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binutils-gdb/ld/ld.texinfo
David MacKenzie de87cdb466 * Make -v only print the version number. 
Share some Texinfo documentation with BFD.
1993-07-08 18:00:11 +00:00

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\input texinfo
@setfilename ld.info
@syncodeindex ky cp
@c @include configdoc.texi
@c @smallbook
@c @cropmarks
@ifinfo
@format
START-INFO-DIR-ENTRY
* Ld:: The GNU linker.
END-INFO-DIR-ENTRY
@end format
@end ifinfo
@ifinfo
This file documents the GNU linker LD.
Copyright (C) 1991, 1992, 1993 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.
@ignore
Permission is granted to process this file through Tex and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).
@end ignore
@end ifinfo
@iftex
@finalout
@setchapternewpage odd
@settitle Using LD, the GNU linker
@titlepage
@title Using ld
@subtitle The GNU linker
@sp 1
@subtitle @code{ld} version 2
@subtitle March 1993
@author Steve Chamberlain and Roland Pesch
@author Cygnus Support
@page
@tex
{\parskip=0pt
\hfill Cygnus Support\par
\hfill steve\@cygnus.com, pesch\@cygnus.com\par
\hfill {\it Using LD, the GNU linker}\par
\hfill Edited by Jeffrey Osier (jeffrey\@cygnus.com), March 1993.\par
}
\global\parindent=0pt % Steve likes it this way.
@end tex
@vskip 0pt plus 1filll
Copyright @copyright{} 1991, 1992, 1993 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.
@end titlepage
@end iftex
@c FIXME: Talk about importance of *order* of args, cmds to linker!
@ifinfo
@node Top
@top Using ld
This file documents the GNU linker ld.
@menu
* Overview:: Overview
* Invocation:: Invocation
* Commands:: Command Language
@ifset GENERIC
* Machine Dependent:: Machine Dependent Features
@end ifset
@ifclear GENERIC
@ifset H8300
* H8/300:: ld and the H8/300
@end ifset
@ifset I960
* i960:: ld and the Intel 960 family
@end ifset
@end ifclear
@ifclear SingleFormat
* BFD:: BFD
@end ifclear
@c Following blank line required for remaining bug in makeinfo conds/menus
* MRI:: MRI Compatible Script Files
* Index:: Index
@end menu
@end ifinfo
@node Overview
@chapter Overview
@cindex GNU linker
@cindex what is this?
@code{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 @code{ld}.
@code{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.
@ifclear SingleFormat
This version of @code{ld} uses the general purpose BFD libraries
to operate on object files. This allows @code{ld} to read, combine, and
write object files in many different formats---for example, COFF or
@code{a.out}. Different formats may be linked together to produce any
available kind of object file. @xref{BFD} for a list of formats
supported on various architectures.
@end ifclear
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,
@code{ld} continues executing, allowing you to identify other errors
(or, in some cases, to get an output file in spite of the error).
@node Invocation
@chapter Invocation
The GNU linker @code{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.
@ifset UsesEnvVars
@menu
* Options:: Command Line Options
* Environment:: Environment Variables
@end menu
@node Options
@section Command Line Options
@end ifset
@cindex command line
@cindex options
Here is a summary of the options you can use on the @code{ld} command
line:
@c FIXME! -relax only avail h8/300, i960. Conditionals screwed in examples.
@smallexample
ld [ -o @var{output} ] @var{objfile}@dots{}
[ -A@var{architecture} ] [ -b @var{input-format} ] [ -Bstatic ]
[ -c @var{MRI-commandfile} ] [ -d | -dc | -dp ]
[ -defsym @var{symbol}=@var{expression} ]
[ -e @var{entry} ] [ -F ] [ -F @var{format} ]
[ -format @var{input-format} ] [ -g ] [ -G @var{size} ] [ --help ] [ -i ]
[ -l@var{archive} ] [ -L@var{searchdir} ] [ -M ] [ -Map @var{mapfile} ]
[ -m @var{emulation} ] [ -N | -n ] [ -noinhibit-exec ]
[ -oformat @var{output-format} ] [ -R @var{filename} ] [ -relax ]
[ -r | -Ur ] [ -S ] [ -s ] [ -T @var{commandfile} ]
[ -Ttext @var{textorg} ] [ -Tdata @var{dataorg} ]
[ -Tbss @var{bssorg} ] [ -t ] [ -u @var{symbol}] [-V] [-v] [ --version ]
[ -y@var{symbol} ] [ -X ] [-x ]
@end smallexample
This plethora of command-line options may seem intimidating, but in
actual practice few of them are used in any particular context.
@cindex standard Unix system
For instance, a frequent use of @code{ld} is to link standard Unix
object files on a standard, supported Unix system. On such a system, to
link a file @code{hello.o}:
@example
ld -o @var{output} /lib/crt0.o hello.o -lc
@end example
This tells @code{ld} to produce a file called @var{output} as the
result of linking the file @code{/lib/crt0.o} with @code{hello.o} and
the library @code{libc.a}, which will come from the standard search
directories. (See the discussion of the @samp{-l} option below.)
The command-line options to @code{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.
@ifclear SingleFormat
The exceptions---which may meaningfully be used more than once---are
@samp{-A}, @samp{-b} (or its synonym @samp{-format}), @samp{-defsym},
@samp{-L}, @samp{-l}, @samp{-R}, and @samp{-u}.
@end ifclear
@ifset SingleFormat
The exceptions---which may meaningfully be used more than once---are
@samp{-A}, @samp{-defsym}, @samp{-L}, @samp{-l}, @samp{-R}, and @samp{-u}.
@end ifset
@cindex object files
The list of object files to be linked together, shown as @var{objfile}@dots{},
may follow, precede, or be mixed in with command-line options, except that
an @var{objfile} argument may not be placed between an option and
its argument.
Usually the linker is invoked with at least one object file, but other
forms of binary input files can also be specified with @samp{-l},
@samp{-R}, and the script command language. If @emph{no} binary input
files at all are specified, the linker does not produce any output, and
issues the message @samp{No input files}.
Option arguments must either follow the option letter without intervening
whitespace, or be given as separate arguments immediately following the
option that requires them.
@table @code
@item @var{objfile}@dots{}
The object files to be linked.
@ifset I960
@cindex architectures
@kindex -A@var{arch}
@item -A@var{architecture}
In the current release of @code{ld}, this option is useful only for the
Intel 960 family of architectures. In that @code{ld} configuration, the
@var{architecture} argument identifies the particular architecture in
the 960 family, enabling some safeguards and modifying the
archive-library search path. @xref{i960,,@code{ld} and the Intel 960
family}, for details.
Future releases of @code{ld} may support similar functionality for
other architecture families.
@end ifset
@ifclear SingleFormat
@cindex binary input format
@kindex -b @var{format}
@cindex input format
@item -b @var{input-format}
@cindex input format
Specify the binary format for input object files that follow this option
on the command line. You don't usually need to specify this, as
@code{ld} is configured to expect as a default input format the most
usual format on each machine. @var{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 @samp{objdump -i}.)
@w{@samp{-format @var{input-format}}} has the same effect, as does the
script command @code{TARGET}. @xref{BFD}.
