Updated, amplified, and reorganized linker manual. Option-flag

[binutils-gdb.git] / ld / ld.texinfo

\input texinfo
@setfilename gld.info
@c $Id$
@syncodeindex ky cp
@ifinfo
This file documents the GNU linker GLD.

Copyright (C) 1991 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.

@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
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that the
section entitled ``GNU General Public License'' is included exactly as
in the original, and provided 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,
except that the section entitled ``GNU General Public License'' may be
included in a translation approved by the author instead of in the
original English.
@end ifinfo
@setchapternewpage odd
@settitle GLD, the GNU linker
@titlepage
@title{gld}
@subtitle{The GNU linker}
@sp 1
@subtitle Second Edition---@code{gld} version 2.0
@subtitle April 1991
@author {Steve Chamberlain, Roland Pesch}
@author {Cygnus Support}
@page

@tex
\def\$#1${{#1}}  % Kluge: collect RCS revision info without $...$
\xdef\manvers{\$Revision$}  % For use in headers, footers too
{\parskip=0pt
\hfill Cygnus Support\par
\hfill {\it GLD, the GNU linker}, \manvers\par
\hfill \TeX{}info \texinfoversion\par
\hfill steve\@cygnus.com, pesch\@cygnus.com\par
}
\global\parindent=0pt % Steve likes it this way.
@end tex

@vskip 0pt plus 1filll
Copyright @copyright{} 1991 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
@c FIXME: Talk about importance of *order* of args, cmds to linker!

@node Top,,,
@ifinfo
This file documents the GNU linker gld.
@end ifinfo

@node Overview,,,
@chapter Overview

@code{gld} 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 @code{gld}.

@code{gld} 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 @code{gld} uses the general purpose @code{bfd} libraries
to operate on object files. This allows @code{gld} 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.

When linking formats with equivalent representations of debugging
information (typically variations on one format), @code{gld} maintains
all debugging information.

@node Invocation,,,
@chapter Command line options

@c FIXME: -D, -N, -z, -f from older GNU linker, but not currently in new;
@c FIXME...steve is currently thinking about whether to add them.  Maybe
@c FIXME...remove from document.
@example
gld [-o @var{output} ] @var{objfiles}@dots{}
    [ -A@var{architecture} ]  [ -b @var{output-format} ]  [ -Bstatic ]  
    [ -c @var{commandfile} ]  [ -D @var{datasize} ]  
    [ -d | -dc | -dp ]  [ -defsym @var{symbol} = @var{expression} ]
    [ -e @var{entry} ]  [ -f @var{fill} ]  [ -F ]  [ -F @var{format} ]
    [ -format @var{output-format} ]  [ -g ]  [ -i ]
    [ -l@var{ar} ]  [ -L@var{searchdir} ]  [ -M | -m ]  
    [ -N | -n | -z ]  [ -noinhibit-exec ]  [ -R @var{filename} ]
    [ -r | -Ur ]  [ -S ]  [ -s ]  
    [ SCRIPT  @dots{}  ENDSCRIPT ]  [ SCRIPT  @dots{}  @@ ]
    [ -T @var{commandfile} ]  
    [ -Ttext @var{textorg} ]  [ -Tdata @var{dataorg} ]  [ -Tbss @var{bssorg} ]
    [ -t ]  [ -u @var{sym}]  [-v]  [ -X ]  [ -x ] 
@end example

This 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 @code{gld} 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
$ gld -o output /lib/crt0.o hello.o -lc
@end example
This tells @code{gld} to produce a file called @code{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.

The command-line options to @code{gld} 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---which may meaningfully be used more than once---
are @code{-L}, @code{-l}, and @code{-u}.
@c FIXME: probably some new opts can be repeated meaningfully too.

The list of object files to be linked together, shown as @var{objfiles},
may follow, precede, or be mixed in with command-line options; save that
an @var{objfiles} argument may not be placed between an option flag and
its argument.

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

@table @code
@item @var{objfiles}@dots{}
The object files @var{objfiles} to be linked; at least one must be specified.

@item -A@var{architecture}
In the current release of @code{gld}, this option is useful only for the
Intel 960 family of architectures.  In that context, the
@var{architecture} 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 selects archive libraries supporting the particular
architecture; its effect in this regard is similar to that of @code{-l}, save
that @code{-A}@var{architecture} triggers a two-level search; first for a
library with exactly the name you specify as @var{architecture}, and if
that fails, for a library named with the @code{-l} convention---i.e.,
@samp{lib@var{architecture}.a}.   

