This file documents the gnu linker ld version 2.15.
This document is distributed under the terms of the GNU Free Documentation License. A copy of the license is included in the section entitled “GNU Free Documentation License”.
ld combines a number of object and archive files, relocates their data and ties up symbol references. Usually the last step in compiling a program is to run ld.
ld accepts Linker Command Language files written in a superset of AT&T's Link Editor Command Language syntax, to provide explicit and total control over the linking process.
This version of ld uses the general purpose BFD libraries
to operate on object files. This allows ld to read, combine, and
write object files in many different formats—for example, COFF or
a.out. Different formats may be linked together to produce any
available kind of object file. See BFD, for more information.
Aside from its flexibility, the gnu linker is more helpful than other linkers in providing diagnostic information. Many linkers abandon execution immediately upon encountering an error; whenever possible, ld continues executing, allowing you to identify other errors (or, in some cases, to get an output file in spite of the error).
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.
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:
-akeyword-Aarchitecture--architecture=architectureFuture releases of ld may support similar functionality for other architecture families.
-b input-format--format=input-formatYou 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-commandfile-d-dc-dpFORCE_COMMON_ALLOCATION has the same effect.
See Miscellaneous Commands.
-e entry--entry=entry-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-init name_init as the
function to call.
-larchive--library=archivelibarchive.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=searchdirIf 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.
-memulationIf 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-o output--output=outputOUTPUT can also specify the output file name.
-O level-q--emit-relocsThis option is currently only supported on ELF platforms.
-r--relocatableOMAGIC.
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--trace-T scriptfile--script=scriptfileld looks for it in the directories
specified by any preceding `-L' options. Multiple `-T'
options accumulate.
-u symbol--undefined=symbolEXTERN linker script command.
-Ur--unique[=SECTION]-v--version-V-x--discard-all-X--discard-locals-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 keyworddlopen.
dldump.
Other keywords are ignored for Solaris compatibility.
-( archives -)--start-group archives --end-groupThe 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--as-needed--no-as-needed-assert keyword-Bdynamic-dy-call_shared-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-Bsymbolic--check-sections--no-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-demangle--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--help--target-help-Map mapfile--no-keep-memory--no-undefined-z defs--allow-multiple-definition-z muldefs--allow-shlib-undefined--no-allow-shlib-undefinedThe 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-mismatch--no-whole-archive--noinhibit-exec-nostdlib--oformat output-formatOUTPUT_FORMAT can also specify the output format, but
this option overrides it. See BFD.
-pie--pic-executable-qmagic-Qy--relaxOn 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 dirLD_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
were not used, search the contents of the environment variable
LD_RUN_PATH. It is for the native linker only.
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.
If the required shared library is not found, the linker will issue a warning and continue with the link.
-shared-Bshareable--sort-common--split-by-file [size]--split-by-reloc [count]--stats--traditional-formatFor 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-Tbss org-Tdata org-Ttext org.bss, .data or
.text as the sectionname.
--unresolved-symbols=methodThe 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--version-script=version-scriptfile--warn-commonThere 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
--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--error-unresolved-symbols--whole-archiveTwo 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-dtagsThe 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 file--dllLIBRARY 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]
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 (see 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.
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. See Implicit Linker Scripts.
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.
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.
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.
In this section we describe the simple linker script commands.
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 the following methods in order, and stopping when one of them succeeds:
ENTRY(symbol) command in a linker script;
start, if defined;
0.
Several linker script commands deal with files.
INCLUDE filenameINCLUDE up to
10 levels deep.
INPUT(file, file, ...)INPUT(file file ...)INPUT command directs the linker to include the named files
in the link, as though they were named on the command line.
For example, if you always want to include subr.o any time you do a link, but you can't be bothered to put it on every link command line, then you can put `INPUT (subr.o)' in your linker script.
In fact, if you like, you can list all of your input files in the linker script, and then invoke the linker with nothing but a `-T' option.
In case a sysroot prefix is configured, and the filename starts with the `/' character, and the script being processed was located inside the sysroot prefix, the filename will be looked for in the sysroot prefix. Otherwise, the linker will try to open the file in the current directory. If it is not found, the linker will search through the archive library search path. See the description of `-L' in Command Line Options.
If you use `INPUT (-lfile)', ld will transform the
name to libfile.a, as with the command line argument
`-l'.
When you use the INPUT command in an implicit linker script, the
files will be included in the link at the point at which the linker
script file is included. This can affect archive searching.
GROUP(file, file, ...)GROUP(file file ...)GROUP command is like INPUT, except that the named
files should all be archives, and they are searched repeatedly until no
new undefined references are created. See the description of `-('
in Command Line Options.
OUTPUT(filename)OUTPUT command names the output file. Using
OUTPUT(filename) in the linker script is exactly like using
`-o filename' on the command line (see Command Line Options). If both are used, the command line option takes
precedence.
You can use the OUTPUT command to define a default name for the
output file other than the usual default of a.out.
SEARCH_DIR(path)SEARCH_DIR command adds path to the list of paths where
ld looks for archive libraries. Using
SEARCH_DIR(path) is exactly like using `-L path'
on the command line (see Command Line Options). If both
are used, then the linker will search both paths. Paths specified using
the command line option are searched first.
STARTUP(filename)STARTUP command is just like the INPUT command, except
that filename will become the first input file to be linked, as
though it were specified first on the command line. This may be useful
when using a system in which the entry point is always the start of the
first file.
A couple of linker script commands deal with object file formats.
OUTPUT_FORMAT(bfdname)OUTPUT_FORMAT(default, big, little)OUTPUT_FORMAT command names the BFD format to use for the
output file (see BFD). Using OUTPUT_FORMAT(bfdname) is
exactly like using `--oformat bfdname' on the command line
(see Command Line Options). If both are used, the command
line option takes precedence.
You can use OUTPUT_FORMAT with three arguments to use different
formats based on the `-EB' and `-EL' command line options.
This permits the linker script to set the output format based on the
desired endianness.
If neither `-EB' nor `-EL' are used, then the output format will be the first argument, default. If `-EB' is used, the output format will be the second argument, big. If `-EL' is used, the output format will be the third argument, little.
For example, the default linker script for the MIPS ELF target uses this command:
OUTPUT_FORMAT(elf32-bigmips, elf32-bigmips, elf32-littlemips)
This says that the default format for the output file is
`elf32-bigmips', but if the user uses the `-EL' command line
option, the output file will be created in the `elf32-littlemips'
format.
TARGET(bfdname)TARGET command names the BFD format to use when reading input
files. It affects subsequent INPUT and GROUP commands.
This command is like using `-b bfdname' on the command line
(see Command Line Options). If the TARGET command
is used but OUTPUT_FORMAT is not, then the last TARGET
command is also used to set the format for the output file. See BFD.
There are a few other linker scripts commands.
ASSERT(exp, message)EXTERN(symbol symbol ...)EXTERN, and you may use EXTERN multiple times. This
command has the same effect as the `-u' command-line option.
FORCE_COMMON_ALLOCATIONINHIBIT_COMMON_ALLOCATIONld omit the assignment of addresses
to common symbols even for a non-relocatable output file.
NOCROSSREFS(section section ...)In certain types of programs, particularly on embedded systems when using overlays, when one section is loaded into memory, another section will not be. Any direct references between the two sections would be errors. For example, it would be an error if code in one section called a function defined in the other section.
The NOCROSSREFS command takes a list of output section names. If
ld detects any cross references between the sections, it reports
an error and returns a non-zero exit status. Note that the
NOCROSSREFS command uses output section names, not input section
names.
OUTPUT_ARCH(bfdarch)objdump program with
the `-f' option.
You may assign a value to a symbol in a linker script. This will define the symbol as a global symbol.
You may assign to a symbol using any of the C assignment operators:
= expression ; += expression ; -= expression ; *= expression ; /= expression ; <<= expression ; >>= expression ; &= expression ; |= expression ;The first case will define symbol to the value of expression. In the other cases, symbol must already be defined, and the value will be adjusted accordingly.
The special symbol name `.' indicates the location counter. You
may only use this within a SECTIONS command.
The semicolon after expression is required.
Expressions are defined below; see Expressions.
You may write symbol assignments as commands in their own right, or as
statements within a SECTIONS command, or as part of an output
section description in a SECTIONS command.
The section of the symbol will be set from the section of the expression; for more information, see Expression Section.
Here is an example showing the three different places that symbol assignments may be used:
floating_point = 0;
SECTIONS
{
.text :
{
*(.text)
_etext = .;
}
_bdata = (. + 3) & ~ 3;
.data : { *(.data) }
}
In this example, the symbol `floating_point' will be defined as zero. The symbol `_etext' will be defined as the address following the last `.text' input section. The symbol `_bdata' will be defined as the address following the `.text' output section aligned upward to a 4 byte boundary.
In some cases, it is desirable for a linker script to define a symbol
only if it is referenced and is not defined by any object included in
the link. For example, traditional linkers defined the symbol
`etext'. However, ANSI C requires that the user be able to use
`etext' as a function name without encountering an error. The
PROVIDE keyword may be used to define a symbol, such as
`etext', only if it is referenced but not defined. The syntax is
PROVIDE(symbol = expression).
Here is an example of using PROVIDE to define `etext':
SECTIONS
{
.text :
{
*(.text)
_etext = .;
PROVIDE(etext = .);
}
}
In this example, if the program defines `_etext' (with a leading underscore), the linker will give a multiple definition error. If, on the other hand, the program defines `etext' (with no leading underscore), the linker will silently use the definition in the program. If the program references `etext' but does not define it, the linker will use the definition in the linker script.
The SECTIONS command tells the linker how to map input sections
into output sections, and how to place the output sections in memory.
The format of the SECTIONS command is:
SECTIONS
{
sections-command
sections-command
...
}
Each sections-command may of be one of the following:
ENTRY command (see Entry command)
The ENTRY command and symbol assignments are permitted inside the
SECTIONS command for convenience in using the location counter in
those commands. This can also make the linker script easier to
understand because you can use those commands at meaningful points in
the layout of the output file.
