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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 section 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).
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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.)
The command-line options to ld
may be specified in any order, and
may be repeated at will. Repeating most options with a different
argument will either have no further effect, or override prior
occurrences (those further to the left on the command line) of that
option. Options which may be meaningfully specified more than once are
noted in the descriptions below.
Non-option arguments are objects files 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 can not recognize the format of an object file, it will
assume that it is a linker script. A script specified in this way
augments the main linker script used for the link (either the default
linker script or the one specified by using ‘-T’). This feature
permits the linker to link against a file 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. See section Command Language.
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, ‘--oformat’ and ‘-oformat’ are equivalent. Arguments to multiple-letter options must either be separated from the option name by an equals sign, or be given as separate arguments immediately following the option that requires them. For example, ‘--oformat srec’ and ‘--oformat=srec’ are equivalent. Unique abbreviations of the names of multiple-letter options are accepted.
-akeyword
This option is supported for HP/UX compatibility. The keyword argument must be one of the strings ‘archive’, ‘shared’, or ‘default’. ‘-aarchive’ is functionally equivalent to ‘-Bstatic’, and the other two keywords are functionally equivalent to ‘-Bdynamic’. This option may be used any number of times.
-b input-format
--format=input-format
ld
may be configured to support more than one kind of object
file. If your ld
is configured this way, you can use the
‘-b’ option to specify the binary format for input object files
that follow this option on the command line. Even when ld
is
configured to support alternative object formats, you don’t usually need
to specify this, as ld
should be configured to expect as a
default input format the most usual format on each machine.
input-format is a text string, the name of a particular format
supported by the BFD libraries. (You can list the available binary
formats with ‘objdump -i’.)
See section BFD.
You may want to use this option if you are linking files with an unusual binary format. You can also use ‘-b’ to switch formats explicitly (when linking object files of different formats), by including ‘-b input-format’ before each group of object files in a particular format.
The default format is taken from the environment variable
GNUTARGET
.
You can also define the input
format from a script, using the command TARGET
; see Option Commands.
-c MRI-commandfile
--mri-script=MRI-commandfile
For compatibility with linkers produced by MRI, ld
accepts script
files written in an alternate, restricted command language, described in
MRI Compatible Script Files. Introduce MRI script files with
the option ‘-c’; use the ‘-T’ option to run linker
scripts written in the general-purpose ld
scripting language.
If MRI-cmdfile does not exist, ld
looks for it in the directories
specified by any ‘-L’ options.
-d
-dc
-dp
These three options are equivalent; multiple forms are supported for
compatibility with other linkers. They
assign space to common symbols even if a relocatable output file is
specified (with ‘-r’). The script command
FORCE_COMMON_ALLOCATION
has the same effect. See section Option Commands.
-e entry
--entry=entry
Use entry as the explicit symbol for beginning execution of your program, rather than the default entry point. See section The Entry Point, for a discussion of defaults and other ways of specifying the entry point.
-E
-export-dynamic
When creating a dynamically linked executable, add all symbols to the
dynamic symbol table. Normally, the dynamic symbol table contains only
symbols which are used by a dynamic object. This option is needed for
some uses of dlopen
.
-F
-Fformat
Ignored. Some older linkers used this option throughout a compilation
toolchain for specifying object-file format for both input and output
object files. The mechanisms ld
uses for this purpose (the
‘-b’ or ‘-format’ options for input files, ‘-oformat’
option or the TARGET
command in linker scripts for output files,
the GNUTARGET
environment variable) are more flexible, but
ld
accepts the ‘-F’ option for compatibility with scripts
written to call the old linker.
--force-exe-suffix
Make sure that an output file has a .exe suffix.
If a successfully built fully linked output file does not have a
.exe
or .dll
suffix, this option forces the linker to copy
the output file to one of the same name with a .exe
suffix. This
option is useful when using unmodified Unix makefiles on a Microsoft
Windows host, since some versions of Windows won’t run an image unless
it ends in a .exe
suffix.
-g
Ignored. Provided for compatibility with other tools.
-Gvalue
--gpsize=value
Set the maximum size of objects to be optimized using the GP register to size. This is only meaningful for object file formats such as MIPS ECOFF which supports putting large and small objects into different sections. This is ignored for other object file formats.
-hname
-soname=name
When creating an ELF shared object, set the internal DT_SONAME field to the specified name. When an executable is linked with a shared object which has a DT_SONAME field, then when the executable is run the dynamic linker will attempt to load the shared object specified by the DT_SONAME field rather than the using the file name given to the linker.
-i
Perform an incremental link (same as option ‘-r’).
-larchive
--library=archive
Add archive file archive to the list of files to link. This
option may be used any number of times. ld
will search its
path-list for occurrences of libarchive.a
for every
archive specified. File extensions other than .a
may be
used on certain systems.
-Lsearchdir
--library-path=searchdir
Add path searchdir to the list of paths that ld
will search
for archive libraries and ld
control scripts. You may use this
option any number of times. The directories are searched in the order
in which they are specified on the command line. Directories specified
on the command line are searched before the default directories. All
-L
options apply to all -l
options, regardless of the
order in which the options appear.
The paths can also be specified in a link script with the
SEARCH_DIR
command. Directories specified this way are searched
at the point in which the linker script appears in the command line.
-memulation
Emulate the emulation linker. You can list the available
emulations with the ‘--verbose’ or ‘-V’ options. The default
depends on how your ld
was configured.
-M
--print-map
Print (to the standard output) a link map—diagnostic information about
where symbols are mapped by ld
, and information on global common
storage allocation.
-n
--nmagic
Set the text segment to be read only, and mark the output as
NMAGIC
if possible.