Future releases of @code{gld} may support similar functionality for
other architecture families.

@item -b @var{output-format}
Specify the desired output-file binary format.  You don't usually need
to specify this.  @code{gld} can determine
the format of @emph{input} files by inspection, and---in the most frequent
case, when all input files have the same format, @code{gld} selects the
same format for output files by default.  

You can use this option if you need to link a variety of object formats
together, or if you wish to force a different output format even though
you have homogeneous input files.

@var{output-format} is a text string, the name of a particular format
supported by the BFD libraries.  @xref{BFD}.

@code{-format @var{output-format}} has the same effect.

@item -Bstatic 
This flag is accepted for command-line compatibility with the SunOS linker,
but has no effect on @code{gld}.

@item -c @var{commandfile}
Directs @code{gld} to read link commands from the file
@var{commandfile}.  These commands will override @code{gld}'s
default link format in its entirety; @var{commandfile} must specify
everything necessary to specify the target format.  @xref{Commands}.

You may also include a script of link commands directly in the command
line by using the @code{SCRIPT} @dots{} @code{ENDSCRIPT} keywords.

@c FIXME: -D in older GNU linker, not necessarily in new
@item -D @var{datasize}
Use this option to specify a target size for the @code{data} segment of
your linked program.  The option is only obeyed if @var{datasize} is
larger than the natural size of the program's @code{data} segment.

@var{datasize} must be an integer specified in hexadecimal.

@code{ld} will simply increase the size of the @code{data} segment,
padding the created gap with zeros (or a fill pattern specified with
@samp{-f}, or using the command language), and reduce the size of the
@code{bss} segment by the same amount.
@c FIXME: double-check this w/Steve.  Open questions: order? Does it
@c FIXME...matter whether -f before or after -D?  What about -c relative
@c FIXME...position? fill cmd in default script? Apparently
@c FIXME...can have multiple fill patterns; which used here?


@item -d 
@itemx -dc
@itemx -dp
These three options are equivalent; multiple forms are supported for
compatibility with other linkers.  Any of them options will force
@code{ld} to assign space to common symbols even if a relocatable output
file is specified (@code{-r}).

@item -defsym @var{symbol} = @var{expression}
Create a global symbol, in the output file, set to the absolute address
given by @var{expression}.  A limited form of arithmetic is supported
for the @var{expression} in this context: you may give a hexadecimal
constant, or use @code{+} and @code{-} to add or subtract hexacedimal
constants.  If you need more elaborate expressions, consider using the
linker command language from a script.

@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.

@c FIXME: -f in older GNU linker, not necessarily in new
@item -f @var{fill}
Sets the default fill pattern for ``holes'' in the output file to
the lowest two bytes of the expression specified.

@item -F
@itemx -F{format}
Some older linkers required the specification of object-file format,
even when all input files were homogeneous, and used this option for
that purpose.  @code{gld} doesn't usually require this information---it
automatically recognizes input-file object format---but it accepts the
option flag for compatibility with old scripts.  

@item -format @var{output-format}
Synonym for @code{-b} @var{output-format}.

@item -g
Accepted, but ignored; provided for compatibility with other tools.

@item -i
Produce an incremental link (same as option @code{-r}).

@item -l@var{ar} 
Add an archive file @var{ar} 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{ar}
specified.

@c FIXME: -l also has a side effect of using the "c++ demangler" if we happen
@c FIXME...to specify -llibg++.  Document?  pesch@@cygnus.com, 24jan91

@item -L@var{searchdir} 
This command adds path @var{searchdir} to the list of paths that
@code{gld} will search for archive libraries.  You may use this option
any number of times.

@c Should we make any attempt to list the standard paths searched
@c without listing?  When hacking on a new system I often want to know
@c this, but this may not be the place... it's not constant across
@c systems, of course, which is what makes it interesting.
@c pesch@@cygnus.com, 24jan91.

@item -M 
@itemx -m
Print (to the standard output file) a link map---diagnostic information
about where symbols are mapped by @code{ld}, and information on global
common storage allocation.

@c FIXME: -N in older GNU linker, not necessarily in new
@item -N 
specifies readable and writable @code{text} and @code{data} sections. If
the output format supports Unix style magic numbers, the output is
marked as @code{OMAGIC}.

