* ld.h (ld_config_type): Add new field warn_once.

[binutils.git] / ld / ld.1

.\" Copyright (c) 1991, 1992 Free Software Foundation
.\" See section COPYING for conditions for redistribution
.TH ld 1 "17 August 1992" "cygnus support" "GNU Development Tools"
.de BP
.sp
.ti \-.2i
\(**
..

.SH NAME
ld \- the GNU linker

.SH SYNOPSIS
.hy 0
.na
.TP
.B ld 
.RB "[\|" \-o "
.I output\c
\&\|] \c
.I objfile\c
\&.\|.\|.
.br
.RB "[\|" \-A\c
.I architecture\c
\&\|] 
.RB "[\|" "\-b\ "\c
.I input-format\c
\&\|] 
.RB "[\|" \-Bstatic "\|]"  
.RB "[\|" "\-c\ "\c
.I commandfile\c
\&\|] 
.RB "[\|" \-d | \-dc | \-dp\c
\|]
.br
.RB "[\|" "\-defsym\ "\c
.I symbol\c
\& = \c
.I expression\c
\&\|]
.RB "[\|" "\-e\ "\c
.I entry\c
\&\|] 
.RB "[\|" \-F "\|]" 
.RB "[\|" "\-F\ "\c
.I format\c
\&\|]
.RB "[\|" "\-format\ "\c
.I input-format\c
\&\|] 
.RB "[\|" \-g "\|]" 
.RB "[\|" \-G\c
.I size\c
\&\|]
.RB "[\|" \-\-help "\|]"
.RB "[\|" \-i "\|]"
.RB "[\|" \-l\c
.I ar\c
\&\|] 
.RB "[\|" \-L\c
.I searchdir\c
\&\|] 
.RB "[\|" \-M "\|]" 
.RB "[\|" \-Map\c
.I mapfile\c
\&\|] 
.RB "[\|" \-m\c
.I emulation\c
\&\|] 
.RB "[\|" \-n | \-N "\|]" 
.RB "[\|" \-noinhibit-exec "\|]" 
.RB "[\|" "\-oformat\ "\c
.I output-format\c
\&\|] 
.RB "[\|" "\-R\ "\c
.I filename\c
\&\|]
.RB "[\|" \-relax "\|]"
.RB "[\|" \-r | \-Ur "\|]" 
.RB "[\|" \-S "\|]" 
.RB "[\|" \-s "\|]" 
.RB "[\|" \-sort\-common "\|]" 
.RB "[\|" "\-T\ "\c
.I commandfile\c
\&\|]  
.RB "[\|" "\-Ttext\ "\c
.I textorg\c
\&\|] 
.RB "[\|" "\-Tdata\ "\c
.I dataorg\c
\&\|] 
.RB "[\|" "\-Tbss\ "\c
.I bssorg\c
\&\|]
.RB "[\|" \-t "\|]" 
.RB "[\|" "\-u\ "\c
.I sym\c
\&]
.RB "[\|" \-V "\|]"
.RB "[\|" \-v "\|]"
.RB "[\|" \-\-version "\|]"
.RB "[\|" \-warn\-common "\|]" 
.RB "[\|" \-warn\-once "\|]" 
.RB "[\|" \-X "\|]" 
.RB "[\|" \-x "\|]" 
.ad b
.hy 1
.SH DESCRIPTION
\c
.B ld\c
\& combines a number of object and archive files, relocates
their data and ties up symbol references. Often the last step in
building a new compiled program to run is a call to \c
.B ld\c
\&.

\c
.B ld\c
\& accepts Linker Command Language files 
to provide explicit and total control over the linking process.
This man page does not describe the command language; see the `\|\c
.B ld\c
\|' entry in `\|\c
.B info\c
\|', or the manual
.I
ld: the GNU linker
\&, for full details on the command language and on other aspects of
the GNU linker. 

This version of \c
.B ld\c
\& uses the general purpose BFD libraries
to operate on object files. This allows \c
.B ld\c
\& to read, combine, and
write object files in many different formats\(em\&for example, COFF or
\c
.B a.out\c
\&.  Different formats may be linked together to produce any
available kind of object file.  You can use `\|\c
.B objdump \-i\c
\|' to get a list of formats supported on various architectures; see 
.BR objdump ( 1 ).

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,
\c
.B ld\c
\& 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 \c
.B ld\c
\& 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 through the command line,
and through environment variables.

