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LD(1)                       General Commands Manual                      LD(1)



NAME
       ld, ld.so - link editor, dynamic link editor

SYNOPSIS
       ld [ -align datum ] [ -assert assertion-keyword ]
            [ -A name ] [ -Bbinding-keyword ] [ -d ] [ -dc ]
            [ -dp ] [ -D hex ] [ -e entry ] [ -lx ] [ -Ldir ]
            [ -M ] [ -n ] [ -N ] [ -o name ] [ -p ] [ -r ] [ -s ] [ -S ]
            [ -t ] [ -T[text] hex ] [ -Tdata hex ] [ -u name ]
            [ -x ] [ -X ] [ -ysym ] [ -z ] filename ...

DESCRIPTION
       ld  combines  object  programs  to create an executable file or another
       object program suitable for further ld processing (with the -r option).
       The  object  modules  on which ld operates are specified on the command
       line, and can be:

              o  simple object files, which typically end in  the  .o  suffix,
                 and are referred to as "dot-oh" files

              o  ar(1V) library archives (.a), or "libraries"

              o  dynamically-bound,  sharable  object  files  (.so),  are also
                 referred to as "shared libraries,"  which  are  created  from
                 previous ld executions.

       Unless  an  output  file  is specified, ld produces a file named a.out.
       This file contains the object files given as input, appropriately  com-
       bined to form an executable file.

OPTIONS
       When  linking  debugging  or  profiling  objects, include the -g or -pg
       option (see cc(1V)), as appropriate, in the ld command.

       Options should appear before filenames, except for abbreviated  library
       names specified with -l options, and some binding control options spec-
       ified by -B (which can appear anywhere in the line).

       -align datum
              Force the global uninitialized data symbol datum (usually a FOR-
              TRAN  common  block) to be page-aligned.  Increase its size to a
              whole number of pages, and place its first byte at the start  of
              a page.

       -assert assertion-keyword
              Check  an assertion about the link editing being performed.  The
              assertion desired is specified by the assertion-keyword  string.
              ld is silent if the assertion holds, else it yields a diagnostic
              and aborts.  Valid assertion-keyword's and their interpretations
              are:

                     definitions    If  the  resulting  program  were run now,
                                    there would be no run-time undefined  sym-
                                    bol diagnostics.  This assertion is set by
                                    default.

                     nodefinitions  Permit the successful linking of a program
                                    with unresolved references.

                     nosymbolic     There  are  no  symbolic  relocation items
                                    remaining to be resolved.

                     pure-text      The resulting load has no relocation items
                                    remaining in its text.

       -A name
              Incremental  loading:  linking is to be done in a manner so that
              the resulting object may be read into an already executing  pro-
              gram.  name is the name of a file whose symbol table is taken as
              a basis on which  to  define  additional  symbols.   Only  newly
              linked  material  is  entered into the text and data portions of
              a.out, but the new symbol table will reflect all symbols defined
              before  and  after  the  incremental  load.   This argument must
              appear before any other object file in the argument  list.   One
              or both of the -T options may be used as well, and will be taken
              to mean that the newly linked segment will commence at the  cor-
              responding  addresses  (which  must  be  a  multiple of the page
              size).  The default value is the old value of _end.

       -Bbinding-keyword
              Specify allowed  binding  times  for  the  items  which  follow.
              Allowed values of binding-keyword are:

              dynamic        Allow  dynamic  binding:  do not resolve symbolic
                             references, allow creation of run-time symbol and
                             relocation   environment.    -Bdynamic   is   the
                             default.   When  -Bdynamic  is  in  effect,   all
                             sharable  objects  encountered until a succeeding
                             -Bstatic may be added dynamically to  the  object
                             being  linked.   Non-sharable  objects  are bound
                             statically.

              nosymbolic     Do not perform symbolic relocation, even if other
                             options imply it.

              static         Bind statically.  Opposite of -Bdynamic.  Implied
                             when either -n or -N  is  specified.   Influences
                             handling  of all objects following its specifica-
                             tion on a command line until the next -Bdynamic.

              symbolic       Force symbolic relocation.  Normally  implied  if
                             an  entry point has been specified with -e, or if
                             dynamic loading is in effect.

       -d     Force common storage for uninitialized variables and other  com-
              mon symbols to be allocated in the current ld run, even when the
              -r flag is present (which would otherwise postpone this  binding
              until the final linking phase).

       -dc    Do -d, but also copy initialized data referenced by this program
              from shared objects.

