ld
version 2
Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 1998 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.
Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.
Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions.
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).
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. Note that specifying a script in this way should only be
used to augment the main linker script; if you want to use some command that
logically can only appear once, such as the SECTIONS
or
MEMORY
command, you must replace the default linker script using
the `-T' option. See 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
-Aarchitecture
--architecture=architecture
ld
, this option is useful only for
the Intel 960 family of architectures. In that ld
configuration,
the architecture argument identifies the particular architecture in
the 960 family, enabling some safeguards and modifying the archive-library
search path. See section ld
and the Intel 960 family, for details. Future releases of ld
may support similar functionality for other architecture families.
-b input-format
--format=input-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
. See section Environment
Variables. You can also define the input format from a script, using the
command TARGET
; see section Option
Commands.
-c MRI-commandfile
--mri-script=MRI-commandfile
ld
accepts
script files written in an alternate, restricted command language, described
in section 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
FORCE_COMMON_ALLOCATION
has the same effect. See section
Option
Commands.
-e entry
--entry=entry
-E
--export-dynamic
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.
-f
--auxiliary name
-F name
--filter name
-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.
--force-exe-suffix
.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
-Gvalue
--gpsize=value
-hname
-soname=name
-i
-larchive
--library=archive
ld
will search its
path-list for occurrences of libarchive.a
for every
archive specified. On systems which support shared libraries,
ld
may also search for libraries with extensions other than
.a
. Specifically, on ELF and SunOS systems, ld
will
search a directory for a library with an extension of .so
before
searching for one with an extension of .a
. By convention, a
.so
extension indicates a shared library. The linker will search
an archive only once, at the location where it is specified on the command
line. If the archive defines a symbol which was undefined in some object which
appeared before the archive on the command line, the linker will include the
appropriate file(s) from the archive. However, an undefined symbol in an
object appearing later on the command line will not cause the linker to search
the archive again. See the -(
option for a way to force the
linker to search archives multiple times. You may list the same archive
multiple times on the command line. This type of archive searching is standard
for Unix linkers. However, if you are using ld
on AIX, note that
it is different from the behaviour of the AIX linker.
-Lsearchdir
--library-path=searchdir
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 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 section Environment
Variables. 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
LDEMULATION
environment variable, if that is defined. Otherwise,
the default emulation depends upon how the linker was configured.
-M
--print-map
-n
--nmagic
NMAGIC
if possible.
-N
--omagic
OMAGIC
.
-o output
--output=output
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
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
-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
ld
processes them.
-T commandfile
--script=commandfile
ld
's default link script (rather than adding to it), so
commandfile must specify everything necessary to describe the
target format. You must use this option if you want to use a command which can
only appear once in a linker script, such as the SECTIONS
or
MEMORY
command. 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
-v
--version
-V
ld
. The -V
option
also lists the supported emulations.
-x
--discard-all
-X
--discard-locals
-y symbol
--trace-symbol=symbol
-Y path
-z keyword
-( archives -)
--start-group archives --end-group
-assert keyword
-Bdynamic
-dy
-call_shared
-l
options which follow
it.
-Bstatic
-dn
-non_shared
-static
-l
options which follow it.
-Bsymbolic
--cref
--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 section Assignment:
Defining Symbols). Note: there should be no white space between
symbol, the equals sign ("="), and
expression.
--dynamic-linker file
-EB
-EL
--embedded-relocs
--help
-Map mapfile
--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-warn-mismatch
ld
will give an error if you try to link together
input files that are mismatched for some reason, perhaps because they have
been compiled for different processors or for different endiannesses. This
option tells ld
that it should silently permit such possible
errors. This option should only be used with care, in cases when you have
taken some special action that ensures that the linker errors are
inappropriate.
--no-whole-archive
--whole-archive
option for
subsequent archive files.
--noinhibit-exec
--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
-Qy
--relax
ld
and the H8/300. See section ld
and the Intel 960 family. On some platforms, the `--relax'
option performs global optimizations that become possible when the linker
resolves addressing in the program, such as relaxing address modes and
synthesizing new instructions in the output object file. On platforms where
this is not supported, `--relax' is accepted, but ignored.