-N
--omagic
Set the text and data sections to be readable and writable. Also, do
not page-align the data segment. If the output format supports Unix
style magic numbers, mark the output as OMAGIC
.
-o output
--output=output
Use output as the name for the program produced by ld
; if this
option is not specified, the name ‘a.out’ is used by default. The
script command OUTPUT
can also specify the output file name.
-r
--relocateable
Generate relocatable output—i.e., generate an output file that can in
turn serve as input to ld
. This is often called partial
linking. As a side effect, in environments that support standard Unix
magic numbers, this option also sets the output file’s magic number to
OMAGIC
.
If this option is not specified, an absolute file is produced. When
linking C++ programs, this option will not resolve references to
constructors; to do that, use ‘-Ur’.
This option does the same thing as ‘-i’.
-R filename
--just-symbols=filename
Read symbol names and their addresses from filename, but do not relocate it or include it in the output. This allows your output file to refer symbolically to absolute locations of memory defined in other programs. You may use this option more than once.
For compatibility with other ELF linkers, if the -R
option is
followed by a directory name, rather than a file name, it is treated as
the -rpath
option.
-s
--strip-all
Omit all symbol information from the output file.
-S
--strip-debug
Omit debugger symbol information (but not all symbols) from the output file.
-t
--trace
Print the names of the input files as ld
processes them.
-T commandfile
--script=commandfile
Read link commands from the file commandfile. These commands
replace ld
’s default link script (rather than adding
to it), so commandfile must specify everything necessary to describe
the target format. See section Command Language. If commandfile does not
exist, ld
looks for it in the directories specified by any
preceding ‘-L’ options. Multiple ‘-T’ options accumulate.
-u symbol
--undefined=symbol
Force symbol to be entered in the output file as an undefined symbol. Doing this may, for example, trigger linking of additional modules from standard libraries. ‘-u’ may be repeated with different option arguments to enter additional undefined symbols.
-v
--version
-V
Display the version number for ld
. The -V
option also
lists the supported emulations.
-x
--discard-all
Delete all local symbols.
-X
--discard-locals
Delete all temporary local symbols. For most targets, this is all local symbols whose names begin with ‘L’.
-y symbol
--trace-symbol=symbol
Print the name of each linked file in which symbol appears. This option may be given any number of times. On many systems it is necessary to prepend an underscore.
This option is useful when you have an undefined symbol in your link but don’t know where the reference is coming from.
-Y path
Add path to the default library search path. This option exists for Solaris compatibility.
-z keyword
This option is ignored for Solaris compatibility.
-( archives -)
--start-group archives --end-group
The archives should be a list of archive files. They may be either explicit file names, or ‘-l’ options.
The specified archives are searched repeatedly until no new undefined references are created. Normally, an archive is searched only once in the order that it is specified on the command line. If a symbol in that archive is needed to resolve an undefined symbol referred to by an object in an archive that appears later on the command line, the linker would not be able to resolve that reference. By grouping the archives, they all be searched repeatedly until all possible references are resolved.
Using this option has a significant performance cost. It is best to use it only when there are unavoidable circular references between two or more archives.
-assert keyword
This option is ignored for SunOS compatibility.
-Bdynamic
-dy
-call_shared
Link against dynamic libraries. This is only meaningful on platforms
for which shared libraries are supported. This option is normally the
default on such platforms. The different variants of this option are
for compatibility with various systems. You may use this option
multiple times on the command line: it affects library searching for
-l
options which follow it.
-Bstatic
-dn
-non_shared
-static
Do not link against shared libraries. This is only meaningful on
platforms for which shared libraries are supported. The different
variants of this option are for compatibility with various systems. You
may use this option multiple times on the command line: it affects
library searching for -l
options which follow it.
-Bsymbolic
When creating a shared library, bind references to global symbols to the definition within the shared library, if any. Normally, it is possible for a program linked against a shared library to override the definition within the shared library. This option is only meaningful on ELF platforms which support shared libraries.
--cref
Output a cross reference table. If a linker map file is being generated, the cross reference table is printed to the map file. Otherwise, it is printed on the standard output.
The format of the table is intentionally simple, so that it may be easily processed by a script if necessary. The symbols are printed out, sorted by name. For each symbol, a list of file names is given. If the symbol is defined, the first file listed is the location of the definition. The remaining files contain references to the symbol.
--defsym symbol=expression
Create a global symbol in the output file, containing the absolute
address given by expression. You may use this option as many
times as necessary to define multiple symbols in the command line. A
limited form of arithmetic is supported for the expression in this
context: you may give a hexadecimal constant or the name of an existing
symbol, or use +
and -
to add or subtract hexadecimal
constants or symbols. If you need more elaborate expressions, consider
using the linker command language from a script (see section Assignment: Symbol Definitions). Note: there should be no
white space between symbol, the equals sign (“<=>”), and
expression.
--dynamic-linker file
Set the name of the dynamic linker. This is only meaningful when generating dynamically linked ELF executables. The default dynamic linker is normally correct; don’t use this unless you know what you are doing.
-EB
Link big-endian objects. This affects the default output format.
-EL
Link little-endian objects. This affects the default output format.
-embedded-relocs
This option is only meaningful when linking MIPS embedded PIC code, generated by the -membedded-pic option to the GNU compiler and assembler. It causes the linker to create a table which may be used at runtime to relocate any data which was statically initialized to pointer values. See the code in testsuite/ld-empic for details.
--help
Print a summary of the command-line options on the standard output and exit.
-Map mapfile
Print to the file mapfile a link map—diagnostic information
about where symbols are mapped by ld
, and information on global
common storage allocation.
