disallow it in object files, by using these two options. Executable images generated with these long section names are slightly non-standard, carrying as they do a string table, and may generate confusing output when examined with non-GNU PE-aware tools, such as file viewers and dumpers. However, GDB relies on the use of PE long section names to find Dwarf-2 debug information sections in an executable image at runtime, and so if neither option is specified on the command-line, `ld' will enable long section names, overriding the default and technically correct behaviour, when it finds the presence of debug information while linking an executable image and not stripping symbols. [This option is valid for all PE targeted ports of the linker] `--enable-stdcall-fixup' `--disable-stdcall-fixup' If the link finds a symbol that it cannot resolve, it will attempt to do "fuzzy linking" by looking for another defined symbol that differs only in the format of the symbol name (cdecl vs stdcall) and will resolve that symbol by linking to the match. For example, the undefined symbol `_foo' might be linked to the function `_foo@12', or the undefined symbol `_bar@16' might be linked to the function `_bar'. When the linker does this, it prints a warning, since it normally should have failed to link, but sometimes import libraries generated from third-party dlls may need this feature to be usable. If you specify `--enable-stdcall-fixup', this feature is fully enabled and warnings are not printed. If you specify `--disable-stdcall-fixup', this feature is disabled and such mismatches are considered to be errors. [This option is specific to the i386 PE targeted port of the linker] `--leading-underscore' `--no-leading-underscore' For most targets default symbol-prefix is an underscore and is defined in target's description. By this option it is possible to disable/enable the default underscore symbol-prefix. `--export-all-symbols' If given, all global symbols in the objects used to build a DLL will be exported by the DLL. Note that this is the default if there otherwise wouldn't be any exported symbols. When symbols are explicitly exported via DEF files or implicitly exported via function attributes, the default is to not export anything else unless this option is given. Note that the symbols `DllMain@12', `DllEntryPoint@0', `DllMainCRTStartup@12', and `impure_ptr' will not be automatically exported. Also, symbols imported from other DLLs will not be re-exported, nor will symbols specifying the DLL's internal layout such as those beginning with `_head_' or ending with `_iname'. In addition, no symbols from `libgcc', `libstd++', `libmingw32', or `crtX.o' will be exported. Symbols whose names begin with `__rtti_' or `__builtin_' will not be exported, to help with C++ DLLs. Finally, there is an extensive list of cygwin-private symbols that are not exported (obviously, this applies on when building DLLs for cygwin targets). These cygwin-excludes are: `_cygwin_dll_entry@12', `_cygwin_crt0_common@8', `_cygwin_noncygwin_dll_entry@12', `_fmode', `_impure_ptr', `cygwin_attach_dll', `cygwin_premain0', `cygwin_premain1', `cygwin_premain2', `cygwin_premain3', and `environ'. [This option is specific to the i386 PE targeted port of the linker] `--exclude-symbols SYMBOL,SYMBOL,...' Specifies a list of symbols which should not be automatically exported. The symbol names may be delimited by commas or colons. [This option is specific to the i386 PE targeted port of the linker] `--exclude-all-symbols' Specifies no symbols should be automatically exported. [This option is specific to the i386 PE targeted port of the linker] `--file-alignment' Specify the file alignment. Sections in the file will always begin at file offsets which are multiples of this number. This defaults to 512. [This option is specific to the i386 PE targeted port of the linker] `--heap RESERVE' `--heap RESERVE,COMMIT' Specify the number of bytes of memory to reserve (and optionally commit) to be used as heap for this program. The default is 1Mb reserved, 4K committed. [This option is specific to the i386 PE targeted port of the linker] `--image-base VALUE' Use VALUE as the base address of your program or dll. This is the lowest memory location that will be used when your program or dll is loaded. To reduce the need to relocate and improve performance of your dlls, each should have a unique base address and not overlap any other dlls. The default is 0x400000 for executables, and 0x10000000 for dlls. [This option is specific to the i386 PE targeted port of the linker] `--kill-at' If given, the stdcall suffixes (@NN) will be stripped from symbols before they are exported. [This option is specific to the i386 PE targeted port of the linker] `--large-address-aware' If given, the appropriate bit in the "Characteristics" field of the COFF header is set to indicate that this executable supports virtual addresses greater than 2 gigabytes. This should be used in conjunction with the /3GB or /USERVA=VALUE megabytes switch in the "[operating systems]" section of the BOOT.INI. Otherwise, this bit has no effect. [This option is specific to PE targeted ports of the