You may want to use this option if you are linking files with an unusual
binary format. You can also use @samp{-b} to switch formats explicitly (when
linking object files of different formats), by including
@samp{-b @var{input-format}} before each group of object files in a
particular format.
The default format is taken from the environment variable
@code{GNUTARGET}.
@ifset UsesEnvVars
@xref{Environment}.
@end ifset
You can also define the input
format from a script, using the command @code{TARGET}; see @ref{Other
Commands}.
@end ifclear
@kindex -Bstatic
@item -Bstatic
Ignored. This option is accepted for command-line compatibility with
the SunOS linker.
@kindex -c @var{MRI-cmdfile}
@cindex compatibility, MRI
@item -c @var{MRI-commandfile}
For compatibility with linkers produced by MRI, @code{ld} accepts script
files written in an alternate, restricted command language, described in
@ref{MRI,,MRI Compatible Script Files}. Introduce MRI script files with
the option @samp{-c}; use the @samp{-T} option to run linker
scripts written in the general-purpose @code{ld} scripting language.
If @var{MRI-cmdfile} does not exist, @code{ld} looks for it in the directories
specified by any @samp{-L} options.
@cindex common allocation
@kindex -d
@item -d
@kindex -dc
@itemx -dc
@kindex -dp
@itemx -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 @samp{-r}). The script command
@code{FORCE_COMMON_ALLOCATION} has the same effect. @xref{Other
Commands}.
@cindex symbols, from command line
@kindex -defsym @var{symbol}=@var{exp}
@item -defsym @var{symbol}=@var{expression}
Create a global symbol in the output file, containing the absolute
address given by @var{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 @var{expression} in this
context: you may give a hexadecimal constant or the name of an existing
symbol, or use @code{+} and @code{-} to add or subtract hexadecimal
constants or symbols. If you need more elaborate expressions, consider
using the linker command language from a script (@pxref{Assignment, ,
Assignment: Symbol Definitions}). @emph{Note:} there should be no
white space between @var{symbol}, the equals sign (``@key{=}''), and
@var{expression}.
@cindex entry point, from command line
@kindex -e @var{entry}
@item -e @var{entry}
Use @var{entry} as the explicit symbol for beginning execution of your
program, rather than the default entry point. @xref{Entry Point}, for a
discussion of defaults and other ways of specifying the
entry point.
@ifclear SingleFormat
@kindex -F
@item -F
@itemx -F@var{format}
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 @code{ld} uses for this purpose (the
@samp{-b} or @samp{-format} options for input files, the @code{TARGET}
command in linker scripts for output files, the @code{GNUTARGET}
environment variable) are more flexible, but @code{ld} accepts the
@samp{-F} option for compatibility with scripts written to call the old
linker.
@kindex -format
@item -format @var{input-format}
Synonym for @samp{-b @var{input-format}}.
@end ifclear
@kindex -g
@item -g
Ignored. Provided for compatibility with other tools.
@kindex -G
@cindex object size
@item -G@var{value}
@itemx -G @var{value}
Set the maximum size of objects to be optimized using the GP register to
@var{size} under MIPS ECOFF. Ignored for other object file formats.
@item --help
@kindex --help
@cindex help
@cindex usage
Print a summary of the command-line options on the standard output and exit.
This option and @samp{--version} begin with two dashes instead of one
for compatibility with other GNU programs. The other options start with
only one dash for compatibility with other linkers.
@kindex -i
@cindex incremental link
@item -i
Perform an incremental link (same as option @samp{-r}).
@cindex archive files, from cmd line
@kindex -l@var{archive}
@item -l@var{ar}
Add archive file @var{archive} to the list of files to link. This
option may be used any number of times. @code{ld} will search its
path-list for occurrences of @code{lib@var{ar}.a} for every @var{archive}
specified.
@cindex search directory, from cmd line
@kindex -L@var{dir}
@item -L@var{searchdir}
Add path @var{searchdir} to the list of paths that @code{ld} will search
for archive libraries and @code{ld} control scripts. You may use this
option any number of times.
@ifset UsesEnvVars
The default set of paths searched (without being specified with
@samp{-L}) depends on which emulation mode @code{ld} is using, and in
some cases also on how it was configured. @xref{Environment}.
@end ifset
The paths can also be specified in a link script with the
@code{SEARCH_DIR} command.
@cindex link map
@kindex -M
@item -M
Print (to the standard output) a link map---diagnostic information
about where symbols are mapped by @code{ld}, and information on global
common storage allocation.
@cindex link map
@kindex -Map
@item -Map @var{mapfile}
Print to the file @var{mapfile} a link map---diagnostic information
about where symbols are mapped by @code{ld}, and information on global
common storage allocation.
@cindex emulation
@kindex -m @var{emulation}
@item -m@var{emulation}
@itemx -m @var{emulation}
Emulate the @var{emulation} linker. You can list the available
emulations with the @samp{-V} option. The
default is the system for which you configured @code{ld}.
@kindex -N
@cindex read/write from cmd line
@kindex OMAGIC
@item -N
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 @code{OMAGIC}.
@item -n
@kindex -n
@cindex read-only text
@kindex NMAGIC
Set the text segment to be read only, and mark the output as
@code{NMAGIC} if possible.
@item -noinhibit-exec
@cindex output file after errors
@kindex -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.
@item -o @var{output}
@kindex -o @var{output}
@cindex naming the output file
Use @var{output} as the name for the program produced by @code{ld}; if this
option is not specified, the name @file{a.out} is used by default. The
script command @code{OUTPUT} can also specify the output file name.
@kindex -oformat
@item -oformat @var{output-format}
Specify the binary format for the output object file. You don't usually
need to specify this, as @code{ld} is configured to produce as a default
output format the most usual format on each machine.
@var{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 @samp{objdump -i}.) The script command
@code{OUTPUT_FORMAT} can also specify the output format, but this option
overrides it. @xref{BFD}.
@item -R @var{filename}
@kindex -R @var{file}
@cindex symbol-only input
On some platforms, this 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.
@item -relax
@kindex -relax
@cindex synthesizing linker
@cindex relaxing addressing modes
An option with machine dependent effects. Currently this option is only
supported on the H8/300.
@ifset H8300
@xref{H8/300,,@code{ld} and the H8/300}.
@end ifset
On some platforms, use 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, @samp{-relax} is accepted, but
ignored.
@item -r
@cindex partial link
@cindex relocatable output
@kindex -r
Generate relocatable output---i.e., generate an output file that can in
turn serve as input to @code{ld}. This is often called @dfn{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
@code{OMAGIC}.
@c ; see @code{-N}.
If this option is not specified, an absolute file is produced. When
linking C++ programs, this option @emph{will not} resolve references to
constructors; to do that, use @samp{-Ur}.
This option does the same as @code{-i}.
@item -S
@kindex -S
@cindex strip debugger symbols
Omit debugger symbol information (but not all symbols) from the output file.
@item -s
@kindex -s
@cindex strip all symbols
Omit all symbol information from the output file.