--no-keep-memory
ld
normally optimizes for speed over memory usage by caching the
symbol tables of input files in memory. This option tells ld
to
instead optimize for memory usage, by rereading the symbol tables as
necessary. This may be required if ld
runs out of memory space
while linking a large executable.
--no-whole-archive
Turn off the effect of the --whole-archive
option for subsequent
archive files.
--noinhibit-exec
Retain the executable output file whenever it is still usable. Normally, the linker will not produce an output file if it encounters errors during the link process; it exits without writing an output file when it issues any error whatsoever.
-oformat output-format
ld
may be configured to support more than one kind of object
file. If your ld
is configured this way, you can use the
‘-oformat’ option to specify the binary format for the output
object file. Even when ld
is configured to support alternative
object formats, you don’t usually need to specify this, as ld
should be configured to produce as a default output format the most
usual format on each machine. output-format is a text string, the
name of a particular format supported by the BFD libraries. (You can
list the available binary formats with ‘objdump -i’.) The script
command OUTPUT_FORMAT
can also specify the output format, but
this option overrides it. See section BFD.
-qmagic
This option is ignored for Linux compatibility.
-Qy
This option is ignored for SVR4 compatibility.
--relax
An option with machine dependent effects.
On some platforms, the ‘--relax’ option performs global optimizations that become possible when the linker resolves addressing in the program, such as relaxing address modes and synthesizing new instructions in the output object file.
--retain-symbols-file filename
Retain only the symbols listed in the file filename, discarding all others. filename is simply a flat file, with one symbol name per line. This option is especially useful in environments where a large global symbol table is accumulated gradually, to conserve run-time memory.
‘-retain-symbols-file’ does not discard undefined symbols, or symbols needed for relocations.
You may only specify ‘-retain-symbols-file’ once in the command line. It overrides ‘-s’ and ‘-S’.
-shared
-Bshareable
Create a shared library. This is currently only supported on ELF, XCOFF
and SunOS platforms. On SunOS, the linker will automatically create a
shared library if the -e
option is not used and there are
undefined symbols in the link.
--sort-common
This option tells ld
to sort the common symbols by size when it
places them in the appropriate output sections. First come all the one
byte symbols, then all the two bytes, then all the four bytes, and then
everything else. This is to prevent gaps between symbols due to
alignment constraints.
--split-by-file
Similar to --split-by-reloc
but creates a new output section for
each input file.
--split-by-reloc count
Trys to creates extra sections in the output file so that no single output section in the file contains more than count relocations. This is useful when generating huge relocatable for downloading into certain real time kernels with the COFF object file format; since COFF cannot represent more than 65535 relocations in a single section. Note that this will fail to work with object file formats which do not support arbitrary sections. The linker will not split up individual input sections for redistribution, so if a single input section contains more than count relocations one output section will contain that many relocations.
--stats
Compute and display statistics about the operation of the linker, such as execution time and memory usage.
-traditional-format
For some targets, the output of ld
is different in some ways from
the output of some existing linker. This switch requests ld
to
use the traditional format instead.
For example, on SunOS, ld
combines duplicate entries in the
symbol string table. This can reduce the size of an output file with
full debugging information by over 30 percent. Unfortunately, the SunOS
dbx
program can not read the resulting program (gdb
has no
trouble). The ‘-traditional-format’ switch tells ld
to not
combine duplicate entries.
-Tbss org
-Tdata org
-Ttext org
Use org as the starting address for—respectively—the
bss
, data
, or the text
segment of the output file.
org must be a single hexadecimal integer;
for compatibility with other linkers, you may omit the leading
‘0x’ usually associated with hexadecimal values.
-Ur
For anything other than C++ programs, this option is equivalent to
‘-r’: it generates relocatable output—i.e., an output file that can in
turn serve as input to ld
. When linking C++ programs, ‘-Ur’
does resolve references to constructors, unlike ‘-r’.
It does not work to use ‘-Ur’ on files that were themselves linked
with ‘-Ur’; once the constructor table has been built, it cannot
be added to. Use ‘-Ur’ only for the last partial link, and
‘-r’ for the others.
--verbose
Display the version number for ld
and list the linker emulations
supported. Display which input files can and cannot be opened. Display
the linker script if using a default builtin script.
-warn-common
Warn when a common symbol is combined with another common symbol or with a symbol definition. Unix linkers allow this somewhat sloppy practice, but linkers on some other operating systems do not. This option allows you to find potential problems from combining global symbols. Unfortunately, some C libraries use this practice, so you may get some warnings about symbols in the libraries as well as in your programs.
There are three kinds of global symbols, illustrated here by C examples:
A definition, which goes in the initialized data section of the output file.
An undefined reference, which does not allocate space. There must be either a definition or a common symbol for the variable somewhere.
A common symbol. If there are only (one or more) common symbols for a variable, it goes in the uninitialized data area of the output file. The linker merges multiple common symbols for the same variable into a single symbol. If they are of different sizes, it picks the largest size. The linker turns a common symbol into a declaration, if there is a definition of the same variable.
The ‘-warn-common’ option can produce five kinds of warnings. Each warning consists of a pair of lines: the first describes the symbol just encountered, and the second describes the previous symbol encountered with the same name. One or both of the two symbols will be a common symbol.
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 if any global constructors are used. This is only useful for a few object file formats. For formats like COFF or ELF, the linker can not detect the use of global constructors.
-warn-multiple-gp
Warn if multiple global pointer values are required in the output file. This is only meaningful for certain processors, such as the Alpha. Specifically, some processors put large-valued constants in a special section. A special register (the global pointer) points into the middle of this section, so that constants can be loaded efficiently via a base-register relative addressing mode. Since the offset in base-register relative mode is fixed and relatively small (e.g., 16 bits), this limits the maximum size of the constant pool. Thus, in large programs, it is often necessary to use multiple global pointer values in order to be able to address all possible constants. This option causes a warning to be issued whenever this case occurs.