.SH OPTIONS
The plethora of command-line options may seem intimidating, but in
actual practice few of them are used in any particular context.
For instance, a frequent use of \c
.B ld\c
\& is to link standard Unix
object files on a standard, supported Unix system.  On such a system, to
link a file \c
.B hello.o\c
\&:
.sp
.br
$\ ld\ \-o\ output\ /lib/crt0.o\ hello.o\ \-lc
.br
.sp
This tells \c
.B ld\c
\& to produce a file called \c
.B output\c
\& as the
result of linking the file \c
.B /lib/crt0.o\c
\& with \c
.B hello.o\c
\& and
the library \c
.B libc.a\c
\& which will come from the standard search
directories.

The command-line options to \c
.B ld\c
\& may be specified in any order, and
may be repeated at will.  For the most part, repeating an option with a
different argument will either have no further effect, or override prior
occurrences (those further to the left on the command line) of an
option.  

The exceptions\(em\&which may meaningfully be used more than once\(em\&are
\c
.B \-A\c
\&, \c
.B \-b\c
\& (or its synonym \c
.B \-format\c
\&), \c
.B \-defsym\c
\&,
\c
.B \-L\c
\&, \c
.B \-l\c
\&, \c
.B \-R\c
\&, and \c
.B \-u\c
\&.

The list of object files to be linked together, shown as \c
.I objfile\c
\&,
may follow, precede, or be mixed in with command-line options; save that
an \c
.I objfile\c
\& argument may not be placed between an option flag 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 \c
.B \-l\c
\&,
\c
.B \-R\c
\&, and the script command language.  If \c
.I no\c
\& binary input
files at all are specified, the linker does not produce any output, and
issues the message `\|\c
.B No input files\c
\|'.

Option arguments must either follow the option letter without intervening
whitespace, or be given as separate arguments immediately following the
option that requires them.

.TP
.BI "-A" "architecture"\c
\&
In the current release of \c
.B ld\c
\&, this option is useful only for the
Intel 960 family of architectures.  In that \c
.B ld\c
\& configuration, the
\c
.I architecture\c
\& argument is 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 \c
.B ld\c
\& command line included `\|\c
.B \-ACA\c
\|' as
well as `\|\c
.B \-ltry\c
\|', the linker would look (in its built-in search
paths, and in any paths you specify with \c
.B \-L\c
\&) for a library with
the names
.sp
.br
try
.br
libtry.a
.br
tryca
.br
libtryca.a
.br
.sp

The first two possibilities would be considered in any event; the last
two are due to the use of `\|\c
.B \-ACA\c
\|'.

Future releases of \c
.B ld\c
\& may support similar functionality for
other architecture families.

You can meaningfully use \c
.B \-A\c
\& more than once on a command line, if
an architecture family allows combination of target architectures; each
use will add another pair of name variants to search for when \c
.B \-l\c
\&
specifies a library.

.TP
.BI "\-b " "input-format"\c
\&
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
\c
.B ld\c
\& is configured to expect as a default input format the most
usual format on each machine.  \c
.I input-format\c
\& is a text string, the
name of a particular format supported by the BFD libraries.  
\c
.B \-format \c
.I input-format\c
\&\c
\& has the same effect, as does the script command
.BR TARGET .

You may want to use this option if you are linking files with an unusual
binary format.  You can also use \c
.B \-b\c
\& to switch formats explicitly (when
linking object files of different formats), by including
\c
.B \-b \c
.I input-format\c
\&\c
\& before each group of object files in a
particular format.  

The default format is taken from the environment variable
.B GNUTARGET\c
\&.  You can also define the input
format from a script, using the command \c
.B TARGET\c
\&.

.TP
.B \-Bstatic 
This flag is accepted for command-line compatibility with the SunOS linker,
but has no effect on \c
.B ld\c
\&.

.TP
.BI "\-c " "commandfile"\c
\&
Directs \c
.B ld\c
\& to read link commands from the file
\c
.I commandfile\c
\&.  These commands will completely override \c
.B ld\c
\&'s
default link format (rather than adding to it); \c
.I commandfile\c
\& must
specify everything necessary to describe the target format.


You may also include a script of link commands directly in the command
line by bracketing it between `\|\c
.B {\c
\|' and `\|\c
.B }\c
\|' characters.

.TP
.B \-d 
.TP
.B \-dc
.TP
.B \-dp
These three options are equivalent; multiple forms are supported for
compatibility with other linkers.  Use any of them to make \c
.B ld\c
\&
assign space to common symbols even if a relocatable output file is
specified (\c
.B \-r\c
\&).  The script command
\c
.B FORCE_COMMON_ALLOCATION\c
\& has the same effect.

.TP
.BI "-defsym " "symbol"\c
\& = \c
.I expression\c
\&
Create a global symbol in the output file, containing the absolute
address given by \c
.I expression\c
\&.  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 \c
.I expression\c
\& in this
context: you may give a hexadecimal constant or the name of an existing
symbol, or use \c
.B +\c
\& and \c
.B \-\c
\& to add or subtract hexadecimal
constants or symbols.  If you need more elaborate expressions, consider
using the linker command language from a script.