@item -Tbss @var{bssorg}
@kindex -Tbss @var{bssorg}
@itemx -Tdata @var{dataorg}
@kindex -Tdata @var{dataorg}
@itemx -Ttext @var{textorg}
@kindex -Ttext @var{textorg}
@cindex segment origins, cmd line
Use @var{org} as the starting address for---respectively---the
@code{bss}, @code{data}, or the @code{text} segment of the output file.
@var{org} must be a single hexadecimal integer;
for compatibility with other linkers, you may omit the leading
@samp{0x} usually associated with hexadecimal values.
@item -T @var{commandfile}
@itemx -T@var{commandfile}
@kindex -T @var{script}
@cindex script files
Read link commands from the file @var{commandfile}. These commands
completely override @code{ld}'s default link format (rather than adding
to it); @var{commandfile} must specify everything necessary to describe
the target format. @xref{Commands}. If @var{commandfile} does not
exist, @code{ld} looks for it in the directories specified by any
preceding @samp{-L} options. Multiple @samp{-T} options accumulate.
@item -t
@kindex -t
@cindex verbose
@cindex input files, displaying
Print the names of the input files as @code{ld} processes them.
@item -u @var{symbol}
@kindex -u @var{symbol}
@cindex undefined symbol
Force @var{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. @samp{-u} may be repeated with different option
arguments to enter additional undefined symbols.
@c Nice idea, but no such command: This option is equivalent
@c to the @code{EXTERN} linker command.
@item -Ur
@kindex -Ur
@cindex constructors
For anything other than C++ programs, this option is equivalent to
@samp{-r}: it generates relocatable output---i.e., an output file that can in
turn serve as input to @code{ld}. When linking C++ programs, @samp{-Ur}
@emph{will} resolve references to constructors, unlike @samp{-r}.
@item -V
@kindex -V
@cindex version
Display the version number for @code{ld} and list the supported emulations.
Display which input files can and can not be opened.
@item -v
@kindex -v
@cindex version
Display the version number for @code{ld}.
@item --version
@kindex --version
Display the version number for @code{ld} and exit.
@item -X
@kindex -X
@cindex local symbols, deleting
@cindex L, deleting symbols beginning
If @samp{-s} or @samp{-S} is also specified, delete only local symbols
beginning with @samp{L}.
@item -x
@kindex -x
@cindex deleting local symbols
If @samp{-s} or @samp{-S} is also specified, delete all local symbols,
not just those beginning with @samp{L}.
@item -y@var{symbol}
@kindex -y@var{symbol}
@cindex symbol tracing
Print the name of each linked file in which @var{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.
@end table
@ifset UsesEnvVars
@node Environment
@section Environment Variables
You can change the behavior of @code{ld} with the environment
variable @code{GNUTARGET}.
@kindex GNUTARGET
@cindex default input format
@code{GNUTARGET} determines the input-file object format if you don't
use @samp{-b} (or its synonym @samp{-format}). Its value should be one
of the BFD names for an input format (@pxref{BFD}). If there is no
@code{GNUTARGET} in the environment, @code{ld} uses the natural format
of the host. If @code{GNUTARGET} is set to @code{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.
@end ifset
@node Commands
@chapter Command Language
@cindex command files
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:
@itemize @bullet
@item
input files
@item
file formats
@item
output file format
@item
addresses of sections
@item
placement of common blocks
@end itemize
You may supply a command file (also known as a link script) to the
linker either explicitly through the @samp{-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 tries to interpret the file
as a command file.
@menu
* Scripts:: Linker Scripts
* Expressions:: Expressions
* MEMORY:: MEMORY Command
* SECTIONS:: SECTIONS Command
* Entry Point:: The Entry Point
* Other Commands:: Other Commands
@end menu
@node Scripts
@section Linker Scripts
The @code{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.
@cindex fundamental script commands
@cindex commands, fundamental
@cindex output file layout
@cindex layout of output file
The most fundamental command of the @code{ld} command language is the
@code{SECTIONS} command (@pxref{SECTIONS}). Every meaningful command
script must have a @code{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 @code{MEMORY} command complements @code{SECTIONS} by describing the
available memory in the target architecture. This command is optional;
if you don't use a @code{MEMORY} command, @code{ld} assumes sufficient
memory is available in a contiguous block for all output.
@xref{MEMORY}.
@cindex comments
You may include comments in linker scripts just as in C: delimited
by @samp{/*} and @samp{*/}. As in C, comments are syntactically
equivalent to whitespace.
@node Expressions
@section Expressions
@cindex expression syntax
@cindex arithmetic
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:
@itemize @bullet
@item
All expressions evaluated as integers and
are of ``long'' or ``unsigned long'' type.
@item
All constants are integers.
@item
All of the C arithmetic operators are provided.
@item
You may reference, define, and create global variables.
@item
You may call special purpose built-in functions.
@end itemize
@menu
* Integers:: Integers
* Symbols:: Symbol Names
* Location Counter:: The Location Counter
* Operators:: Operators
* Evaluation:: Evaluation
* Assignment:: Assignment: Defining Symbols
* Built-ins:: Built-In Functions
@end menu
@node Integers
@subsection Integers
@cindex integer notation
@cindex octal integers
An octal integer is @samp{0} followed by zero or more of the octal
digits (@samp{01234567}).
@example
_as_octal = 0157255;
@end example
@cindex decimal integers
A decimal integer starts with a non-zero digit followed by zero or
more digits (@samp{0123456789}).
@example
_as_decimal = 57005;
@end example
@cindex hexadecimal integers
@kindex 0x
A hexadecimal integer is @samp{0x} or @samp{0X} followed by one or
more hexadecimal digits chosen from @samp{0123456789abcdefABCDEF}.
@example
_as_hex = 0xdead;
@end example
@cindex negative integers
To write a negative integer, use
the prefix operator @samp{-}; @pxref{Operators}.
@example
_as_neg = -57005;
@end example
@cindex scaled integers
@cindex K and M integer suffixes
@cindex M and K integer suffixes
@cindex suffixes for integers
@cindex integer suffixes
Additionally the suffixes @code{K} and @code{M} may be used to scale a
constant by
@c TEXI2ROFF-KILL
@ifinfo
@c END TEXI2ROFF-KILL
@code{1024} or @code{1024*1024}
@c TEXI2ROFF-KILL
@end ifinfo
@tex
${\rm 1024}$ or ${\rm 1024}^2$
@end tex
@c END TEXI2ROFF-KILL
respectively. For example, the following all refer to the same quantity:
@example
_fourk_1 = 4K;
_fourk_2 = 4096;
_fourk_3 = 0x1000;
@end example
@node Symbols
@subsection Symbol Names
@cindex symbol names
@cindex names
@cindex quoted symbol names
@kindex "
Unless quoted, symbol names start with a letter, underscore, point or
hyphen and may include any letters, underscores, digits, points,
and minus signs. 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:
@example
"SECTION" = 9;
"with a space" = "also with a space" + 10;
@end example
@node Location Counter
@subsection The Location Counter
@kindex .
@cindex dot
@cindex location counter
@cindex current output location
The special linker variable @dfn{dot} @samp{.} always contains the
current output location counter. Since the @code{.} always refers to
a location in an output section, it must always appear in an
expression within a @code{SECTIONS} command. The @code{.} 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 @code{.} symbol will cause the location counter to be moved.