@item -n 
sets the text segment to be read only, and @code{NMAGIC} is written
if possible.

@item -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 for even after non-fatal errors.

@item -o @var{output}
@var{output} is a name for the program produced by @code{ld}; if this
option is not specified, the name @samp{a.out} is used by default.

@item -R @var{filename}
Read symbol names and their addresses from @var{filename}, but do not
relocate it or include it in the output.  This allows your output file
to refer symbolically to absolute locations of memory defined in other
programs.
@c FIXME: -R accurate?  Motivation?  Kernel memory, shared mem?

@item -r 
@cindex partial link
Generates relocatable output---i.e., generate an output file that can in
turn serve as input to @code{gld}.  This is often called @dfn{partial
linking}.  As a side effect, this option also sets the output file's
magic number to @code{OMAGIC}; see @samp{-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;
@samp{-Ur} is an alternative.

@item -S 
Omits debugger symbol information (but not all symbols) from the output file.

@item -s 
Omits all symbol information from the output file.

@item SCRIPT  @dots  @@
@itemx SCRIPT  @dots  ENDSCRIPT
You can, if you wish, include a script of linker commands directly in
the command line instead of referring to it via an input file.  When the
keyword @code{SCRIPT} occurs on the command line, the linker switches to
interpreting the command language until the end of the list of commands
is reached---flagged with either an at sign @samp{@@} or with the
keyword @code{ENDSCRIPT}.  Other command-line options will not be
recognized while parsing the script.  @xref{Commands} for a description
of the command language.

@item -Tbss @var{bssorg}
@itemx -Tdata @var{dataorg}
@itemx -Ttext @var{textorg}
Use @var{textorg} as the starting address for---respectively---the
@code{bss}, @code{data}, or the @code{text} segment of the output file.
@var{textorg} must be a hexadecimal integer.

@item -T @var{commandfile}
@itemx -T@var{commandfile}
Equivalent to @code{-c @var{commandfile}}; supported for compatibility with
other tools.  

@item -t 
Prints names of input files as @code{ld} processes them.

@item -u @var{sym}
Forces @var{sym} to be entered in the output file as an undefined symbol.
This may, for example, trigger linking of additional modules from
standard libraries.  @code{-u} may be repeated with different option
arguments to enter additional undefined symbols. This option is equivalent
to the @code{EXTERN} linker command.

@item -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{gld}.  When linking C++ programs, @samp{-Ur}
@emph{will} resolve references to constructors, unlike @samp{-r}.

@item -v
@cindex version
@cindex verbose
``Verbose'' switch: display informative messages, including the version
numbers for @code{gld} and BFD, information on files opened, and BFD
subroutine calls.

@item -X 
If @code{-s} or @code{-S} is also specified, delete only local symbols
beginning with @samp{L}.

@item -x
If @code{-s} or @code{-S} is also specified, delete all local symbols,
not just those beginning with @samp{L}.

@c FIXME: -z in older GNU linker, not necessarily in new
@item -z
Specifies a read-only, demand pageable, and shared @code{text} segment.
If the output format supports Unix-style magic numbers, @code{-z} also
marks the output as @code{ZMAGIC}, the default.

@c FIXME: why is following here?.  Is it useful to say '-z -r' for
@c FIXME...instance, or is this just a ref to other ways of setting
@c FIXME...magic no?
Specifying a relocatable output file (@code{-r}) will also set the magic
number to @code{OMAGIC}.

See description of @samp{-N}.

@end table

@node Commands,,,
@chapter Command Language
@c FIXME: is this a good place to talk about LDEMULATION env var?
@c FIXME...Apparently some commands "subtly different" depending on
@c FIXME...whether this set to eg "link960", "gld960", "gld".  What is
@c FIXME...full set of possibilities, what is default?  Config-dep?


The command language allows explicit control over the link process,
allowing complete specification of the mapping between the linker's
input files and its output.  This includes:
@itemize @bullet
@item input files 
@item file formats
@item output file format
@item addresses of sections
@item placement of common blocks
@end itemize

A command file may be supplied to the linker, either explicitly through
the @code{-c} 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.

@node Scripts,,,
@section Linker Scripts
The @code{gld} command language is a collection of statements; some are
simple keywords setting a particular flag, some are used to select and
group input files or name output files; and two particular statement
types have a fundamental and pervasive impact on the linking process.

