The linker is meant to cover a broad range of situations, and to be as compatible as possible with other linkers. As a result, you have many choices to control its behavior.
The linker supports a plethora of command-line options, but in practice few of them are used in any particular context. For example, to link a file hello.o:
This tells the linker to produce a file called output as the result of linking the file crt0.o with hello.o and the library libc.a, which will come from the standard search directories. (See the discussion of the -l option below.)
The command-line options to the linker may be specified in any order, and may be repeated at will. Repeating most options with a different argument will either have no further effect, or override prior occurrences (those further to the left on the command line) of that option. Options that may be meaningfully specified more than once are noted in the descriptions below.
Non-option arguments are objects files that are to be linked together. They may follow, precede, or be mixed in with command-line options, except that an object file argument may not be placed between an option and its argument.
Usually the linker is invoked with at least one object file, but you can specify other forms of binary input files using -l, -R, and the script command language. If no binary input files at all are specified, the linker does not produce any output, and issues the message "No input files".
If the linker can not recognize the format of an object file, it will assume that it is a linker script. A script specified in this way augments the main linker script used for the link (either the default linker script or the one specified by using -T). This feature permits the linker to link against a file that 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 should only be used to augment the main linker script; if you want to use some command that logically can only appear once, such as the SECTIONS or MEMORY command, you must replace the default linker script using the -T option. See Chapter 11.
For options whose names are a single letter, option arguments either must follow the option letter without intervening white-space, or be given as separate arguments immediately following the option that requires them.
For options whose names are multiple letters, either one dash or two can precede the option name; for example, --oformat and --oformat are equivalent. Arguments to multiple-letter options must either be separated from the option name by an equals sign, or be given as separate arguments immediately following the option that requires them. For example, --oformat srec and --oformat=srec are equivalent. Unique abbreviations of the names of multiple-letter options are accepted.
The linker is configured to support more than one kind of object file. You can use the -b option to specify the binary format for input object files that follow this option on the command line. You don't usually need to specify this, as the linker 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 Appendix A.
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 GNUPREFIX. See Section 10.2. You can also define the input format from a script, using the command PREFIX; see Section 11.7.
These three options are equivalent; multiple forms are supported for compatibility with other linkers. They assign space to common symbols even if a relocatable output file is specified (with -r). The script command FORCE_COMMON_ALLOCATION has the same effect. See Section 11.7.
Use entry as the explicit symbol for beginning execution of your program, rather than the default entry point. See Section 11.5, for a discussion of defaults and other ways of specifying the entry point.
When creating a dynamically linked executable, add all symbols to the dynamic symbol table. Normally, the dynamic symbol table contains only symbols that are used by a dynamic object. This option is needed for some uses of dlopen.
When creating an ELF shared object, set the internal DT_AUXILIARY field to the specified name. This tells the dynamic linker that the symbol table of the shared object should be used as an auxiliary filter on the symbol table of the shared object name.
If you later link a program against this filter object, then, when you run the program, the dynamic linker will see the DT_AUXILIARY field. If the dynamic linker resolves any symbols from the filter object, it will first check whether there is a definition in the shared object 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.
When creating an ELF shared object, set the internal DT_FILTER field to the specified name. This tells the dynamic linker that the symbol table of the shared object that is being created should be used as a filter on the symbol table of the shared object name.
If you later link a program against this filter object, then, when you run the program, the dynamic linker will see the DT_FILTER field. The dynamic linker will resolve symbols according to the symbol table of the filter object as usual, but it will actually link to the definitions found in the shared object 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 tool chain for specifying object-file format for both input and output object files. The XGC linker uses other mechanisms for this purpose: the -b, --format, --oformat options, the PREFIX command in linker scripts, and the GNUPREFIX environment variable. The linker will ignore the -F option when not creating an ELF shared object.
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.
Ignored. Provided for compatibility with other tools.