@cindex holes
This may be used to create holes in the output section. The location
counter may never be moved backwards.
@example
SECTIONS
@{
output :
@{
file1(.text)
. = . + 1000;
file2(.text)
. += 1000;
file3(.text)
@} = 0x1234;
@}
@end example
@noindent
In the previous example, @code{file1} is located at the beginning of the
output section, then there is a 1000 byte gap. Then @code{file2}
appears, also with a 1000 byte gap following before @code{file3} is
loaded. The notation @samp{= 0x1234} specifies what data to write in
the gaps (@pxref{Section Options}).
@node Operators
@subsection Operators
@cindex Operators for arithmetic
@cindex arithmetic operators
@cindex precedence in expressions
The linker recognizes the standard C set of arithmetic operators, with
the standard bindings and precedence levels:
@c TEXI2ROFF-KILL
@ifinfo
@c END TEXI2ROFF-KILL
@example
precedence associativity Operators Notes
(highest)
1 left ! - ~ (1)
2 left * / %
3 left + -
4 left >> <<
5 left == != > < <= >=
6 left &
7 left |
8 left &&
9 left ||
10 right ? :
11 right &= += -= *= /= (2)
(lowest)
@end example
Notes:
(1) Prefix operators
(2) @xref{Assignment}
@c TEXI2ROFF-KILL
@end ifinfo
@tex
\vskip \baselineskip
%"lispnarrowing" is the extra indent used generally for @example
\hskip\lispnarrowing\vbox{\offinterlineskip
\hrule
\halign
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height2pt&\omit&&\omit&&\omit&\cr
&Precedence&& Associativity &&{\rm Operators}&\cr
height2pt&\omit&&\omit&&\omit&\cr
\noalign{\hrule}
height2pt&\omit&&\omit&&\omit&\cr
&highest&&&&&\cr
% '176 is tilde, '~' in tt font
&1&&left&&\qquad- \char'176\ !\qquad\dag&\cr
&2&&left&&* / \%&\cr
&3&&left&&+ -&\cr
&4&&left&&>> <<&\cr
&5&&left&&== != > < <= >=&\cr
&6&&left&&\&&\cr
&7&&left&&|&\cr
&8&&left&&{\&\&}&\cr
&9&&left&&||&\cr
&10&&right&&? :&\cr
&11&&right&&\qquad\&= += -= *= /=\qquad\ddag&\cr
&lowest&&&&&\cr
height2pt&\omit&&\omit&&\omit&\cr}
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@end tex
@iftex
{
@obeylines@parskip=0pt@parindent=0pt
@dag@quad Prefix operators.
@ddag@quad @xref{Assignment}.
}
@end iftex
@c END TEXI2ROFF-KILL
@node Evaluation
@subsection Evaluation
@cindex lazy evaluation
@cindex expression evaluation order
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.
@node Assignment
@subsection Assignment: Defining Symbols
@cindex assignment in scripts
@cindex symbol definition, scripts
@cindex variables, defining
You may create global symbols, and assign values (addresses) to global
symbols, using any of the C assignment operators:
@table @code
@item @var{symbol} = @var{expression} ;
@itemx @var{symbol} &= @var{expression} ;
@itemx @var{symbol} += @var{expression} ;
@itemx @var{symbol} -= @var{expression} ;
@itemx @var{symbol} *= @var{expression} ;
@itemx @var{symbol} /= @var{expression} ;
@end table
Two things distinguish assignment from other operators in @code{ld}
expressions.
@itemize @bullet
@item
Assignment may only be used at the root of an expression;
@samp{a=b+3;} is allowed, but @samp{a+b=3;} is an error.
@kindex ;
@cindex semicolon
@item
You must place a trailing semicolon (``@key{;}'') at the end of an
assignment statement.
@end itemize
Assignment statements may appear:
@itemize @bullet
@item
as commands in their own right in an @code{ld} script; or
@item
as independent statements within a @code{SECTIONS} command; or
@item
as part of the contents of a section definition in a
@code{SECTIONS} command.
@end itemize
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 (@pxref{SECTIONS}).
@cindex absolute and relocatable symbols
@cindex relocatable and absolute symbols
@cindex symbols, relocatable and absolute
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
@code{ABSOLUTE}. For example, to create an absolute symbol whose address
is the last byte of an output section named @code{.data}:
@example
SECTIONS@{ @dots{}
.data :
@{
*(.data)
_edata = ABSOLUTE(.) ;
@}
@dots{} @}
@end example
The linker tries to put off the evaluation of an assignment until all
the terms in the source expression are known (@pxref{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 @dfn{dot}, @samp{.} 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
@example
SECTIONS @{ @dots{}
text 9+this_isnt_constant :
@{ @dots{}
@}
@dots{} @}
@end example
@kindex Non constant expression
@noindent
will cause the error message ``@code{Non constant expression for initial
address}''.
@node Built-ins
@subsection Built-In Functions
@cindex functions in expression language
The command language includes a number of built-in
functions for use in link script expressions.
@table @code
@item ABSOLUTE(@var{exp})
@kindex ABSOLUTE(@var{exp})
@cindex expression, absolute
Return the absolute (non-relocatable, as opposed to non-negative) value
of the expression @var{exp}. Primarily useful to assign an absolute
value to a symbol within a section definition, where symbol values are
normally section-relative.
@item ADDR(@var{section})
@kindex ADDR(@var{section})
@cindex section address
Return the absolute address of the named @var{section}. Your script must
previously have defined the location of that section. In the following
example, @code{symbol_1} and @code{symbol_2} are assigned identical
values:
@example
SECTIONS@{ @dots{}
.output1 :
@{
start_of_output_1 = ABSOLUTE(.);
@dots{}
@}
.output :
@{
symbol_1 = ADDR(.output1);
symbol_2 = start_of_output_1;
@}
@dots{} @}
@end example
@item ALIGN(@var{exp})
@kindex ALIGN(@var{exp})
@cindex rounding up location counter
Return the result of the current location counter (@code{.}) aligned to
the next @var{exp} boundary. @var{exp} must be an expression whose
value is a power of two. This is equivalent to
@example
(. + @var{exp} - 1) & ~(@var{exp} - 1)
@end example
@code{ALIGN} doesn't change the value of the location counter---it just
does arithmetic on it. As an example, to align the output @code{.data}
section to the next @code{0x2000} byte boundary after the preceding
section and to set a variable within the section to the next
@code{0x8000} boundary after the input sections:
@example
SECTIONS@{ @dots{}
.data ALIGN(0x2000): @{
*(.data)
variable = ALIGN(0x8000);
@}
@dots{} @}
@end example
@noindent
The first use of @code{ALIGN} in this example specifies the location of
a section because it is used as the optional @var{start} attribute of a
section definition (@pxref{Section Options}). The second use simply
defines the value of a variable.
The built-in @code{NEXT} is closely related to @code{ALIGN}.