The most fundamental command of the @code{gld} 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; if it is not present,
sufficient memory is assumed to be available in a contiguous block for
all output.  @xref{MEMORY}.

@node Expressions,,,
@section Expressions
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 Global variables may be referenced, defined and created.
@item Built in functions may be called.
@end itemize

@node Integers,,,
@subsection Integers
An octal integer is @samp{0} followed by zero or more of the octal
digits (@samp{01234567}).
@example
@end example

A decimal integer starts with a non-zero digit followed by zero or
more digits (@samp{0123456789}).
@example
_as_octal = 0157255;
@end example

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

Decimal integers have the usual values.  To denote a negative integer, use
the prefix operator @samp{-}; @pxref{Operators}.
@example
_as_decimal = 57005;
_as_neg = -57005;
@end example

Additionally the suffixes @code{K} and @code{M} may be used to scale a
constant by 
@tex
${\rm 1024}$ or ${\rm 1024}^2$
@end tex
@ifinfo
1024 or 1024*1024
@end ifinfo
respectively. For example, the following all refer to the same quantity:@refill

@example
        _4k_1 = 4K;
        _4k_2 = 4096;
        _4k_3 = 0x1000;
@end example

@node Symbols,,,
@subsection Symbol Names
Unless quoted, symbol names start with a letter, underscore, point or
minus sign 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

@subsection The Location Counter
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.
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)
                . -= 32;
                file4(.text)
                @} = 0x1234;
        @}
@end example
In the previous example, @code{file1} is located at the beginning of
the output section, then there is a 1000 byte gap, filled with 0x1234.
Then @code{file2} appears, also with a 1000 byte gap following before
@code{file3} is loaded. Then the first 32 bytes of @code{file4} are
placed over the last 32 bytes of @code{file3}.

@node Operators,,,
@subsection Operators
The linker recognizes  the standard C set of arithmetic operators, with
the standard bindings and precedence levels:
@c FIXME: distinguish somehow between prefix, infix in operator table!
@c FIXME: is it fair to include assignments below?  Don't they
@c FIXME...require trailing ; when no other exprs do?
@ifinfo
@example
precedence      associativity   Operators
(highest)
1               left            !  -  ~
2               left            *  /  %
3               left            +  -
4               left            >>  <<
5               left            ==  !=  >  <  <=  >=
6               left            &
7               left            |
8               left            &&
9               left            ||
10              right           ? :
11              right           &=  +=  -=  *=  /=
(lowest)
@end example
@end ifinfo
@c FIXME: simplify, debug TeX form of this table!
@tex

\vbox{\offinterlineskip
\hrule
\halign
{\vrule#&\hfil#\hfil&\vrule#&\hfil#\hfil&\vrule#&\hfil#\hfil&\vrule#\cr
height2pt&&&&&\cr
&Level&&  associativity  &&Operators&\cr
height2pt&&&&&\cr
\noalign{\hrule}
height2pt&&&&&\cr
&highest&&&&&\cr
&1&&left&&$ ! -         ~$&\cr
height2pt&&&&&\cr
&2&&left&&*        /        \%&\cr
height2pt&&&&&\cr
&3&&left&&+        -&\cr
height2pt&&&&&\cr
&4&&left&&$>> <<$&\cr
height2pt&&&&&\cr
&5&&left&&$==        !=        > < <= >=$&\cr
height2pt&&&&&\cr
&6&&left&&\&&\cr
height2pt&&&&&\cr
&7&&left&&|&\cr
height2pt&&&&&\cr
&8&&left&&{\&\&}&\cr
height2pt&&&&&\cr
&9&&left&&||&\cr
height2pt&&&&&\cr
&10&&right&&? :&\cr
height2pt&&&&&\cr
&11&&right&&$${\&=        +=        -=        *=        /=}&\cr
&lowest&&&&&\cr
height2pt&&&&&\cr}
\hrule}
@end tex

@node Evaluation,,,
@subsection Evaluation

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

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} ;
@end table

Two things distinguish assignment from other operators in @code{gld}
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.
@item A trailing semicolon is required at the end of an assignment
statement. 
@end itemize

Assignment statements may appear:
@itemize @bullet
@item as commands in their own right in a @code{gld} 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}).