.TP
.BI "-e " "entry"\c
\& 
Use \c
.I entry\c
\& as the explicit symbol for beginning execution of your
program, rather than the default entry point.  for a
discussion of defaults and other ways of specifying the
entry point.

.TP
.B \-F
.TP
.BI "-F" "format"\c
\&
Some older linkers used this option throughout a compilation toolchain
for specifying object-file format for both input and output object
files.  \c
.B ld\c
\&'s mechanisms (the \c
.B \-b\c
\& or \c
.B \-format\c
\& options
for input files, the \c
.B TARGET\c
\& command in linker scripts for output
files, the \c
.B GNUTARGET\c
\& environment variable) are more flexible, but
but it accepts (and ignores) the \c
.B \-F\c
\& option flag for compatibility
with scripts written to call the old linker.

.TP
.BI "\-format " "input\-format"\c
\&
Synonym for \c
.B \-b\c
\& \c
.I input\-format\c
\&.

.TP
.B \-g
Accepted, but ignored; provided for compatibility with other tools.

.TP
.BI "\-G " "size"\c
Set the maximum size of objects to be optimized using the GP register
to
.I size
under MIPS ECOFF.  Ignored for other object file formats.

.TP
.B \-\-help
Print a summary of the command-line options on the standard output and exit.
This option and
.B \-\-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.

.TP
.B \-i
Perform an incremental link (same as option \c
.B \-r\c
\&).

.TP
.BI "\-l" "ar"\c
\& 
Add an archive file \c
.I ar\c
\& to the list of files to link.  This 
option may be used any number of times.  \c
.B ld\c
\& will search its
path-list for occurrences of \c
.B lib\c
.I ar\c
\&.a\c
\& for every \c
.I ar\c
\&
specified.

.TP
.BI "\-L" "searchdir"\c
\& 
This command adds path \c
.I searchdir\c
\& to the list of paths that
\c
.B ld\c
\& will search for archive libraries.  You may use this option
any number of times.

The default set of paths searched (without being specified with
\c
.B \-L\c
\&) depends on what emulation mode \c
.B ld\c
\& is using, and in
some cases also on how it was configured.    The
paths can also be specified in a link script with the \c
.B SEARCH_DIR\c
\&
command.

.TP
.B \-M 
Print (to the standard output file) a link map\(em\&diagnostic information
about where symbols are mapped by \c
.B ld\c
\&, and information on global
common storage allocation.

.TP
.BI "\-Map " "mapfile"\c
Print to the file
.I mapfile
a link map\(em\&diagnostic information
about where symbols are mapped by \c
.B ld\c
\&, and information on global
common storage allocation.

.TP
.BI "\-m " "emulation"\c
Emulate the
.I emulation
linker.  You can list the available emulations with the
.I \-V
option.  This option overrides the compiled-in default, which is the
system for which you configured
.BR ld .

.TP
.B \-N 
specifies readable and writable \c
.B text\c
\& and \c
.B data\c
\& sections. If
the output format supports Unix style magic numbers, the output is
marked as \c
.B OMAGIC\c
\&.

When you use the `\|\c
.B \-N\c
\&\|' option, the linker does not page-align the
data segment.

.TP
.B \-n 
sets the text segment to be read only, and \c
.B NMAGIC\c
\& is written
if possible.

.TP
.B \-noinhibit\-exec
Normally, the linker will not produce an output file if it encounters
errors during the link process.  With this flag, you can specify that
you wish the output file retained even after non-fatal errors.

.TP
.BI "\-o " "output"\c
\&
.I output\c
\&
\c
.I output\c
\& is a name for the program produced by \c
.B ld\c
\&; if this
option is not specified, the name `\|\c
.B a.out\c
\|' is used by default.  The
script command \c
.B OUTPUT\c
\& can also specify the output file name.

.TP
.BI "\-oformat " "output\-format"\c
\&
Specify the binary format for the output object file.
You don't usually need to specify this, as
\c
.B ld\c
\& is configured to produce as a default output format the most
usual format on each machine.  \c
.I output-format\c
\& is a text string, the
name of a particular format supported by the BFD libraries.  
The script command
.B OUTPUT_FORMAT
can also specify the output format, but this option overrides it.

.TP
.BI "\-R " "filename"\c
\&
.I file\c
\&
Read symbol names and their addresses from \c
.I filename\c
\&, but do not
relocate it or include it in the output.  This allows your output file
to refer symbolically to absolute locations of memory defined in other
programs.

.TP
.B \-relax
An option with machine dependent effects.  Currently this option is only
supported on the H8/300.