--retain-symbols-file filename
-rpath dir
-rpath
arguments are concatenated and passed to the
runtime linker, which uses them to locate shared objects at runtime. The
-rpath
option is also used when locating shared objects which are
needed by shared objects explicitly included in the link; see the description
of the -rpath-link
option. If -rpath
is not used
when linking an ELF executable, the contents of the environment variable
LD_RUN_PATH
will be used if it is defined. The
-rpath
option may also be used on SunOS. By default, on SunOS,
the linker will form a runtime search patch out of all the -L
options it is given. If a -rpath
option is used, the runtime
search path will be formed exclusively using the -rpath
options,
ignoring the -L
options. This can be useful when using gcc, which
adds many -L
options which may be on NFS mounted filesystems. For
compatibility with other ELF linkers, if the -R
option is
followed by a directory name, rather than a file name, it is treated as the
-rpath
option.
-rpath-link DIR
ld -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-relocateable 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. The linker uses the
following search paths to locate required shared libraries.
-rpath-link
options.
-rpath
options. The difference
between -rpath
and -rpath-link
is that directories
specified by -rpath
options are included in the executable and
used at runtime, whereas the -rpath-link
option is only
effective at link time.
-rpath
and rpath-link
options were not used, search the contents of the environment variable
LD_RUN_PATH
.
-rpath
option was not used, search any
directories specified using -L
options.
LD_LIBRARY_PATH
.
-shared
-Bshareable
-e
option is not
used and there are undefined symbols in the link.
--sort-common
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
--split-by-reloc
but creates a new output section
for each input file.
--split-by-reloc count
--stats
--traditional-format
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
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
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
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.
--version-script=version-scriptfile
--warn-common
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-align
SECTIONS
command does not specify a start address for the section
(see section Specifying
Output Sections).
--whole-archive
--whole-archive
option, include every object file in the archive
in the link, rather than searching the archive for the required object files.
This is normally used to turn an archive file into a shared library, forcing
every object to be included in the resulting shared library. This option may
be used more than once.
--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 (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
. You can change the behavior of ld
with the environment variables
GNUTARGET
and LDEMULATION
.
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 section 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.
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 linker script) to the linker
either explicitly through the `-T' option, or implicitly as an
ordinary file. Normally you should use the `-T' option. An implicit
linker script should only be used when you want to augment, rather than replace,
the default linker script; typically an implicit linker script would consist
only of INPUT
or GROUP
commands.
If the linker opens a file which it cannot recognize as a supported object or archive format, nor as a linker script, it reports an error.
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.
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:
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;
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.
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).
@vfill
The linker recognizes the standard C set of arithmetic operators, with the standard bindings and precedence levels: { @obeylines@parskip=0pt@parindent=0pt @dag@quad Prefix operators. @ddag@quad See section Assignment: Defining Symbols. }
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.
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)
.
The command language includes a number of built-in functions for use in link 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; } ... }
LOADADDR(section)
ADDR
, but it may be different if the
AT
keyword is used in the section definition (see section Optional
Section Attributes).
ALIGN(exp)
.
) 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)
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)
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_HEADERS
sizeof_headers
MAX(exp1, exp2)
MIN(exp1, exp2)
Semicolons (";") are required in the following places. In all other places they can appear for aesthetic reasons but are otherwise ignored.
Assignment
PHDRS
PHDRS
statement. See
section ELF Program
Headers 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
(attr)
ALIRWX
" that
match section attributes. If you omit the attribute list, you may omit the
parentheses around it as well. The attributes currently supported are:
Letter
'
Section Attribute
R
'
W
'
X
'
A
'
I
'
L
'
I
.
!
'
origin
ORIGIN
may be abbreviated to org
or
o
(but not, for example, `ORG').
len
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. The rom
memory region
will get all sections without an explicit memory register that are either
read-only or contain code, while the ram
memory region will get the
sections.