-warn-once
Only warn once for each undefined symbol, rather than once per module which refers to it.
--whole-archive
For each archive mentioned on the command line after the
--whole-archive
option, include every object file in the archive
in the link, rather than searching the archive for the required object
files. This is normally used to turn an archive file into a shared
library, forcing every object to be included in the resulting shared
library. This option may be used more than once.
--wrap symbol
Use a wrapper function for symbol. Any undefined reference to
symbol will be resolved to __wrap_symbol
. Any
undefined reference to __real_symbol
will be resolved to
symbol.
This can be used to provide a wrapper for a system function. The
wrapper function should be called __wrap_symbol
. If it
wishes to call the system function, it should call
__real_symbol
.
Here is a trivial example:
void * __wrap_malloc (int c) { printf ("malloc called with %ld\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
.
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The command language provides explicit control over the link process, allowing complete specification of the mapping between the linker’s input files and its output. It controls:
You may supply a command file (also known as a link script) to the linker either explicitly through the ‘-T’ option, or implicitly as an ordinary file. If the linker opens a file which it cannot recognize as a supported object or archive format, it reports an error.
3.1 Linker Scripts | ||
3.2 Expressions | ||
3.3 Memory Layout | MEMORY Command | |
3.4 Specifying Output Sections | SECTIONS Command | |
3.5 ELF Program Headers | PHDRS Command | |
3.6 The Entry Point | ||
3.7 Option Commands |
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The ld
command language is a collection of statements; some are
simple keywords setting a particular option, some are used to select and
group input files or name output files; and two statement
types have a fundamental and pervasive impact on the linking process.
The most fundamental command of the ld
command language is the
SECTIONS
command (see section Specifying Output Sections). Every meaningful command
script must have a SECTIONS
command: it specifies a
“picture” of the output file’s layout, in varying degrees of detail.
No other command is required in all cases.
The MEMORY
command complements SECTIONS
by describing the
available memory in the target architecture. This command is optional;
if you don’t use a MEMORY
command, ld
assumes sufficient
memory is available in a contiguous block for all output.
See section Memory Layout.
You may include comments in linker scripts just as in C: delimited by ‘/*’ and ‘*/’. As in C, comments are syntactically equivalent to whitespace.
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Many useful commands involve arithmetic expressions. The syntax for expressions in the command language is identical to that of C expressions, with the following features:
3.2.1 Integers | ||
3.2.2 Symbol Names | ||
3.2.3 The Location Counter | ||
3.2.4 Operators | ||
3.2.5 Evaluation | ||
3.2.6 Assignment: Defining Symbols | ||
3.2.7 Arithmetic Functions | Built-In Functions | |
3.2.8 Semicolons | Semicolon Usage |
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An octal integer is ‘0’ followed by zero or more of the octal digits (‘01234567’).
_as_octal = 0157255;
A decimal integer starts with a non-zero digit followed by zero or more digits (‘0123456789’).
_as_decimal = 57005;
A hexadecimal integer is ‘0x’ or ‘0X’ followed by one or more hexadecimal digits chosen from ‘0123456789abcdefABCDEF’.
_as_hex = 0xdead;
To write a negative integer, use the prefix operator ‘-’ (see section Operators).
_as_neg = -57005;
Additionally the suffixes K
and M
may be used to scale a
constant by
respectively. For example, the following all refer to the same quantity:
_fourk_1 = 4K; _fourk_2 = 4096; _fourk_3 = 0x1000;
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Unless quoted, symbol names start with a letter, underscore, or point and may include any letters, underscores, digits, points, 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.
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The special linker variable dot ‘.’ always contains the
current output location counter. Since the .
always refers to
a location in an output section, it must always appear in an
expression within a SECTIONS
command. The .
symbol
may appear anywhere that an ordinary symbol is allowed in an
expression, but its assignments have a side effect. Assigning a value
to the .
symbol 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) } = 0x1234; }
In the previous example, file1
is located at the beginning of the
output section, then there is a 1000 byte gap. Then file2
appears, also with a 1000 byte gap following before file3
is
loaded. The notation ‘= 0x1234’ specifies what data to write in
the gaps (see section Optional Section Attributes).
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The linker recognizes the standard C set of arithmetic operators, with the standard bindings and precedence levels:
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The linker uses “lazy evaluation” for expressions; it only calculates an expression when absolutely necessary. The linker needs the value of the start address, and the 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 command file. 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.
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You may create global symbols, and assign values (addresses) to global symbols, using any of the C assignment operators:
symbol = expression ;
symbol &= expression ;
symbol += expression ;
symbol -= expression ;
symbol *= expression ;
symbol /= expression ;
Two things distinguish assignment from other operators in ld
expressions.
Assignment statements may appear:
ld
script; or
SECTIONS
command; or
SECTIONS
command.
The first two cases are equivalent in effect—both define a symbol with an absolute address. The last case defines a symbol whose address is relative to a particular section (see section Specifying Output Sections).
When a linker expression is evaluated and assigned to a variable, it is given either an absolute or a relocatable type. An absolute expression type is one in which the symbol contains the value that it will have in the output file; a relocatable expression type is one in which the value is expressed as a fixed offset from the base of a section.