       -dp    Force an alias definition of undefined procedure  entry  points.
              Used with dynamic binding to improve sharing and the locality of
              run-time relocations.

       -D hex Pad the data segment with zero-valued bytes to make it hex bytes
              long.

       -e entry
              Define  the  entry point: the entry argument is made the name of
              the entry point of the loaded program.  Implies -Bsymbolic.

       -lx[.v]
              This option is an abbreviation  for  the  library  name  libx.a,
              where  x  is  a  string.  ld searches for libraries first in any
              directories specified with -L  options,  then  in  the  standard
              directories  /lib,  /usr/lib,  and /usr/local/lib.  A library is
              searched when its name is encountered, so the placement of a  -l
              is  significant.  If a dynamically loadable object is found, and
              -Bdynamic is in effect at that point on the command  line,  then
              ld prepares to access the object for relocation at run-time.  In
              such a case, the optional .v suffix can be used  to  indicate  a
              specific library version.

       -Ldir  Add  dir  to  the  list  of  directories  in which to search for
              libraries.  Directories specified with -L  are  searched  before
              the  standard  directories,  /lib, /usr/lib, and /usr/local/lib.
              When building a program in which one or more objects are  loaded
              when  -Bdynamic  is in effect, those directories specified by -L
              options will be "remembered" for use at  execution  time.   This
              permits the construction of software that uses shared objects as
              libraries not residing in  the  standard  locations  and  avoids
              requiring  the  specification of the LD_LIBRARY_PATH environment
              variable in order to execute  such  software.   Note  that  such
              directories  are  retained  in exactly the form specified in the
              option,  which  means  that  relative  directory  specifications
              (i.e., not beginning with "/") will be evaluated relative to the
              current directory when the program is run, not just  during  the
              operation of ld.

       -M     Produce  a  primitive  load  map, listing the names of the files
              which will be loaded.

       -n     Arrange (by giving the output file a 0410 "magic  number")  that
              when the output file is executed, the text portion will be read-
              only with the data areas placed at the  beginning  of  the  next
              address  boundary  following  the  end  of  the  text.   Implies
              -Bstatic.

       -N     Do not make the text portion  read-only.   (Use  "magic  number"
              0407.)  Implies -Bstatic.

       -o name
              name is made the name of the ld output file, instead of a.out.

       -p     Arrange  for  the data segment to begin on a page boundary, even
              if the text is not shared (with the -N option).

       -r     Generate relocation bits in the output file so that  it  can  be
              the  subject  of  another ld run.  This flag also prevents final
              definitions from being given to common symbols,  and  suppresses
              the "undefined symbol" diagnostics.

       -s     Strip  the  output, that is, remove the symbol table and reloca-
              tion bits to save space (but impair the usefulness of the debug-
              gers).  This information can also be removed by strip(1).

       -S     Strip the output by removing all symbols except locals and glob-
              als.

       -t     Trace: display the name of each file as it is processed.

       -T[text]hex
              Start the text segment at location hex.  Specifying  -T  is  the
              same as using the -Ttext option.

       -Tdata hex
              Start  the data segment at location hex.  This option is only of
              use to programmers wishing to write code for  PROMs,  since  the
              resulting code cannot be executed by the system.

       -u name
              Enter  name  as an undefined symbol.  This is useful for loading
              wholly from a library, since initially the symbol table is empty
              and  an  unresolved  reference is needed to force the loading of
              the first routine.

       -x     Preserve only global (non-.globl) symbols in the  output  symbol
              table;  only  enter  external  symbols.   This option saves some
              space in the output file.

       -X     Record local symbols, except for those whose names begin with L.
              This option is used by cc to discard internally generated labels
              while retaining symbols local to routines.

       -ysym  Display each file in which sym appears, its type and whether the
              file  defines  or references it.  Many such options may be given
              to trace many symbols.  It is usually  necessary  to  begin  sym
              with  an  `_', as external C, FORTRAN and Pascal variables begin
              with underscores.

       -z     Arrange for the process demand paged  from  the  resulting  exe-
              cutable  file  (0413  "magic  number").   This  is  the default.
              Results in a (32-byte) header on the  output  file  followed  by
              text  and  data  segments, each of which has a multiple of page-
              size bytes (being padded out with null characters in the file if
              necessary).   With this format the first few BSS segment symbols
              may actually end up in the data segment; this is to avoid  wast-
              ing  the  space  resulting  from rounding the data segment size.
              Implies -Bdynamic.

