Node: Options, Next: Environment, Up: Invocation
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.)
Some of the command-line options to ld may be specified at any point in the command line. However, options which refer to files, such as -l or -T, cause the file to be read at the point at which the option appears in the command line, relative to the object files and other file options. Repeating non-file 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 object files or archives 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 cannot 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 that specifying a script in this way merely augments the main linker script; use the -T option to replace the default linker script entirely. See Scripts.
For options whose names are a single letter, option arguments must either follow the option letter without intervening whitespace, or be given as separate arguments immediately following the option that requires them.
For options whose names are multiple letters, either one dash or two can precede the option name; for example, -trace-symbol and --trace-symbol are equivalent. Note--there is one exception to this rule. Multiple letter options that start with a lower case 'o' can only be preceeded by two dashes. This is to reduce confusion with the -o option. So for example -omagic sets the output file name to magic whereas --omagic sets the NMAGIC flag on the output.
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, --trace-symbol foo and --trace-symbol=foo are equivalent. Unique abbreviations of the names of multiple-letter options are accepted.
Note--if the linker is being invoked indirectly, via a compiler driver (e.g. gcc) then all the linker command line options should be prefixed by -Wl, (or whatever is appropriate for the particular compiler driver) like this:
gcc -Wl,--startgroup foo.o bar.o -Wl,--endgroup
This is important, because otherwise the compiler driver program may silently drop the linker options, resulting in a bad link.
Here is a table of the generic command line switches accepted by the GNU linker:
-akeywordarchive, 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=architectureld, 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. See ld and the Intel 960 family, for details.
Future releases of ld may support similar functionality for other architecture families.
-b input-format--format=input-formatld 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.) See 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. See Environment. You can also define the input format from a script, using the command TARGET; see Format Commands.
-c MRI-commandfile--mri-script=MRI-commandfileld accepts script files written in an alternate, restricted command language, described in MRI Compatible Script Files. 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-r). The script command FORCE_COMMON_ALLOCATION has the same effect. See Miscellaneous Commands.-e entry--entry=entry0x for base 16, or a leading 0 for base 8). See Entry Point, for a discussion of defaults and other ways of specifying the entry point.-E--export-dynamicIf you do not use this option, the dynamic symbol table will normally contain only those symbols which are referenced by some dynamic object mentioned in the link.
If you use dlopen to load a dynamic object which needs to refer back to the symbols defined by the program, rather than some other dynamic object, then you will probably need to use this option when linking the program itself.
You can also use the version script to control what symbols should be added to the dynamic symbol table if the output format supports it. See the description of --version-script in VERSION.
-EB-EL-f--auxiliary nameIf you later link a program against this filter object, then, when you run the program, the dynamic linker will see the DT_AUXILIARY field. If the dynamic linker resolves any symbols from the filter object, it will first check whether there is a definition in the shared object name. If there is one, it will be used instead of the definition in the filter object. The shared object name need not exist. Thus the shared object name may be used to provide an alternative implementation of certain functions, perhaps for debugging or for machine specific performance.
This option may be specified more than once. The DT_AUXILIARY entries will be created in the order in which they appear on the command line.
-F name--filter nameIf you later link a program against this filter object, then, when you run the program, the dynamic linker will see the DT_FILTER field. The dynamic linker will resolve symbols according to the symbol table of the filter object as usual, but it will actually link to the definitions found in the shared object name. Thus the filter object can be used to select a subset of the symbols provided by the object name.
Some older linkers used the -F option throughout a compilation toolchain for specifying object-file format for both input and output object files. The GNU linker uses other mechanisms for this purpose: the -b, --format, --oformat options, the TARGET command in linker scripts, and the GNUTARGET environment variable. The GNU linker will ignore the -F option when not creating an ELF shared object.
-fini name_fini as the function to call.-g-Gvalue--gpsize=value-hname-soname=name-i-r).-init name_init as the function to call.-larchive--library=archiveld will search its path-list for occurrences of libarchive.a for every archive specified.
On systems which support shared libraries, ld may also search for libraries with extensions other than .a. Specifically, on ELF and SunOS systems, ld will search a directory for a library with an extension of .so before searching for one with an extension of .a. By convention, a .so extension indicates a shared library.