The type of the expression is controlled by its position in the script
file. A symbol assigned within a section definition is created relative
to the base of the section; a symbol assigned in any other place is
created as an absolute symbol. Since a symbol created within a
section definition is relative to the base of the section, it
will remain relocatable if relocatable output is requested. A symbol
may be created with an absolute value even when assigned to within a
section definition by using the absolute assignment function
ABSOLUTE
. For example, to create an absolute symbol whose address
is the last byte of an output section named .data
:
SECTIONS{ … .data : { *(.data) _edata = ABSOLUTE(.) ; } … }
The linker tries to put off the evaluation of an assignment until all the terms in the source expression are known (see section Evaluation). For instance, 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 dot, ‘.’ must be evaluated during 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 : { … } … }
will cause the error message “Non constant expression for initial
address
”.
In some cases, it is desirable for a linker script to define a symbol
only if it is referenced, and only if it 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)
.
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The command language includes a number of built-in functions for use in link script expressions.
ABSOLUTE(exp)
Return the absolute (non-relocatable, as opposed to non-negative) value of the expression exp. Primarily useful to assign an absolute value to a symbol within a section definition, where symbol values are normally section-relative.
ADDR(section)
Return the absolute address of the named section. Your script must
previously have defined the location of that section. In the following
example, 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(exp)
Return the result of the current location counter (.
) aligned to
the next exp boundary. exp must be an expression whose
value is a power of two. This is equivalent to
(. + exp - 1) & ~(exp - 1)
ALIGN
doesn’t change the value of the location counter—it just
does arithmetic on it. As an example, to align the output .data
section to the next 0x2000
byte boundary after the preceding
section and to set 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 start attribute of a
section definition (see section Optional Section Attributes). The second use simply
defines the value of a variable.
The built-in NEXT
is closely related to ALIGN
.
DEFINED(symbol)
Return 1 if symbol is in the linker global symbol table and is
defined, otherwise return 0. You can use this function to provide default
values for symbols. For example, the following command-file fragment shows how
to set a global symbol begin
to the first location in the
.text
section—but if a symbol called begin
already
existed, its value is preserved:
SECTIONS{ … .text : { begin = DEFINED(begin) ? begin : . ; … } … }
NEXT(exp)
Return the next unallocated address that is a multiple of exp.
This function is closely related to ALIGN(exp)
; unless you
use the MEMORY
command to define discontinuous memory for the
output file, the two functions are equivalent.
SIZEOF(section)
Return the size in bytes of the named section, if that section has
been allocated. In the following example, symbol_1
and
symbol_2
are assigned identical values:
SECTIONS{ … .output { .start = . ; … .end = . ; } symbol_1 = .end - .start ; symbol_2 = SIZEOF(.output); … }
SIZEOF_HEADERS
sizeof_headers
Return the size in bytes of the output file’s headers. You can use this number as the start address of the first section, if you choose, to facilitate paging.
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Semicolons (“<;>”) are required in the following places. In all other places they can appear for aesthetic reasons but are otherwise ignored.
Assignment
Semicolons must appear at the end of assignment expressions. See section Assignment: Defining Symbols
PHDRS
Semicolons must appear at the end of a PHDRS
statement.
See section ELF Program Headers
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The linker’s default configuration permits allocation of all available memory.
You can override this configuration by using the MEMORY
command. The
MEMORY
command describes the location and size of blocks of
memory in the target. By using it carefully, you can describe which
memory regions may be used by the linker, and which memory regions it
must avoid. The linker does not shuffle sections to fit into the
available regions, but does move the requested sections into the correct
regions and issue errors when the regions become too full.
A command file 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 … }
name
is a name used internally by the linker to refer to the region. Any symbol name may be used. The region names are stored in a separate name space, and will not conflict with symbols, file names or section names. Use distinct names to specify multiple regions.
(attr)
is an optional list of attributes, permitted for compatibility with the
AT&T linker but not used by ld
beyond checking that the
attribute list is valid. Valid attribute lists must be made up of the
characters “LIRWX
”. If you omit the attribute list, you may
omit the parentheses around it as well.
origin
is the start address of the region in physical memory. It is
an expression that must evaluate to a constant before
memory allocation is performed. The keyword ORIGIN
may be
abbreviated to org
or o
(but not, for example, ‘ORG’).
len
is the size in bytes of the region (an expression).
The keyword LENGTH
may be abbreviated to len
or l
.
For example, to specify that memory has two regions available for
allocation—one starting at 0 for 256 kilobytes, and the other
starting at 0x40000000
for four megabytes:
MEMORY { rom : ORIGIN = 0, LENGTH = 256K ram : org = 0x40000000, l = 4M }
Once you have defined a region of memory named mem, you can direct
specific output sections there by using a command ending in
‘>mem’ within the SECTIONS
command (see section Optional Section Attributes). If the combined output sections directed to a region are too
big for the region, the linker will issue an error message.
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The SECTIONS
command controls exactly where input sections are
placed into output sections, their order in the output file, and to
which output sections they are allocated.
You may use at most one SECTIONS
command in a script file,
but you can have as many statements within it as you wish. Statements
within the SECTIONS
command can do one of three things:
You can also use the first two operations—defining the entry point and
defining symbols—outside the SECTIONS
command: see section The Entry Point, and Assignment: Defining Symbols. They are permitted here as well for
your convenience in reading the script, so that symbols and the entry
point can be defined at meaningful points in your output-file layout.
If you do not use a SECTIONS
command, the linker places 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.
3.4.1 Section Definitions | ||
3.4.2 Section Placement | ||
3.4.3 Section Data Expressions | ||
3.4.4 Optional Section Attributes |
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The most frequently used statement in the SECTIONS
command is
the section definition, which specifies the
properties of an output section: its location, alignment, contents,
fill pattern, and target memory region. Most of
these specifications are optional; the simplest form of a section
definition is
SECTIONS { … secname : { contents } … }
secname is the name of the output section, and contents a specification of what goes there—for example, a list of input files or sections of input files (see section Section Placement). As you might assume, the whitespace shown is optional. You do need the colon ‘:’ and the braces ‘{}’, however.
secname 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 any name which does not conform to the standard
ld
symbol name syntax must be quoted.