USAGE
   Command Line Processing
       In general, options should  appear  ahead  of  the  list  of  files  to
       process.   Unless  otherwise  specified, the effect of an option covers
       all of ld operations, independent of that  option's  placement  on  the
       command line.  Exceptions to this rule include some of the binding con-
       trol options specified by `-B' and the abbreviated library-names speci-
       fied by `-l'.  These may appear anywhere, and their influence is depen-
       dent upon their location.  Some options may be obtained  from  environ-
       ment  variables, such options are interpreted before any on the command
       line (see ENVIRONMENT, below).

   Object File Processing
       The files specified on the command line  are  processed  in  the  order
       listed.   Information  is extracted from each file, and concatenated to
       form the output.  The specific processing performed  on  a  given  file
       depends  upon whether it is a simple object file, a library archive, or
       a shared library.

       Simple object (.o) files are concatenated to the  output  as  they  are
       encountered.

       Library  archive  (.a) files are searched exactly once each, as each is
       encountered; only those archive entries matching an unresolved external
       reference are extracted and concatenated to the output.  If a member of
       an archive references a symbol defined by another member of  that  same
       archive,  the member making the reference must appear before the member
       containing the definition.

       On Sun386i, a library contains a dictionary of symbols,  On  other  Sun
       systems,  processing  library  archives through ranlib(1) provides this
       dictionary.  In addition, you can use lorder(1),  in  combination  with
       tsort(1)  to  place library members in calling order (see lorder(1) for
       details), or both (for fastest symbol lookup).  The first member of  an
       archived  processed by ranlib has the reserved name of __.SYMDEF, which
       ld takes to be the dictionary of all symbols defined by members of  the
       archive.

       Sharable  objects  (.so)  are scanned for symbol definitions and refer-
       ences, but are not normally included in the output from ld,  except  in
       cases  where  a  shared library exports initialized data structures and
       the -dc option is in effect.  However, the occurrence of each  sharable
       object file in the ld command line is noted in the resulting executable
       file; this notation is utilized by an  execution-time  variant  of  ld,
       ld.so,  for  deferred and dynamic loading and binding during execution.
       See Execution-Time Loading, below, for details.

       The -l option specifies a short name for an object file or archive used
       as  a  library.   The full name of the object file is derived by adding
       the prefix lib and a suffix of either .a  or  .so[.v]  to  indicate  an
       ar(1V)  archive or a shared library, respectively.  The specific suffix
       used is determined through rules discussed in  Binding  and  Relocation
       Semantics, below.

       ld  searches  for the desired object file through a list of directories
       specified by -L options, the environment variable LD_LIBRARY_PATH,  and
       finally,  the  built-in  list  of  standard  library directories: /lib,
       /usr/lib, and /usr/local/lib.

   Binding and Relocation Semantics
       The manner in which ld processes a given object file  is  dependent  in
       part  upon  the "binding mode" in which it is operating at the time the
       file is encountered.  This binding mode is specified by  the  -B  flag,
       which takes the keyword arguments:

              dynamic   Allow  dynamic binding, do not resolve symbolic refer-
                        ences, and allow creation of execution-time symbol and
                        relocation information.  This is the default setting.

              static    Force static binding, implied by options that generate
                        non-sharable executable formats.

       -Bdynamic and -Bstatic may be specified several times, and may be  used
       to  toggle  each  other  on  and  off.   Like -l, the influence of each
       depends upon its location within the command line.  When  -Bdynamic  is
       in  effect,  -l  searches  may  be satisfied by the first occurrence of
       either form of library (.so or .a), but if both  are  encountered,  the
       .so  form  is preferred.  When -Bstatic is in effect, ld refuses to use
       any .so libraries it encounters; it continue searching for the .a form.
       Furthermore,  an  explicit  request to load a .so file is treated as an
       error.

       After ld has processed all input files and command  line  options,  the
       form  of the output it produces is based on the information provided in
       both.  ld first tries to reduce all  symbolic  references  to  relative
       numerical  offsets  within  the  executable it is building.  To perform
       this "symbolic reduction," ld must be able to determine that:

              o  all information relating to the program has been provided, in
                 particular, no .so is to be added at execution time; and/or

              o  the program has an entry point, and symbolic reduction can be
                 performed for all symbols having definitions existing in  the
                 material provided.

       It  should  be  noted  that  uninitialized "common" areas (for example,
       uninitialized C globals) are allocated by the link editor after it  has
       collected all references.  In particular, this allocation can not occur
       in a program that still requires the addition of information  contained
       in  a  .so  file,  as the missing information may affect the allocation
       process.  Initialized "commons" however,  are allocated within the exe-
       cutable in which their definition appears.