Output section descriptions and overlay descriptions are described below.
If you do not use a SECTIONS command in your linker script, the
linker will place each input section into an identically named output
section in the order that the sections are first encountered in the
input files. If all input sections are present in the first file, for
example, the order of sections in the output file will match the order
in the first input file. The first section will be at address zero.
The full description of an output section looks like this:
section [address] [(type)] :
[AT(lma)] [SUBALIGN(subsection_align)]
{
output-section-command
output-section-command
...
} [>region] [AT>lma_region] [:phdr :phdr ...] [=fillexp]
Most output sections do not use most of the optional section attributes.
The whitespace around section is required, so that the section name is unambiguous. The colon and the curly braces are also required. The line breaks and other white space are optional.
Each output-section-command may be one of the following:
The name of the output section is section. section must
meet the constraints of your output format. In formats which only
support a limited number of sections, such as a.out, the name
must be one of the names supported by the format (a.out, for
example, allows only `.text', `.data' or `.bss'). If the
output format supports any number of sections, but with numbers and not
names (as is the case for Oasys), the name should be supplied as a
quoted numeric string. A section name may consist of any sequence of
characters, but a name which contains any unusual characters such as
commas must be quoted.
The output section name `/DISCARD/' is special; Output Section Discarding.
The address is an expression for the VMA (the virtual memory address) of the output section. If you do not provide address, the linker will set it based on region if present, or otherwise based on the current value of the location counter.
If you provide address, the address of the output section will be set to precisely that. If you provide neither address nor region, then the address of the output section will be set to the current value of the location counter aligned to the alignment requirements of the output section. The alignment requirement of the output section is the strictest alignment of any input section contained within the output section.
For example,
.text . : { *(.text) }
and
.text : { *(.text) }
are subtly different. The first will set the address of the `.text' output section to the current value of the location counter. The second will set it to the current value of the location counter aligned to the strictest alignment of a `.text' input section.
The address may be an arbitrary expression; Expressions. For example, if you want to align the section on a 0x10 byte boundary, so that the lowest four bits of the section address are zero, you could do something like this:
.text ALIGN(0x10) : { *(.text) }
This works because ALIGN returns the current location counter
aligned upward to the specified value.
Specifying address for a section will change the value of the location counter.
The most common output section command is an input section description.
The input section description is the most basic linker script operation. You use output sections to tell the linker how to lay out your program in memory. You use input section descriptions to tell the linker how to map the input files into your memory layout.
An input section description consists of a file name optionally followed by a list of section names in parentheses.
The file name and the section name may be wildcard patterns, which we describe further below (see Input Section Wildcards).
The most common input section description is to include all input sections with a particular name in the output section. For example, to include all input `.text' sections, you would write:
*(.text)
Here the `*' is a wildcard which matches any file name. To exclude a list of files from matching the file name wildcard, EXCLUDE_FILE may be used to match all files except the ones specified in the EXCLUDE_FILE list. For example:
(*(EXCLUDE_FILE (*crtend.o *otherfile.o) .ctors))
will cause all .ctors sections from all files except crtend.o and otherfile.o to be included.
There are two ways to include more than one section:
*(.text .rdata)
*(.text) *(.rdata)
The difference between these is the order in which the `.text' and `.rdata' input sections will appear in the output section. In the first example, they will be intermingled, appearing in the same order as they are found in the linker input. In the second example, all `.text' input sections will appear first, followed by all `.rdata' input sections.
You can specify a file name to include sections from a particular file. You would do this if one or more of your files contain special data that needs to be at a particular location in memory. For example:
data.o(.data)
If you use a file name without a list of sections, then all sections in the input file will be included in the output section. This is not commonly done, but it may by useful on occasion. For example:
data.o
When you use a file name which does not contain any wild card
characters, the linker will first see if you also specified the file
name on the linker command line or in an INPUT command. If you
did not, the linker will attempt to open the file as an input file, as
though it appeared on the command line. Note that this differs from an
INPUT command, because the linker will not search for the file in
the archive search path.
In an input section description, either the file name or the section name or both may be wildcard patterns.
The file name of `*' seen in many examples is a simple wildcard pattern for the file name.
The wildcard patterns are like those used by the Unix shell.
When a file name is matched with a wildcard, the wildcard characters will not match a `/' character (used to separate directory names on Unix). A pattern consisting of a single `*' character is an exception; it will always match any file name, whether it contains a `/' or not. In a section name, the wildcard characters will match a `/' character.
File name wildcard patterns only match files which are explicitly
specified on the command line or in an INPUT command. The linker
does not search directories to expand wildcards.
If a file name matches more than one wildcard pattern, or if a file name appears explicitly and is also matched by a wildcard pattern, the linker will use the first match in the linker script. For example, this sequence of input section descriptions is probably in error, because the data.o rule will not be used:
.data : { *(.data) }
.data1 : { data.o(.data) }
Normally, the linker will place files and sections matched by wildcards
in the order in which they are seen during the link. You can change
this by using the SORT keyword, which appears before a wildcard
pattern in parentheses (e.g., SORT(.text*)). When the
SORT keyword is used, the linker will sort the files or sections
into ascending order by name before placing them in the output file.
If you ever get confused about where input sections are going, use the `-M' linker option to generate a map file. The map file shows precisely how input sections are mapped to output sections.
This example shows how wildcard patterns might be used to partition files. This linker script directs the linker to place all `.text' sections in `.text' and all `.bss' sections in `.bss'. The linker will place the `.data' section from all files beginning with an upper case character in `.DATA'; for all other files, the linker will place the `.data' section in `.data'.
SECTIONS {
.text : { *(.text) }
.DATA : { [A-Z]*(.data) }
.data : { *(.data) }
.bss : { *(.bss) }
}
A special notation is needed for common symbols, because in many object file formats common symbols do not have a particular input section. The linker treats common symbols as though they are in an input section named `COMMON'.
You may use file names with the `COMMON' section just as with any other input sections. You can use this to place common symbols from a particular input file in one section while common symbols from other input files are placed in another section.
In most cases, common symbols in input files will be placed in the `.bss' section in the output file. For example:
.bss { *(.bss) *(COMMON) }
Some object file formats have more than one type of common symbol. For example, the MIPS ELF object file format distinguishes standard common symbols and small common symbols. In this case, the linker will use a different special section name for other types of common symbols. In the case of MIPS ELF, the linker uses `COMMON' for standard common symbols and `.scommon' for small common symbols. This permits you to map the different types of common symbols into memory at different locations.
You will sometimes see `[COMMON]' in old linker scripts. This notation is now considered obsolete. It is equivalent to `*(COMMON)'.
When link-time garbage collection is in use (`--gc-sections'),
it is often useful to mark sections that should not be eliminated.
This is accomplished by surrounding an input section's wildcard entry
with KEEP(), as in KEEP(*(.init)) or
KEEP(SORT(*)(.ctors)).
The following example is a complete linker script. It tells the linker to read all of the sections from file all.o and place them at the start of output section `outputa' which starts at location `0x10000'. All of section `.input1' from file foo.o follows immediately, in the same output section. All of section `.input2' from foo.o goes into output section `outputb', followed by section `.input1' from foo1.o. All of the remaining `.input1' and `.input2' sections from any files are written to output section `outputc'.
SECTIONS {
outputa 0x10000 :
{
all.o
foo.o (.input1)
}
outputb :
{
foo.o (.input2)
foo1.o (.input1)
}
outputc :
{
*(.input1)
*(.input2)
}
}
You can include explicit bytes of data in an output section by using
BYTE, SHORT, LONG, QUAD, or SQUAD as
an output section command. Each keyword is followed by an expression in
parentheses providing the value to store (see Expressions). The
value of the expression is stored at the current value of the location
counter.
The BYTE, SHORT, LONG, and QUAD commands
store one, two, four, and eight bytes (respectively). After storing the
bytes, the location counter is incremented by the number of bytes
stored.
For example, this will store the byte 1 followed by the four byte value of the symbol `addr':
BYTE(1)
LONG(addr)
When using a 64 bit host or target, QUAD and SQUAD are the
same; they both store an 8 byte, or 64 bit, value. When both host and
target are 32 bits, an expression is computed as 32 bits. In this case
QUAD stores a 32 bit value zero extended to 64 bits, and
SQUAD stores a 32 bit value sign extended to 64 bits.
If the object file format of the output file has an explicit endianness, which is the normal case, the value will be stored in that endianness. When the object file format does not have an explicit endianness, as is true of, for example, S-records, the value will be stored in the endianness of the first input object file.
Note—these commands only work inside a section description and not between them, so the following will produce an error from the linker:
SECTIONS { .text : { *(.text) } LONG(1) .data : { *(.data) } }
whereas this will work:
SECTIONS { .text : { *(.text) ; LONG(1) } .data : { *(.data) } }
You may use the FILL command to set the fill pattern for the
current section. It is followed by an expression in parentheses. Any
otherwise unspecified regions of memory within the section (for example,
gaps left due to the required alignment of input sections) are filled
with the value of the expression, repeated as
necessary. A FILL statement covers memory locations after the
point at which it occurs in the section definition; by including more
than one FILL statement, you can have different fill patterns in
different parts of an output section.
This example shows how to fill unspecified regions of memory with the value `0x90':
FILL(0x90909090)
The FILL command is similar to the `=fillexp' output
section attribute, but it only affects the
part of the section following the FILL command, rather than the
entire section. If both are used, the FILL command takes
precedence. See Output Section Fill, for details on the fill
expression.
There are a couple of keywords which can appear as output section commands.
CREATE_OBJECT_SYMBOLSCREATE_OBJECT_SYMBOLS command appears.
This is conventional for the a.out object file format. It is not normally used for any other object file format.
CONSTRUCTORSCONSTRUCTORS command tells the
linker to place constructor information in the output section where the
CONSTRUCTORS command appears. The CONSTRUCTORS command is
ignored for other object file formats.