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 relocateable 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 @code{SECTION} specification 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 @code{SECTION} specification 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 @code{SECTION} specification by using the absolute assignment
function @code{ABSOLUTE} For example, to create an absolute symbol
whose address is the last byte of the output section @code{.data}:
@example
.data : 
        @{
                *(.data)
                _edata = ABSOLUTE(.) ;
        @} 
@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, attempting to use a
script like the following
@example
SECTIONS @{
      text 9+this_isnt_constant: 
                @{ @dots{}
                @}
        @}
@end example
will get the error message ``@code{Non constant expression for initial
address}''. 

@node Builtins,,,
@subsection Built in Functions
The command language provides built in functions for use in
expressions in link scripts.
@itemize @bullet 
@item @code{ALIGN(@var{exp})}
returns 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 @samp{(. + @var{exp} -1)
& ~(@var{exp}-1)}.  As an example, to align the output @code{.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:
@example
        .data ALIGN(0x2000) :@{
                *(.data)
                variable = ALIGN(0x8000);
        @}
@end example

@item @code{ADDR(@var{section name})}
returns the absolute address of the named section.  Your script must
previously have defined the location of that section. In the following
example the @code{symbol_1} and @code{symbol_2} are assigned identical
values:
@example
        .output1:
                @{ 
                start_of_output_1 $= .;
                ... 
                @}
        .output:
                @{
                symbol_1 = ADDR(.output1);
                symbol_2 = start_of_output_1;
                @}
@end example

@item @code{SIZEOF(@var{section name})}
returns the size in bytes of the named section, if the section has
been allocated.  In the following example the @code{symbol_1} and
@code{symbol_2} are assigned identical values:
@example
        .output @{
                .start = . ;
                ...
                .end = .;
                @}
        symbol_1 = .end - .start;
        symbol_2 = SIZEOF(.output);
@end example

@item @code{DEFINED(@var{symbol name})}
Returns 1 if the symbol is in the linker global symbol table and is
defined, otherwise it returns 0. For example, this command-file fragment
shows how to set a global symbol @code{begin} to the first location in
the @code{.text} section---but only if no symbol called @code{begin}
existed:
@example
        .text: @{
                begin = DEFINED(begin) ? begin : . ;
                ...
        @}
@end example
@end itemize

@node MEMORY,,,
@section MEMORY Command
The linker's default configuration permits allocation of all memory.
You can override this 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.  

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}
       .
       .
       .
      @}
@end example
@table @code
@item @var{name}
is a name used internally by the linker to refer to the region. Any
symbol name may be used.  The region names are stored in a separate
name space, and will not conflict with symbols, filenames or section
names.  Use distinct names to specify multiple regions.
@item (@var{attr})
is an optional list of attributes, parsed for compatibility with the
AT@&T linker but ignored by the both the AT@&T and the GNU linker.
Valid attribute lists must be made up of the characters ``@code{RWXL}''.
If you omit the attribute list, you may omit the parentheses around it
as well. 
@item @var{origin}
is the start address of the region in physical memory.  It is expressed as
an expression, which 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}
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 256k, and the other starting at
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.  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
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 @samp{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.

@node Section Definition,,,
@subsection Section Definitions
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 can all be specified.  Most of
these specifications are optional; the simplest form of a section
definition is
@example
SECTIONS
@{
.
.
.
@var{secname} : @{
                 @var{contents}
                @}
.
.
.
@}
@end example
@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
(in the case of @code{a.out}, @code{.text}, @code{.data} or @code{.bss}). If
the output format supports any number of sections, but with numbers and
not names (in the case of IEEE), 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{gld} symbol
name syntax must be quoted.

@node Section Contents,,,
@subsection Section Contents
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 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}( @var{section} )
@itemx @var{filename}( @var{section}, @var{section}, @dots{} )
@itemx @var{filename}( @var{section} @var{section} @dots{} )
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{filename}
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.  Since multiple statements may be
present in the contents of a section definition, you can specify a list
of particular files by name:
@example
.data: @{ afile.o bfile.o cfile.o @}
@end example

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{section})
@itemx * (@var{section}, @var{section}, @dots{})
@itemx * (@var{section} @var{section} @dots{})
Instead of explicitly naming particular input files in a link control
script, you can refer to @emph{all} files from the @code{gld} command
line: use @samp{*} instead of a particular filename before the
parenthesized input-file section list.  