See section Symbol Names.
The special secname ‘/DISCARD/’ may be used to discard input sections. Any sections which are assigned to an output section named ‘/DISCARD/’ are not included in the final link output.
The linker will not create output sections which do not have any contents. This is for convenience when referring to input sections that may or may not exist. 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.
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In a section definition, you can specify the contents of an output section by listing particular input files, by listing particular input-file sections, or by a combination of the two. You can also place arbitrary data in the section, and define symbols relative to the beginning of the section.
The contents of a section definition may include any of the following kinds of statement. You can include as many of these as you like in a single section definition, separated from one another by whitespace.
filename
You may simply name a particular input file to be placed in the current output section; all sections from that file are placed in the current section definition. If the file name has already been mentioned in another section definition, with an explicit section name list, then only those sections which have not yet been allocated are used.
To specify a list of particular files by name:
.data : { afile.o bfile.o cfile.o }
The example also illustrates that multiple statements can be included in the contents of a section definition, since each file name is a separate statement.
filename( section )
filename( section , section, … )
filename( section section … )
You can name one or more sections from your input files, for insertion in the current output section. If you wish to specify a list of input-file sections inside the parentheses, you may separate the section names by either commas or whitespace.
* (section)
* (section, section, …)
* (section section …)
Instead of explicitly naming particular input files in a link control
script, you can refer to all files from the ld
command
line: use ‘*’ instead of a particular file name before the
parenthesized input-file section list.
If you have already explicitly included some files by name, ‘*’ refers to all remaining files—those whose places in the output file have not yet been defined.
For example, to copy sections 1
through 4
from an Oasys file
into the .text
section of an a.out
file, and sections 13
and 14
into the .data
section:
SECTIONS { .text :{ *("1" "2" "3" "4") } .data :{ *("13" "14") } }
‘[ section … ]’ used to be accepted as an alternate way to specify named sections from all unallocated input files. Because some operating systems (VMS) allow brackets in file names, that notation is no longer supported.
filename( COMMON )
*( COMMON )
Specify where in your output file to place uninitialized data
with this notation. *(COMMON)
by itself refers to all
uninitialized data from all input files (so far as it is not yet
allocated); filename(COMMON)
refers to uninitialized data
from a particular file. Both are special cases of the general
mechanisms for specifying where to place input-file sections:
ld
permits you to refer to uninitialized data as if it
were in an input-file section named COMMON
, regardless of the
input file’s format.
For example, the following command script arranges the output file into
three consecutive sections, named .text
, .data
, and
.bss
, taking the input for each from the correspondingly named
sections of all the input files:
SECTIONS { .text : { *(.text) } .data : { *(.data) } .bss : { *(.bss) *(COMMON) } }
The following example reads all of the sections from file all.o
and places 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) } }
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The foregoing statements arrange, in your output file, data originating
from your input files. You can also place data directly in an output
section from the link command script. Most of these additional
statements involve expressions (see section Expressions). Although these
statements are shown separately here for ease of presentation, no such
segregation is needed within a section definition in the SECTIONS
command; you can intermix them freely with any of the statements we’ve
just described.
CREATE_OBJECT_SYMBOLS
Create a symbol for each input file
in the current section, set to the address of the first byte of
data written from that input file. For instance, with a.out
files it is conventional to have a symbol for each input file. You can
accomplish this by defining the output .text
section as follows:
SECTIONS { .text 0x2020 : { CREATE_OBJECT_SYMBOLS *(.text) _etext = ALIGN(0x2000); } … }
If sample.ld
is a file containing this script, and a.o
,
b.o
, c.o
, and d.o
are four input files with
contents like the following—
/* a.c */ afunction() { } int adata=1; int abss;
‘ld -M -T sample.ld a.o b.o c.o d.o’ would create a map like this, containing symbols matching the object file names:
00000000 A __DYNAMIC 00004020 B _abss 00004000 D _adata 00002020 T _afunction 00004024 B _bbss 00004008 D _bdata 00002038 T _bfunction 00004028 B _cbss 00004010 D _cdata 00002050 T _cfunction 0000402c B _dbss 00004018 D _ddata 00002068 T _dfunction 00004020 D _edata 00004030 B _end 00004000 T _etext 00002020 t a.o 00002038 t b.o 00002050 t c.o 00002068 t d.o
symbol = expression ;
symbol f= expression ;
symbol is any symbol name (see section Symbol Names). “f=”
refers to any of the operators &= += -= *= /=
which combine
arithmetic and assignment.
When you assign a value to a symbol within a particular section definition, the value is relative to the beginning of the section (see section Assignment: Defining Symbols). If you write
SECTIONS { abs = 14 ; … .data : { … rel = 14 ; … } abs2 = 14 + ADDR(.data); … }
abs
and rel
do not have the same value; rel
has the
same value as abs2
.
BYTE(expression)
SHORT(expression)
LONG(expression)
QUAD(expression)
By including one of these four statements in a section definition, you
can explicitly place one, two, four, or eight bytes (respectively) at
the current address of that section. QUAD
is only supported when
using a 64 bit host or target.
Multiple-byte quantities are represented in whatever byte order is appropriate for the output file format (see section BFD).
FILL(expression)
Specify the “fill pattern” for the current section. Any otherwise
unspecified regions of memory within the section (for example, regions
you skip over by assigning a new value to the location counter ‘.’)
are filled with the two least significant bytes from the
expression argument. A FILL
statement covers memory
locations after the point 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.