@item DEFINED(@var{symbol})
@kindex DEFINED(@var{symbol})
@cindex symbol defaults
Return 1 if @var{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 @code{begin} to the first location in the
@code{.text} section---but if a symbol called @code{begin} already
existed, its value is preserved:
@smallexample
SECTIONS@{ @dots{}
.text : @{
begin = DEFINED(begin) ? begin : . ;
@dots{}
@}
@dots{} @}
@end smallexample
@item NEXT(@var{exp})
@kindex NEXT(@var{exp})
@cindex unallocated address, next
Return the next unallocated address that is a multiple of @var{exp}.
This function is closely related to @code{ALIGN(@var{exp})}; unless you
use the @code{MEMORY} command to define discontinuous memory for the
output file, the two functions are equivalent.
@item SIZEOF(@var{section})
@kindex SIZEOF(@var{section})
@cindex section size
Return the size in bytes of the named @var{section}, if that section has
been allocated. In the following example, @code{symbol_1} and
@code{symbol_2} are assigned identical values:
@c What does it return if the section hasn't been allocated? 0?
@example
SECTIONS@{ @dots{}
.output @{
.start = . ;
@dots{}
.end = . ;
@}
symbol_1 = .end - .start ;
symbol_2 = SIZEOF(.output);
@dots{} @}
@end example
@item SIZEOF_HEADERS
@kindex SIZEOF_HEADERS
@cindex header size
@itemx sizeof_headers
@kindex 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.
@end table
@node MEMORY
@section MEMORY Command
@kindex MEMORY
@cindex regions of memory
@cindex discontinuous memory
@cindex allocating memory
The linker's default configuration permits allocation of all available memory.
You can override this configuration by using the @code{MEMORY} command. The
@code{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.
The command files may contain at most one use of the @code{MEMORY}
command; however, you can define as many blocks of memory within it as
you wish. The syntax is:
@example
MEMORY
@{
@var{name} (@var{attr}) : ORIGIN = @var{origin}, LENGTH = @var{len}
@dots{}
@}
@end example
@table @code
@item @var{name}
@cindex naming memory regions
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.
@item (@var{attr})
@cindex memory region attributes
is an optional list of attributes, permitted for compatibility with the
AT&T linker but not used by @code{ld} beyond checking that the
attribute list is valid. Valid attribute lists must be made up of the
characters ``@code{LIRWX}''. If you omit the attribute list, you may
omit the parentheses around it as well.
@item @var{origin}
@kindex ORIGIN =
@kindex o =
@kindex org =
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 @code{ORIGIN} may be
abbreviated to @code{org} or @code{o}.
@item @var{len}
@kindex LENGTH =
@kindex len =
@kindex l =
is the size in bytes of the region (an expression).
The keyword @code{LENGTH} may be abbreviated to @code{len} or @code{l}.
@end table
For example, to specify that memory has two regions available for
allocation---one starting at 0 for 256 kilobytes, and the other
starting at @code{0x40000000} for four megabytes:
@example
MEMORY
@{
rom : ORIGIN = 0, LENGTH = 256K
ram : org = 0x40000000, l = 4M
@}
@end example
Once you have defined a region of memory named @var{mem}, you can direct
specific output sections there by using a command ending in
@samp{>@var{mem}} within the @code{SECTIONS} command (@pxref{Section
Options}). If the combined output sections directed to a region are too
big for the region, the linker will issue an error message.
@node SECTIONS
@section SECTIONS Command
@kindex SECTIONS
The @code{SECTIONS} command controls exactly where input sections are
placed into output sections, their order and to which output sections
they are allocated.
You may use at most one @code{SECTIONS} command in a commands file,
but you can have as many statements within it as you wish. Statements
within the @code{SECTIONS} command can do one of three things:
@itemize @bullet
@item
define the entry point;
@item
assign a value to a symbol;
@item
describe the placement of a named output section, and what input
sections make it up.
@end itemize
The first two possibilities---defining the entry point, and defining
symbols---can also be done outside the @code{SECTIONS} command:
@pxref{Entry Point}, @pxref{Assignment}. They are permitted here as
well for your convenience in reading the script, so that symbols or the
entry point can be defined at meaningful points in your output-file
layout.
When no @code{SECTIONS} command is specified, the default action
of the linker is to place each input section into an identically named
output section in the order that the sections are first encountered in
the input files; if all input sections are present in the first file,
for example, the order of sections in the output file will match the
order in the first input file.
@menu
* Section Definition:: Section Definitions
* Section Contents:: Section Contents
* Section Options:: Optional Section Attributes
@end menu
@node Section Definition
@subsection Section Definitions
@cindex section definition
The most frequently used statement in the @code{SECTIONS} command is
the @dfn{section definition}, which you can use to specify 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
@example
SECTIONS @{ @dots{}
@var{secname} : @{
@var{contents}
@}
@dots{} @}
@end example
@cindex naming output sections
@noindent
@var{secname} is the name of the output section, and @var{contents} a
specification of what goes there---for example, a list of input files or
sections of input files. As you might assume, the whitespace shown is
optional. You do need the colon @samp{:} and the braces @samp{@{@}},
however.
@var{secname} must meet the constraints of your output format. In
formats which only support a limited number of sections, such as
@code{a.out}, the name must be one of the names supported by the format
(@code{a.out}, for example, allows only @code{.text}, @code{.data} or
@code{.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 characters, but any name which does not conform to the standard
@code{ld} symbol name syntax must be quoted.
@xref{Symbols, , Symbol Names}.
@node Section Contents
@subsection Section Contents
@cindex contents of a section
In a section definition, you can specify the contents of an output section by
listing particular object 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 @var{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.
@table @code
@item @var{filename}
@kindex @var{filename}
@cindex input files, section defn
@cindex files, including in output sections
You may simply name a particular input file to be placed in the current
output section; @emph{all} sections from that file are placed in the
current section definition. To specify a list of particular files by
name:
@example
.data : @{ afile.o bfile.o cfile.o @}
@end example
@noindent
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.
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.
@item @var{filename}( @var{section} )
@itemx @var{filename}( @var{section}, @var{section}, @dots{} )
@itemx @var{filename}( @var{section} @var{section} @dots{} )
@kindex @var{filename}(@var{section})
@cindex files and sections, section defn
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.
@item * (@var{section})
@itemx * (@var{section}, @var{section}, @dots{})
@itemx * (@var{section} @var{section} @dots{}
@cindex input sections to output section
@kindex *(@var{section})
Instead of explicitly naming particular input files in a link control
script, you can refer to @emph{all} files from the @code{ld} command
line: use @samp{*} instead of a particular file name before the
parenthesized input-file section list.
For example, to copy sections @code{1} through @code{4} from an Oasys file
into the @code{.text} section of an @code{a.out} file, and sections @code{13}
and @code{14} into the @code{.data} section:
@example
SECTIONS @{
.text :@{
*("1" "2" "3" "4")
@}
.data :@{
*("13" "14")
@}
@}
@end example
If you have already explicitly included some files by name, @samp{*}
refers to all @emph{remaining} files---those whose places in the output
file have not yet been defined.