On some platforms, use this option to perform global optimizations that
become possible when the linker resolves addressing in your program, such
as relaxing address modes and synthesizing new instructions in the
output object file.  

On platforms where this is not supported, `\|\c
.B \-relax\c
\&\|' is accepted, but has no effect.

.TP
.B \-r 
Generates relocatable output\(em\&i.e., generate an output file that can in
turn serve as input to \c
.B ld\c
\&.  This is often called \c
.I partial
linking\c
\&.  As a side effect, in environments that support standard Unix
magic numbers, this option also sets the output file's magic number to
\c
.B OMAGIC\c
\&.
If this option is not specified, an absolute file is produced.  When
linking C++ programs, this option \c
.I will not\c
\& resolve references to
constructors; \c
.B \-Ur\c
\& is an alternative. 

This option does the same as \c
.B \-i\c
\&.

.TP
.B \-S 
Omits debugger symbol information (but not all symbols) from the output file.

.TP
.B \-s 
Omits all symbol information from the output file.

.TP
.B \-sort\-common
Normally, when
.B ld
places the global common symbols in the appropriate output sections,
it sorts them by size.  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.  This option disables that sorting.

.TP
.BI "\-Tbss " "org"\c
.TP
.BI "\-Tdata " "org"\c
.TP
.BI "\-Ttext " "org"\c
Use \c
.I org\c
\& as the starting address for\(em\&respectively\(em\&the
\c
.B bss\c
\&, \c
.B data\c
\&, or the \c
.B text\c
\& segment of the output file.
\c
.I textorg\c
\& must be a hexadecimal integer.

.TP
.BI "\-T " "commandfile"\c
\&
.TP
.BI "\-T" "commandfile"\c
Equivalent to \c
.B \-c \c
.I commandfile\c
\&\c
\&; supported for compatibility with
other tools.  

.TP
.B \-t 
Prints names of input files as \c
.B ld\c
\& processes them.

.TP
.BI "\-u " "sym"
Forces \c
.I sym\c
\& to be entered in the output file as an undefined symbol.
This may, for example, trigger linking of additional modules from
standard libraries.  \c
.B \-u\c
\& may be repeated with different option
arguments to enter additional undefined symbols.

.TP
.B \-Ur 
For anything other than C++ programs, this option is equivalent to
\c
.B \-r\c
\&: it generates relocatable output\(em\&i.e., an output file that can in
turn serve as input to \c
.B ld\c
\&.  When linking C++ programs, \c
.B \-Ur\c
\&
\c
.I will\c
\& resolve references to constructors, unlike \c
.B \-r\c
\&.

.TP
.B \-V
Display the version number for \c
.B ld
and list the supported emulations.
Display which input files can and can not be opened.

.TP
.B \-v
Display the version number for \c
.B ld\c
\&.

.TP
.B \-\-version
Display the version number for \c
.B ld
and exit.

.TP
.B \-warn\-common
Warn when a common symbol is combined with another common symbol or with
a symbol definition.  Unix linkers allow this somewhat sloppy practice,
but linkers on some other operating systems do not.  This option allows
you to find potential problems from combining global symbols.

.TP
.B \-warn\-once
Only warn once for each undefined symbol, rather than once per module
which refers to it.

.TP
.B \-X 
If \c
.B \-s\c
\& or \c
.B \-S\c
\& is also specified, delete only local symbols
beginning with `\|\c
.B L\c
\|'.

.TP
.B \-x
If \c
.B \-s\c
\& or \c
.B \-S\c
\& is also specified, delete all local symbols,
not just those beginning with `\|\c
.B L\c
\|'.

.PP

.SH ENVIRONMENT
\c
You can change the behavior of
.B ld\c
\& with the environment variable \c
.B GNUTARGET\c
\&.

\c
.B GNUTARGET\c
\& determines the input-file object format if you don't
use \c
.B \-b\c
\& (or its synonym \c
.B \-format\c
\&).  Its value should be one
of the BFD names for an input format.  If there is no
\c
.B GNUTARGET\c
\& in the environment, \c
.B ld\c
\& uses the natural format
of the host. If \c
.B GNUTARGET\c
\& is set to \c
.B default\c
\& 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 flag 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.

.PP

.SH "SEE ALSO"

.BR objdump ( 1 )
.br
.br
.RB "`\|" ld "\|' and `\|" binutils "\|'"
entries in
.B info\c
.br
.I 
ld: the GNU linker\c
, Steve Chamberlain and Roland Pesch;
.I
The GNU Binary Utilities\c
, Roland H. Pesch.

.SH COPYING
Copyright (c) 1991, 1992 Free Software Foundation, Inc.
.PP
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.
.PP
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.
.PP
Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that this permission notice may be included in
translations approved by the Free Software Foundation instead of in
the original English.