The symbol __CTOR_LIST__ marks the start of the global
constructors, and the symbol __DTOR_LIST marks the end. The
first word in the list is the number of entries, followed by the address
of each constructor or destructor, followed by a zero word. The
compiler must arrange to actually run the code. For these object file
formats gnu C++ normally calls constructors from a subroutine
__main; a call to __main is automatically inserted into
the startup code for main. gnu C++ normally runs
destructors either by using atexit, or directly from the function
exit.
For object file formats such as COFF or ELF which support
arbitrary section names, gnu C++ will normally arrange to put the
addresses of global constructors and destructors into the .ctors
and .dtors sections. Placing the following sequence into your
linker script will build the sort of table which the gnu C++
runtime code expects to see.
__CTOR_LIST__ = .;
LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2)
*(.ctors)
LONG(0)
__CTOR_END__ = .;
__DTOR_LIST__ = .;
LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2)
*(.dtors)
LONG(0)
__DTOR_END__ = .;
If you are using the gnu C++ support for initialization priority,
which provides some control over the order in which global constructors
are run, you must sort the constructors at link time to ensure that they
are executed in the correct order. When using the CONSTRUCTORS
command, use `SORT(CONSTRUCTORS)' instead. When using the
.ctors and .dtors sections, use `*(SORT(.ctors))' and
`*(SORT(.dtors))' instead of just `*(.ctors)' and
`*(.dtors)'.
Normally the compiler and linker will handle these issues automatically, and you will not need to concern yourself with them. However, you may need to consider this if you are using C++ and writing your own linker scripts.
The linker will not create output section which do not have any contents. This is for convenience when referring to input sections that may or may not be present in any of the input files. For example:
.foo { *(.foo) }
will only create a `.foo' section in the output file if there is a `.foo' section in at least one input file.
If you use anything other than an input section description as an output section command, such as a symbol assignment, then the output section will always be created, even if there are no matching input sections.
The special output section name `/DISCARD/' may be used to discard input sections. Any input sections which are assigned to an output section named `/DISCARD/' are not included in the output file.
We showed above that the full description of an output section looked like this:
section [address] [(type)] :
[AT(lma)] [SUBALIGN(subsection_align)]
{
output-section-command
output-section-command
...
} [>region] [AT>lma_region] [:phdr :phdr ...] [=fillexp]
We've already described section, address, and output-section-command. In this section we will describe the remaining section attributes.
Each output section may have a type. The type is a keyword in parentheses. The following types are defined:
NOLOADDSECTCOPYINFOOVERLAYThe linker normally sets the attributes of an output section based on the input sections which map into it. You can override this by using the section type. For example, in the script sample below, the `ROM' section is addressed at memory location `0' and does not need to be loaded when the program is run. The contents of the `ROM' section will appear in the linker output file as usual.
SECTIONS {
ROM 0 (NOLOAD) : { ... }
...
}
Every section has a virtual address (VMA) and a load address (LMA); see Basic Script Concepts. The address expression which may appear in an output section description sets the VMA (see Output Section Address).
The linker will normally set the LMA equal to the VMA. You can change
that by using the AT keyword. The expression lma that
follows the AT keyword specifies the load address of the
section.
Alternatively, with `AT>lma_region' expression, you may specify a memory region for the section's load address. See MEMORY. Note that if the section has not had a VMA assigned to it then the linker will use the lma_region as the VMA region as well. See Output Section Region.
This feature is designed to make it easy to build a ROM image. For
example, the following linker script creates three output sections: one
called `.text', which starts at 0x1000, one called
`.mdata', which is loaded at the end of the `.text' section
even though its VMA is 0x2000, and one called `.bss' to hold
uninitialized data at address 0x3000. The symbol _data is
defined with the value 0x2000, which shows that the location
counter holds the VMA value, not the LMA value.
SECTIONS
{
.text 0x1000 : { *(.text) _etext = . ; }
.mdata 0x2000 :
AT ( ADDR (.text) + SIZEOF (.text) )
{ _data = . ; *(.data); _edata = . ; }
.bss 0x3000 :
{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;}
}
The run-time initialization code for use with a program generated with this linker script would include something like the following, to copy the initialized data from the ROM image to its runtime address. Notice how this code takes advantage of the symbols defined by the linker script.
extern char _etext, _data, _edata, _bstart, _bend;
char *src = &_etext;
char *dst = &_data;
/* ROM has data at end of text; copy it. */
while (dst < &_edata) {
*dst++ = *src++;
}
/* Zero bss */
for (dst = &_bstart; dst< &_bend; dst++)
*dst = 0;
You can force input section alignment within an output section by using SUBALIGN. The value specified overrides any alignment given by input sections, whether larger or smaller.
You can assign a section to a previously defined region of memory by using `>region'. See MEMORY.
Here is a simple example:
MEMORY { rom : ORIGIN = 0x1000, LENGTH = 0x1000 }
SECTIONS { ROM : { *(.text) } >rom }
You can assign a section to a previously defined program segment by
using `:phdr'. See PHDRS. If a section is assigned to
one or more segments, then all subsequent allocated sections will be
assigned to those segments as well, unless they use an explicitly
:phdr modifier. You can use :NONE to tell the
linker to not put the section in any segment at all.
Here is a simple example:
PHDRS { text PT_LOAD ; }
SECTIONS { .text : { *(.text) } :text }
You can set the fill pattern for an entire section by using
`=fillexp'. fillexp is an expression
(see Expressions). Any otherwise unspecified regions of memory
within the output section (for example, gaps left due to the required
alignment of input sections) will be filled with the value, repeated as
necessary. If the fill expression is a simple hex number, ie. a string
of hex digit starting with `0x' and without a trailing `k' or `M', then
an arbitrarily long sequence of hex digits can be used to specify the
fill pattern; Leading zeros become part of the pattern too. For all
other cases, including extra parentheses or a unary +, the fill
pattern is the four least significant bytes of the value of the
expression. In all cases, the number is big-endian.
You can also change the fill value with a FILL command in the
output section commands; (see Output Section Data).
Here is a simple example:
SECTIONS { .text : { *(.text) } =0x90909090 }
An overlay description provides an easy way to describe sections which are to be loaded as part of a single memory image but are to be run at the same memory address. At run time, some sort of overlay manager will copy the overlaid sections in and out of the runtime memory address as required, perhaps by simply manipulating addressing bits. This approach can be useful, for example, when a certain region of memory is faster than another.
Overlays are described using the OVERLAY command. The
OVERLAY command is used within a SECTIONS command, like an
output section description. The full syntax of the OVERLAY
command is as follows:
OVERLAY [start] : [NOCROSSREFS] [AT ( ldaddr )]
{
secname1
{
output-section-command
output-section-command
...
} [:phdr...] [=fill]
secname2
{
output-section-command
output-section-command
...
} [:phdr...] [=fill]
...
} [>region] [:phdr...] [=fill]
Everything is optional except OVERLAY (a keyword), and each
section must have a name (secname1 and secname2 above). The
section definitions within the OVERLAY construct are identical to
those within the general SECTIONS contruct (see SECTIONS),
except that no addresses and no memory regions may be defined for
sections within an OVERLAY.
The sections are all defined with the same starting address. The load
addresses of the sections are arranged such that they are consecutive in
memory starting at the load address used for the OVERLAY as a
whole (as with normal section definitions, the load address is optional,
and defaults to the start address; the start address is also optional,
and defaults to the current value of the location counter).
If the NOCROSSREFS keyword is used, and there any references
among the sections, the linker will report an error. Since the sections
all run at the same address, it normally does not make sense for one
section to refer directly to another. See NOCROSSREFS.
For each section within the OVERLAY, the linker automatically
defines two symbols. The symbol __load_start_secname is
defined as the starting load address of the section. The symbol
__load_stop_secname is defined as the final load address of
the section. Any characters within secname which are not legal
within C identifiers are removed. C (or assembler) code may use these
symbols to move the overlaid sections around as necessary.
At the end of the overlay, the value of the location counter is set to the start address of the overlay plus the size of the largest section.
Here is an example. Remember that this would appear inside a
SECTIONS construct.
OVERLAY 0x1000 : AT (0x4000)
{
.text0 { o1/*.o(.text) }
.text1 { o2/*.o(.text) }
}
This will define both `.text0' and `.text1' to start at
address 0x1000. `.text0' will be loaded at address 0x4000, and
`.text1' will be loaded immediately after `.text0'. The
following symbols will be defined: __load_start_text0,
__load_stop_text0, __load_start_text1,
__load_stop_text1.
C code to copy overlay .text1 into the overlay area might look
like the following.
extern char __load_start_text1, __load_stop_text1;
memcpy ((char *) 0x1000, &__load_start_text1,
&__load_stop_text1 - &__load_start_text1);
Note that the OVERLAY command is just syntactic sugar, since
everything it does can be done using the more basic commands. The above
example could have been written identically as follows.
.text0 0x1000 : AT (0x4000) { o1/*.o(.text) }
__load_start_text0 = LOADADDR (.text0);
__load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0);
.text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) { o2/*.o(.text) }
__load_start_text1 = LOADADDR (.text1);
__load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1);
. = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1));
The linker's default configuration permits allocation of all available
memory. You can override this by using the MEMORY command.
The MEMORY command describes the location and size of blocks of
memory in the target. You can use it to describe which memory regions
may be used by the linker, and which memory regions it must avoid. You
can then assign sections to particular memory regions. The linker will
set section addresses based on the memory regions, and will warn about
regions that become too full. The linker will not shuffle sections
around to fit into the available regions.
A linker script may contain at most one use of the MEMORY
command. However, you can define as many blocks of memory within it as
you wish. The syntax is:
MEMORY
{
name [(attr)] : ORIGIN = origin, LENGTH = len
...
}
The name is a name used in the linker script to refer to the region. The region name has no meaning outside of the linker script. Region names are stored in a separate name space, and will not conflict with symbol names, file names, or section names. Each memory region must have a distinct name.