@item [ @var{section} ]
@itemx [ @var{section}, @var{section}, @dots{} ]
@itemx [ @var{section} @var{section} @dots{} ]
@kindex [ @var{sections} ]
This is an alternate notation to specify named sections from all
unallocated input files; its effect is exactly the same as that of
@samp{* (@var{section}@dots{})}
@item @var{filename}@code{( COMMON )}
@itemx ( COMMON )
@kindex ( COMMON )
@cindex uninitialized data
@cindex commons in output
Specify where in your output file to place uninitialized data
with this notation. @code{*(COMMON)} by itself refers to all
uninitialized data from all input files (so far as it is not yet
allocated); @var{filename}@code{(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:
@code{ld} permits you to refer to uninitialized data as if it
were in an input-file section named @code{COMMON}, regardless of the
input file's format.
@end table
For example, the following command script arranges the output file into
three consecutive sections, named @code{.text}, @code{.data}, and
@code{.bss}, taking the input for each from the correspondingly named
sections of all the input files:
@example
SECTIONS @{
.text : @{ *(.text) @}
.data : @{ *(.data) @}
.bss : @{ *(.bss) *(COMMON) @}
@}
@end example
The following example reads all of the sections from file @code{all.o}
and places them at the start of output section @code{outputa} which
starts at location @code{0x10000}. All of section @code{.input1} from
file @code{foo.o} follows immediately, in the same output section. All
of section @code{.input2} from @code{foo.o} goes into output section
@code{outputb}, followed by section @code{.input1} from @code{foo1.o}.
All of the remaining @code{.input1} and @code{.input2} sections from any
files are written to output section @code{outputc}.
@example
SECTIONS @{
outputa 0x10000 :
@{
all.o
foo.o (.input1)
@}
outputb :
@{
foo.o (.input2)
foo1.o (.input1)
@}
outputc :
@{
*(.input1)
*(.input2)
@}
@}
@end example
There are still more kinds of statements permitted in the contents of
output section definitions. The foregoing statements permitted you to
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; @pxref{Expressions}. Although these statements are shown
separately here for ease of presentation, no such segregation is needed
within a section definition in the @code{SECTIONS} command; you can
intermix them freely with any of the statements we've just described.
@table @code
@item CREATE_OBJECT_SYMBOLS
@kindex CREATE_OBJECT_SYMBOLS
@cindex input filename symbols
@cindex filename symbols
Create a symbol for each input file
in the current section, set to the address of the first byte of
data written from the input file. For instance, with @code{a.out}
files it is conventional to have a symbol for each input file. You can
accomplish this by defining the output @code{.text} section as follows:
@example
SECTIONS @{
.text 0x2020 :
@{
CREATE_OBJECT_SYMBOLS
*(.text)
_etext = ALIGN(0x2000);
@}
@dots{}
@}
@end example
If @code{objsym} is a file containing this script, and @code{a.o},
@code{b.o}, @code{c.o}, and @code{d.o} are four input files with
contents like the following---
@example
/* a.c */
afunction() @{ @}
int adata=1;
int abss;
@end example
@noindent
@samp{ld -M sample a.o b.o c.o d.o} would create a map like this,
containing symbols matching the object file names:
@example
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
@end example
@item @var{symbol} = @var{expression} ;
@kindex @var{symbol} = @var{expression} ;
@itemx @var{symbol} @var{f}= @var{expression} ;
@kindex @var{symbol} @var{f}= @var{expression} ;
@var{symbol} is any symbol name (@pxref{Symbols}). ``@var{f}=''
refers to any of the operators @code{&= += -= *= /=} which combine
arithmetic and assignment.
@cindex assignment, in section defn
When you assign a value to a symbol within a particular section
definition, the value is relative to the beginning of the section
(@pxref{Assignment}). If you write
@example
SECTIONS @{
abs = 14 ;
@dots{}
.data : @{ @dots{} rel = 14 ; @dots{} @}
abs2 = 14 + ADDR(.data);
@dots{}
@}
@end example
@c FIXME: Try above example!
@noindent
@code{abs} and @code{rel} do not have the same value; @code{rel} has the
same value as @code{abs2}.
@item BYTE(@var{expression})
@kindex BYTE(@var{expression})
@itemx SHORT(@var{expression})
@kindex SHORT(@var{expression})
@itemx LONG(@var{expression})
@kindex LONG(@var{expression})
@cindex direct output
By including one of these three statements in a section definition, you
can explicitly place one, two, or four bytes (respectively) at the
current address of that section.
@ifclear SingleFormat
Multiple-byte quantities are represented in whatever byte order is
appropriate for the output file format (@pxref{BFD}).
@end ifclear
@item FILL(@var{expression})
@kindex FILL(@var{expression})
@cindex holes, filling
@cindex unspecified memory
Specifies 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 @samp{.})
are filled with the two least significant bytes from the
@var{expression} argument. A @code{FILL} statement covers memory
locations @emph{after} the point it occurs in the section definition; by
including more than one @code{FILL} statement, you can have different
fill patterns in different parts of an output section.
@end table
@node Section Options
@subsection Optional Section Attributes
@cindex section defn, full syntax
Here is the full syntax of a section definition, including all the
optional portions:
@smallexample
SECTIONS @{
@dots{}
@var{secname} @var{start} BLOCK(@var{align}) (NOLOAD) : @{ @var{contents} @} =@var{fill} >@var{region}
@dots{}
@}
@end smallexample
@var{secname} and @var{contents} are required. @xref{Section
Definition}, and @pxref{Section Contents} for details on @var{contents}.
The remaining elements---@var{start}, @code{BLOCK(@var{align)}},
@code{(NOLOAD)} @code{=@var{fill}}, and @code{>@var{region}}---are all
optional.
@table @code
@item @var{start}
@cindex start address, section
@cindex section start
@cindex section address
You can force the output section to be loaded at a specified address by
specifying @var{start} immediately following the section name.
@var{start} can be represented as any expression. The following
example generates section @var{output} at location
@code{0x40000000}:
@example
SECTIONS @{
@dots{}
output 0x40000000: @{
@dots{}
@}
@dots{}
@}
@end example
@item BLOCK(@var{align})
@kindex BLOCK(@var{align})
@cindex section alignment
@cindex aligning sections
You can include @code{BLOCK()} specification to advance
the location counter @code{.} prior to the beginning of the section, so
that the section will begin at the specified alignment. @var{align} is
an expression.
@item (NOLOAD)
@kindex NOLOAD
@cindex prevent unnecessary loading
Use @samp{(NOLOAD)} to prevent a section from being loaded into memory
each time it is accessed. For example, in the script sample below, the
@code{ROM} segment is addressed at memory location @samp{0} and does not
need to be loaded into each object file:
@example
SECTIONS @{
ROM 0 (NOLOAD) : @{ @dots{} @}
@dots{}
@}
@end example
@item =@var{fill}
@kindex =@var{fill}
@cindex section fill pattern
@cindex fill pattern, entire section
Including
@code{=@var{fill}} in a section definition specifies the initial fill
value for that section.
You may use any expression to specify @var{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 @code{FILL} statement in the
@var{contents} of a section definition.
@item >@var{region}
@kindex >@var{region}
@cindex section, assigning to memory region
@cindex memory regions and sections
Assign this section to a previously defined region of memory.
@xref{MEMORY}.
