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GNU Info File | 1996-07-15 | 47.2 KB | 1,196 lines

This is Info file ld.info, produced by Makeinfo-1.55 from the input
file ./ld.texinfo.

START-INFO-DIR-ENTRY
* Ld: (ld).                       The GNU linker.
END-INFO-DIR-ENTRY

   This file documents the GNU linker LD.

   Copyright (C) 1991, 92, 93, 94, 95, 1996 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.


File: ld.info,  Node: Arithmetic Functions,  Next: Semicolons,  Prev: Assignment,  Up: Expressions

Arithmetic Functions
--------------------

   The command language includes a number of built-in functions for use
in link script expressions.
`ABSOLUTE(EXP)'
     Return the absolute (non-relocatable, as opposed to non-negative)
     value of the expression EXP.  Primarily useful to assign an
     absolute value to a symbol within a section definition, where
     symbol values are normally section-relative.

`ADDR(SECTION)'
     Return the absolute address of the named SECTION.  Your script must
     previously have defined the location of that section. In the
     following example, `symbol_1' and `symbol_2' are assigned identical
     values:
          SECTIONS{ ...
            .output1 :
              {
              start_of_output_1 = ABSOLUTE(.);
              ...
              }
            .output :
              {
              symbol_1 = ADDR(.output1);
              symbol_2 = start_of_output_1;
              }
          ... }

`ALIGN(EXP)'
     Return the result of the current location counter (`.') aligned to
     the next EXP boundary.  EXP must be an expression whose value is a
     power of two.  This is equivalent to
          (. + EXP - 1) & ~(EXP - 1)

     `ALIGN' doesn't change the value of the location counter--it just
     does arithmetic on it.  As an example, to align the output `.data'
     section to the next `0x2000' byte boundary after the preceding
     section and to set a variable within the section to the next
     `0x8000' boundary after the input sections:
          SECTIONS{ ...
            .data ALIGN(0x2000): {
              *(.data)
              variable = ALIGN(0x8000);
            }
          ... }

     The first use of `ALIGN' in this example specifies the location of
     a section because it is used as the optional START attribute of a
     section definition (*note Section Options::.).  The second use
     simply defines the value of a variable.

     The built-in `NEXT' is closely related to `ALIGN'.

`DEFINED(SYMBOL)'
     Return 1 if SYMBOL is in the linker global symbol table and is
     defined, otherwise return 0.  You can use this function to provide
     default values for symbols.  For example, the following
     command-file fragment shows how to set a global symbol `begin' to
     the first location in the `.text' section--but if a symbol called
     `begin' already existed, its value is preserved:

          SECTIONS{ ...
            .text : {
              begin = DEFINED(begin) ? begin : . ;
              ...
            }
          ... }

`NEXT(EXP)'
     Return the next unallocated address that is a multiple of EXP.
     This function is closely related to `ALIGN(EXP)'; unless you use
     the `MEMORY' command to define discontinuous memory for the output
     file, the two functions are equivalent.

`SIZEOF(SECTION)'
     Return the size in bytes of the named SECTION, if that section has
     been allocated.  In the following example, `symbol_1' and
     `symbol_2' are assigned identical values:
          SECTIONS{ ...
            .output {
              .start = . ;
              ...
              .end = . ;
              }
            symbol_1 = .end - .start ;
            symbol_2 = SIZEOF(.output);
          ... }

`SIZEOF_HEADERS'
`sizeof_headers'
     Return the size in bytes of the output file's headers.  You can
     use this number as the start address of the first section, if you
     choose, to facilitate paging.


File: ld.info,  Node: Semicolons,  Prev: Arithmetic Functions,  Up: Expressions

Semicolons
----------

   Semicolons (";") are required in the following places.  In all other
places they can appear for aesthetic reasons but are otherwise ignored.

`Assignment'
     Semicolons must appear at the end of assignment expressions.
     *Note Assignment::

`PHDRS'
     Semicolons must appear at the end of a `PHDRS' statement.  *Note
     PHDRS::


File: ld.info,  Node: MEMORY,  Next: SECTIONS,  Prev: Expressions,  Up: Commands

Memory Layout
=============

   The linker's default configuration permits allocation of all
available memory.  You can override this configuration by using the
`MEMORY' command.  The `MEMORY' command describes the location and size
of blocks of memory in the target.  By using it carefully, you can
describe which memory regions may be used by the linker, and which
memory regions it must avoid.  The linker does not shuffle sections to
fit into the available regions, but does move the requested sections
into the correct regions and issue errors when the regions become too
full.

   A command file may contain at most one use of the `MEMORY' command;
however, you can define as many blocks of memory within it as you wish.
The syntax is:

     MEMORY
       {
         NAME (ATTR) : ORIGIN = ORIGIN, LENGTH = LEN
         ...
       }

`NAME'
     is a name used internally by the linker to refer to the region. Any
     symbol name may be used.  The region names are stored in a separate
     name space, and will not conflict with symbols, file names or
     section names.  Use distinct names to specify multiple regions.