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Here is the full syntax of a section definition, including all the optional portions:
SECTIONS { … secname start BLOCK(align) (NOLOAD) : AT ( ldadr ) { contents } >region :phdr =fill … }
secname and contents are required. See section Section Definitions, and Section Placement, for details on
contents. The remaining elements—start,
BLOCK(align)
, (NOLOAD)
, AT ( ldadr )
,
>region
, :phdr
, and =fill
—are
all optional.
start
You can force the output section to be loaded at a specified address by
specifying start immediately following the section name.
start can be represented as any expression. The following
example generates section output at location
0x40000000
:
SECTIONS { … output 0x40000000: { … } … }
BLOCK(align)
You can include BLOCK()
specification to advance
the location counter .
prior to the beginning of the section, so
that the section will begin at the specified alignment. align is
an expression.
(NOLOAD)
Use ‘(NOLOAD)’ to prevent a section from being loaded into memory
each time it is accessed. For example, in the script sample below, the
ROM
segment is addressed at memory location ‘0’ and does not
need to be loaded into each object file:
SECTIONS { ROM 0 (NOLOAD) : { … } … }
AT ( ldadr )
The expression ldadr that follows the AT
keyword specifies
the load address of the section. The default (if you do not use the
AT
keyword) is to make the load address the same as the
relocation address. This feature is designed to make it easy to build a
ROM image. For example, this SECTIONS
definition creates two
output sections: one called ‘.text’, which starts at 0x1000
,
and one called ‘.mdata’, which is loaded at the end of the
‘.text’ section even though its relocation address is
0x2000
. The symbol _data
is defined with the value
0x2000
:
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 C programs, usually crt0
)
for use with a ROM generated this way has to include something like
the following, to copy the initialized data from the ROM image to its runtime
address:
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;
>region
Assign this section to a previously defined region of memory. See section Memory Layout.
:phdr
Assign this section to a segment described by a program header.
See section ELF Program Headers. 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. To
prevent a section from being assigned to a segment when it would
normally default to one, use :NONE
.
=fill
Including =fill
in a section definition specifies the
initial fill value for that section. You may use any expression to
specify fill. Any unallocated holes in the current output section
when written to the output file will be filled with the two least
significant bytes of the value, repeated as necessary. You can also
change the fill value with a FILL
statement in the contents
of a section definition.
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The ELF object file format uses program headers, which are read by
the system loader and describe how the program should be loaded into
memory. These program headers must be set correctly in order to run the
program on a native ELF system. The linker will create reasonable
program headers by default. However, in some cases, it is desirable to
specify the program headers more precisely; the PHDRS
command may
be used for this purpose. When the PHDRS
command is used, the
linker will not generate any program headers itself.
The PHDRS
command is only meaningful when generating an ELF
output file. It is ignored in other cases. This manual does not
describe the details of how the system loader interprets program
headers; for more information, see the ELF ABI. The program headers of
an ELF file may be displayed using the ‘-p’ option of the
objdump
command.
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 does not get put into the output file.
Certain program header types describe segments of memory which are
loaded from the file by the system loader. In the linker script, the
contents of these segments are specified by directing allocated output
sections to be placed in the segment. To do this, the command
describing the output section in the SECTIONS
command should use
‘:name’, where name is the name of the program header
as it appears in the PHDRS
command. See section Optional Section Attributes.
It is normal for certain sections to appear in more than one segment. This merely implies that one segment of memory contains another. This is specified by repeating ‘:name’, using it once for each program header in which the section is to appear.
If a section is placed in one or more segments using ‘:name’,
then all subsequent allocated sections which do not specify
‘:name’ are placed in the same segments. This is for
convenience, since generally a whole set of contiguous sections will be
placed in a single segment. To prevent a section from being assigned to
a segment when it would normally default to one, use :NONE
.
The FILEHDR
and PHDRS
keywords which may appear after the
program header type also indicate contents of the segment of memory.
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)Indicates an unused program header.
PT_LOAD
(1)Indicates that this program header describes a segment to be loaded from the file.
PT_DYNAMIC
(2)Indicates a segment where dynamic linking information can be found.
PT_INTERP
(3)Indicates a segment where the name of the program interpreter may be found.
PT_NOTE
(4)Indicates a segment holding note information.
PT_SHLIB
(5)A reserved program header type, defined but not specified by the ELF ABI.
PT_PHDR
(6)Indicates a segment where the program headers may be found.
An expression giving the numeric type of the program header. This may be used for types not defined above.
It is possible to specify that a segment should be loaded at a
particular address in memory. This is done using an AT
expression. This is identical to the AT
command used in the
SECTIONS
command (see section Optional Section Attributes). Using the AT
command for a program header overrides any information in the
SECTIONS
command.
Normally the segment flags are set based on the sections. The
FLAGS
keyword may be used 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 the use 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 … }
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The linker command language includes a command specifically for defining the first executable instruction in an output file (its entry point). Its argument is a symbol name:
ENTRY(symbol)
Like symbol assignments, the ENTRY
command may be placed either
as an independent command in the command file, or among the section
definitions within the SECTIONS
command—whatever makes the most
sense for your layout.
ENTRY
is only one of several ways of choosing the entry point.
You may indicate it in any of the following ways (shown in descending
order of priority: methods higher in the list override methods lower down).
ENTRY(symbol)
command in a linker control script;
start
, if present;
.text
section, if present;
0
.
For example, you can use these rules to generate an entry point with an
assignment statement: if no symbol start
is defined within your
input files, you can simply define it, assigning it an appropriate
value—
start = 0x2020;
The example shows an absolute address, but you can use any expression.