The attr string is an optional list of attributes that specify whether to use a particular memory region for an input section which is not explicitly mapped in the linker script. As described in SECTIONS, if you do not specify an output section for some input section, the linker will create an output section with the same name as the input section. If you define region attributes, the linker will use them to select the memory region for the output section that it creates.
The attr string must consist only of the following characters:
If a unmapped section matches any of the listed attributes other than `!', it will be placed in the memory region. The `!' attribute reverses this test, so that an unmapped section will be placed in the memory region only if it does not match any of the listed attributes.
The origin is an expression for the start address of the memory
region. The expression must evaluate to a constant before memory
allocation is performed, which means that you may not use any section
relative symbols. The keyword ORIGIN may be abbreviated to
org or o (but not, for example, ORG).
The len is an expression for the size in bytes of the memory
region. As with the origin expression, the expression must
evaluate to a constant before memory allocation is performed. The
keyword LENGTH may be abbreviated to len or l.
In the following example, we specify that there are two memory regions available for allocation: one starting at `0' for 256 kilobytes, and the other starting at `0x40000000' for four megabytes. The linker will place into the `rom' memory region every section which is not explicitly mapped into a memory region, and is either read-only or executable. The linker will place other sections which are not explicitly mapped into a memory region into the `ram' memory region.
MEMORY
{
rom (rx) : ORIGIN = 0, LENGTH = 256K
ram (!rx) : org = 0x40000000, l = 4M
}
Once you define a memory region, you can direct the linker to place specific output sections into that memory region by using the `>region' output section attribute. For example, if you have a memory region named `mem', you would use `>mem' in the output section definition. See Output Section Region. If no address was specified for the output section, the linker will set the address to the next available address within the memory region. If the combined output sections directed to a memory region are too large for the region, the linker will issue an error message.
The ELF object file format uses program headers, also knows as
segments. The program headers describe how the program should be
loaded into memory. You can print them out by using the objdump
program with the `-p' option.
When you run an ELF program on a native ELF system, the system loader reads the program headers in order to figure out how to load the program. This will only work if the program headers are set correctly. This manual does not describe the details of how the system loader interprets program headers; for more information, see the ELF ABI.
The linker will create reasonable program headers by default. However,
in some cases, you may need to specify the program headers more
precisely. You may use the PHDRS command for this purpose. When
the linker sees the PHDRS command in the linker script, it will
not create any program headers other than the ones specified.
The linker only pays attention to the PHDRS command when
generating an ELF output file. In other cases, the linker will simply
ignore PHDRS.
This is the syntax of the PHDRS command. The words PHDRS,
FILEHDR, AT, and FLAGS are keywords.
PHDRS
{
name type [ FILEHDR ] [ PHDRS ] [ AT ( address ) ]
[ FLAGS ( flags ) ] ;
}
The name is used only for reference in the SECTIONS command
of the linker script. It is not put into the output file. Program
header names are stored in a separate name space, and will not conflict
with symbol names, file names, or section names. Each program header
must have a distinct name.
Certain program header types describe segments of memory which the system loader will load from the file. In the linker script, you specify the contents of these segments by placing allocatable output sections in the segments. You use the `:phdr' output section attribute to place a section in a particular segment. See Output Section Phdr.
It is normal to put certain sections in more than one segment. This merely implies that one segment of memory contains another. You may repeat `:phdr', using it once for each segment which should contain the section.
If you place a section in one or more segments using `:phdr',
then the linker will place all subsequent allocatable sections which do
not specify `:phdr' in the same segments. This is for
convenience, since generally a whole set of contiguous sections will be
placed in a single segment. You can use :NONE to override the
default segment and tell the linker to not put the section in any
segment at all.
You may use the FILEHDR and PHDRS keywords appear after
the program header type to further describe the contents of the segment.
The FILEHDR keyword means that the segment should include the ELF
file header. The PHDRS keyword means that the segment should
include the ELF program headers themselves.
The type may be one of the following. The numbers indicate the value of the keyword.
PT_NULL (0)PT_LOAD (1)PT_DYNAMIC (2)PT_INTERP (3)PT_NOTE (4)PT_SHLIB (5)PT_PHDR (6)You can specify that a segment should be loaded at a particular address
in memory by using an AT expression. This is identical to the
AT command used as an output section attribute (see Output Section LMA). The AT command for a program header overrides the
output section attribute.
The linker will normally set the segment flags based on the sections
which comprise the segment. You may use the FLAGS keyword to
explicitly specify the segment flags. The value of flags must be
an integer. It is used to set the p_flags field of the program
header.
Here is an example of PHDRS. This shows a typical set of program
headers used on a native ELF system.
PHDRS
{
headers PT_PHDR PHDRS ;
interp PT_INTERP ;
text PT_LOAD FILEHDR PHDRS ;
data PT_LOAD ;
dynamic PT_DYNAMIC ;
}
SECTIONS
{
. = SIZEOF_HEADERS;
.interp : { *(.interp) } :text :interp
.text : { *(.text) } :text
.rodata : { *(.rodata) } /* defaults to :text */
...
. = . + 0x1000; /* move to a new page in memory */
.data : { *(.data) } :data
.dynamic : { *(.dynamic) } :data :dynamic
...
}
The linker supports symbol versions when using ELF. Symbol versions are only useful when using shared libraries. The dynamic linker can use symbol versions to select a specific version of a function when it runs a program that may have been linked against an earlier version of the shared library.
You can include a version script directly in the main linker script, or you can supply the version script as an implicit linker script. You can also use the `--version-script' linker option.
The syntax of the VERSION command is simply
VERSION { version-script-commands }
The format of the version script commands is identical to that used by Sun's linker in Solaris 2.5. The version script defines a tree of version nodes. You specify the node names and interdependencies in the version script. You can specify which symbols are bound to which version nodes, and you can reduce a specified set of symbols to local scope so that they are not globally visible outside of the shared library.
The easiest way to demonstrate the version script language is with a few examples.
VERS_1.1 {
global:
foo1;
local:
old*;
original*;
new*;
};
VERS_1.2 {
foo2;
} VERS_1.1;
VERS_2.0 {
bar1; bar2;
} VERS_1.2;
This example version script defines three version nodes. The first version node defined is `VERS_1.1'; it has no other dependencies. The script binds the symbol `foo1' to `VERS_1.1'. It reduces a number of symbols to local scope so that they are not visible outside of the shared library; this is done using wildcard patterns, so that any symbol whose name begins with `old', `original', or `new' is matched. The wildcard patterns available are the same as those used in the shell when matching filenames (also known as “globbing”).
Next, the version script defines node `VERS_1.2'. This node depends upon `VERS_1.1'. The script binds the symbol `foo2' to the version node `VERS_1.2'.
Finally, the version script defines node `VERS_2.0'. This node depends upon `VERS_1.2'. The scripts binds the symbols `bar1' and `bar2' are bound to the version node `VERS_2.0'.
When the linker finds a symbol defined in a library which is not specifically bound to a version node, it will effectively bind it to an unspecified base version of the library. You can bind all otherwise unspecified symbols to a given version node by using `global: *;' somewhere in the version script.
The names of the version nodes have no specific meaning other than what they might suggest to the person reading them. The `2.0' version could just as well have appeared in between `1.1' and `1.2'. However, this would be a confusing way to write a version script.
Node name can be omited, provided it is the only version node in the version script. Such version script doesn't assign any versions to symbols, only selects which symbols will be globally visible out and which won't.
{ global: foo; bar; local: *; };
When you link an application against a shared library that has versioned symbols, the application itself knows which version of each symbol it requires, and it also knows which version nodes it needs from each shared library it is linked against. Thus at runtime, the dynamic loader can make a quick check to make sure that the libraries you have linked against do in fact supply all of the version nodes that the application will need to resolve all of the dynamic symbols. In this way it is possible for the dynamic linker to know with certainty that all external symbols that it needs will be resolvable without having to search for each symbol reference.
The symbol versioning is in effect a much more sophisticated way of doing minor version checking that SunOS does. The fundamental problem that is being addressed here is that typically references to external functions are bound on an as-needed basis, and are not all bound when the application starts up. If a shared library is out of date, a required interface may be missing; when the application tries to use that interface, it may suddenly and unexpectedly fail. With symbol versioning, the user will get a warning when they start their program if the libraries being used with the application are too old.
There are several GNU extensions to Sun's versioning approach. The first of these is the ability to bind a symbol to a version node in the source file where the symbol is defined instead of in the versioning script. This was done mainly to reduce the burden on the library maintainer. You can do this by putting something like:
__asm__(".symver original_foo,foo@VERS_1.1");
in the C source file. This renames the function `original_foo' to be an alias for `foo' bound to the version node `VERS_1.1'. The `local:' directive can be used to prevent the symbol `original_foo' from being exported. A `.symver' directive takes precedence over a version script.
The second GNU extension is to allow multiple versions of the same function to appear in a given shared library. In this way you can make an incompatible change to an interface without increasing the major version number of the shared library, while still allowing applications linked against the old interface to continue to function.
To do this, you must use multiple `.symver' directives in the source file. Here is an example:
__asm__(".symver original_foo,foo@");
__asm__(".symver old_foo,foo@VERS_1.1");
__asm__(".symver old_foo1,foo@VERS_1.2");
__asm__(".symver new_foo,foo@@VERS_2.0");
In this example, `foo@' represents the symbol `foo' bound to the unspecified base version of the symbol. The source file that contains this example would define 4 C functions: `original_foo', `old_foo', `old_foo1', and `new_foo'.
When you have multiple definitions of a given symbol, there needs to be some way to specify a default version to which external references to this symbol will be bound. You can do this with the `foo@@VERS_2.0' type of `.symver' directive. You can only declare one version of a symbol as the default in this manner; otherwise you would effectively have multiple definitions of the same symbol.
If you wish to bind a reference to a specific version of the symbol within the shared library, you can use the aliases of convenience (i.e., `old_foo'), or you can use the `.symver' directive to specifically bind to an external version of the function in question.