`(ATTR)'
     is an optional list of attributes, permitted for compatibility
     with the AT&T linker but not used by `ld' beyond checking that the
     attribute list is valid.  Valid attribute lists must be made up of
     the characters "`LIRWX'".  If you omit the attribute list, you may
     omit the parentheses around it as well.

`ORIGIN'
     is the start address of the region in physical memory.  It is an
     expression that must evaluate to a constant before memory
     allocation is performed. The keyword `ORIGIN' may be abbreviated
     to `org' or `o' (but not, for example, `ORG').

`LEN'
     is the size in bytes of the region (an expression).  The keyword
     `LENGTH' may be abbreviated to `len' or `l'.

   For example, to specify that memory has two regions available for
allocation--one starting at 0 for 256 kilobytes, and the other starting
at `0x40000000' for four megabytes:

     MEMORY
       {
       rom : ORIGIN = 0, LENGTH = 256K
       ram : org = 0x40000000, l = 4M
       }

   Once you have defined a region of memory named MEM, you can direct
specific output sections there by using a command ending in `>MEM'
within the `SECTIONS' command (*note Section Options::.).  If the
combined output sections directed to a region are too big for the
region, the linker will issue an error message.


File: ld.info,  Node: SECTIONS,  Next: PHDRS,  Prev: MEMORY,  Up: Commands

Specifying Output Sections
==========================

   The `SECTIONS' command controls exactly where input sections are
placed into output sections, their order in the output file, and to
which output sections they are allocated.

   You may use at most one `SECTIONS' command in a script file, but you
can have as many statements within it as you wish.  Statements within
the `SECTIONS' command can do one of three things:

   * define the entry point;

   * assign a value to a symbol;

   * describe the placement of a named output section, and which input
     sections go into it.

   You can also use the first two operations--defining the entry point
and defining symbols--outside the `SECTIONS' command: *note Entry
Point::., and *Note Assignment::.  They are permitted here as well for
your convenience in reading the script, so that symbols and the entry
point can be defined at meaningful points in your output-file layout.

   If you do not use a `SECTIONS' command, the linker places each input
section into an identically named output section in the order that the
sections are first encountered in the input files.  If all input
sections are present in the first file, for example, the order of
sections in the output file will match the order in the first input
file.

* Menu:

* Section Definition::          Section Definitions
* Section Placement::           Section Placement
* Section Data Expressions::    Section Data Expressions
* Section Options::             Optional Section Attributes


File: ld.info,  Node: Section Definition,  Next: Section Placement,  Up: SECTIONS

Section Definitions
-------------------

   The most frequently used statement in the `SECTIONS' command is the
"section definition", which specifies the properties of an output
section: its location, alignment, contents, fill pattern, and target
memory region.  Most of these specifications are optional; the simplest
form of a section definition is
     SECTIONS { ...
       SECNAME : {
         CONTENTS
       }
     ... }

SECNAME is the name of the output section, and CONTENTS a specification
of what goes there--for example, a list of input files or sections of
input files (*note Section Placement::.).  As you might assume, the
whitespace shown is optional.  You do need the colon `:' and the braces
`{}', however.

   SECNAME must meet the constraints of your output format.  In formats
which only support a limited number of sections, such as `a.out', the
name must be one of the names supported by the format (`a.out', for
example, allows only `.text', `.data' or `.bss'). If the output format
supports any number of sections, but with numbers and not names (as is
the case for Oasys), the name should be supplied as a quoted numeric
string.  A section name may consist of any sequence of characters, but
any name which does not conform to the standard `ld' symbol name syntax
must be quoted.  *Note Symbol Names: Symbols.

   The special SECNAME `/DISCARD/' may be used to discard input
sections.  Any sections which are assigned to an output section named
`/DISCARD/' are not included in the final link output.

   The linker will not create output sections which do not have any
contents.  This is for convenience when referring to input sections that
may or may not exist.  For example,
     .foo { *(.foo) }
   will only create a `.foo' section in the output file if there is a
`.foo' section in at least one input file.


File: ld.info,  Node: Section Placement,  Next: Section Data Expressions,  Prev: Section Definition,  Up: SECTIONS

Section Placement
-----------------

   In a section definition, you can specify the contents of an output
section by listing particular input files, by listing particular
input-file sections, or by a combination of the two.  You can also place
arbitrary data in the section, and define symbols relative to the
beginning of the section.

   The CONTENTS of a section definition may include any of the
following kinds of statement.  You can include as many of these as you
like in a single section definition, separated from one another by
whitespace.