linker] `--major-image-version VALUE' Sets the major number of the "image version". Defaults to 1. [This option is specific to the i386 PE targeted port of the linker] `--major-os-version VALUE' Sets the major number of the "os version". Defaults to 4. [This option is specific to the i386 PE targeted port of the linker] `--major-subsystem-version VALUE' Sets the major number of the "subsystem version". Defaults to 4. [This option is specific to the i386 PE targeted port of the linker] `--minor-image-version VALUE' Sets the minor number of the "image version". Defaults to 0. [This option is specific to the i386 PE targeted port of the linker] `--minor-os-version VALUE' Sets the minor number of the "os version". Defaults to 0. [This option is specific to the i386 PE targeted port of the linker] `--minor-subsystem-version VALUE' Sets the minor number of the "subsystem version". Defaults to 0. [This option is specific to the i386 PE targeted port of the linker] `--output-def FILE' The linker will create the file FILE which will contain a DEF file corresponding to the DLL the linker is generating. This DEF file (which should be called `*.def') may be used to create an import library with `dlltool' or may be used as a reference to automatically or implicitly exported symbols. [This option is specific to the i386 PE targeted port of the linker] `--out-implib FILE' The linker will create the file FILE which will contain an import lib corresponding to the DLL the linker is generating. This import lib (which should be called `*.dll.a' or `*.a' may be used to link clients against the generated DLL; this behaviour makes it possible to skip a separate `dlltool' import library creation step. [This option is specific to the i386 PE targeted port of the linker] `--enable-auto-image-base' Automatically choose the image base for DLLs, unless one is specified using the `--image-base' argument. By using a hash generated from the dllname to create unique image bases for each DLL, in-memory collisions and relocations which can delay program execution are avoided. [This option is specific to the i386 PE targeted port of the linker] `--disable-auto-image-base' Do not automatically generate a unique image base. If there is no user-specified image base (`--image-base') then use the platform default. [This option is specific to the i386 PE targeted port of the linker] `--dll-search-prefix STRING' When linking dynamically to a dll without an import library, search for `.dll' in preference to `lib.dll'. This behaviour allows easy distinction between DLLs built for the various "subplatforms": native, cygwin, uwin, pw, etc. For instance, cygwin DLLs typically use `--dll-search-prefix=cyg'. [This option is specific to the i386 PE targeted port of the linker] `--enable-auto-import' Do sophisticated linking of `_symbol' to `__imp__symbol' for DATA imports from DLLs, and create the necessary thunking symbols when building the import libraries with those DATA exports. Note: Use of the 'auto-import' extension will cause the text section of the image file to be made writable. This does not conform to the PE-COFF format specification published by Microsoft. Note - use of the 'auto-import' extension will also cause read only data which would normally be placed into the .rdata section to be placed into the .data section instead. This is in order to work around a problem with consts that is described here: http://www.cygwin.com/ml/cygwin/2004-09/msg01101.html Using 'auto-import' generally will 'just work' - but sometimes you may see this message: "variable '' can't be auto-imported. Please read the documentation for ld's `--enable-auto-import' for details." This message occurs when some (sub)expression accesses an address ultimately given by the sum of two constants (Win32 import tables only allow one). Instances where this may occur include accesses to member fields of struct variables imported from a DLL, as well as using a constant index into an array variable imported from a DLL. Any multiword variable (arrays, structs, long long, etc) may trigger this error condition. However, regardless of the exact data type of the offending exported variable, ld will always detect it, issue the warning, and exit. There are several ways to address this difficulty, regardless of the data type of the exported variable: One way is to use -enable-runtime-pseudo-reloc switch. This leaves the task of adjusting references in your client code for runtime environment, so this method works only when runtime environment supports this feature. A second solution is to force one of the 'constants' to be a variable - that is, unknown and un-optimizable at compile time. For arrays, there are two possibilities: a) make the indexee (the array's address) a variable, or b) make the 'constant' index a variable. Thus: extern type extern_array[]; extern_array[1] --> { volatile type *t=extern_array; t[1] } or extern type extern_array[]; extern_array[1] --> { volatile int t=1; extern_array[t] } For structs (and most other multiword data types) the only option is to make the struct itself (or the long long, or the ...) variable: extern struct s extern_struct; extern_struct.field --> { volatile struct s *t=&extern_struct; t->field } or extern long long extern_ll; extern_ll --> { volatile long long * local_ll=&extern_ll; *local_ll } A third method of dealing with this difficulty is to abandon 'auto-import' for the offending symbol and mark it with `__declspec(dllimport)'. However, in practise that requires using compile-time #defines to indicate whether you are building a DLL, building client code that will link to the DLL, or merely building/linking to a static library. In making the choice between the various methods of resolving the 'direct address with constant offset' problem, you should consider typical real-world usage: Original: --foo.h extern int arr[]; --foo.c #include "foo.h" void main(int argc, char **argv){ printf("%d\n",arr[1]); } Solution 1: --foo.h extern int arr[]; --foo.c #include "foo.h" void main(int argc, char **argv){ /* This workaround is for win32 and cygwin; do not "optimize" */ volatile int *parr = arr; printf("%d\n",parr[1]); } Solution 2: --foo.h /* Note: auto-export is assumed (no __declspec(dllexport)) */ #if (defined(_WIN32) || defined(__CYGWIN__)) && \ !(defined(FOO_BUILD_DLL) || defined(FOO_STATIC)) #define FOO_IMPORT __declspec(dllimport) #else #define FOO_IMPORT #endif extern FOO_IMPORT int arr[]; --foo.c #include "foo.h" void main(int argc, char **argv){ printf("%d\n",arr[1]); } A fourth way to avoid this problem is to re-code your library to use a functional interface rather than a data interface for the offending variables (e.g. set_foo() and get_foo() accessor functions). [This option is specific to the i386 PE targeted port of the linker] `--disable-auto-import' Do not attempt to do sophisticated linking of `_symbol' to `__imp__symbol' for DATA imports from DLLs. [This option is specific to the i386 PE targeted port of the linker] `--enable-runtime-pseudo-reloc' If your code contains expressions described in -enable-auto-import section, that is, DATA imports from DLL with non-zero offset, this switch will create a vector of 'runtime pseudo relocations' which can be used by runtime environment to adjust references to such data in your client code. [This option is specific to the i386 PE targeted port of the linker] `--disable-runtime-pseudo-reloc' Do not create pseudo relocations for non-zero offset DATA imports from DLLs. This is the default. [This option is specific to the i386 PE targeted port of the linker] `--enable-extra-pe-debug' Show additional debug info related to auto-import symbol thunking. [This option is specific to the i386 PE targeted port of the linker] `--section-alignment' Sets the section alignment. Sections in memory will always begin at addresses which are a multiple of this number. Defaults to 0x1000. [This option is specific to the i386 PE targeted port of the linker] `--stack RESERVE' `--stack RESERVE,COMMIT' Specify the number of bytes of memory to reserve (and optionally commit) to be used as stack for this program. The default is 2Mb reserved, 4K committed. [This option is specific to the i386 PE targeted port of the linker] `--subsystem WHICH' `--subsystem WHICH:MAJOR' `--subsystem WHICH:MAJOR.MINOR' Specifies the subsystem under which your program will execute. The legal values for WHICH are `native', `windows', `console', `posix', and `xbox'. You may optionally set the subsystem version also. Numeric values are also accepted for WHICH. [This option is specific to the i386 PE targeted port of the linker] The following options set flags in the `DllCharacteristics' field of the PE file header: [These options are specific to PE targeted ports of the linker] `--dynamicbase' The image base address may be relocated using address space layout randomization (ASLR). This feature was introduced with MS Windows Vista for i386 PE targets. `--forceinteg' Code integrity checks are enforced. `--nxcompat' The image is compatible with the Data Execution Prevention. This feature was introduced with MS Windows XP SP2 for i386 PE targets. `--no-isolation' Although the image understands isolation, do not isolate the image. `--no-seh' The image does not use SEH. No SE handler may be called from this image. `--no-bind' Do not bind this image. `--wdmdriver' The driver uses the MS Windows Driver Model. `--tsaware' The image is Terminal Server aware. 2.1.2 Options specific to Motorola 68HC11 and 68HC12 targets ------------------------------------------------------------ The 68HC11 and 68HC12 linkers support specific options to control the memory bank switching mapping and trampoline code generation. `--no-trampoline' This option disables the generation of trampoline. By default a trampoline is generated for each far function which is called using a `jsr' instruction (this happens when a pointer to a far function is taken). `--bank-window NAME' This option indicates to the linker the name of the memory region in the `MEMORY' specification that describes the memory bank window. The definition of such region is then used by the linker to compute paging and addresses within the memory window. 