MEMORY { rom (rx) : ORIGIN = 0, LENGTH = 256K ram (!rx) : 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.
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 section 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.
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). The whitespace around secname is required, so that the section name is unambiguous. The other 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.
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
.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 ...
)
* (section)
* (section, section, ...)
* (section section ...)
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 )
*(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. In any place where you may use a specific file or section name, you may also use a wildcard pattern. The linker handles wildcards much as the Unix shell does. A `*' character matches any number of characters. A `?' character matches any single character. The sequence `[chars]' will match a single instance of any of the chars; the `-' character may be used to specify a range of characters, as in `[a-z]' to match any lower case letter. A `\' character may be used to quote the following character.
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. In a section name, the wildcard characters will match a `/' character.
Wildcards only match files which are explicitly specified on the command line. The linker does not search directories to expand wildcards. However, if you specify a simple file name--a name with no wildcard characters--in a linker script, and the file name is not also specified on the command line, the linker will attempt to open the file as though it appeared on the command line.
In the following example, the 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) } }
This example shows how wildcard patterns might be used to partition files.
All .text
sections are placed in .text
, and all
.bss
sections are placed in .bss
. For all files
beginning with an upper case character, the .data
section is placed
into .DATA
; for all other files, the .data
section is
placed into .data
.
SECTIONS { .text : { *(.text) } .DATA : { [A-Z]*(.data) } .data : { *(.data) } .bss : { *(.bss) } }
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
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 ;
&= += -=
*= /=
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)
SQUAD(expression)
QUAD
and SQUAD
are the same. When
both host and target are 32 bits, QUAD
uses an unsigned 32 bit
value, and SQUAD
sign extends the value. Both will use the
correct endianness when writing out the value. Multiple-byte quantities are
represented in whatever byte order is appropriate for the output file format
(see section BFD).
FILL(expression)
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. 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 Section
Placement, for details on contents. The remaining
elements---start, BLOCK(align)
,
(NOLOAD)
, AT ( ldadr )
,
>region
, :phdr
, and
=fill
---are all optional.
start
0x40000000
: SECTIONS { ... output 0x40000000: { ... } ... }
BLOCK(align)
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)
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) : { ... } ... }
AT ( ldadr )
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
:phdr
:phdr
modifier. To
prevent a section from being assigned to a segment when it would normally
default to one, use :NONE
.
=fill
=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. The OVERLAY
command 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.
The OVERLAY
command is used within a SECTIONS
command. It appears as follows:
OVERLAY start : [ NOCROSSREFS ] AT ( ldaddr ) { secname1 { contents } :phdr =fill secname2 { contents } :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 section Specifying
Output 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
.
).
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 section Option
Commands.
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 .
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 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)
PT_LOAD
(1)
PT_DYNAMIC
(2)
PT_INTERP
(3)
PT_NOTE
(4)
PT_SHLIB
(5)
PT_PHDR
(6)
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 ... }
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 ;
The linker command script includes a command specifically for specifying a version script, and is only meaningful for ELF platforms that support shared libraries. A version script can be build directly into the linker script that you are using, or you can supply the version script as just another input file to the linker at the time that you link. The command script syntax is:
VERSION { version script contents }
The version script can also be specified to the linker by means of the `--version-script' linker command line option. Version scripts are only meaningful when creating shared libraries.
The format of the version script itself is identical to that used by Sun's linker in Solaris 2.5. Versioning is done by defining a tree of version nodes with the names and interdependencies specified in the version script. The version script can specify which symbols are bound to which version nodes, and it 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;
In this example, three version nodes are defined. `VERS_1.1' is the first version node defined, and has no other dependencies. The symbol `foo1' is bound to this version node, and a number of symbols that have appeared within various object files are reduced in scope to local so that they are not visible outside of the shared library.
Next, the node `VERS_1.2' is defined. It depends upon `VERS_1.1'. The symbol `foo2' is bound to this version node.
Finally, the node `VERS_2.0' is defined. It depends upon `VERS_1.2'. The symbols `bar1' and `bar2' are bound to this version node.