`FILENAME'
     You may simply name a particular input file to be placed in the
     current output section; *all* sections from that file are placed
     in the current section definition.  If the file name has already
     been mentioned in another section definition, with an explicit
     section name list, then only those sections which have not yet
     been allocated are used.

     To specify a list of particular files by name:
          .data : { afile.o bfile.o cfile.o }

     The example also illustrates that multiple statements can be
     included in the contents of a section definition, since each file
     name is a separate statement.

`FILENAME( SECTION )'
`FILENAME( SECTION , SECTION, ... )'
`FILENAME( SECTION SECTION ... )'
     You can name one or more sections from your input files, for
     insertion in the current output section.  If you wish to specify a
     list of input-file sections inside the parentheses, you may
     separate the section names by either commas or whitespace.

`* (SECTION)'
`* (SECTION, SECTION, ...)'
`* (SECTION SECTION ...)'
     Instead of explicitly naming particular input files in a link
     control script, you can refer to *all* files from the `ld' command
     line: use `*' instead of a particular file name before the
     parenthesized input-file section list.

     If you have already explicitly included some files by name, `*'
     refers to all *remaining* files--those whose places in the output
     file have not yet been defined.

     For example, to copy sections `1' through `4' from an Oasys file
     into the `.text' section of an `a.out' file, and sections `13' and
     `14' into the `.data' section:
          SECTIONS {
            .text :{
              *("1" "2" "3" "4")
            }

            .data :{
              *("13" "14")
            }
          }

     `[ SECTION ... ]' used to be accepted as an alternate way to
     specify named sections from all unallocated input files.  Because
     some operating systems (VMS) allow brackets in file names, that
     notation is no longer supported.

`FILENAME`( COMMON )''
`*( COMMON )'
     Specify where in your output file to place uninitialized data with
     this notation.  `*(COMMON)' by itself refers to all uninitialized
     data from all input files (so far as it is not yet allocated);
     FILENAME`(COMMON)' refers to uninitialized data from a particular
     file.  Both are special cases of the general mechanisms for
     specifying where to place input-file sections: `ld' permits you to
     refer to uninitialized data as if it were in an input-file section
     named `COMMON', regardless of the input file's format.

   For example, the following command script arranges the output file
into three consecutive sections, named `.text', `.data', and `.bss',
taking the input for each from the correspondingly named sections of
all the input files:

     SECTIONS {
       .text : { *(.text) }
       .data : { *(.data) }
       .bss :  { *(.bss)  *(COMMON) }
     }

   The following example reads all of the sections from file `all.o'
and places them at the start of output section `outputa' which starts
at location `0x10000'. All of section `.input1' from file `foo.o'
follows immediately, in the same output section.  All of section
`.input2' from `foo.o' goes into output section `outputb', followed by
section `.input1' from `foo1.o'.  All of the remaining `.input1' and
`.input2' sections from any files are written to output section
`outputc'.

     SECTIONS {
       outputa 0x10000 :
         {
         all.o
         foo.o (.input1)
         }
       outputb :
         {
         foo.o (.input2)
         foo1.o (.input1)
         }
       outputc :
         {
         *(.input1)
         *(.input2)
         }
     }


File: ld.info,  Node: Section Data Expressions,  Next: Section Options,  Prev: Section Placement,  Up: SECTIONS

Section Data Expressions
------------------------

   The foregoing statements arrange, in your output file, data
originating from your input files.  You can also place data directly in
an output section from the link command script.  Most of these
additional statements involve expressions (*note Expressions::.).
Although these statements are shown separately here for ease of
presentation, no such segregation is needed within a section definition
in the `SECTIONS' command; you can intermix them freely with any of the
statements we've just described.

`CREATE_OBJECT_SYMBOLS'
     Create a symbol for each input file in the current section, set to
     the address of the first byte of data written from that input
     file.  For instance, with `a.out' files it is conventional to have
     a symbol for each input file.  You can accomplish this by defining
     the output `.text' section as follows:
          SECTIONS {
            .text 0x2020 :
               {
              CREATE_OBJECT_SYMBOLS
              *(.text)
              _etext = ALIGN(0x2000);
              }
            ...
          }

     If `sample.ld' is a file containing this script, and `a.o', `b.o',
     `c.o', and `d.o' are four input files with contents like the
     following--
          /* a.c */

          afunction() { }
          int adata=1;
          int abss;

     `ld -M -T sample.ld a.o b.o c.o d.o' would create a map like this,
     containing symbols matching the object file names:
          00000000 A __DYNAMIC
          00004020 B _abss
          00004000 D _adata
          00002020 T _afunction
          00004024 B _bbss
          00004008 D _bdata
          00002038 T _bfunction
          00004028 B _cbss
          00004010 D _cdata
          00002050 T _cfunction
          0000402c B _dbss
          00004018 D _ddata
          00002068 T _dfunction
          00004020 D _edata
          00004030 B _end
          00004000 T _etext
          00002020 t a.o
          00002038 t b.o
          00002050 t c.o
          00002068 t d.o

`SYMBOL = EXPRESSION ;'
`SYMBOL F= EXPRESSION ;'
     SYMBOL is any symbol name (*note Symbols::.).  "F=" refers to any
     of the operators `&= += -= *= /=' which combine arithmetic and
     assignment.

