This file documents the GNU linker gld.

Overview
********

The gld command 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.

The gld command accepts Linker Command Language files in a superset of AT+T's Link Editor Command Language syntax, to provide explict 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 and write any of the formats supported by bfd, different formats may be linked together producing any available object file.

Supported formats:

  * 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

  * s-records

When linking similar formats, gld maintains all debugging information.

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

       gld [ -Bstatic ]  [ -D DATASIZE ]
           [ -c FILENAME ]
           [ -d ]  | [ -dc ] | [ -dp ]
           [ -i ]
           [ -e ENTRY ]  [ -l ARCH ]  [ -L SEARCHDIR ]  [ -M ]
           [ -N | -n | -z ]  [ -noinhibit-exec ]  [ -r ]  [ -S ]  [ -s ]
           [ -f FILL ]
           [ -T TEXTORG ]  [ -Tdata DATAORG ]  [ -t ]  [ -u SYM]
           [ -X ]  [ -x ]
           [-o OUTPUT ] OBJFILES...

Command-line options to GNU 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 present several times are -L, -l, and -u.

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.

-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 linkage commands from the file COMMANDFILE.

-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, and reduce the size of the bss segment to match.

-d
     Force ld to assign space to common symbols even if a relocatable output file is specified (-r).

-dc | -dp
     This flags is accepted for command-line compatibility with the SunOS linker, but has no effect on gld.

-e ENTRY
     Use ENTRY as the explicit symbol for beginning execution of your program, rather than the default entry point. If this symbol is not specified, the symbol start is used as the entry address. If there is no symbol called start, then the entry address is set to the first address in the first output section (usually the text section).

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

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

-l ARCH
     Add an archive file ARCH 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 libARCH.a for every ARCH specified.

-L SEARCHDIR
     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 read and writable text and data sections. If the output format supports Unix style magic numbers, then OMAGIC is set.

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

-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
     Generates relocatable output---i.e., generate an output file that can in turn serve as input to gld.  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.

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

-s
     Omits all symbol information from the output file.

-T TEXTORG
-Ttext TEXTORG
     Use TEXTORG as the starting address for the text segment of the output file.  Both forms of this option are equivalent.  The option argument must be a hexadecimal integer.

-Tdata DATAORG
     Use DATAORG as the starting address for the data segment of the output file.  The option argument must be a hexadecimal integer.

-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.

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

-z
     -z sets ZMAGIC, the default: the text segment is read-only, demand pageable, and shared.

     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 linkage process, allowing specification of:

input files

file formats

output file format

addresses of sections

placement of common blocks

and more

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 does not have a reasonable object or archive format, it tries to read the file as if it were a command file.

Structure
=========

To be added

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.

Build in functions may be called.

Expressions
===========

The linker has a practice of ``lazy evaluation'' for expressions; it only calculates an expression when absolutely necessary. For instance, when the linker reads in the command file it has to know the values of the start address and the length of the memory regions for linkage to continue, so these values are worked out, but other values (such as symbol values) are not known or needed until after storage allocation. They are evaluated later, when the other information, such as the sizes of output sections are available for use in the symbol assignment expression.

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(.) ;
             }

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.  To specify a symbol which contains odd characters or has the same name as a keyword surround it in double quotes:

             ``SECTION'' = 9;
             ``with a space'' = ``also with a space'' + 10;

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).

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

Integers have the usual values.  To denote a negative integer, use the unary operator - discussed under expressions.

Additionally the suffixes K and M may be used to multiply the previous constant by 1024 or respectively.

             _as_decimal = 57005;
             _as_hex = 0xdead;
             _as_octal = 0157255;

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

Operators
---------

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

Built in Functions
==================

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

ALIGN(EXP)
     returns the result of the current location counter (dot) aligned to the next EXP boundary, where EXP 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 if the section has already been bound. In the following examples 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. This example shows the setting of a global symbol begin to the first location in the .text section, only if there is no other symbol called begin already:

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

MEMORY Directive
================

The linker's default configuration is for all memory to be allocatable.  This state may be overridden by using the MEMORY directive.  The MEMORY directive describes the location and size of blocks of memory in the target.  Careful use can describe memory regions which may or may not be used by the linker. The linker does not shuffle sections to fit into the available regions, but does move the requested sections into the correct regions and issue errors when the regions become too full.  The syntax is:

             MEMORY
                     {

                     }

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.

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.

ORIGIN
     is the start address of the region in physical memory expressed as standard linker 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 as a standard linker 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 : ORIGIN= 0x40000000, LENGTH = 4M
                     }

If the combined output sections directed to a region are too big for the region the linker will emit an error message.

SECTIONS Directive
==================

The SECTIONS directive controls exactly where input sections are placed into output sections, their order and to which output sections they are allocated.

When no SECTIONS directives are 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 appear in the first file, and then the order of the files.

The syntax of the SECTIONS directive is:

        SECTIONS
        {
        }

NAME
     controls the name of the output section. 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. 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'')
                          }
                  }

FILL EXPRESSION
     If present this expression sets the fill value. 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.