For example, to copy sections @code{1} through @code{4} from a 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
SECTION @{
        .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{} ]
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 ]
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{gld} 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 its 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
instructs the linker to create a symbol for each input file and place it
into the current section, set with 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);
                @}
        .
        .
        .
        @}
@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{gld -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 FORCE_COMMON_ALLOCATION
@c FIXME!  I don't know what this does.

@item @var{symbol} = @var{expression} ;
@itemx @var{symbol} @var{f}= @var{expression} ;
@var{symbol} is any symbol name (@pxref{Symbols}).  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 ;
.
.
.
  .data: { @dots{} rel = 14 ; @dots{} }
  abs2 = 14 + ADDR(.data);
.
.
.
}
@end example
@c FIXME!  Try above example!
@noindent
@code{abs} and @var{rel} do not have the same value; @code{rel} has the
same value as @code{abs2}.

``@var{f}='' here refers to any of the operators @code{&=  +=  -=  *=
/=} which combine arithmetic and assignment.

@item BYTE(@var{expression})
@itemx SHORT(@var{expression})
@itemx LONG(@var{expression})
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.  Multiple-byte quantities are
represented in whatever byte order is appropriate for the output file
format (@pxref{BFD}). 

@item FILL(@var{expression})
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
Here is the full syntax of a section definition, including all the
optional portions:

@example
SECTIONS
@{
.
.
.
@var{secname} @var{start} BLOCK(@var{align}) : @var{contents} =@var{fill} >@var{region}
.
.
.
@}
@end example

@var{secname} and @var{contents} are required.  @xref{Section
Definition}, and @pxref{Section Contents} for the details of
@var{contents}.  @var{start}, @code{BLOCK(@var{align)}},
@code{=@var{fill}}, and @code{>@var{region}} are all optional.

@table @code
@item @var{start} 
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 @{
        .
        .
        .
        output 0x40000000: @{
               @dots{}
          @}
        .
        .
        .
@}
@end example

@item BLOCK(@var{align})
@c FIXME!  Fill in BLOCK(align) description

@item =@var{fill}
You may use any expression to specify @var{fill}.  Including
@code{=@var{fill}} in a section definition specifies the initial fill
value for that section. 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}
@c FIXME! Fill in >region description

@end table

@node Entry Point,,,
@section The Entry Point
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.

@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 @code{-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 also generate an entry point with an assignment statement:
if no symbol @code{start} is  defined within your input files, you can
simply assign 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
@itemx NOFLOAT
Declare to the linker whether or not floating point support is
available.  The default assumption is @code{NOFLOAT}.
@c FIXME: So what?  What does it do once it knows FLOAT or NOFLOAT?

@item HLL ( @var{file}, @var{file}, @dots{} )
@itemx HLL ( @var{file} @var{file} @dots{} )

@item INPUT ( @var{file}, @var{file}, @dots{} )
@itemx INPUT ( @var{file} @var{file} @dots{} )

@item MAP ( @var{name} )

@item OUTPUT ( @var{filename} )

@item SEARCH_DIR ( @var{pathname} )

@item STARTUP ( @var{name} )

@item SYSLIB ( @var{file}, @var{file}, @dots{} )
@itemx SYSLIB ( @var{file} @var{file} @dots{} )

@item TARGET ( @var{format} )

@end table

@node BFD,,,
@chapter BFD

The linker accesses object and archive files using the @code{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 @code{bfd} back end and adding it to the library.

Formats currently supported:
@itemize @bullet
@item 
Sun3 68k @code{a.out}
@item 
IEEE-695 68k Object Module Format
@item 
Oasys 68k Binary Relocatable Object File Format
@item 
Sun4 sparc @code{a.out}
@item 
88k bcs coff
@item 
i960 coff little endian
@item 
i960 coff big endian
@item 
i960 @code{b.out} little endian
@item 
i960 @code{b.out} big endian
@end itemize

As with most implementations, @code{bfd} is a compromise between
several conflicting requirements. The major factor influencing
@code{bfd} design was efficiency, any time used converting between
formats is time which would not have been spent had @code{bfd} not
been involved. This is partly offset by abstraction payback; since
@code{bfd} simplifies applications and back ends, more time and care
may be spent optimizing algorithms for a greater speed.

One minor artifact of the @code{bfd} solution which the
user should be aware of is the potential for information loss.
There are two places where useful information can be lost using the 
@code{bfd} mechanism; during conversion and during output. @xref{BFD
information loss}.