The linker will search an archive only once, at the location where it is specified on the command line. If the archive defines a symbol which was undefined in some object which appeared before the archive on the command line, the linker will include the appropriate file(s) from the archive. However, an undefined symbol in an object appearing later on the command line will not cause the linker to search the archive again.
See the -( option for a way to force the linker to search archives multiple times.
You may list the same archive multiple times on the command line.
This type of archive searching is standard for Unix linkers. However, if you are using ld on AIX, note that it is different from the behaviour of the AIX linker.
-Lsearchdir--library-path=searchdirld 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.
If searchdir begins with =, then the = will be replaced by the sysroot prefix, a path specified when the linker is configured.
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. See 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--verbose or -V options.
If the -m option is not used, the emulation is taken from the LDEMULATION environment variable, if that is defined.
Otherwise, the default emulation depends upon how the linker was configured.
-M--print-map-n--nmagicNMAGIC if possible.-N--omagicOMAGIC. Note: Although a writable text section is allowed for PE-COFF targets, it does not conform to the format specification published by Microsoft.--no-omagic-N option. It sets the text section to be read-only, and forces the data segment to be page-aligned. Note - this option does not enable linking against shared libraries. Use -Bdynamic for this.-o output--output=outputld; 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.-O levelld optimizes the output. This might take significantly longer and therefore probably should only be enabled for the final binary.-q--emit-relocsThis option is currently only supported on ELF platforms.
-r--relocatableld. 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.
When an input file does not have the same format as the output file, partial linking is only supported if that input file does not contain any relocations. Different output formats can have further restrictions; for example some a.out-based formats do not support partial linking with input files in other formats at all.
This option does the same thing as -i.
-R filename--just-symbols=filenameFor 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-S--strip-debug-t--traceld processes them.-T scriptfile--script=scriptfileld's default linker script (rather than adding to it), so commandfile must specify everything necessary to describe the output file. See Scripts. If scriptfile does not exist in the current directory, ld looks for it in the directories specified by any preceding -L options. Multiple -T options accumulate.-u symbol--undefined=symbol-u may be repeated with different option arguments to enter additional undefined symbols. This option is equivalent to the EXTERN linker script command.-Ur-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.--unique[=SECTION]-v--version-Vld. The -V option also lists the supported emulations.-x--discard-all-X--discard-localsL.-y symbol--trace-symbol=symbolThis option is useful when you have an undefined symbol in your link but don't know where the reference is coming from.
-Y path-z keywordcombrelocdefsinitfirstinterposeloadfltrmuldefsnocombrelocnocopyrelocnodefaultlibnodeletenodlopendlopen.nodumpdldump.noworiginOther keywords are ignored for Solaris compatibility.
-( archives -)--start-group archives --end-group-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.
--accept-unknown-input-arch--no-accept-unknown-input-arch--accept-unknown-input-arch option has been added to restore the old behaviour.--as-needed--no-as-needed--as-needed option. Normally, the linker will add a DT_NEEDED tag for each dynamic library mentioned on the command line, regardless of whether the library is actually needed. --as-needed causes DT_NEEDED tags to only be emitted for libraries that satisfy some reference from regular objects. --no-as-needed restores the default behaviour.-assert keyword-Bdynamic-dy-call_shared-l options which follow it.-BgroupDF_1_GROUP flag in the DT_FLAGS_1 entry in the dynamic section. This causes the runtime linker to handle lookups in this object and its dependencies to be performed only inside the group. --unresolved-symbols=report-all is implied. This option is only meaningful on ELF platforms which support shared libraries.-Bstatic-dn-non_shared-static-l options which follow it. This option also implies --unresolved-symbols=report-all.-Bsymbolic--check-sections--no-check-sections--check-sections.--crefThe 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-commonINHIBIT_COMMON_ALLOCATION has the same effect. See 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+ and - to add or subtract hexadecimal constants or symbols. If you need more elaborate expressions, consider using the linker command language from a script (see Assignment: Symbol Definitions). Note: there should be no white space between symbol, the equals sign ("<=>"), and expression.--demangle[=style]--no-demangleCOLLECT_NO_DEMANGLE is set. These options may be used to override the default.--dynamic-linker file--embedded-relocs--fatal-warnings--force-exe-suffixIf 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.
