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.

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.

This file documents the GNU linker gld.

Overview
********

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 gld.

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 gld uses the general purpose bfd libraries to operate on object files. This allows gld to read, combine, and write object files in many different formats---for example, COFF or a.out.  Different formats may be linked together to produce any available kind of object file.  See BFD for a list of formats supported on various architectures.

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

Command line options
********************

     gld [-o OUTPUT ] OBJFILES...
         [ -AARCHITECTURE ]  [ -b OUTPUT-FORMAT ]  [ -Bstatic ]
         [ -c COMMANDFILE ]  [ -D DATASIZE ]
         [ -d | -dc | -dp ]  [ -defsym SYMBOL = EXPRESSION ]
         [ -e ENTRY ]  [ -f FILL ]  [ -F ]  [ -F FORMAT ]
         [ -format OUTPUT-FORMAT ]  [ -g ]  [ -i ]
         [ -lAR ]  [ -LSEARCHDIR ]  [ -M | -m ]
         [ -N | -n | -z ]  [ -noinhibit-exec ]  [ -R FILENAME ]
         [ -r | -Ur ]  [ -S ]  [ -s ]
         [ SCRIPT  ...  ENDSCRIPT ]  [ SCRIPT  ...  @ ]
         [ -T COMMANDFILE ]
         [ -Ttext TEXTORG ]  [ -Tdata DATAORG ]  [ -Tbss BSSORG ]
         [ -t ]  [ -u SYM]  [-v]  [ -X ]  [ -x ]

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 gld is to link standard Unix object files on a standard, supported Unix system.  On such a system, to link a file hello.o:

     $ gld -o output /lib/crt0.o hello.o -lc

This tells gld to produce a file called output as the result of linking the file /lib/crt0.o with hello.o and the library libc.a which will come from the standard search directories.

The command-line options to 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 -L, -l, and -u.

The list of object files to be linked together, shown as OBJFILES, may follow, precede, or be mixed in with command-line options; save that an 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.

OBJFILES...
     The object files OBJFILES to be linked; at least one must be specified.

-AARCHITECTURE
     In the current release of gld, this option is useful only for the Intel 960 family of architectures.  In that context, the 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 -l, save that -AARCHITECTURE triggers a two-level search; first for a library with exactly the name you specify as ARCHITECTURE, and if that fails, for a library named with the -l convention---i.e., libARCHITECTURE.a.

     Future releases of gld may support similar functionality for other architecture families.

-b OUTPUT-FORMAT
     Specify the desired output-file binary format.  You don't usually need to specify this.  gld can determine the format of input files by inspection, and---in the most frequent case, when all input files have the same format, 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.

     OUTPUT-FORMAT is a text string, the name of a particular format supported by the BFD libraries.  See BFD.

     -format OUTPUT-FORMAT has the same effect.

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

-c COMMANDFILE
     Directs gld to read link commands from the file COMMANDFILE.  These commands will override gld's default link format in its entirety; COMMANDFILE must specify everything necessary to specify the target format.  See Commands.

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

-D DATASIZE
     Use this option to specify a target size for the data segment of your linked program.  The option is only obeyed if DATASIZE is larger than the natural size of the program's data segment.

     DATASIZE must be an integer specified in hexadecimal.

     ld will simply increase the size of the data segment, padding the created gap with zeros (or a fill pattern specified with -f, or using the command language), and reduce the size of the bss segment by the same amount.

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

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

-e ENTRY
     Use ENTRY as the explicit symbol for beginning execution of your program, rather than the default entry point. See Entry Point, for a discussion of defaults and other ways of specifying the entry point.

-f FILL
     Sets the default fill pattern for ``holes'' in the output file to the lowest two bytes of the expression specified.

-F
-Fformat
     Some older linkers required the specification of object-file format, even when all input files were homogeneous, and used this option for that purpose.  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.

-format OUTPUT-FORMAT
     Synonym for -b OUTPUT-FORMAT.