@end table
@node Entry Point
@section The Entry Point
@kindex ENTRY(@var{symbol})
@cindex start of execution
@cindex first instruction
The linker command language includes a command specifically for
defining the first executable instruction in an output file (its
@dfn{entry point}). Its argument is a symbol name:
@example
ENTRY(@var{symbol})
@end example
Like symbol assignments, the @code{ENTRY} command may be placed either
as an independent command in the command file, or among the section
definitions within the @code{SECTIONS} command---whatever makes the most
sense for your layout.
@cindex entry point, defaults
@code{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).
@itemize @bullet
@item
the @samp{-e} @var{entry} command-line option;
@item
the @code{ENTRY(@var{symbol}} command in a linker control script;
@item
the value of the symbol @code{start}, if present;
@item
the value of the symbol @code{_main}, if present;
@item
the address of the first byte of the @code{.text} section, if present;
@item
The address @code{0}.
@end itemize
For example, you can use these rules to generate an entry point with an
assignment statement: if no symbol @code{start} is defined within your
input files, you can simply define it, assigning it an appropriate
value---
@example
start = 0x2020;
@end example
@noindent
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 @code{start}:
@example
start = other_symbol ;
@end example
@node Other Commands
@section Other Commands
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.
@table @code
@item FLOAT
@kindex FLOAT
@itemx NOFLOAT
@kindex NOFLOAT
These keywords were used in some older linkers to request a particular
math subroutine library. @code{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
@code{FLOAT} and @code{NOFLOAT} are accepted and ignored.
@item FORCE_COMMON_ALLOCATION
@kindex FORCE_COMMON_ALLOCATION
@cindex common allocation
This command has the same effect as the @samp{-d} command-line option:
to make @code{ld} assign space to common symbols even if a relocatable
output file is specified (@samp{-r}).
@item INPUT ( @var{file}, @var{file}, @dots{} )
@kindex INPUT ( @var{files} )
@itemx INPUT ( @var{file} @var{file} @dots{} )
@cindex binary input files
Use this command to include binary input files in the link, without
including them in a particular section definition. Files specified this
way are treated identically to object files listed on the command line.
@ignore
@item MAP ( @var{name} )
@kindex MAP ( @var{name} )
@c MAP(...) appears to look for an F in the arg, ignoring all other
@c chars; if it finds one, it sets "map_option_f" to true. But nothing
@c checks map_option_f. Apparently a stub for the future...
@end ignore
@item OUTPUT ( @var{filename} )
@kindex OUTPUT ( @var{filename} )
@cindex naming the output file
Use this command to name the link output file @var{filename}. The
effect of @code{OUTPUT(@var{filename})} is identical to the effect of
@w{@samp{-o @var{filename}}}, and whichever is encountered last will
control the name actually used to name the output file. In particular,
you can use this command to supply a default output-file name other than
@code{a.out}.
@ifclear SingleFormat
@item OUTPUT_ARCH ( @var{bfdname} )
@kindex OUTPUT_ARCH ( @var{bfdname} )
@cindex machine architecture, output
Specify a particular output machine architecture, with one of the names
used by the BFD back-end routines (@pxref{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 @code{OUTPUT_FORMAT}
command.
@item OUTPUT_FORMAT ( @var{bfdname} )
@kindex OUTPUT_FORMAT ( @var{bfdname} )
@cindex format, output file
Specify a particular output format, with one of the names used by the
BFD back-end routines (@pxref{BFD}). This selection will only affect
the output file; the related command @code{TARGET} affects primarily
input files.
@end ifclear
@item SEARCH_DIR ( @var{path} )
@kindex SEARCH_DIR ( @var{path} )
@cindex path for libraries
@cindex search path, libraries
Add @var{path} to the list of paths where @code{ld} looks for
archive libraries. @code{SEARCH_DIR(@var{path})} has the same
effect as @samp{-L@var{path}} on the command line.
@item STARTUP ( @var{filename} )
@kindex STARTUP ( @var{filename} )
@cindex first input file
Ensure that @var{filename} is the first input file used in the link
process.
@ifclear SingleFormat
@item TARGET ( @var{format} )
@cindex input file format
@kindex TARGET ( @var{format} )
Change the input-file object code format (like the command-line option
@samp{-b} or its synonym @samp{-format}). The argument @var{format} is
one of the strings used by BFD to name binary formats. In the current
@code{ld} implementation, if @code{TARGET} is specified but
@code{OUTPUT_FORMAT} is not, the last @code{TARGET} argument is also
used as the default format for the @code{ld} output file.
@xref{BFD}.
@kindex GNUTARGET
If you don't use the @code{TARGET} command, @code{ld} uses the value of
the environment variable @code{GNUTARGET}, if available, to select the
output file format. If that variable is also absent, @code{ld} uses
the default format configured for your machine in the BFD libraries.
@end ifclear
@end table
@ifset GENERIC
@node Machine Dependent
@chapter Machine Dependent Features
@cindex machine dependencies
@code{ld} has additional features on some platforms; the following
sections describe them. Machines where @code{ld} has no additional
functionality are not listed.
@menu
* H8/300:: @code{ld} and the H8/300
* i960:: @code{ld} and the Intel 960 family
@end menu
@end ifset
@c FIXME! This could use @up/@down, but there seems to be a conflict
@c between those and node-defaulting.
@ifset H8300
@ifclear GENERIC
@up
@end ifclear
@node H8/300
@section @code{ld} and the H8/300
@cindex H8/300 support
For the H8/300, @code{ld} can perform these global optimizations when
you specify the @samp{-relax} command-line option.
@table @emph
@item relaxing address modes
@cindex relaxing on H8/300
@code{ld} finds all @code{jsr} and @code{jmp} instructions whose
targets are within eight bits, and turns them into eight-bit
program-counter relative @code{bsr} and @code{bra} instructions,
respectively.
@item synthesizing instructions
@cindex synthesizing on H8/300
@c FIXME: specifically mov.b, or any mov instructions really?
@code{ld} finds all @code{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 @samp{mov.b @code{@@}@var{aa}:16} into
@samp{mov.b @code{@@}@var{aa}:8} whenever the address @var{aa} is in the
top page of memory).
@end table
@ifclear GENERIC
@down
@end ifclear
@end ifset
@ifset I960
@ifclear GENERIC
@up
@end ifclear
@node i960
@section @code{ld} and the Intel 960 family
@cindex i960 support
You can use the @samp{-A@var{architecture}} 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 @code{ld} command line included @w{@samp{-ACA}} as
well as @w{@samp{-ltry}}, the linker would look (in its built-in search
paths, and in any paths you specify with @samp{-L}) for a library with
the names
@example
try
libtry.a
tryca
libtryca.a
@end example
@noindent
The first two possibilities would be considered in any event; the last
two are due to the use of @w{@samp{-ACA}}.
You can meaningfully use @samp{-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 @w{@samp{-l}}
specifies a library.