     When you assign a value to a symbol within a particular section
     definition, the value is relative to the beginning of the section
     (*note Assignment::.).  If you write

          SECTIONS {
            abs = 14 ;
            ...
            .data : { ... rel = 14 ; ... }
            abs2 = 14 + ADDR(.data);
            ...
          }

     `abs' and `rel' do not have the same value; `rel' has the same
     value as `abs2'.

`BYTE(EXPRESSION)'
`SHORT(EXPRESSION)'
`LONG(EXPRESSION)'
`QUAD(EXPRESSION)'
     By including one of these four statements in a section definition,
     you can explicitly place one, two, four, or eight bytes
     (respectively) at the current address of that section.  `QUAD' is
     only supported when using a 64 bit host or target.

     Multiple-byte quantities are represented in whatever byte order is
     appropriate for the output file format (*note BFD::.).

`FILL(EXPRESSION)'
     Specify the "fill pattern" for the current section.  Any otherwise
     unspecified regions of memory within the section (for example,
     regions you skip over by assigning a new value to the location
     counter `.') are filled with the two least significant bytes from
     the EXPRESSION argument.  A `FILL' statement covers memory
     locations *after* the point it occurs in the section definition; by
     including more than one `FILL' statement, you can have different
     fill patterns in different parts of an output section.


File: ld.info,  Node: Section Options,  Prev: Section Data Expressions,  Up: SECTIONS

Optional Section Attributes
---------------------------

   Here is the full syntax of a section definition, including all the
optional portions:

     SECTIONS {
     ...
     SECNAME START BLOCK(ALIGN) (NOLOAD) : AT ( LDADR )
       { CONTENTS } >REGION :PHDR =FILL
     ...
     }

   SECNAME and CONTENTS are required.  *Note Section Definition::, and
*Note Section Placement::, for details on CONTENTS.  The remaining
elements--START, `BLOCK(ALIGN)', `(NOLOAD)', `AT ( LDADR )', `>REGION',
`:PHDR', and `=FILL'--are all optional.

`START'
     You can force the output section to be loaded at a specified
     address by specifying START immediately following the section name.
     sTART can be represented as any expression. The following example
     generates section OUTPUT at location `0x40000000':

          SECTIONS {
            ...
            output 0x40000000: {
              ...
              }
            ...
          }

`BLOCK(ALIGN)'
     You can include `BLOCK()' specification to advance the location
     counter `.' prior to the beginning of the section, so that the
     section will begin at the specified alignment.  ALIGN is an
     expression.

`(NOLOAD)'
     Use `(NOLOAD)' to prevent a section from being loaded into memory
     each time it is accessed.  For example, in the script sample
     below, the `ROM' segment is addressed at memory location `0' and
     does not need to be loaded into each object file:

          SECTIONS {
            ROM  0  (NOLOAD)  : { ... }
            ...
          }

`AT ( LDADR )'
     The expression LDADR that follows the `AT' keyword specifies the
     load address of the section.  The default (if you do not use the
     `AT' keyword) is to make the load address the same as the
     relocation address.  This feature is designed to make it easy to
     build a ROM image.  For example, this `SECTIONS' definition
     creates two output sections: one called `.text', which starts at
     `0x1000', and one called `.mdata', which is loaded at the end of
     the `.text' section even though its relocation address is
     `0x2000'.  The symbol `_data' is defined with the value `0x2000':

          SECTIONS
            {
            .text 0x1000 : { *(.text) _etext = . ; }
            .mdata 0x2000 :
              AT ( ADDR(.text) + SIZEOF ( .text ) )
              { _data = . ; *(.data); _edata = . ;  }
            .bss 0x3000 :
              { _bstart = . ;  *(.bss) *(COMMON) ; _bend = . ;}
          }

     The run-time initialization code (for C programs, usually `crt0')
     for use with a ROM generated this way has to include something like
     the following, to copy the initialized data from the ROM image to
     its runtime address:

          char *src = _etext;
          char *dst = _data;

          /* ROM has data at end of text; copy it. */
          while (dst < _edata) {
            *dst++ = *src++;
          }

          /* Zero bss */
          for (dst = _bstart; dst< _bend; dst++)
            *dst = 0;

`>REGION'
     Assign this section to a previously defined region of memory.
     *Note MEMORY::.

`:PHDR'
     Assign this section to a segment described by a program header.
     *Note PHDRS::.  If a section is assigned to one or more segments,
     then all subsequent allocated sections will be assigned to those
     segments as well, unless they use an explicitly `:PHDR' modifier.
     To prevent a section from being assigned to a segment when it would
     normally default to one, use `:NONE'.