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

-i
     Produce an incremental link (same as option -r).

-lAR
     Add an archive file AR to the list of files to link.  This option may be used any number of times.  ld will search its path-list for occurrences of libAR.a for every AR specified.

-LSEARCHDIR
     This command adds path SEARCHDIR to the list of paths that gld will search for archive libraries.  You may use this option any number of times.

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

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

-n
     sets the text segment to be read only, and NMAGIC is written if possible.

-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.

-o OUTPUT
     OUTPUT is a name for the program produced by ld; if this option is not specified, the name a.out is used by default.

-R FILENAME
     Read symbol names and their addresses from 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.

-r
     Generates relocatable output---i.e., generate an output file that can in turn serve as input to gld.  This is often called partial linking.  As a side effect, this option also sets the output file's magic number to OMAGIC; see -N. If this option is not specified, an absolute file is produced.  When linking C++ programs, this option will not resolve references to constructors; -Ur is an alternative.

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

-s
     Omits all symbol information from the output file.

SCRIPT  ... @
SCRIPT  ... 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 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 @ or with the keyword ENDSCRIPT.  Other command-line options will not be recognized while parsing the script.  See Commands for a description of the command language.

-Tbss BSSORG
-Tdata DATAORG
-Ttext TEXTORG
     Use TEXTORG as the starting address for---respectively---the bss, data, or the text segment of the output file. TEXTORG must be a hexadecimal integer.

-T COMMANDFILE
-TCOMMANDFILE
     Equivalent to -c COMMANDFILE; supported for compatibility with other tools.

-t
     Prints names of input files as ld processes them.

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

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

-v
     ``Verbose'' switch: display informative messages, including the version numbers for gld and BFD, information on files opened, and BFD subroutine calls.

-X
     If -s or -S is also specified, delete only local symbols beginning with L.

-x
     If -s or -S is also specified, delete all local symbols, not just those beginning with L.

-z
     Specifies a read-only, demand pageable, and shared text segment. If the output format supports Unix-style magic numbers, -z also marks the output as ZMAGIC, the default.

     Specifying a relocatable output file (-r) will also set the magic number to OMAGIC.

     See description of -N.

Command Language
****************

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:

  * input files

  * file formats

  * output file format

  * addresses of sections

  * placement of common blocks

A command file may be supplied to the linker, either explicitly through the -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.

Linker Scripts
==============

The 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 gld command language is the SECTIONS command (see SECTIONS).  Every meaningful command script must have a SECTIONS command: it specifies a ``picture'' of the output file's layout, in varying degrees of detail. No other command is required in all cases.

The MEMORY command complements SECTIONS by describing the available memory in the target architecture; if it is not present, sufficient memory is assumed to be available in a contiguous block for all output.  See MEMORY.

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:

  * All expressions evaluated as integers and are of ``long'' or ``unsigned long'' type.

  * All constants are integers.

  * All of the C arithmetic operators are provided.

  * Global variables may be referenced, defined and created.

  * Built in functions may be called.

Integers
--------

An octal integer is 0 followed by zero or more of the octal digits (01234567).

A decimal integer starts with a non-zero digit followed by zero or more digits (0123456789).

     _as_octal = 0157255;

A hexadecimal integer is 0x or 0X followed by one or more hexadecimal digits chosen from 0123456789abcdefABCDEF.

     _as_hex = 0xdead;

Decimal integers have the usual values.  To denote a negative integer, use the prefix operator -; see Operators.

     _as_decimal = 57005;
     _as_neg = -57005;

Additionally the suffixes K and M may be used to scale a constant by 1024 or 1024*1024 respectively. For example, the following all refer to the same quantity:

             _4k_1 = 4K;
             _4k_2 = 4096;
             _4k_3 = 0x1000;

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:

             "SECTION" = 9;
             "with a space" = "also with a space" + 10;

The Location Counter
--------------------

The special linker variable dot . always contains the current output location counter. Since the . always refers to a location in an output section, it must always appear in an expression within a SECTIONS command. The . symbol may appear anywhere that an ordinary symbol is allowed in an expression, but its assignments have a side effect. Assigning a value to the . symbol will cause the location counter to be moved. This may be used to create holes in the output section.  The location counter may never be moved backwards.