2.1.3 Options specific to Motorola 68K target --------------------------------------------- The following options are supported to control handling of GOT generation when linking for 68K targets. `--got=TYPE' This option tells the linker which GOT generation scheme to use. TYPE should be one of `single', `negative', `multigot' or `target'. For more information refer to the Info entry for `ld'.  File: ld.info, Node: Environment, Prev: Options, Up: Invocation 2.2 Environment Variables ========================= You can change the behaviour of `ld' with the environment variables `GNUTARGET', `LDEMULATION' and `COLLECT_NO_DEMANGLE'. `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. `LDEMULATION' determines the default emulation if you don't use the `-m' option. The emulation can affect various aspects of linker behaviour, particularly the default linker script. You can list the available emulations with the `--verbose' or `-V' options. If the `-m' option is not used, and the `LDEMULATION' environment variable is not defined, the default emulation depends upon how the linker was configured. Normally, the linker will default to demangling symbols. However, if `COLLECT_NO_DEMANGLE' is set in the environment, then it will default to not demangling symbols. This environment variable is used in a similar fashion by the `gcc' linker wrapper program. The default may be overridden by the `--demangle' and `--no-demangle' options.  File: ld.info, Node: Scripts, Next: Machine Dependent, Prev: Invocation, Up: Top 3 Linker Scripts **************** Every link is controlled by a "linker script". This script is written in the linker command language. The main purpose of the linker script is to describe how the sections in the input files should be mapped into the output file, and to control the memory layout of the output file. Most linker scripts do nothing more than this. However, when necessary, the linker script can also direct the linker to perform many other operations, using the commands described below. The linker always uses a linker script. If you do not supply one yourself, the linker will use a default script that is compiled into the linker executable. You can use the `--verbose' command line option to display the default linker script. Certain command line options, such as `-r' or `-N', will affect the default linker script. You may supply your own linker script by using the `-T' command line option. When you do this, your linker script will replace the default linker script. You may also use linker scripts implicitly by naming them as input files to the linker, as though they were files to be linked. *Note Implicit Linker Scripts::. * Menu: * Basic Script Concepts:: Basic Linker Script Concepts * Script Format:: Linker Script Format * Simple Example:: Simple Linker Script Example * Simple Commands:: Simple Linker Script Commands * Assignments:: Assigning Values to Symbols * SECTIONS:: SECTIONS Command * MEMORY:: MEMORY Command * PHDRS:: PHDRS Command * VERSION:: VERSION Command * Expressions:: Expressions in Linker Scripts * Implicit Linker Scripts:: Implicit Linker Scripts  File: ld.info, Node: Basic Script Concepts, Next: Script Format, Up: Scripts 3.1 Basic Linker Script Concepts ================================ We need to define some basic concepts and vocabulary in order to describe the linker script language. The linker combines input files into a single output file. The output file and each input file are in a special data format known as an "object file format". Each file is called an "object file". The output file is often called an "executable", but for our purposes we will also call it an object file. Each object file has, among other things, a list of "sections". We sometimes refer to a section in an input file as an "input section"; similarly, a section in the output file is an "output section". Each section in an object file has a name and a size. Most sections also have an associated block of data, known as the "section contents". A section may be marked as "loadable", which mean that the contents should be loaded into memory when the output file is run. A section with no contents may be "allocatable", which means that an area in memory should be set aside, but nothing in particular should be loaded there (in some cases this memory must be zeroed out). A section which is neither loadable nor allocatable typically contains some sort of debugging information. Every loadable or allocatable output section has two addresses. The first is the "VMA", or virtual memory address. This is the address the section will have when the output file is run. The second is the "LMA", or load memory address. This is the address at which the section will be loaded. In most cases the two addresses will be the same. An example of when they might be different is when a data section is loaded into ROM, and then copied into RAM when the program starts up (this technique is often used to initialize global variables in a ROM based system). In this case the ROM address would be the LMA, and the RAM address would be the VMA. You can see the sections in an object file by using the `objdump' program with the `-h' option. Every object file also has a list of "symbols", known as the "symbol table". A symbol may be defined or undefined. Each symbol has a name, and each defined symbol has an address, among other information. If you compile a C or C++ program into an object file, you will get a defined symbol for every defined function and global or static variable. Every undefined function or global variable which is referenced in the input file will become an undefined symbol. You can see the symbols in an object file by using the `nm' program, or by using the `objdump' program with the `-t' option.  