@ifclear GENERIC
@down
@end ifclear
@end ifset
@ifclear SingleFormat
@node BFD
@chapter BFD
@cindex back end
@cindex object file management
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. You can use @code{objdump -i}
(@pxref{objdump,,objdump,binutils.info,The GNU Binary Utilities}) to
list all the formats available for each architecture under BFD. This
was the list of formats, and of architectures supported for each format,
as of the time this manual was prepared:
@cindex formats available
@cindex architectures available
@example
BFD header file version 0.18
a.out-i386
(header big endian, data big endian)
m68k:68020
a29k
sparc
i386
a.out-sunos-big
(header big endian, data big endian)
m68k:68020
a29k
sparc
i386
b.out.big
(header big endian, data little endian)
i960:core
b.out.little
(header little endian, data little endian)
i960:core
coff-a29k-big
(header big endian, data big endian)
a29k
coff-h8300
(header big endian, data big endian)
H8/300
coff-i386
(header little endian, data little endian)
i386
coff-Intel-big
(header big endian, data little endian)
i960:core
coff-Intel-little
(header little endian, data little endian)
i960:core
coff-m68k
(header big endian, data big endian)
m68k:68020
coff-m88kbcs
(header big endian, data big endian)
m88k:88100
ecoff-bigmips
(header big endian, data big endian)
mips
ecoff-littlemips
(header little endian, data little endian)
mips
elf-big
(header big endian, data big endian)
m68k:68020
vax
i960:core
a29k
sparc
mips
i386
m88k:88100
H8/300
rs6000:6000
elf-little
(header little endian, data little endian)
m68k:68020
vax
i960:core
a29k
sparc
mips
i386
m88k:88100
H8/300
rs6000:6000
ieee
(header big endian, data big endian)
m68k:68020
vax
i960:core
a29k
sparc
mips
i386
m88k:88100
H8/300
rs6000:6000
srec
(header big endian, data big endian)
m68k:68020
vax
i960:core
a29k
sparc
mips
i386
m88k:88100
H8/300
rs6000:6000
@end example
@cindex BFD requirements
@cindex requirements for BFD
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}.
@menu
* BFD outline:: How it works: an outline of BFD
* BFD information loss:: Information Loss
* Mechanism:: Mechanism
@end menu
@node BFD outline
@section How it works: an outline of BFD
@cindex opening object files
@include ../bfd/doc/bfdsumm.texi
@end ifclear
@node MRI
@appendix MRI Compatible Script Files
@cindex MRI compatibility
To aid users making the transition to @sc{gnu} @code{ld} from the MRI
linker, @code{ld} can use MRI compatible linker scripts as an
alternative to the more general-purpose linker scripting language
described in @ref{Commands,,Command Language}. MRI compatible linker
scripts have a much simpler command set than the scripting language
otherwise used with @code{ld}. @sc{gnu} @code{ld} supports the most
commonly used MRI linker commands; these commands are described here.
You can specify a file containing an MRI-compatible script using the
@samp{-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, @code{ld}
issues a warning message, but continues processing the script.
Lines beginning with @samp{*} are comments.
You can write these commands using all upper-case letters, or all
lower case; for example, @samp{chip} is the same as @samp{CHIP}.
The following list shows only the upper-case form of each command.
@table @code
@item ABSOLUTE @var{secname}
@item ABSOLUTE @var{secname}, @var{secname}, @dots{} @var{secname}
@cindex @code{ABSOLUTE} (MRI)
Normally, @code{ld} includes in the output file all sections from all
the input files. However, in an MRI-compatible script, you can use the
@code{ABSOLUTE} command to restrict the sections that will be present in
your output program. If the @code{ABSOLUTE} command is used at all in a
script, then only the sections named explicitly in @code{ABSOLUTE}
commands will appear in the linker output. You can still use other
input sections (whatever you select on the command line, or using
@code{LOAD}) to resolve addresses in the output file.
@item ALIAS @var{out-secname}, @var{in-secname}
@cindex @code{ALIAS} (MRI)
Use this command to place the data from input section @var{in-secname}
in a section called @var{out-secname} in the linker output file.
@var{in-secname} may be an integer.
@item BASE @var{expression}
@cindex @code{BASE} (MRI)
Use the value of @var{expression} as the lowest address (other than
absolute addresses) in the output file.
@item CHIP @var{expression}
@itemx CHIP @var{expression}, @var{expression}
@cindex @code{CHIP} (MRI)
This command does nothing; it is accepted only for compatibility.
@item END
@cindex @code{END} (MRI)
This command does nothing whatever; it's only accepted for compatibility.
@item FORMAT @var{output-format}
@cindex @code{FORMAT} (MRI)
Similar to the @code{OUTPUT_FORMAT} command in the more general linker
language, but restricted to one of these output formats:
@enumerate
@item
S-records, if @var{output-format} is @samp{S}
@item
IEEE, if @var{output-format} is @samp{IEEE}
@item
COFF (the @samp{coff-m68k} variant in BFD), if @var{output-format} is
@samp{COFF}
@end enumerate
@item LIST @var{anything}@dots{}
@cindex @code{LIST} (MRI)
Print (to the standard output file) a link map, as produced by the
@code{ld} command-line option @samp{-M}.
The keyword @code{LIST} may be followed by anything on the
same line, with no change in its effect.
@item LOAD @var{filename}
@item LOAD @var{filename}, @var{filename}, @dots{} @var{filename}
@cindex @code{LOAD} (MRI)
Include one or more object file @var{filename} in the link; this has the
same effect as specifying @var{filename} directly on the @code{ld}
command line.
@item NAME @var{output-name}
@cindex @code{NAME} (MRI)
@var{output-name} is the name for the program produced by @code{ld}; the
MRI-compatible command @code{NAME} is equivalent to the command-line
option @samp{-o} or the general script language command @code{OUTPUT}.
@item ORDER @var{secname}, @var{secname}, @dots{} @var{secname}
@itemx ORDER @var{secname} @var{secname} @var{secname}
@cindex @code{ORDER} (MRI)
Normally, @code{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 @code{ORDER} command. The
sections you list with @code{ORDER} will appear first in your output
file, in the order specified.
@item PUBLIC @var{name}=@var{expression}
@itemx PUBLIC @var{name},@var{expression}
@itemx PUBLIC @var{name} @var{expression}
@cindex @code{PUBLIC} (MRI)
Supply a value (@var{expression}) for external symbol
@var{name} used in the linker input files.
@item SECT @var{secname}, @var{expression}
@itemx SECT @var{secname}=@var{expression}
@itemx SECT @var{secname} @var{expression}
@cindex @code{SECT} (MRI)
You can use any of these three forms of the @code{SECT} command to
specify the start address (@var{expression}) for section @var{secname}.
If you have more than one @code{SECT} statement for the same
@var{secname}, only the @emph{first} sets the start address.
@end table
@node Index
@unnumbered Index
@printindex cp
@tex
% I think something like @colophon should be in texinfo. In the
% meantime:
\long\def\colophon{\hbox to0pt{}\vfill
\centerline{The body of this manual is set in}
\centerline{\fontname\tenrm,}
\centerline{with headings in {\bf\fontname\tenbf}}
\centerline{and examples in {\tt\fontname\tentt}.}
\centerline{{\it\fontname\tenit\/} and}
\centerline{{\sl\fontname\tensl\/}}
\centerline{are used for emphasis.}\vfill}
\page\colophon
% Blame: pesch@cygnus.com, 28mar91.
@end tex
@contents
@bye
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