Set the maximum size of objects to be optimized using the GP register to size. This is only meaningful for object file formats such as ECOFF, which supports putting large and small objects into different sections. This is ignored for other object file formats.
When creating an ELF shared object, set the internal DT_SOGNAT field to the specified name. When an executable is linked with a shared object that has a DT_SOGNAT field, then when the executable is run the dynamic linker will attempt to load the shared object specified by the DT_SOGNAT field rather than the using the file name given to the linker.
Perform an incremental link (same as option -r).
Add archive file archive to the list of files to link. This option may be used any number of times. The linker will search its path-list for occurrences of libarchive.a for every archive specified.
On systems that support shared libraries, the linker may also search for libraries with extensions other than .a. Specifically, on ELF and Sun-OS systems, the linker 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 that 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.
Add path searchdir to the list of paths that the linker will search for archive libraries and the linker control scripts. You may use this option any number of times. The directories are searched in the order in which they are specified on the command line. Directories specified on the command line are searched before the default directories. All -L options apply to all -l options, regardless of the order in which the options appear.
The default set of paths searched (without being specified with -L) depends on which emulation mode the linker is using, and in some cases also on how it was configured. See Section 10.2.
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.
Emulate the emulation linker. You can list the available emulations with the --verbose or -V options. The default depends on how the linker was configured.
Print (to the standard output) a link map—diagnostic information about where symbols are mapped by the linker, and information on global common storage allocation.
Set the text segment to be read only, and mark the output as NMAGIC if possible.
Set the text and data sections to be readable and writable. Also, do not page-align the data segment. If the output format supports UNIX style magic numbers, mark the output as OMAGIC.
Use output as the name for the program produced by the linker; 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.
Generate relocatable output—that is, generate an output file that can in turn serve as input to the linker. This is often called partial linking. As a side effect, in environments that support standard UNIX magic numbers, this option also sets the output file's magic number to OMAGIC. If this option is not specified, an absolute file is produced. When linking C++ programs, this option will not resolve references to constructors; to do that, use -Ur.
This option does the same thing as -i.
Read symbol names and their addresses from filename, but do not relocate it or include it in the output. This allows your output file to refer symbolically to absolute locations of memory defined in other programs. You may use this option more than once.
For compatibility with other ELF linkers, if the -R option is followed by a directory name, rather than a file name, it is treated as the -rpath option.
Omit all symbol information from the output file.
Omit debugger symbol information (but not all symbols) from the output file.
Print the names of the input files as the linker processes them.
Read link commands from the file commandfile. These commands replace the linker's default link script (rather than adding to it), so commandfile must specify everything necessary to describe the target format. You must use this option if you want to use a command that can only appear once in a linker script, such as the SECTIONS or MEMORY command. See Chapter 11. If commandfile does not exist, the linker looks for it in the directories specified by any preceding -L options. Multiple -T options accumulate.
Force symbol to be entered in the output file as an undefined symbol. Doing this may, for example, trigger linking of additional modules from standard libraries. -u may be repeated with different option arguments to enter additional undefined symbols.
Display the version number for the linker. The -V option also lists the supported emulations.
Delete all local symbols.
Delete all temporary local symbols. For most targets, this is all local symbols whose names begin with "L".
Print the name of each linked file in which symbol appears. This option may be given any number of times. On many systems it is necessary to prepend an underscore.
This option is useful when you have an undefined symbol in your link but don't know where the reference is coming from.
Add path to the default library search path. This option exists for Solaris compatibility.
This option is ignored for Solaris compatibility.
The archives should be a list of archive files. They may be either explicit file names, or -l options.
The specified archives are searched repeatedly until no new undefined references are created. Normally, an archive is searched only once in the order that it is specified on the command line. If a symbol in that archive is needed to resolve an undefined symbol referred to by an object in an archive that appears later on the command line, the linker would not be able to resolve that reference. By grouping the archives, they all be searched repeatedly until all possible references are resolved.