You can also specify the language in the version script:
VERSION extern "lang" { version-script-commands }
The supported `lang's are `C', `C++', and `Java'. The linker will iterate over the list of symbols at the link time and demangle them according to `lang' before matching them to the patterns specified in `version-script-commands'.
The syntax for expressions in the linker script language is identical to that of C expressions. All expressions are evaluated as integers. All expressions are evaluated in the same size, which is 32 bits if both the host and target are 32 bits, and is otherwise 64 bits.
You can use and set symbol values in expressions.
The linker defines several special purpose builtin functions for use in expressions.
As in C, the linker considers an integer beginning with `0' to be octal, and an integer beginning with `0x' or `0X' to be hexadecimal. The linker considers other integers to be decimal.
In addition, you can use the suffixes K and M to scale a
constant by
1024 or 1024*1024
respectively. For example, the following all refer to the same quantity:
_fourk_1 = 4K;
_fourk_2 = 4096;
_fourk_3 = 0x1000;
Unless quoted, symbol names start with a letter, underscore, or period and may include letters, digits, underscores, periods, and hyphens. Unquoted symbol names must not conflict with any keywords. You can specify a symbol which contains odd characters or has the same name as a keyword by surrounding the symbol name in double quotes:
"SECTION" = 9;
"with a space" = "also with a space" + 10;
Since symbols can contain many non-alphabetic characters, it is safest to delimit symbols with spaces. For example, `A-B' is one symbol, whereas `A - B' is an expression involving subtraction.
The special linker variable dot `.' always contains the
current output location counter. Since the . always refers to a
location in an output section, it may only appear in an expression
within a SECTIONS command. The . symbol may appear
anywhere that an ordinary symbol is allowed in an expression.
Assigning a value to . will cause the location counter to be
moved. This may be used to create holes in the output section. The
location counter may never be moved backwards.
SECTIONS
{
output :
{
file1(.text)
. = . + 1000;
file2(.text)
. += 1000;
file3(.text)
} = 0x12345678;
}
In the previous example, the `.text' section from file1 is located at the beginning of the output section `output'. It is followed by a 1000 byte gap. Then the `.text' section from file2 appears, also with a 1000 byte gap following before the `.text' section from file3. The notation `= 0x12345678' specifies what data to write in the gaps (see Output Section Fill).
Note: . actually refers to the byte offset from the start of the
current containing object. Normally this is the SECTIONS
statement, whose start address is 0, hence . can be used as an
absolute address. If . is used inside a section description
however, it refers to the byte offset from the start of that section,
not an absolute address. Thus in a script like this:
SECTIONS
{
. = 0x100
.text: {
*(.text)
. = 0x200
}
. = 0x500
.data: {
*(.data)
. += 0x600
}
}
The `.text' section will be assigned a starting address of 0x100
and a size of exactly 0x200 bytes, even if there is not enough data in
the `.text' input sections to fill this area. (If there is too
much data, an error will be produced because this would be an attempt to
move . backwards). The `.data' section will start at 0x500
and it will have an extra 0x600 bytes worth of space after the end of
the values from the `.data' input sections and before the end of
the `.data' output section itself.
The linker recognizes the standard C set of arithmetic operators, with the standard bindings and precedence levels:
precedence associativity Operators Notes
(highest)
1 left ! - ~ (1)
2 left * / %
3 left + -
4 left >> <<
5 left == != > < <= >=
6 left &
7 left |
8 left &&
9 left ||
10 right ? :
11 right &= += -= *= /= (2)
(lowest)
Notes: (1) Prefix operators (2) See Assignments.
The linker evaluates expressions lazily. It only computes the value of an expression when absolutely necessary.
The linker needs some information, such as the value of the start address of the first section, and the origins and lengths of memory regions, in order to do any linking at all. These values are computed as soon as possible when the linker reads in the linker script.
However, other values (such as symbol values) are not known or needed until after storage allocation. Such values are evaluated later, when other information (such as the sizes of output sections) is available for use in the symbol assignment expression.
The sizes of sections cannot be known until after allocation, so assignments dependent upon these are not performed until after allocation.
Some expressions, such as those depending upon the location counter `.', must be evaluated during section allocation.
If the result of an expression is required, but the value is not available, then an error results. For example, a script like the following
SECTIONS
{
.text 9+this_isnt_constant :
{ *(.text) }
}
will cause the error message `non constant expression for initial address'.
When the linker evaluates an expression, the result is either absolute or relative to some section. A relative expression is expressed as a fixed offset from the base of a section.
The position of the expression within the linker script determines whether it is absolute or relative. An expression which appears within an output section definition is relative to the base of the output section. An expression which appears elsewhere will be absolute.
A symbol set to a relative expression will be relocatable if you request relocatable output using the `-r' option. That means that a further link operation may change the value of the symbol. The symbol's section will be the section of the relative expression.
A symbol set to an absolute expression will retain the same value through any further link operation. The symbol will be absolute, and will not have any particular associated section.
You can use the builtin function ABSOLUTE to force an expression
to be absolute when it would otherwise be relative. For example, to
create an absolute symbol set to the address of the end of the output
section `.data':
SECTIONS
{
.data : { *(.data) _edata = ABSOLUTE(.); }
}
If `ABSOLUTE' were not used, `_edata' would be relative to the `.data' section.
The linker script language includes a number of builtin functions for use in linker script expressions.
ABSOLUTE(exp)ADDR(section)symbol_1 and symbol_2 are assigned
identical values:
SECTIONS { ...
.output1 :
{
start_of_output_1 = ABSOLUTE(.);
...
}
.output :
{
symbol_1 = ADDR(.output1);
symbol_2 = start_of_output_1;
}
... }
ALIGN(align)ALIGN(exp,align).) or arbitrary expression aligned
to the next align boundary. The single operand ALIGN
doesn't change the value of the location counter—it just does
arithmetic on it. The two operand ALIGN allows an arbitrary
expression to be aligned upwards (ALIGN(align) is
equivalent to ALIGN(., align)).
Here is an example which aligns the output .data section to the
next 0x2000 byte boundary after the preceding section and sets a
variable within the section to the next 0x8000 boundary after the
input sections:
SECTIONS { ...
.data ALIGN(0x2000): {
*(.data)
variable = ALIGN(0x8000);
}
... }
The first use of ALIGN in this example specifies the location of
a section because it is used as the optional address attribute of
a section definition (see Output Section Address). The second use
of ALIGN is used to defines the value of a symbol.
The builtin function NEXT is closely related to ALIGN.
BLOCK(exp)ALIGN, for compatibility with older linker
scripts. It is most often seen when setting the address of an output
section.
DATA_SEGMENT_ALIGN(maxpagesize, commonpagesize) (ALIGN(maxpagesize) + (. & (maxpagesize - 1)))
or
(ALIGN(maxpagesize) + (. & (maxpagesize - commonpagesize)))
depending on whether the latter uses fewer commonpagesize sized pages
for the data segment (area between the result of this expression and
DATA_SEGMENT_END) than the former or not.
If the latter form is used, it means commonpagesize bytes of runtime
memory will be saved at the expense of up to commonpagesize wasted
bytes in the on-disk file.
This expression can only be used directly in SECTIONS commands, not in
any output section descriptions and only once in the linker script.
commonpagesize should be less or equal to maxpagesize and should
be the system page size the object wants to be optimized for (while still
working on system page sizes up to maxpagesize).
Example:
. = DATA_SEGMENT_ALIGN(0x10000, 0x2000);
DATA_SEGMENT_END(exp)DATA_SEGMENT_ALIGN
evaluation purposes.
. = DATA_SEGMENT_END(.);
DEFINED(symbol) SECTIONS { ...
.text : {
begin = DEFINED(begin) ? begin : . ;
...
}
...
}
LOADADDR(section)ADDR, but it may be different if the AT
attribute is used in the output section definition (see Output Section LMA).
MAX(exp1, exp2)MIN(exp1, exp2)NEXT(exp)ALIGN(exp); unless you
use the MEMORY command to define discontinuous memory for the
output file, the two functions are equivalent.
SIZEOF(section)symbol_1 and symbol_2 are assigned identical values:
SECTIONS{ ...
.output {
.start = . ;
...
.end = . ;
}
symbol_1 = .end - .start ;
symbol_2 = SIZEOF(.output);
... }
SIZEOF_HEADERSsizeof_headersWhen producing an ELF output file, if the linker script uses the
SIZEOF_HEADERS builtin function, the linker must compute the
number of program headers before it has determined all the section
addresses and sizes. If the linker later discovers that it needs
additional program headers, it will report an error `not enough
room for program headers'. To avoid this error, you must avoid using
the SIZEOF_HEADERS function, or you must rework your linker
script to avoid forcing the linker to use additional program headers, or
you must define the program headers yourself using the PHDRS
command (see PHDRS).
If you specify a linker input file which the linker can not recognize as an object file or an archive file, it will try to read the file as a linker script. If the file can not be parsed as a linker script, the linker will report an error.
An implicit linker script will not replace the default linker script.
Typically an implicit linker script would contain only symbol
assignments, or the INPUT, GROUP, or VERSION
commands.
Any input files read because of an implicit linker script will be read at the position in the command line where the implicit linker script was read. This can affect archive searching.
ld has additional features on some platforms; the following sections describe them. Machines where ld has no additional functionality are not listed.
For the H8/300, ld can perform these global optimizations when you specify the `--relax' command-line option.
jsr and jmp instructions whose
targets are within eight bits, and turns them into eight-bit
program-counter relative bsr and bra instructions,
respectively.
mov.b instructions which use the
sixteen-bit absolute address form, but refer to the top
page of memory, and changes them to use the eight-bit address form.
(That is: the linker turns `mov.b @aa:16' into
`mov.b @aa:8' whenever the address aa is in the
top page of memory).
You can use the `-Aarchitecture' command line option to specify one of the two-letter names identifying members of the 960 family; the option specifies the desired output target, and warns of any incompatible instructions in the input files. It also modifies the linker's search strategy for archive libraries, to support the use of libraries specific to each particular architecture, by including in the search loop names suffixed with the string identifying the architecture.