--no-gc-sections--gc-sections-r, nor should it be used with dynamic linking. The default behaviour (of not performing this garbage collection) can be restored by specifying --no-gc-sections on the command line.--help--target-help-Map mapfile-M option, above.--no-keep-memoryld 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--[no-]allow-shlib-undefined controls the behaviour for reporting unresolved references found in shared libraries being linked in.--allow-multiple-definition-z muldefs--allow-shlib-undefined--no-allow-shlib-undefined--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 reason that --allow-shlib-undefined is the default is that the shared library being specified at link time may not be the same as the one that is available at load time, so the symbols might actually be resolvable at load time. Plus there are some systems, (eg BeOS) where undefined symbols in shared libraries is normal. (The kernel patches them at load time to select which function is most appropriate for the current architecture. This is used for example to dynamically select an appropriate memset function). Apparently it is also normal for HPPA shared libraries to have undefined symbols.
--no-undefined-version--no-warn-mismatchld 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-whole-archive--whole-archive option for subsequent archive files.--noinhibit-exec-nostdlib--oformat output-formatld 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. See BFD.-pie--pic-executable-qmagic-Qy--relaxld and the H8/300. See ld and the Intel 960 family. See ld and Xtensa Processors.
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 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.
--retain-symbols-file filename--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-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 DIRld -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.
-rpath-link options.-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. It is for the native linker only.-rpath and rpath-link options were not used, search the contents of the environment variable LD_RUN_PATH. It is for the native linker only.-rpath option was not used, search any directories specified using -L options.LD_LIBRARY_PATH.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./lib and /usr/lib./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-e option is not used and there are undefined symbols in the link.--sort-commonld 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 byte, then all the four byte, and then everything else. This is to prevent gaps between symbols due to alignment constraints.--split-by-file [size]--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]--stats--traditional-formatld 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=org0x 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.bss, .data or .text as the sectionname.--unresolved-symbols=methodmethod:
ignore-allreport-allignore-in-object-filesignore-in-shared-libsThe 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--verboseld 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--warn-commonThere are three kinds of global symbols, illustrated here by C examples:
int i = 1;extern int i;int i;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.
file(section): warning: common of `symbol'
overridden by definition
file(section): warning: defined here
file(section): warning: definition of `symbol'
overriding common
file(section): warning: common is here
file(section): warning: multiple common
of `symbol'
file(section): warning: previous common is here
file(section): warning: common of `symbol'
overridden by larger common
file(section): warning: larger common is here
file(section): warning: common of `symbol'
overriding smaller common
file(section): warning: smaller common is here
--warn-constructors--warn-multiple-gp--warn-once--warn-section-alignSECTIONS command does not specify a start address for the section (see SECTIONS).--warn-unresolved-symbols--unresolved-symbols) it will normally generate an error. This option makes it generate a warning instead.--error-unresolved-symbols--whole-archive--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__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.
--enable-new-dtags--disable-new-dtags--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.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--base-file filedlltool. [This is an i386 PE specific option]--dll-shared or specify a LIBRARY in a given .def file. [This option is specific to the i386 PE targeted port of the linker]--enable-stdcall-fixup--disable-stdcall-fixup_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]--export-all-symbolsDllMain@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,...--exclude-libs lib,lib,...--exclude-libs ALL excludes symbols in all archive libraries from automatic export. Symbols explicitly listed in a .def file are still exported, regardless of this option. [This option is specific to the i386 PE targeted port of the linker]--file-alignment--heap reserve--heap reserve,commit--image-base value--kill-at--major-image-version value--major-os-version value--major-subsystem-version value--minor-image-version value--minor-os-version value--minor-subsystem-version value--output-def file*.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*.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--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--image-base) then use the platform default. [This option is specific to the i386 PE targeted port of the linker]--dll-search-prefix string<string><basename>.dll in preference to lib<basename>.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_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.
Using 'auto-import' generally will 'just work' - but sometimes you may see this message:
"variable '<var>' 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_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--disable-runtime-pseudo-reloc--enable-extra-pe-debug--section-alignment--stack reserve--stack reserve,commit--subsystem which--subsystem which:major--subsystem which:major.minornative, windows, console, and posix. You may optionally set the subsystem version also. [This option is specific to the i386 PE targeted port of the linker]