Using this option has a significant performance cost. It is best to use it only when there are unavoidable circular references between two or more archives.
This option is ignored for Sun-OS compatibility.
Link against dynamic libraries. This is only meaningful on platforms for which shared libraries are supported. This option is normally the default on such platforms. The different variants of this option are for compatibility with various systems. You may use this option multiple times on the command line: it affects library searching for -l options that follow it.
Do not link against shared libraries. This is only meaningful on platforms for which shared libraries are supported. The different variants of this option are for compatibility with various systems. You may use this option multiple times on the command line: it affects library searching for -l options that follow it.
When creating a shared library, bind references to global symbols to the definition within the shared library, if any. Normally, it is possible for a program linked against a shared library to override the definition within the shared library. This option is only meaningful on ELF platforms that support shared libraries.
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.
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 (see Section 11.2.6). Note: there should be no white space between symbol, the equals sign (=), and expression.
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.
Link big-endian objects. This affects the default output format.
Link little-endian objects. This affects the default output format.
Print a summary of the command-line options on the standard output and exit.
Print to the file mapfile a link map — diagnostic information about where symbols are mapped by the linker, and information on global common storage allocation.
The linker normally optimizes for speed over memory usage by caching the symbol tables of input files in memory. This option tells the linker to instead optimize for memory usage, by rereading the symbol tables as necessary. This may be required if the linker runs out of memory space while linking a large executable.
Turn off the effect of the --whole-archive option for subsequent archive files.
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.
The linker may be configured to support more than one kind of object file. If the linker is configured this way, you can use the --oformat option to specify the binary format for the output object file. Even when the linker is configured to support alternative object formats, you don't usually need to specify this, as the linker 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 Appendix A.
This option is ignored for Linux compatibility.
This option is ignored for SVR4 compatibility.
An option with machine dependent effects.
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.
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 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.
This option tells the linker to sort the common symbols by size when it places them in the appropriate output sections. First come all the one byte symbols, then all the two bytes, then all the four bytes, and then everything else. This is to prevent gaps between symbols due to alignment constraints.
Similar to --split-by-reloc but creates a new output section for each input file.
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 for down-loading 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 that 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.
Compute and display statistics about the operation of the linker, such as execution time and memory usage.
For some targets, the output of the linker is different in some ways from the output of some existing linker. This switch requests the linker to use the traditional format instead.
For example, on Sun-OS, the linker 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 Sun-OS dbx program can not read the resulting program (gdb has no trouble). The --traditional-format switch tells the linker to not combine duplicate entries.
Use org as the starting address for—respectively—the bss, data, or the text segment of the output file. org must be a single hexadecimal integer; for compatibility with other linkers, you may omit the leading "0x" usually associated with hexadecimal values.
For anything other than C++ programs, this option is equivalent to -r: it generates relocatable output—that is, an output file that can in turn serve as input to the linker. 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.
Display the version number for the linker and list the linker emulations supported. Display which input files can and cannot be opened. Display the linker script if using a default built in script.
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 meaningful on ELF platforms that support shared libraries. See Section 11.6.
Warn when a common symbol is combined with another common symbol or with a symbol definition. UNIX linkers allow this somewhat sloppy practice, but linkers on some other operating systems do not. This option allows you to find potential problems from combining global symbols. Unfortunately, some C libraries use this practice, so you may get some warnings about symbols in the libraries as well as in your programs.
There are three kinds of global symbols, illustrated here by C examples:
A definition, which goes in the initialized data section of the output file.
An undefined reference, which does not allocate space. There must be either a definition or a common symbol for the variable somewhere.
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.
Turning a common symbol into a reference, because there is already a definition for the symbol.
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.
Merging a common symbol with a previous same-sized common symbol.
Merging a common symbol with a previous larger common symbol.
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.
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 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.
Only warn once for each undefined symbol, rather than once per module that refers to it.
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 (see Section 11.4).
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.
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:
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.