For example, if your ld command line included `-ACA' as well as `-ltry', the linker would look (in its built-in search paths, and in any paths you specify with `-L') for a library with the names
try
libtry.a
tryca
libtryca.a
The first two possibilities would be considered in any event; the last two are due to the use of `-ACA'.
You can meaningfully use `-A' more than once on a command line, since the 960 architecture family allows combination of target architectures; each use will add another pair of name variants to search for when `-l' specifies a library.
ld supports the `--relax' option for the i960 family. If
you specify `--relax', ld finds all balx and
calx instructions whose targets are within 24 bits, and turns
them into 24-bit program-counter relative bal and cal
instructions, respectively. ld also turns cal
instructions into bal instructions when it determines that the
target subroutine is a leaf routine (that is, the target subroutine does
not itself call any subroutines).
For the ARM, ld will generate code stubs to allow functions calls betweem ARM and Thumb code. These stubs only work with code that has been compiled and assembled with the `-mthumb-interwork' command line option. If it is necessary to link with old ARM object files or libraries, which have not been compiled with the -mthumb-interwork option then the `--support-old-code' command line switch should be given to the linker. This will make it generate larger stub functions which will work with non-interworking aware ARM code. Note, however, the linker does not support generating stubs for function calls to non-interworking aware Thumb code.
The `--thumb-entry' switch is a duplicate of the generic `--entry' switch, in that it sets the program's starting address. But it also sets the bottom bit of the address, so that it can be branched to using a BX instruction, and the program will start executing in Thumb mode straight away.
When generating a shared library, ld will by default generate import stubs suitable for use with a single sub-space application. The `--multi-subspace' switch causes ld to generate export stubs, and different (larger) import stubs suitable for use with multiple sub-spaces.
Long branch stubs and import/export stubs are placed by ld in stub sections located between groups of input sections. `--stub-group-size' specifies the maximum size of a group of input sections handled by one stub section. Since branch offsets are signed, a stub section may serve two groups of input sections, one group before the stub section, and one group after it. However, when using conditional branches that require stubs, it may be better (for branch prediction) that stub sections only serve one group of input sections. A negative value for `N' chooses this scheme, ensuring that branches to stubs always use a negative offset. Two special values of `N' are recognized, `1' and `-1'. These both instruct ld to automatically size input section groups for the branch types detected, with the same behaviour regarding stub placement as other positive or negative values of `N' respectively.
Note that `--stub-group-size' does not split input sections. A single input section larger than the group size specified will of course create a larger group (of one section). If input sections are too large, it may not be possible for a branch to reach its stub.
ld and MMIXFor MMIX, there is a choice of generating ELF object files or
mmo object files when linking. The simulator mmix
understands the mmo format. The binutils objcopy utility
can translate between the two formats.
There is one special section, the `.MMIX.reg_contents' section.
Contents in this section is assumed to correspond to that of global
registers, and symbols referring to it are translated to special symbols,
equal to registers. In a final link, the start address of the
`.MMIX.reg_contents' section corresponds to the first allocated
global register multiplied by 8. Register $255 is not included in
this section; it is always set to the program entry, which is at the
symbol Main for mmo files.
Symbols with the prefix __.MMIX.start., for example
__.MMIX.start..text and __.MMIX.start..data are special;
there must be only one each, even if they are local. The default linker
script uses these to set the default start address of a section.
Initial and trailing multiples of zero-valued 32-bit words in a section, are left out from an mmo file.
ld and MSP430For the MSP430 it is possible to select the MPU architecture. The flag `-m [mpu type]' will select an appropriate linker script for selected MPU type. (To get a list of known MPUs just pass `-m help' option to the linker).
The linker will recognize some extra sections which are MSP430 specific:
`.vectors'`.bootloader'`.infomem'`.infomemnobits'`.noinit'The last two sections are used by gcc.
The `--format' switch allows selection of one of the various TI COFF versions. The latest of this writing is 2; versions 0 and 1 are also supported. The TI COFF versions also vary in header byte-order format; ld will read any version or byte order, but the output header format depends on the default specified by the specific target.
This section describes some of the win32 specific ld issues. See Command Line Options for detailed decription of the command line options mentioned here.
If, however, `--export-all-symbols' is not given explicitly on the command line, then the default auto-export behavior will be disabled if either of the following are true:
gcc -o <output> <objectfiles> <dll name>.def
Using a DEF file turns off the normal auto-export behavior, unless the `--export-all-symbols' option is also used.
Here is an example of a DEF file for a shared library called `xyz.dll':
LIBRARY "xyz.dll" BASE=0x10000000
EXPORTS
foo
bar
_bar = bar
This example defines a base address and three symbols. The third symbol is an alias for the second. For the complete format specification see ld/deffilep.y in the binutils sources.
While linking a shared dll, ld is able to create a DEF file
with the `--output-def <file>' command line option.
__declspec(dllexport) int a_variable
__declspec(dllexport) void a_function(int with_args)
All such symbols will be exported from the DLL. If, however, any of the object files in the DLL contain symbols decorated in this way, then the normal auto-export behavior is disabled, unless the `--export-all-symbols' option is also used.
Note that object files that wish to access these symbols must not decorate them with dllexport. Instead, they should use dllimport, instead:
__declspec(dllimport) int a_variable
__declspec(dllimport) void a_function(int with_args)
This complicates the structure of library header files, because when included by the library itself the header must declare the variables and functions as dllexport, but when included by client code the header must declare them as dllimport. There are a number of idioms that are typically used to do this; often client code can omit the __declspec() declaration completely. See `--enable-auto-import' and `automatic data imports' for more imformation.
auto-import of variables does not always work flawlessly without additional assistance. Sometimes, you will see this message
"variable '<var>' can't be auto-imported. Please read the
documentation for ld's --enable-auto-import for details."
The `--enable-auto-import' documentation explains why this error occurs, and several methods that can be used to overcome this difficulty. One of these methods is the runtime pseudo-relocs feature, described below.
For complex variables imported from DLLs (such as structs or classes), object files typically contain a base address for the variable and an offset (addend) within the variable–to specify a particular field or public member, for instance. Unfortunately, the runtime loader used in win32 environments is incapable of fixing these references at runtime without the additional information supplied by dllimport/dllexport decorations. The standard auto-import feature described above is unable to resolve these references.
The `--enable-runtime-pseudo-relocs' switch allows these references to be resolved without error, while leaving the task of adjusting the references themselves (with their non-zero addends) to specialized code provided by the runtime environment. Recent versions of the cygwin and mingw environments and compilers provide this runtime support; older versions do not. However, the support is only necessary on the developer's platform; the compiled result will run without error on an older system.
`--enable-runtime-pseudo-relocs' is not the default; it must be explicitly enabled as needed.
Linking directly to a dll uses no extra command-line switches other than `-L' and `-l', because ld already searches for a number of names to match each library. All that is needed from the developer's perspective is an understanding of this search, in order to force ld to select the dll instead of an import library.
For instance, when ld is called with the argument `-lxxx' it will attempt to find, in the first directory of its search path,
libxxx.dll.a
xxx.dll.a
libxxx.a
cygxxx.dll (*)
libxxx.dll
xxx.dll
before moving on to the next directory in the search path.
(*) Actually, this is not `cygxxx.dll' but in fact is `<prefix>xxx.dll', where `<prefix>' is set by the ld option `--dll-search-prefix=<prefix>'. In the case of cygwin, the standard gcc spec file includes `--dll-search-prefix=cyg', so in effect we actually search for `cygxxx.dll'.
Other win32-based unix environments, such as mingw or pw32, may use other `<prefix>'es, although at present only cygwin makes use of this feature. It was originally intended to help avoid name conflicts among dll's built for the various win32/un*x environments, so that (for example) two versions of a zlib dll could coexist on the same machine.
The generic cygwin/mingw path layout uses a `bin' directory for applications and dll's and a `lib' directory for the import libraries (using cygwin nomenclature):
bin/
cygxxx.dll
lib/
libxxx.dll.a (in case of dll's)
libxxx.a (in case of static archive)
Linking directly to a dll without using the import library can be done two ways:
1. Use the dll directly by adding the `bin' path to the link line
gcc -Wl,-verbose -o a.exe -L../bin/ -lxxx
However, as the dll's often have version numbers appended to their names (`cygncurses-5.dll') this will often fail, unless one specifies `-L../bin -lncurses-5' to include the version. Import libs are generally not versioned, and do not have this difficulty.
2. Create a symbolic link from the dll to a file in the `lib' directory according to the above mentioned search pattern. This should be used to avoid unwanted changes in the tools needed for making the app/dll.
ln -s bin/cygxxx.dll lib/[cyg|lib|]xxx.dll[.a]
Then you can link without any make environment changes.
gcc -Wl,-verbose -o a.exe -L../lib/ -lxxx
This technique also avoids the version number problems, because the following is perfectly legal
bin/
cygxxx-5.dll
lib/
libxxx.dll.a -> ../bin/cygxxx-5.dll
Linking directly to a dll without using an import lib will work even when auto-import features are exercised, and even when `--enable-runtime-pseudo-relocs' is used.
Given the improvements in speed and memory usage, one might justifiably wonder why import libraries are used at all. There are two reasons:
1. Until recently, the link-directly-to-dll functionality did not work with auto-imported data.
2. Sometimes it is necessary to include pure static objects within the import library (which otherwise contains only bfd's for indirection symbols that point to the exports of a dll). Again, the import lib for the cygwin kernel makes use of this ability, and it is not possible to do this without an import lib.
So, import libs are not going away. But the ability to replace
true import libs with a simple symbolic link to (or a copy of)
a dll, in most cases, is a useful addition to the suite of tools
binutils makes available to the win32 developer. Given the
massive improvements in memory requirements during linking, storage
requirements, and linking speed, we expect that many developers
will soon begin to use this feature whenever possible.