OPTIONS
     the OPTIONS parameter is a list of optional arguments specifying attributes of the output section, they may be taken from the following list:

     ADDR EXPRESSION
          forces the output section to be loaded at a specified address. The address is specified as a standard linker expression. The following example generates section OUTPUT at location 0x40000000:

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

          Since the built in function ALIGN references the location counter implicitly, a section may be located on a certain boundary by using the ALIGN function in the expression. For example, to locate the .data section on the next 8k boundary after the end of the .text section:

                       SECTIONS {
                               .text {
                                       ...
                               }
                               .data ALIGN(4K) {
                                       ...
                               }
                       }

STATEMENTS
     is a list of file names, input sections and assignments. These statements control what is placed into the output section. The syntax of a single STATEMENT is one of:

     SYMBOL  [ $= | += | -= | *= | /= ]  EXPRESSION ;

          Global symbols may be created and have their values (addresses) altered using the assignment statement. The linker tries to put off the evaluation of an assignment until all the terms in the source expression are known; 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: eg

                       SECTIONS {
                             text 9+this_isnt_constant:
                                       {
                                       }
                               }
                       testscript:21: Non constant expression for initial address

     CREATE_OBJECT_SYMBOLS
          causes the linker to create a symbol for each input file and place it into the specified section set with the value 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.

                       SECTIONS {
                               .text 0x2020 :
                                        {
                                       CREATE_OBJECT_SYMBOLS
                                       *(.text)
                                       _etext = ALIGN(0x2000);
                                       }
                               }

          Supplied with four object files, a.o, b.o, c.o, and d.o a run of gld could create a map:

               From functions like :
               a.c:
                       afunction() { }
                       int adata=1;
                       int abss;

               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

     FILENAME ( SECTION NAME LIST )
          This command allocates all the named sections from the input object file supplied into the output section at the current point. Sections are written in the order they appear in the list so:

                       SECTIONS {
                               .text 0x2020 :
                                       {
                                       a.o(.data)
                                       b.o(.data)
                                       *(.text)
                                       }
                               .data :
                                       {
                                       *(.data)
                                       }
                               .bss :
                                       {
                                       *(.bss)
                                       COMMON
                                       }
                       }

          will produce a map:

                       insert here

     * ( SECTION NAME LIST )
          This command causes all sections from all input files which have not yet been assigned output sections to be assigned the current output section.

     FILENAME [COMMON]
          This allocates all the common symbols from the specified file and places them into the current output section.

     * [COMMON]
          This allocates all the common symbols from the files which have not yet had their common symbols allocated and places them into the current output section.

     FILENAME
          A filename alone within a SECTIONS statement will cause all the input sections from the file to be placed into the current output section at the current location. If the file name has been mentioned before with a section name list then only those sections which have not yet been allocated are noted.

          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 the data from section .input1 from file foo.o is placed next into the same output section.  All of section .input2 is read from foo.o and placed into output section outputb.  Next all of section .input1 is read from foo1.o. All of the remaining .input1 and .input2 sections from any files are written to output section output3.

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

Using the Location Counter
==========================

The special linker variable dot, . always contains the current output location counter. Since the dot always refers to a location in an output section, it must always appear in an expression within a SECTIONS directive. The dot symbol may appear anywhere that an ordinary symbol may appear in an expression, but its assignments have a side effect. Assigning a value to the dot 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.

Command Language Syntax
=======================

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

The linker chooses the first executable instruction in an output file from a list of possibilities, in order:

  * The value of the symbol provided to the command line with the -e option, when present.

  * The value of the symbol provided in the ENTRY directive, if present.

  * 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 value 0.

If the symbol start is not defined within the set of input files to a link, it may be generated by a simple assignment expression. eg.

             start = 0x2020;

Section Attributes
==================

Allocation of Sections into Memory
==================================

Defining Symbols
================

Examples of operation
*********************

The simplest case is linking standard Unix object files on a standard Unix system supported by the linker. To link a file hello.o:

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

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

Partial Linking
***************

Specifying the -r on the command line causes gld to perform a partial link.

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 information lossage. There are two places where useful information can be lost using the bfd mechanism; during conversion and during output.

How it works
============

When an object file is opened, bfd tries to automatically determine the format of the input object file, a descriptor is built in memory with pointers to routines 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 the canonical form.  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 output format.

Information Leaks
=================

Information lost during output.
     The output formats supported by bfd do not provide identical facilities, and information which may be described in one form has no where to go in another format. One example of this would be alignment information in b.out. There is no where 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 is lost. (Note that in this case the linker has 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. It is possible to circumvent this problem by describing the desired input section 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 the 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 when linking big endian COFF to little endian COFF, or from a.out to b.out. When a mixture of formats are linked, the information is only lost from the files with a different format to the destination.

Mechanism
=========

The smallest amount of information is preserved when there is a small union 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 will help the user appreciate what is possible to be maintained between conversions.

file level 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 together at the same time.

section level
     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.

symbol level
     Each symbol contains a pointer to the object file which originally defined it, its name, value and various flags bits. When a symbol table is read in all symbols are relocated to make them relative to the base of the section they were defined in, so each symbol points to the containing section. Each symbol also has a varying amount of hidden data to contain private data for the back end. Since the symbol points to the original file, the symbol private data format is accessible. Operations may be done to a list of symbols of wildly different formats without problems.

     Normal global and simple local symbols are maintained on output, so an output file, no matter the format will retain symbols pointing to functions, globals, statics and commons.  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. (Note that gdb does support stabs 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 who's line number is being described. The rest of the list is tuples offsets into the section and line indexes. Any format which can simply derive this information can pass it without lossage between formats (COFF, IEEE and Oasys).