`=FILL'
     Including `=FILL' in a section definition specifies the initial
     fill value for that section.  You may use any expression to
     specify FILL.  Any unallocated holes in the current output section
     when written to the output file will be filled with the two least
     significant bytes of the value, repeated as necessary.  You can
     also change the fill value with a `FILL' statement in the CONTENTS
     of a section definition.


File: ld.info,  Node: PHDRS,  Next: Entry Point,  Prev: SECTIONS,  Up: Commands

ELF Program Headers
===================

   The ELF object file format uses "program headers", which are read by
the system loader and describe how the program should be loaded into
memory.  These program headers must be set correctly in order to run the
program on a native ELF system.  The linker will create reasonable
program headers by default.  However, in some cases, it is desirable to
specify the program headers more precisely; the `PHDRS' command may be
used for this purpose.  When the `PHDRS' command is used, the linker
will not generate any program headers itself.

   The `PHDRS' command is only meaningful when generating an ELF output
file.  It is ignored in other cases.  This manual does not describe the
details of how the system loader interprets program headers; for more
information, see the ELF ABI.  The program headers of an ELF file may
be displayed using the `-p' option of the `objdump' command.

   This is the syntax of the `PHDRS' command.  The words `PHDRS',
`FILEHDR', `AT', and `FLAGS' are keywords.

     PHDRS
     {
       NAME TYPE [ FILEHDR ] [ PHDRS ] [ AT ( ADDRESS ) ]
             [ FLAGS ( FLAGS ) ] ;
     }

   The NAME is used only for reference in the `SECTIONS' command of the
linker script.  It does not get put into the output file.

   Certain program header types describe segments of memory which are
loaded from the file by the system loader.  In the linker script, the
contents of these segments are specified by directing allocated output
sections to be placed in the segment.  To do this, the command
describing the output section in the `SECTIONS' command should use
`:NAME', where NAME is the name of the program header as it appears in
the `PHDRS' command.  *Note Section Options::.

   It is normal for certain sections to appear in more than one segment.
This merely implies that one segment of memory contains another.  This
is specified by repeating `:NAME', using it once for each program
header in which the section is to appear.

   If a section is placed in one or more segments using `:NAME', then
all subsequent allocated sections which do not specify `:NAME' are
placed in the same segments.  This is for convenience, since generally
a whole set of contiguous sections will be placed in a single segment.
To prevent a section from being assigned to a segment when it would
normally default to one, use `:NONE'.

   The `FILEHDR' and `PHDRS' keywords which may appear after the
program header type also indicate contents of the segment of memory.
The `FILEHDR' keyword means that the segment should include the ELF
file header.  The `PHDRS' keyword means that the segment should include
the ELF program headers themselves.

   The TYPE may be one of the following.  The numbers indicate the
value of the keyword.

`PT_NULL' (0)
     Indicates an unused program header.

`PT_LOAD' (1)
     Indicates that this program header describes a segment to be
     loaded from the file.

`PT_DYNAMIC' (2)
     Indicates a segment where dynamic linking information can be found.

`PT_INTERP' (3)
     Indicates a segment where the name of the program interpreter may
     be found.

`PT_NOTE' (4)
     Indicates a segment holding note information.

`PT_SHLIB' (5)
     A reserved program header type, defined but not specified by the
     ELF ABI.

`PT_PHDR' (6)
     Indicates a segment where the program headers may be found.

EXPRESSION
     An expression giving the numeric type of the program header.  This
     may be used for types not defined above.

   It is possible to specify that a segment should be loaded at a
particular address in memory.  This is done using an `AT' expression.
This is identical to the `AT' command used in the `SECTIONS' command
(*note Section Options::.).  Using the `AT' command for a program
header overrides any information in the `SECTIONS' command.

   Normally the segment flags are set based on the sections.  The
`FLAGS' keyword may be used to explicitly specify the segment flags.
The value of FLAGS must be an integer.  It is used to set the `p_flags'
field of the program header.

   Here is an example of the use of `PHDRS'.  This shows a typical set
of program headers used on a native ELF system.

     PHDRS
     {
       headers PT_PHDR PHDRS ;
       interp PT_INTERP ;
       text PT_LOAD FILEHDR PHDRS ;
       data PT_LOAD ;
       dynamic PT_DYNAMIC ;
     }

     SECTIONS
     {
       . = SIZEOF_HEADERS;
       .interp : { *(.interp) } :text :interp
       .text : { *(.text) } :text
       .rodata : { *(.rodata) } /* defaults to :text */
       ...
       . = . + 0x1000; /* move to a new page in memory */
       .data : { *(.data) } :data
       .dynamic : { *(.dynamic) } :data :dynamic
       ...
     }


File: ld.info,  Node: Entry Point,  Next: Option Commands,  Prev: PHDRS,  Up: Commands

The Entry Point
===============

   The linker command language includes a command specifically for
defining the first executable instruction in an output file (its "entry
point").  Its argument is a symbol name:
     ENTRY(SYMBOL)

   Like symbol assignments, the `ENTRY' command may be placed either as
an independent command in the command file, or among the section
definitions within the `SECTIONS' command--whatever makes the most
sense for your layout.