             SECTIONS
             {
                     output :
                     {
                     file1(.text)
                     . = . + 1000;
                     file2(.text)
                     . += 1000;
                     file3(.text)
                     . -= 32;
                     file4(.text)
                     } = 0x1234;
             }

In the previous example, file1 is located at the beginning of the output section, then there is a 1000 byte gap, filled with 0x1234. Then file2 appears, also with a 1000 byte gap following before file3 is loaded. Then the first 32 bytes of file4 are placed over the last 32 bytes of file3.

Operators
---------

The linker recognizes  the standard C set of arithmetic operators, with the standard bindings and precedence levels:

     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)

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.

Assignment: Defining Symbols
----------------------------

You may create global symbols, and assign values (addresses) to global symbols, using any of the C assignment operators:

SYMBOL  = EXPRESSION ;
SYMBOL += EXPRESSION ;
SYMBOL -= EXPRESSION ;
SYMBOL *= EXPRESSION ;
SYMBOL /= EXPRESSION ;

Two things distinguish assignment from other operators in gld expressions.

  * Assignment may only be used at the root of an expression; a=b+3; is allowed, but a+b=3; is an error.

  * A trailing semicolon is required at the end of an assignment statement.

Assignment statements may appear:

  * as commands in their own right in a gld script; or

  * as independent statements within a SECTIONS command; or

  * as part of the contents of a section definition in a SECTIONS command.

The first two cases are equivalent in effect---both define a symbol with an absolute address; the last case defines a symbol whose address is relative to a particular section (see 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 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 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 SECTION specification by using the absolute assignment function ABSOLUTE For example, to create an absolute symbol whose address is the last byte of the output section .data:

     .data :
             {
                     *(.data)
                     _edata = ABSOLUTE(.) ;
             }

The linker tries to put off the evaluation of an assignment until all the terms in the source expression are known (see Evaluation). For instance the sizes of sections cannot be known until after allocation, so assignments dependent upon these are not performed until after allocation. Some expressions, such as those depending upon the location counter dot, . must be evaluated during allocation. If the result of an expression is required, but the value is not available, then an error results.  For example, attempting to use a script like the following

     SECTIONS {
           text 9+this_isnt_constant:
                     { ...
                     }
             }

will get the error message ``Non constant expression for initial address''.

Built in Functions
------------------

The command language provides built in functions for use in expressions in link scripts.

  * ALIGN(EXP) returns 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).  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:

                  .data ALIGN(0x2000) :{
                          *(.data)
                          variable = ALIGN(0x8000);
                  }

  * ADDR(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 symbol_1 and symbol_2 are assigned identical values:

                  .output1:
                          {
                          start_of_output_1 $= .;
                          ...
                          }
                  .output:
                          {
                          symbol_1 = ADDR(.output1);
                          symbol_2 = start_of_output_1;
                          }

  * SIZEOF(SECTION NAME) returns the size in bytes of the named section, if the section has been allocated.  In the following example the symbol_1 and symbol_2 are assigned identical values:

                  .output {
                          .start = . ;
                          ...
                          .end = .;
                          }
                  symbol_1 = .end - .start;
                  symbol_2 = SIZEOF(.output);

  * DEFINED(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 begin to the first location in the .text section---but only if no symbol called begin existed:

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

MEMORY Command
==============

The linker's default configuration permits allocation of all memory. You can override this 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.

Command files 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, filenames or section names.  Use distinct names to specify multiple regions.