LIBRARY "xyz.dll" BASE=0x61000000
EXPORTS
foo
_foo = foo
The line `_foo = foo' maps the symbol `foo' to `_foo'.
Another method for creating a symbol alias is to create it in the source code using the "weak" attribute:
void foo () { /* Do something. */; }
void _foo () __attribute__ ((weak, alias ("foo")));
See the gcc manual for more information about attributes and weak
symbols.
LIBRARY "xyz.dll" BASE=0x61000000
EXPORTS
_foo = foo
The line `_foo = foo' maps the exported symbol `foo' to `_foo'.
Note: using a DEF file disables the default auto-export behavior, unless the `--export-all-symbols' command line option is used. If, however, you are trying to rename symbols, then you should list all desired exports in the DEF file, including the symbols that are not being renamed, and do not use the `--export-all-symbols' option. If you list only the renamed symbols in the DEF file, and use `--export-all-symbols' to handle the other symbols, then the both the new names and the original names for the renamed symbols will be exported. In effect, you'd be aliasing those symbols, not renaming them, which is probably not what you wanted.
ld and Xtensa ProcessorsThe default ld behavior for Xtensa processors is to interpret
SECTIONS commands so that lists of explicitly named sections in a
specification with a wildcard file will be interleaved when necessary to
keep literal pools within the range of PC-relative load offsets. For
example, with the command:
SECTIONS
{
.text : {
*(.literal .text)
}
}
ld may interleave some of the .literal
and .text sections from different object files to ensure that the
literal pools are within the range of PC-relative load offsets. A valid
interleaving might place the .literal sections from an initial
group of files followed by the .text sections of that group of
files. Then, the .literal sections from the rest of the files
and the .text sections from the rest of the files would follow.
The non-interleaved order can still be specified as:
SECTIONS
{
.text : {
*(.literal) *(.text)
}
}
The Xtensa version of ld enables the --relax option by default to attempt to reduce space in the output image by combining literals with identical values. It also provides the --no-relax option to disable this optimization. When enabled, the relaxation algorithm ensures that a literal will only be merged with another literal when the new merged literal location is within the offset range of all of its uses.
The relaxation mechanism will also attempt to optimize
assembler-generated “longcall” sequences of
L32R/CALLXn when the target is known to fit into a
CALLn instruction encoding. The current optimization
converts the sequence into NOP/CALLn and removes the
literal referenced by the L32R instruction.
The linker accesses object and archive files using the BFD libraries.
These libraries allow the linker to use the same routines to operate on
object files whatever the object file format. A different object file
format can be supported simply by creating a new BFD back end and adding
it to the library. To conserve runtime memory, however, the linker and
associated tools are usually configured to support only a subset of the
object file formats available. You can use objdump -i
(see objdump) to
list all the formats available for your configuration.
As with most implementations, BFD is a compromise between several conflicting requirements. The major factor influencing BFD design was efficiency: any time used converting between formats is time which would not have been spent had BFD not been involved. This is partly offset by abstraction payback; since BFD simplifies applications and back ends, more time and care may be spent optimizing algorithms for a greater speed.
One minor artifact of the BFD solution which you should bear in mind is the potential for information loss. There are two places where useful information can be lost using the BFD mechanism: during conversion and during output. See BFD information loss.
When an object file is opened, BFD subroutines automatically determine the format of the input object file. They then build a descriptor in memory with pointers to routines that will be used to access elements of the object file's data structures.
As different information from the object files is required, BFD reads from different sections of the file and processes them. For example, a very common operation for the linker is processing symbol tables. Each BFD back end provides a routine for converting between the object file's representation of symbols and an internal canonical format. When the linker asks for the symbol table of an object file, it calls through a memory pointer to the routine from the relevant BFD back end which reads and converts the table into a canonical form. The linker then operates upon the canonical form. When the link is finished and the linker writes the output file's symbol table, another BFD back end routine is called to take the newly created symbol table and convert it into the chosen output format.
Information can be lost during output. The output formats
supported by BFD do not provide identical facilities, and
information which can be described in one form has nowhere to go in
another format. One example of this is alignment information in
b.out. There is nowhere in an a.out format file to store
alignment information on the contained data, so when a file is linked
from b.out and an a.out image is produced, alignment
information will not propagate to the output file. (The linker will
still use the alignment information internally, so the link is performed
correctly).
Another example is COFF section names. COFF files may contain an
unlimited number of sections, each one with a textual section name. If
the target of the link is a format which does not have many sections (e.g.,
a.out) or has sections without names (e.g., the Oasys format), the
link cannot be done simply. You can circumvent this problem by
describing the desired input-to-output section mapping with the linker command
language.
Information can be lost during canonicalization. The BFD internal canonical form of the external formats is not exhaustive; there are structures in input formats for which there is no direct representation internally. This means that the BFD back ends cannot maintain all possible data richness through the transformation between external to internal and back to external formats.
This limitation is only a problem when an application reads one
format and writes another. Each BFD back end is responsible for
maintaining as much data as possible, and the internal BFD
canonical form has structures which are opaque to the BFD core,
and exported only to the back ends. When a file is read in one format,
the canonical form is generated for BFD and the application. At the
same time, the back end saves away any information which may otherwise
be lost. If the data is then written back in the same format, the back
end routine will be able to use the canonical form provided by the
BFD core as well as the information it prepared earlier. Since
there is a great deal of commonality between back ends,
there is no information lost when
linking or copying big endian COFF to little endian COFF, or a.out to
b.out. When a mixture of formats is linked, the information is
only lost from the files whose format differs from the destination.
The greatest potential for loss of information occurs when there is the least overlap between the information provided by the source format, that stored by the canonical format, and that needed by the destination format. A brief description of the canonical form may help you understand which kinds of data you can count on preserving across conversions.
ZMAGIC
file would have both the demand pageable bit and the write protected
text bit set. The byte order of the target is stored on a per-file
basis, so that big- and little-endian object files may be used with one
another.
ld can
operate on a collection of symbols of wildly different formats without
problems.
Normal global and simple local symbols are maintained on output, so an
output file (no matter its format) will retain symbols pointing to
functions and to global, static, and common variables. Some symbol
information is not worth retaining; in a.out, type information is
stored in the symbol table as long symbol names. This information would
be useless to most COFF debuggers; the linker has command line switches
to allow users to throw it away.
There is one word of type information within the symbol, so if the
format supports symbol type information within symbols (for example, COFF,
IEEE, Oasys) and the type is simple enough to fit within one word
(nearly everything but aggregates), the information will be preserved.
Your bug reports play an essential role in making ld reliable.
Reporting a bug may help you by bringing a solution to your problem, or it may not. But in any case the principal function of a bug report is to help the entire community by making the next version of ld work better. Bug reports are your contribution to the maintenance of ld.
In order for a bug report to serve its purpose, you must include the information that enables us to fix the bug.
If you are not sure whether you have found a bug, here are some guidelines:
A number of companies and individuals offer support for gnu products. If you obtained ld from a support organization, we recommend you contact that organization first.
You can find contact information for many support companies and individuals in the file etc/SERVICE in the gnu Emacs distribution.
Otherwise, send bug reports for ld to `bug-binutils@gnu.org'.
The fundamental principle of reporting bugs usefully is this: report all the facts. If you are not sure whether to state a fact or leave it out, state it!
Often people omit facts because they think they know what causes the problem and assume that some details do not matter. Thus, you might assume that the name of a symbol you use in an example does not matter. Well, probably it does not, but one cannot be sure. Perhaps the bug is a stray memory reference which happens to fetch from the location where that name is stored in memory; perhaps, if the name were different, the contents of that location would fool the linker into doing the right thing despite the bug. Play it safe and give a specific, complete example. That is the easiest thing for you to do, and the most helpful.
Keep in mind that the purpose of a bug report is to enable us to fix the bug if it is new to us. Therefore, always write your bug reports on the assumption that the bug has not been reported previously.
Sometimes people give a few sketchy facts and ask, “Does this ring a bell?” This cannot help us fix a bug, so it is basically useless. We respond by asking for enough details to enable us to investigate. You might as well expedite matters by sending them to begin with.
To enable us to fix the bug, you should include all these things:
Without this, we will not know whether there is any point in looking for the bug in the current version of ld.
BFD library.
gcc-2.7”.
If we were to try to guess the arguments, we would probably guess wrong and then we might not encounter the bug.
If the source files were assembled using gas or compiled using
gcc, then it may be OK to send the source files rather than the
object files. In this case, be sure to say exactly what version of
gas or gcc was used to produce the object files. Also say
how gas or gcc were configured.
Of course, if the bug is that ld gets a fatal signal, then we will certainly notice it. But if the bug is incorrect output, we might not notice unless it is glaringly wrong. You might as well not give us a chance to make a mistake.
Even if the problem you experience is a fatal signal, you should still say so explicitly. Suppose something strange is going on, such as, your copy of ld is out of synch, or you have encountered a bug in the C library on your system. (This has happened!) Your copy might crash and ours would not. If you told us to expect a crash, then when ours fails to crash, we would know that the bug was not happening for us. If you had not told us to expect a crash, then we would not be able to draw any conclusion from our observations.
diff with the `-u', `-c', or
`-p' option. Always send diffs from the old file to the new file.
If you even discuss something in the ld source, refer to it by
context, not by line number.
The line numbers in our development sources will not match those in your sources. Your line numbers would convey no useful information to us.
Here are some things that are not necessary:
Often people who encounter a bug spend a lot of time investigating which changes to the input file will make the bug go away and which changes will not affect it.
This is often time consuming and not very useful, because the way we will find the bug is by running a single example under the debugger with breakpoints, not by pure deduction from a series of examples. We recommend that you save your time for something else.
Of course, if you can find a simpler example to report instead of the original one, that is a convenience for us. Errors in the output will be easier to spot, running under the debugger will take less time, and so on.
However, simplification is not vital; if you do not want to do this, report the bug anyway and send us the entire test case you used.