   `ENTRY' is only one of several ways of choosing the entry point.
You may indicate it in any of the following ways (shown in descending
order of priority: methods higher in the list override methods lower
down).
   * the `-e' ENTRY command-line option;

   * the `ENTRY(SYMBOL)' command in a linker control script;

   * the value of the symbol `start', if present;

   * the address of the first byte of the `.text' section, if present;

   * The address `0'.

   For example, you can use these rules to generate an entry point with
an assignment statement: if no symbol `start' is defined within your
input files, you can simply define it, assigning it an appropriate
value--

     start = 0x2020;

The example shows an absolute address, but you can use any expression.
For example, if your input object files use some other symbol-name
convention for the entry point, you can just assign the value of
whatever symbol contains the start address to `start':

     start = other_symbol ;


File: ld.info,  Node: Option Commands,  Prev: Entry Point,  Up: Commands

Option 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.

`CONSTRUCTORS'
     When linking using the `a.out' object file format, the linker uses
     an unusual set construct to support C++ global constructors and
     destructors.  When linking object file formats which do not support
     arbitrary sections, such as `ECOFF' and `XCOFF', the linker will
     automatically recognize C++ global constructors and destructors by
     name.  For these object file formats, the `CONSTRUCTORS' command
     tells the linker where this information should be placed.  The
     `CONSTRUCTORS' command is ignored for other object file formats.

     The symbol `__CTOR_LIST__' marks the start of the global
     constructors, and the symbol `__DTOR_LIST' marks the end.  The
     first word in the list is the number of entries, followed by the
     address of each constructor or destructor, followed by a zero
     word.  The compiler must arrange to actually run the code.  For
     these object file formats GNU C++ calls constructors from a
     subroutine `__main'; a call to `__main' is automatically inserted
     into the startup code for `main'.  GNU C++ runs destructors either
     by using `atexit', or directly from the function `exit'.

     For object file formats such as `COFF' or `ELF' which support
     multiple sections, GNU C++ will normally arrange to put the
     addresses of global constructors and destructors into the `.ctors'
     and `.dtors' sections.  Placing the following sequence into your
     linker script will build the sort of table which the GNU C++
     runtime code expects to see.

                __CTOR_LIST__ = .;
                LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2)
                *(.ctors)
                LONG(0)
                __CTOR_END__ = .;
                __DTOR_LIST__ = .;
                LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2)
                *(.dtors)
                LONG(0)
                __DTOR_END__ = .;

     Normally the compiler and linker will handle these issues
     automatically, and you will not need to concern yourself with
     them.  However, you may need to consider this if you are using C++
     and writing your own linker scripts.

`FLOAT'
`NOFLOAT'
     These keywords were used in some older linkers to request a
     particular math subroutine library.  `ld' doesn't use the
     keywords, assuming instead that any necessary subroutines are in
     libraries specified using the general mechanisms for linking to
     archives; but to permit the use of scripts that were written for
     the older linkers, the keywords `FLOAT' and `NOFLOAT' are accepted
     and ignored.

`FORCE_COMMON_ALLOCATION'
     This command has the same effect as the `-d' command-line option:
     to make `ld' assign space to common symbols even if a relocatable
     output file is specified (`-r').

`INPUT ( FILE, FILE, ... )'
`INPUT ( FILE FILE ... )'
     Use this command to include binary input files in the link, without
     including them in a particular section definition.  Specify the
     full name for each FILE, including `.a' if required.

     `ld' searches for each FILE through the archive-library search
     path, just as for files you specify on the command line.  See the
     description of `-L' in *Note Command Line Options: Options.

     If you use `-lFILE', `ld' will transform the name to `libFILE.a'
     as with the command line argument `-l'.

`GROUP ( FILE, FILE, ... )'
`GROUP ( FILE FILE ... )'
     This command is like `INPUT', except that the named files should
     all be archives, and they are searched repeatedly until no new
     undefined references are created.  See the description of `-(' in
     *Note Command Line Options: Options.

`OUTPUT ( FILENAME )'
     Use this command to name the link output file FILENAME.  The
     effect of `OUTPUT(FILENAME)' is identical to the effect of
     `-o FILENAME', which overrides it.  You can use this command to
     supply a default output-file name other than `a.out'.