For example, if your input object files use some other symbol-name
convention for the entry point, you can just assign the value of
whatever symbol contains the start address to start
:
start = other_symbol ;
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The command language includes a number of other commands that you can use for specialized purposes. They are similar in purpose to command-line options.
CONSTRUCTORS
When linking using the a.out
object file format, the linker uses
an unusual set construct to support C++ global constructors and
destructors. When linking object file formats which do not support
arbitrary sections, such as ECOFF
and XCOFF
, the linker
will automatically recognize C++ global constructors and destructors by
name. For these object file formats, the CONSTRUCTORS
command
tells the linker where this information should be placed. 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++ calls constructors from a subroutine __main
;
a call to __main
is automatically inserted into the startup code
for main
. GNU C++ runs destructors either by using
atexit
, or directly from the function exit
.
For object file formats such as COFF
or ELF
which support
multiple sections, 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__ = .;
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.
FLOAT
NOFLOAT
These keywords were used in some older linkers to request a particular
math subroutine library. ld
doesn’t use the keywords, assuming
instead that any necessary subroutines are in libraries specified using
the general mechanisms for linking to archives; but to permit the use of
scripts that were written for the older linkers, the keywords
FLOAT
and NOFLOAT
are accepted and ignored.
FORCE_COMMON_ALLOCATION
This command has the same effect as the ‘-d’ command-line option:
to make ld
assign space to common symbols even if a relocatable
output file is specified (‘-r’).
INPUT ( file, file, … )
INPUT ( file file … )
Use this command to include binary input files in the link, without including them in a particular section definition. Specify the full name for each file, including ‘.a’ if required.
ld
searches for each file through the archive-library
search path, just as for files you specify on the command line.
See the description of ‘-L’ in @ref{Options,,Command Line
Options}.
If you use ‘-lfile’, ld
will transform the name to
libfile.a
as with the command line argument ‘-l’.
GROUP ( file, file, … )
GROUP ( file file … )
This 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
@ref{Options,,Command Line Options}.
OUTPUT ( filename )
Use this command to name the link output file filename. The
effect of OUTPUT(filename)
is identical to the effect of
‘-o filename’, which overrides it. You can use this
command to supply a default output-file name other than a.out
.
OUTPUT_ARCH ( bfdname )
Specify a particular output machine architecture, with one of the names
used by the BFD back-end routines (see section BFD). This command is often
unnecessary; the architecture is most often set implicitly by either the
system BFD configuration or as a side effect of the OUTPUT_FORMAT
command.
OUTPUT_FORMAT ( bfdname )
When ld
is configured to support multiple object code formats,
you can use this command to specify a particular output format.
bfdname is one of the names used by the BFD back-end routines
(see section BFD). The effect is identical to the effect of the
‘-oformat’ command-line option. This selection affects only
the output file; the related command TARGET
affects primarily
input files.
SEARCH_DIR ( path )
Add path to the list of paths where ld
looks for
archive libraries. SEARCH_DIR(path)
has the same
effect as ‘-Lpath’ on the command line.
STARTUP ( filename )
Ensure that filename is the first input file used in the link process.
TARGET ( format )
When ld
is configured to support multiple object code formats,
you can use this command to change the input-file object code format
(like the command-line option ‘-b’ or its synonym ‘-format’).
The argument format is one of the strings used by BFD to name
binary formats. If TARGET
is specified but OUTPUT_FORMAT
is not, the last TARGET
argument is also used as the default
format for the ld
output file. See section BFD.
If you don’t use the TARGET
command, ld
uses the value of
the environment variable GNUTARGET
, if available, to select the
output file format. If that variable is also absent, ld
uses
the default format configured for your machine in the BFD libraries.
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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 in The GNU Binary Utilities) 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. @xref{BFD information loss}.
4.1 How it works: an outline of BFD |
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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 Command Language. 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 secname
ABSOLUTE secname, secname, … secname
Normally, ld
includes in the output file all sections from all
the input files. However, in an MRI-compatible script, you can use the
ABSOLUTE
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-secname
Use this command to place the data from input section in-secname in a section called out-secname in the linker output file.
in-secname may be an integer.
ALIGN secname = expression
Align the section called secname to expression. The expression should be a power of two.
BASE expression
Use the value of expression as the lowest address (other than absolute addresses) in the output file.
CHIP expression
CHIP expression, expression
This command does nothing; it is accepted only for compatibility.
END
This command does nothing whatever; it’s only accepted for compatibility.
FORMAT output-format
Similar to the OUTPUT_FORMAT
command in the more general linker
language, but restricted to one of these output formats:
LIST anything…
Print (to the standard output file) a link map, as produced by the
ld
command-line option ‘-M’.
The keyword LIST
may be followed by anything on the
same line, with no change in its effect.
LOAD filename
LOAD filename, filename, … filename
Include one or more object file filename in the link; this has the
same effect as specifying filename directly on the ld
command line.
NAME output-name
output-name is the name for the program produced by ld
; the
MRI-compatible command NAME
is equivalent to the command-line
option ‘-o’ or the general script language command OUTPUT
.
ORDER secname, secname, … secname
ORDER secname secname secname
Normally, ld
orders the sections in its output file in the
order in which they first appear in the input files. In an MRI-compatible
script, you can override this ordering with the ORDER
command. The
sections you list with ORDER
will appear first in your output
file, in the order specified.
PUBLIC name=expression
PUBLIC name,expression
PUBLIC name expression
Supply a value (expression) for external symbol name used in the linker input files.
SECT secname, expression
SECT secname=expression
SECT secname expression
You can use any of these three forms of the SECT
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.
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