A patch for the bug does help us if it is a good one. But do not omit the necessary information, such as the test case, on the assumption that a patch is all we need. We might see problems with your patch and decide to fix the problem another way, or we might not understand it at all.
Sometimes with a program as complicated as ld it is very hard to construct an example that will make the program follow a certain path through the code. If you do not send us the example, we will not be able to construct one, so we will not be able to verify that the bug is fixed.
And if we cannot understand what bug you are trying to fix, or why your patch should be an improvement, we will not install it. A test case will help us to understand.
Such guesses are usually wrong. Even we cannot guess right about such things without first using the debugger to find the facts.
To aid users making the transition to gnu ld from the MRI linker, ld can use MRI compatible linker scripts as an alternative to the more general-purpose linker scripting language described in Scripts. MRI compatible linker scripts have a much simpler command set than the scripting language otherwise used with ld. gnu ld supports the most commonly used MRI linker commands; these commands are described here.
In general, MRI scripts aren't of much use with the a.out object
file format, since it only has three sections and MRI scripts lack some
features to make use of them.
You can specify a file containing an MRI-compatible script using the `-c' command-line option.
Each command in an MRI-compatible script occupies its own line; each command line starts with the keyword that identifies the command (though blank lines are also allowed for punctuation). If a line of an MRI-compatible script begins with an unrecognized keyword, ld issues a warning message, but continues processing the script.
Lines beginning with `*' are comments.
You can write these commands using all upper-case letters, or all lower case; for example, `chip' is the same as `CHIP'. The following list shows only the upper-case form of each command.
ABSOLUTE secnameABSOLUTE secname, secname, ... secnameABSOLUTE command to restrict the sections that will be present in
your output program. If the ABSOLUTE command is used at all in a
script, then only the sections named explicitly in ABSOLUTE
commands will appear in the linker output. You can still use other
input sections (whatever you select on the command line, or using
LOAD) to resolve addresses in the output file.
ALIAS out-secname, in-secnamein-secname may be an integer.
ALIGN secname = expressionBASE expressionCHIP expressionCHIP expression, expressionENDFORMAT output-formatOUTPUT_FORMAT command in the more general linker
language, but restricted to one of these output formats:
LIST anything...The keyword LIST may be followed by anything on the
same line, with no change in its effect.
LOAD filenameLOAD filename, filename, ... filenameNAME output-nameNAME is equivalent to the command-line
option `-o' or the general script language command OUTPUT.
ORDER secname, secname, ... secnameORDER secname secname secnameORDER command. The
sections you list with ORDER will appear first in your output
file, in the order specified.
PUBLIC name=expressionPUBLIC name,expressionPUBLIC name expressionSECT secname, expressionSECT secname=expressionSECT secname expressionSECT command to
specify the start address (expression) for section secname.
If you have more than one SECT statement for the same
secname, only the first sets the start address.
Copyright (C) 2000, Free Software Foundation, Inc.
59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
The purpose of this License is to make a manual, textbook, or other written document “free” in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.
This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.
We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.
This License applies to any manual or other work that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you.”
A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.
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If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.
": Symbols-(: Options--accept-unknown-input-arch: Options--add-stdcall-alias: Options--allow-multiple-definition: Options--allow-shlib-undefined: Options--architecture=arch: Options--as-needed: Options--auxiliary: Options--base-file: Options--check-sections: Options--cref: Options--defsym symbol=exp: Options--demangle[=style]: Options--disable-auto-image-base: Options--disable-auto-import: Options--disable-new-dtags: Options--disable-runtime-pseudo-reloc: Options--disable-stdcall-fixup: Options--discard-all: Options--discard-locals: Options--dll: Options--dll-search-prefix: Options--dynamic-linker file: Options--embedded-relocs: Options--emit-relocs: Options--enable-auto-image-base: Options--enable-auto-import: Options--enable-extra-pe-debug: Options--enable-new-dtags: Options--enable-runtime-pseudo-reloc: Options--enable-stdcall-fixup: Options--entry=entry: Options--error-unresolved-symbols: Options--exclude-libs: Options--exclude-symbols: Options--export-all-symbols: Options--export-dynamic: Options--fatal-warnings: Options--file-alignment: Options--filter: Options--force-exe-suffix: Options--format=format: Options--format=version: TI COFF--gc-sections: Options--gpsize: Options--heap: Options--help: Options--image-base: Options--just-symbols=file: Options--kill-at: Options--library-path=dir: Options--library=archive: Options--major-image-version: Options--major-os-version: Options--major-subsystem-version: Options--minor-image-version: Options--minor-os-version: Options--minor-subsystem-version: Options--mri-script=MRI-cmdfile: Options--multi-subspace: HPPA ELF32--nmagic: Options--no-accept-unknown-input-arch: Options--no-allow-shlib-undefined: Options--no-as-needed: Options--no-check-sections: Options--no-define-common: Options--no-demangle: Options--no-gc-sections: Options--no-keep-memory: Options--no-omagic: Options--no-relax: Xtensa--no-undefined: Options--no-undefined-version: Options--no-warn-mismatch: Options--no-whole-archive: Options--noinhibit-exec: Options--oformat: Options--omagic: Options--out-implib: Options--output-def: Options--output=output: Options--pic-executable: Options--print-map: Options--relax: Options--relax on Xtensa: Xtensa--relocatable: Options--script=script: Options--section-alignment: Options--section-start sectionname=org: Options--sort-common: Options--split-by-file: Options--split-by-reloc: Options--stack: Options--stats: Options--strip-all: Options--strip-debug: Options--stub-group-size=N: HPPA ELF32--subsystem: Options--support-old-code: ARM--target-help: Options--thumb-entry=entry: ARM--trace: Options--trace-symbol=symbol: Options--traditional-format: Options--undefined=symbol: Options--unique[=SECTION]: Options--unresolved-symbols: Options--verbose: Options--version: Options--version-script=version-scriptfile: Options--warn-common: Options--warn-constructors: Options--warn-multiple-gp: Options--warn-once: Options--warn-section-align: Options--warn-unresolved-symbols: Options--whole-archive: Options--wrap: Options-Aarch: Options-akeyword: Options-assert keyword: Options-b format: Options-Bdynamic: Options-Bgroup: Options-Bshareable: Options-Bstatic: Options-Bsymbolic: Options-c MRI-cmdfile: Options-call_shared: Options-d: Options-dc: Options-dn: Options-dp: Options-dy: Options-E: Options-e entry: Options-EB: Options-EL: Options-F: Options-f: Options-fini: Options-G: Options-g: Options-hname: Options-i: Options-Ifile: Options-init: Options-larchive: Options-Ldir: Options-M: Options-m emulation: Options-Map: Options-N: Options-n: Options-non_shared: Options-nostdlib: Options-O level: Options-o output: Options-pie: Options-q: Options-qmagic: Options-Qy: Options-r: Options-R file: Options-rpath: Options-rpath-link: Options-s: Options-S: Options-shared: Options-soname=name: Options-static: Options-t: Options-T script: Options-Tbss org: Options-Tdata org: Options-Ttext org: Options-u symbol: Options-Ur: Options-V: Options-v: Options-X: Options-x: Options-Y path: Options-y symbol: Options-z defs: Options-z keyword: Options-z muldefs: Options.: Location Counter:phdr: Output Section Phdr=fillexp: Output Section Fill>region: Output Section RegionABSOLUTE (MRI): MRIABSOLUTE(exp): Builtin FunctionsADDR(section): Builtin FunctionsALIAS (MRI): MRIALIGN (MRI): MRIALIGN(align): Builtin FunctionsALIGN(exp,align): Builtin FunctionsASSERT: Miscellaneous CommandsAT(lma): Output Section LMAAT>lma_region: Output Section LMABASE (MRI): MRIBLOCK(exp): Builtin FunctionsBYTE(expression): Output Section DataCHIP (MRI): MRICOLLECT_NO_DEMANGLE: EnvironmentCONSTRUCTORS: Output Section KeywordsCREATE_OBJECT_SYMBOLS: Output Section KeywordsDATA_SEGMENT_ALIGN(maxpagesize, commonpagesize): Builtin FunctionsDATA_SEGMENT_END(exp): Builtin FunctionsDEFINED(symbol): Builtin FunctionsEND (MRI): MRIENTRY(symbol): Entry PointEXTERN: Miscellaneous CommandsFILEHDR: PHDRSFILL(expression): Output Section DataFORCE_COMMON_ALLOCATION: Miscellaneous CommandsFORMAT (MRI): MRIGNUTARGET: EnvironmentGROUP(files): File CommandsINCLUDE filename: File CommandsINHIBIT_COMMON_ALLOCATION: Miscellaneous CommandsINPUT(files): File Commandsl =: MEMORYLDEMULATION: Environmentlen =: MEMORYLENGTH =: MEMORYLIST (MRI): MRILOAD (MRI): MRILOADADDR(section): Builtin FunctionsLONG(expression): Output Section DataMAX: Builtin FunctionsMEMORY: MEMORYMIN: Builtin FunctionsNAME (MRI): MRINEXT(exp): Builtin FunctionsNOCROSSREFS(sections): Miscellaneous CommandsNOLOAD: Output Section Typeo =: MEMORYobjdump -i: BFDORDER (MRI): MRIorg =: MEMORYORIGIN =: MEMORYOUTPUT(filename): File CommandsOUTPUT_ARCH(bfdarch): Miscellaneous CommandsOUTPUT_FORMAT(bfdname): Format CommandsOVERLAY: Overlay DescriptionPHDRS: PHDRSPUBLIC (MRI): MRIQUAD(expression): Output Section DataSEARCH_DIR(path): File CommandsSECT (MRI): MRISECTIONS: SECTIONSSHORT(expression): Output Section DataSIZEOF(section): Builtin FunctionsSIZEOF_HEADERS: Builtin FunctionsSQUAD(expression): Output Section DataSTARTUP(filename): File CommandsSUBALIGN(subsection_align): Forced Input AlignmentTARGET(bfdname): Format CommandsVERSION {script text}: VERSION