ld - Unix, Linux Command

ld accepts Linker Command Language files written in
a superset of AT&T’s Link Editor Command Language syntax,
to provide explicit and total control over the linking process.


This man page does not describe the command language; see the
ld entry in info for full details on the command
language and on other aspects of the GNU linker.


This version of ld uses the general purpose BFD libraries
to operate on object files. This allows ld to read, combine, and
write object files in many different formats---for example, COFF or
a.out.  Different formats may be linked together to produce any
available kind of object file. 


Aside from its flexibility, the GNU linker is more helpful than other
linkers in providing diagnostic information.  Many linkers abandon
execution immediately upon encountering an error; whenever possible,
ld continues executing, allowing you to identify other errors
(or, in some cases, to get an output file in spite of the error).


The GNU linker ld is meant to cover a broad range of situations,
and to be as compatible as possible with other linkers.  As a result,
you have many choices to control its behavior.




        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.


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 preceded 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:



Options in file are separated by whitespace.  A whitespace
character may be included in an option by surrounding the entire
option in either single or double quotes.  Any character (including a
backslash) may be included by prefixing the character to be included
with a backslash.  The file may itself contain additional
@file options; any such options will be processed recursively.



Future releases of ld may support similar functionality for
other architecture families.



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.



You can also define the input format from a script, using the command
TARGET;



If 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 dynamic list to control what symbols should
be added to the dynamic symbol table if the output format supports it.
See the description of --dynamic-list.



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.



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
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.



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.



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. 



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.



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.



Note - symbols whose values are computed by an expression which
involves a reference to a previous value of the same symbol may not
have correct result displayed in the link map.  This is because the
linker discards intermediate results and only retains the final value
of an expression.  Under such circumstances the linker will display
the final value enclosed by square brackets.  Thus for example a
linker script containing:





           foo = 1
           foo = foo * 4
           foo = foo + 8



will produce the following output in the link map if the -M
option is used:





           0x00000001                foo = 0x1
           [0x0000000c]                foo = (foo * 0x4)
           [0x0000000c]                foo = (foo + 0x8)



See Expressions for more information about expressions in linker
scripts.



This option is currently only supported on ELF platforms.



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.



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.



This option is useful when you have an undefined symbol in your link but
don’t know where the reference is coming from.



Other keywords are ignored for Solaris compatibility.



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.



The format of the dynamic list is the same as the version node without
scope and node name.  See VERSION for more information.



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.



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.



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.



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.



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 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.



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.



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:



If the required shared library is not found, the linker will issue a
warning and continue with the link.



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.



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.



There are three kinds of global symbols, illustrated here by C examples:



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



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.



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.



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.


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.



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]


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.  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.


Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1
or any later version published by the Free Software Foundation;
with no Invariant Sections, with no Front-Cover Texts, and with no
Back-Cover Texts.  A copy of the license is included in the
section entitled GNU Free Documentation License.











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