@node BFD outline,,,
@section How it works: an outline of BFD
When an object file is opened, @code{bfd} subroutines automatically
determine the format of the input object file, and 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
@code{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 @code{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 the memory pointer to the relevant @code{bfd}
back end routine which reads and converts the table into a canonical
form.  The linker then operates upon the common form. When the link is
finished and the linker writes the symbol table of the output file,
another @code{bfd} back end routine is called which takes the newly
created symbol table and converts it into the chosen output format.

@node BFD information loss,,,
@section Information Loss
@emph{Information can be lost during output.} The output formats
supported by @code{bfd} do not provide identical facilities, and
information which may be described in one form has nowhere to go in
another format. One example of this is alignment information in
@code{b.out}. There is nowhere in an @code{a.out} format file to store
alignment information on the contained data, so when a file is linked
from @code{b.out} and an @code{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 (eg
@code{a.out}) or has sections without names (eg 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 command
language.

@emph{Information lost during canonicalization.}  The @code{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 @code{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 using the linker to read one
format and write another. Each @code{bfd} back end is responsible for
maintaining as much data as possible, and the internal @code{bfd}
canonical form has structures which are opaque to the @code{bfd} core,
and exported only to the back ends. When a file is read in one format,
the canonical form is generated for @code{bfd} and the linker. At the
same time, the back end saves away any information which may otherwise
be lost. If the data is then written back to the same back end, the back
end routine will be able to use the canonical form provided by the
@code{bfd} core as well as the information it prepared earlier.  Since
there is a great deal of commonality between back ends, this mechanism
is very useful. There is no information lost for this reason when
linking big endian COFF to little endian COFF, or from @code{a.out} to
@code{b.out}.  When a mixture of formats is linked, the information is
only lost from the files whose format differs from the destination.

@node Mechanism,,,
@section Mechanism 
The greatest potential for loss of information is when there is least
overlap between the information provided by the source format, that
stored by the canonical format and the information needed by the
destination format. A brief description of the canonical form may help
you appreciate what kinds of data you can count on preserving across
conversions.

@table @emph
@item files
Information on target machine architecture, particular implementation
and format type are stored on a per-file basis. Other information
includes a demand pageable bit and a write protected bit.  Note that
information like Unix magic numbers is not stored here---only the magic
numbers' meaning, so a @code{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
both big- and little-endian object files may be linked with one another.

@item sections
Each section in the input file contains the name of the section, the
original address in the object file, various flags, size and alignment
information and pointers into other @code{bfd} data structures.

@item symbols
Each symbol contains a pointer to the object file which originally
defined it, its name, value and various flag bits. When a symbol table
is read in, all symbols are relocated to make them relative to the base
of the section where they were defined, so that each symbol points to
its containing section. Each symbol also has a varying amount of hidden
data to contain private data for the BFD back end. Since the symbol
points to the original file, the private data format for that symbol is
accessible.  @code{gld} 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 @code{a.out} type information is
stored in the symbol table as long symbol names. This information would
be useless to most COFF debuggers and may be thrown away with
appropriate command line switches. (The GNU debugger @code{gdb} does
support @code{a.out} style debugging information in COFF).

There is one word of type information within the symbol, so if the
format supports symbol type information within symbols - (eg COFF,
IEEE, Oasys) and the type is simple enough to fit within one word
(nearly everything but aggregates) the information will be preserved.

@item relocation level
@c FIXME: I don't understand "relocation record" from this so I can't
@c FIXME...improve the explanation to make it clear...
Each canonical relocation record contains a pointer to the symbol to
relocate to, the offset of the data to relocate, the section the data
is in and a pointer to a relocation type descriptor. Relocation is
performed effectively by message passing through the relocation type
descriptor and symbol pointer. It allows relocations to be performed
on output data using a relocation method only available in one of the
input formats. For instance, Oasys provides a byte relocation format.
A relocation record requesting this relocation type would point
indirectly to a routine to perform this, so the relocation may be
performed on a byte being written to a COFF file, even though 68k COFF
has no such relocation type.

@item line numbers
Line numbers have to be relocated along with the symbol information.
Each symbol with an associated list of line number records points to the
first record of the list.  The head of a line number list consists of a
pointer to the symbol, which allows divination of the address of the
function whose line number is being described. The rest of the list is
made up of pairs: offsets into the section and line numbers. Any format
which can simply derive this information can pass it successfully
between formats (COFF, IEEE and Oasys).
@end table

@contents
@bye