File: ld.info, Node: Script Format, Next: Simple Example, Prev: Basic Script Concepts, Up: Scripts 3.2 Linker Script Format ======================== Linker scripts are text files. You write a linker script as a series of commands. Each command is either a keyword, possibly followed by arguments, or an assignment to a symbol. You may separate commands using semicolons. Whitespace is generally ignored. Strings such as file or format names can normally be entered directly. If the file name contains a character such as a comma which would otherwise serve to separate file names, you may put the file name in double quotes. There is no way to use a double quote character in a file name. You may include comments in linker scripts just as in C, delimited by `/*' and `*/'. As in C, comments are syntactically equivalent to whitespace.  File: ld.info, Node: Simple Example, Next: Simple Commands, Prev: Script Format, Up: Scripts 3.3 Simple Linker Script Example ================================ Many linker scripts are fairly simple. The simplest possible linker script has just one command: `SECTIONS'. You use the `SECTIONS' command to describe the memory layout of the output file. The `SECTIONS' command is a powerful command. Here we will describe a simple use of it. Let's assume your program consists only of code, initialized data, and uninitialized data. These will be in the `.text', `.data', and `.bss' sections, respectively. Let's assume further that these are the only sections which appear in your input files. For this example, let's say that the code should be loaded at address 0x10000, and that the data should start at address 0x8000000. Here is a linker script which will do that: SECTIONS { . = 0x10000; .text : { *(.text) } . = 0x8000000; .data : { *(.data) } .bss : { *(.bss) } } You write the `SECTIONS' command as the keyword `SECTIONS', followed by a series of symbol assignments and output section descriptions enclosed in curly braces. The first line inside the `SECTIONS' command of the above example sets the value of the special symbol `.', which is the location counter. If you do not specify the address of an output section in some other way (other ways are described later), the address is set from the current value of the location counter. The location counter is then incremented by the size of the output section. At the start of the `SECTIONS' command, the location counter has the value `0'. The second line defines an output section, `.text'. The colon is required syntax which may be ignored for now. Within the curly braces after the output section name, you list the names of the input sections which should be placed into this output section. The `*' is a wildcard which matches any file name. The expression `*(.text)' means all `.text' input sections in all input files. Since the location counter is `0x10000' when the output section `.text' is defined, the linker will set the address of the `.text' section in the output file to be `0x10000'. The remaining lines define the `.data' and `.bss' sections in the output file. The linker will place the `.data' output section at address `0x8000000'. After the linker places the `.data' output section, the value of the location counter will be `0x8000000' plus the size of the `.data' output section. The effect is that the linker will place the `.bss' output section immediately after the `.data' output section in memory. The linker will ensure that each output section has the required alignment, by increasing the location counter if necessary. In this example, the specified addresses for the `.text' and `.data' sections will probably satisfy any alignment constraints, but the linker may have to create a small gap between the `.data' and `.bss' sections. That's it! That's a simple and complete linker script.  File: ld.info, Node: Simple Commands, Next: Assignments, Prev: Simple Example, Up: Scripts 3.4 Simple Linker Script Commands ================================= In this section we describe the simple linker script commands. * Menu: * Entry Point:: Setting the entry point * File Commands:: Commands dealing with files * Format Commands:: Commands dealing with object file formats * REGION_ALIAS:: Assign alias names to memory regions * Miscellaneous Commands:: Other linker script commands  File: ld.info, Node: Entry Point, Next: File Commands, Up: Simple Commands 3.4.1 Setting the Entry Point ----------------------------- The first instruction to execute in a program is called the "entry point". You can use the `ENTRY' linker script command to set the entry point. The argument is a symbol name: ENTRY(SYMBOL) There are several ways to set the entry point. The linker will set the entry point by trying each of th