`OUTPUT_ARCH ( BFDNAME )'
     Specify a particular output machine architecture, with one of the
     names used by the BFD back-end routines (*note BFD::.).  This
     command is often unnecessary; the architecture is most often set
     implicitly by either the system BFD configuration or as a side
     effect of the `OUTPUT_FORMAT' command.

`OUTPUT_FORMAT ( BFDNAME )'
     When `ld' is configured to support multiple object code formats,
     you can use this command to specify a particular output format.
     bFDNAME is one of the names used by the BFD back-end routines
     (*note BFD::.).  The effect is identical to the effect of the
     `-oformat' command-line option.  This selection affects only the
     output file; the related command `TARGET' affects primarily input
     files.

`SEARCH_DIR ( PATH )'
     Add PATH to the list of paths where `ld' looks for archive
     libraries.  `SEARCH_DIR(PATH)' has the same effect as `-LPATH' on
     the command line.

`STARTUP ( FILENAME )'
     Ensure that FILENAME is the first input file used in the link
     process.

`TARGET ( FORMAT )'
     When `ld' is configured to support multiple object code formats,
     you can use this command to change the input-file object code
     format (like the command-line option `-b' or its synonym
     `-format').  The argument FORMAT is one of the strings used by BFD
     to name binary formats.  If `TARGET' is specified but
     `OUTPUT_FORMAT' is not, the last `TARGET' argument is also used as
     the default format for the `ld' output file.  *Note BFD::.

     If you don't use the `TARGET' command, `ld' uses the value of the
     environment variable `GNUTARGET', if available, to select the
     output file format.  If that variable is also absent, `ld' uses
     the default format configured for your machine in the BFD
     libraries.


File: ld.info,  Node: Machine Dependent,  Next: BFD,  Prev: Commands,  Up: Top

Machine Dependent Features
**************************

   `ld' has additional features on some platforms; the following
sections describe them.  Machines where `ld' has no additional
functionality are not listed.

* Menu:

* H8/300::                      `ld' and the H8/300
* i960::                        `ld' and the Intel 960 family


File: ld.info,  Node: H8/300,  Next: i960,  Up: Machine Dependent

`ld' and the H8/300
===================

   For the H8/300, `ld' can perform these global optimizations when you
specify the `-relax' command-line option.

*relaxing address modes*
     `ld' finds all `jsr' and `jmp' instructions whose targets are
     within eight bits, and turns them into eight-bit program-counter
     relative `bsr' and `bra' instructions, respectively.

*synthesizing instructions*
     `ld' finds all `mov.b' instructions which use the sixteen-bit
     absolute address form, but refer to the top page of memory, and
     changes them to use the eight-bit address form.  (That is: the
     linker turns `mov.b `@'AA:16' into `mov.b `@'AA:8' whenever the
     address AA is in the top page of memory).


File: ld.info,  Node: i960,  Prev: H8/300,  Up: Machine Dependent

`ld' and the Intel 960 family
=============================

   You can use the `-AARCHITECTURE' command line option to specify one
of the two-letter names identifying members of the 960 family; the
option specifies the desired output target, and warns of any
incompatible instructions in the input files.  It also modifies the
linker's search strategy for archive libraries, to support the use of
libraries specific to each particular architecture, by including in the
search loop names suffixed with the string identifying the architecture.

   For example, if your `ld' command line included `-ACA' as well as
`-ltry', the linker would look (in its built-in search paths, and in
any paths you specify with `-L') for a library with the names

     try
     libtry.a
     tryca
     libtryca.a

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

   You can meaningfully use `-A' more than once on a command line, since
the 960 architecture family allows combination of target architectures;
each use will add another pair of name variants to search for when `-l'
specifies a library.

   `ld' supports the `-relax' option for the i960 family.  If you
specify `-relax', `ld' finds all `balx' and `calx' instructions whose
targets are within 24 bits, and turns them into 24-bit program-counter
relative `bal' and `cal' instructions, respectively.  `ld' also turns
`cal' instructions into `bal' instructions when it determines that the
target subroutine is a leaf routine (that is, the target subroutine does
not itself call any subroutines).


File: ld.info,  Node: BFD,  Next: MRI,  Prev: Machine Dependent,  Up: Top

BFD
***

   The linker accesses object and archive files using the BFD libraries.
These libraries allow the linker to use the same routines to operate on
object files whatever the object file format.  A different object file
format can be supported simply by creating a new BFD back end and adding
it to the library.  To conserve runtime memory, however, the linker and
associated tools are usually configured to support only a subset of the
object file formats available.  You can use `objdump -i' (*note
objdump: (binutils.info)objdump.) to list all the formats available for
your configuration.

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

   One minor artifact of the BFD solution which you should bear in mind
is the potential for information loss.  There are two places where
useful information can be lost using the BFD mechanism: during
conversion and during output. *Note BFD information loss::.