(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 ``RWXL''. 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 expressed as an expression, which must evaluate to a constant before memory allocation is performed. The keyword ORIGIN may be abbreviated to org or o.

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 256k, 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.  If the combined output sections directed to a region are too big for the region, the linker will issue an error message.

SECTIONS Command
================

The 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 SECTIONS command in a commands 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 what input sections make it up.

The first two possibilities---defining the entry point, and defining symbols---can also be done outside the SECTIONS command: see Entry Point, see 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 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.

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

The most frequently used statement in the SECTIONS command is the 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

     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.  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 (in the case of a.out, .text, .data or .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 gld symbol name syntax must be quoted.

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 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( 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.

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.  Since multiple statements may be present in the contents of a section definition, you can specify a list of particular files by name:

          .data: { afile.o bfile.o cfile.o }

     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.

* (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 gld command line: use * instead of a particular filename before the parenthesized input-file section list.

     For example, to copy sections 1 through 4 from a Oasys file into the .text section of an a.out file, and sections 13 and 14 into the .data section:

          SECTION {
                  .text :{
                          *("1" "2" "3" "4")
                  }

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

     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.

[ SECTION ]
[ SECTION, SECTION, ... ]
[ SECTION SECTION ... ]
     This is an alternate notation to specify named sections from all unallocated input files; its effect is exactly the same as that of * (SECTION...)

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: gld 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 its 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)
                     }
             }

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; see 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
     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 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 objsym 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;

     gld -M sample 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

FORCE_COMMON_ALLOCATION

SYMBOL = EXPRESSION ;
SYMBOL F= EXPRESSION ;
     SYMBOL is any symbol name (see Symbols).  When you assign a value to a symbol within a particular section definition, the value is relative to the beginning of the section (see 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.

     ``F='' here refers to any of the operators &=  +=  -=  *= /= which combine arithmetic and assignment.

BYTE(EXPRESSION)
SHORT(EXPRESSION)
LONG(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 (see BFD).

FILL(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 .) 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.

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

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

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

SECNAME and CONTENTS are required.  See Section Definition, and see Section Contents for the details of CONTENTS.  START, BLOCK(ALIGN), =FILL, and >REGION 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)

=FILL
     You may use any expression to specify FILL.  Including =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 FILL statement in the CONTENTS of a section definition.

>REGION

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 value of the symbol _main, if present;

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

  * The address 0.

For example, you can also generate an entry point with an assignment statement: if no symbol start is  defined within your input files, you can simply assign 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;

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.

FLOAT
NOFLOAT
     Declare to the linker whether or not floating point support is available.  The default assumption is NOFLOAT.

HLL ( FILE, FILE, ... )
HLL ( FILE FILE ... )

INPUT ( FILE, FILE, ... )
INPUT ( FILE FILE ... )

MAP ( NAME )

OUTPUT ( FILENAME )

SEARCH_DIR ( PATHNAME )

STARTUP ( NAME )

SYSLIB ( FILE, FILE, ... )
SYSLIB ( FILE FILE ... )

TARGET ( FORMAT )

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.

Formats currently supported:

  * Sun3 68k a.out

  * IEEE-695 68k Object Module Format

  * Oasys 68k Binary Relocatable Object File Format

  * Sun4 sparc a.out

  * 88k bcs coff

  * i960 coff little endian

  * i960 coff big endian

  * i960 b.out little endian

  * i960 b.out big endian

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 the user should be aware of is the potential for information loss. There are two places where useful information can be lost using the bfd mechanism; during conversion and during output. See BFD information loss.

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

When an object file is opened, 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 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 the memory pointer to the relevant 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 bfd back end routine is called which takes the newly created symbol table and converts it into the chosen output format.

Information Loss
================

Information can be lost during output. The output formats supported by 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 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 (eg 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.

Information 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 using the linker to read one format and write 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 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 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 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.

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.

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 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.

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 bfd data structures.

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.  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 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 gdb does support 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.

relocation level
     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.

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).