Symbols defined in the library which aren't specifically bound to a version node are effectively bound to an unspecified base version of the library. It is possible to bind all otherwise unspecified symbols to a given version node using `global: *' somewhere in the version script.
Lexically 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.
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. This can be done by putting something like:
__asm__(".symver original_foo,foo@VERS_1.1");
in the C source file. This renamed 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.
The second GNU extension is to allow multiple versions of the same function to appear in a given shared library. In this way an incompatible change to an interface can take place without increasing the major version number of the shared library, while still allowing applications linked against the old interface to continue to function.
This can only be accomplished by using multiple `.symver' directives in the assembler. An example of this would be:
__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. This can be accomplished with the `foo@@VERS_2.0' type of `.symver' directive. Only one version of a symbol can be declared '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.
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
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
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
ld
assign space to common symbols even if a
relocatable output file is specified (`-r').
INCLUDE filename
-L
option. You can nest calls to INCLUDE
up to
10 levels deep.
INPUT ( file, file, ... )
INPUT ( file file ... )
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 section 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 ... )
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
section Command Line
Options.
OUTPUT ( filename )
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 )
OUTPUT_FORMAT
command.
OUTPUT_FORMAT ( bfdname )
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 )
ld
looks for
archive libraries. SEARCH_DIR(path)
has the same
effect as `-Lpath' on the command line.
STARTUP ( filename )
TARGET ( format )
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.
NOCROSSREFS ( section section ... )
ld
to issue an error about
any references among certain sections. In certain types of programs,
particularly on embedded systems, 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 section names. If ld
detects any cross
references between the sections, it reports an error and returns a non-zero
exit status. The NOCROSSREFS
command uses output section names,
defined in the SECTIONS
command. It does not use the names of
input sections. ld
has additional features on some platforms;
the following sections describe them. Machines where ld
has no
additional functionality are not listed.
ld
and the H8/300For the H8/300, ld
can perform these global
optimizations when you specify the `--relax' command-line option.
ld
finds all jsr
and
jmp
instructions whose targets are within eight bits, and turns
them into eight-bit program-counter relative bsr
and
bra
instructions, respectively.
ld
finds all 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). ld
and the Intel 960 familyYou 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).
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 section `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. See section 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 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:
ld
bug.
Reliable linkers never crash.
ld
produces an error message for valid input, that is a
bug.
ld
does not produce an error message for invalid input,
that may be a bug. In the general case, the linker can not verify that object
files are correct.
ld
are welcome in any case. 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.
In any event, we also recommend that you send bug reports for ld
to `bug-gnu-utils@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?" Those bug reports are useless, and we urge everyone to refuse to respond to them except to chide the sender to report bugs properly.
To enable us to fix the bug, you should include all these things:
ld
. ld
announces it if you start
it with the `--version' argument. Without this, we will not know
whether there is any point in looking for the bug in the current version of
ld
.
ld
source, including
any patches made to the BFD
library.
ld
---e.g.
"gcc-2.7
".
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.
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.
ld
source, send us
context diffs, as generated by 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:
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.
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
section 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
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
ALIGN secname = expression
BASE expression
CHIP expression
CHIP expression, expression
END
FORMAT output-format
OUTPUT_FORMAT
command in the more general
linker language, but restricted to one of these output formats:
LIST anything...
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
ld
command line.
NAME output-name
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
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
SECT secname, expression
SECT secname=expression
SECT secname expression
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.
Jump to: " - * - - - . - 0 - : - ; - = - > - [ - a - b - c - d - e - f - g - h - i - k - l - m - n - o - p - q - r - s - t - u - v - w
--relax
on i960
[section...]
,
not supported ABSOLUTE
(MRI)
ALIAS
(MRI)
ALIGN
(MRI)
BASE
(MRI)
ld
CHIP
(MRI)
END
(MRI)
FORMAT
(MRI)
ld
bugs, reporting
LIST
(MRI)
LOAD
(MRI)
NAME
(MRI)
ORDER
(MRI)
PUBLIC
(MRI) ld
SECT
(MRI)
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