* Menu:

* BFD outline::                 How it works: an outline of BFD


File: ld.info,  Node: BFD outline,  Up: BFD

How it works: an outline of BFD
===============================

   When an object file is opened, BFD subroutines automatically
determine the format of the input object file.  They then build a
descriptor in memory with pointers to routines that will be used to
access elements of the object file's data structures.

   As different information from the the object files is required, BFD
reads from different sections of the file and processes them.  For
example, a very common operation for the linker is processing symbol
tables.  Each BFD back end provides a routine for converting between
the object file's representation of symbols and an internal canonical
format. When the linker asks for the symbol table of an object file, it
calls through a memory pointer to the routine from the relevant BFD
back end which reads and converts the table into a canonical form.  The
linker then operates upon the canonical form. When the link is finished
and the linker writes the output file's symbol table, another BFD back
end routine is called to take the newly created symbol table and
convert it into the chosen output format.

* Menu:

* BFD information loss::    Information Loss
* Canonical format::        The BFD    canonical object-file format


File: ld.info,  Node: BFD information loss,  Next: Canonical format,  Up: BFD outline

Information Loss
----------------

   *Information can be lost during output.* The output formats
supported by BFD do not provide identical facilities, and information
which can be described in one form has nowhere to go in another format.
One example of this is alignment information in `b.out'. There is
nowhere in an `a.out' format file to store alignment information on the
contained data, so when a file is linked from `b.out' and an `a.out'
image is produced, alignment information will not propagate to the
output file. (The linker will still use the alignment information
internally, so the link is performed correctly).

   Another example is COFF section names. COFF files may contain an
unlimited number of sections, each one with a textual section name. If
the target of the link is a format which does not have many sections
(e.g., `a.out') or has sections without names (e.g., the Oasys format),
the link cannot be done simply. You can circumvent this problem by
describing the desired input-to-output section mapping with the linker
command language.

   *Information can be lost during canonicalization.* The BFD internal
canonical form of the external formats is not exhaustive; there are
structures in input formats for which there is no direct representation
internally.  This means that the BFD back ends cannot maintain all
possible data richness through the transformation between external to
internal and back to external formats.

   This limitation is only a problem when an application reads one
format and writes another.  Each BFD back end is responsible for
maintaining as much data as possible, and the internal BFD canonical
form has structures which are opaque to the BFD core, and exported only
to the back ends. When a file is read in one format, the canonical form
is generated for BFD and the application. At the same time, the back
end saves away any information which may otherwise be lost. If the data
is then written back in the same format, the back end routine will be
able to use the canonical form provided by the BFD core as well as the
information it prepared earlier.  Since there is a great deal of
commonality between back ends, there is no information lost when
linking or copying big endian COFF to little endian COFF, or `a.out' to
`b.out'.  When a mixture of formats is linked, the information is only
lost from the files whose format differs from the destination.


File: ld.info,  Node: Canonical format,  Prev: BFD information loss,  Up: BFD outline

The BFD canonical object-file format
------------------------------------

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

*files*
     Information stored on a per-file basis includes target machine
     architecture, particular implementation format type, a demand
     pageable bit, and a write protected bit.  Information like Unix
     magic numbers is not stored here--only the magic numbers' meaning,
     so a `ZMAGIC' file would have both the demand pageable bit and the
     write protected text bit set.  The byte order of the target is
     stored on a per-file basis, so that big- and little-endian object
     files may be used with one another.

*sections*
     Each section in the input file contains the name of the section,
     the section's original address in the object file, size and
     alignment information, various flags, and pointers into other BFD
     data structures.

*symbols*
     Each symbol contains a pointer to the information for the object
     file which originally defined it, its name, its value, and various
     flag bits.  When a BFD back end reads in a symbol table, it
     relocates all symbols to make them relative to the base of the
     section where they were defined.  Doing this ensures that each
     symbol points to its containing section.  Each symbol also has a
     varying amount of hidden private data for the BFD back end.  Since
     the symbol points to the original file, the private data format
     for that symbol is accessible.  `ld' can operate on a collection
     of symbols of wildly different formats without problems.

     Normal global and simple local symbols are maintained on output,
     so an output file (no matter its format) will retain symbols
     pointing to functions and to global, static, and common variables.
     Some symbol information is not worth retaining; in `a.out', type
     information is stored in the symbol table as long symbol names.
     This information would be useless to most COFF debuggers; the
     linker has command line switches to allow users to throw it away.

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

*relocation level*
     Each canonical BFD 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 by passing messages through
     the relocation type descriptor and the symbol pointer. Therefore,
     relocations can be performed on output data using a relocation
     method that is 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 68k COFF file, even though 68k COFF has no
     such relocation type.

*line numbers*
     Object formats can contain, for debugging purposes, some form of
     mapping between symbols, source line numbers, and addresses in the
     output file.  These addresses 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 finding out 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).