       After  ld has performed all the symbolic reductions it can, it attempts
       to transform all relative references to absolute addresses.  ld is able
       to  perform this "relative reduction" only if it has been provided some
       absolute address, either implicitly through  the  specification  of  an
       entry  point, or explicitly through ld command-line options.  If, after
       performing all the reductions it can, there are no further  relocations
       or  definitions  to  perform,  then ld has produced a completely linked
       executable.

   Execution-Time Loading
       In the event that one or more reductions can not be completed, the exe-
       cutable will require further link editing at execution time in order to
       be usable.  Such executables contain an data structure identified  with
       the  symbol __DYNAMIC.  An incompletely linked "main" program should be
       linked with a "bootstrap" routine that invokes ld.so,  which  uses  the
       information  contained  in the main program's __DYNAMIC to assemble the
       rest of the executables constituting the entire  program.   A  standard
       Sun  compilation  driver (such as cc(1V)) automatically includes such a
       module in each "main" executable.

       When ld.so is given control on program startup, it finds all .so  files
       specified when the program was constructed (and all .so's on which they
       depend), and loads them into the address space.  The algorithm by which
       such  files are found mimics that used when ld is run, and like ld, can
       be influenced by the setting of  LD_LIBRARY_PATH  and  any  -L  options
       specified  to  ld when the program was built.  ld.so then completes all
       remaining relocations, with the exception  of  procedure  call  reloca-
       tions;  failure to resolve a given non-procedural relocation results in
       termination of the program with an appropriate diagnostic.

       Procedure relocations are resolved when the referencing instruction  is
       first  executed.  It should be noted that it is possible for "undefined
       symbol" diagnostics to be produced during program execution if a  given
       target is not defined when referenced.

       Although  it is possible for binding errors to occur at execution-time,
       such an occurrence generally indicates something wrong in  the  mainte-
       nance  of  shared objects.  ld's -assert  nodefinitions function (on by
       default) checks at ld-time whether or  not  an  execution-time  binding
       error would occur.

   Version Handling for Shared Libraries
       To  allow  the independent evolution of .so's used as libraries and the
       programs which use them, ld's handling of .so files  found  through  -l
       options involves the retention and management of version control infor-
       mation.  The .so files used as such "shared libraries"  are  post-fixed
       with  a  Dewey-decimal  format  string  describing  the  version of the
       "library" contained in the file.

       The first decimal component is called  the  library's  "major  version"
       number,  and  the second component its "minor version" number.  When ld
       records a .so used as a library, it also records these two  numbers  in
       the  database  used  by  ld.so  at execution time.  In turn, ld.so uses
       these numbers to decide which of multiple versions of a  given  library
       is "best" or whether any of the available versions are acceptable.  The
       rules are:

              o  Major Versions Identical: the major version used at execution
                 time  must exactly match the version found at ld-time.  Fail-
                 ure to find an instance of the library with a matching  major
                 version  causes  a  diagnostic to be issued and the program's
                 execution to be terminated.

              o  Highest Minor Version: in the presence of multiple  instances
                 of libraries that match the desired major version, ld.so uses
                 the highest minor version it finds.  However, if the  highest
                 minor  version  found at execution time is less than the ver-
                 sion found at ld-time, a warning diagnostic is  issued;  pro-
                 gram execution continues.

       The  semantics  of  version numbers are such that major version numbers
       should be changed whenever  interfaces  are  changed.   Minor  versions
       should  be changed to reflect compatible updates to libraries, and pro-
       grams will silently favor  the  highest  compatible  version  they  can
       obtain.

   Special Symbols
       A number of symbols have special meanings to ld and programs should not
       define these symbols.  The symbols described below are  those  actually
       seen  by  ld.  Note: C and several other languages prepend symbols they
       use with `_'.

       _etext The first location after the text of the program.

       _edata The first location after initialized data.

       _end   The first location after all data.

       __DYNAMIC
              Identifies an ld-produced data structure.  It is defined with  a
              non-zero  value in executables which require execution-time link
              editing.  By convention, if defined, it is the first  symbol  in
              the symbol table associated with an a.out file.

       __GLOBAL_OFFSET_TABLE_
              A  position-independent  reference to an ld-constructed table of
              addresses.  This table is  constructed  from  "position-indepen-
              dent" data references occurring in objects that have been assem-
              bled with the assembler's -k flag (invoked on behalf of C compi-
              lations  performed  with  the  -pic flag).  A related table (for
              which no symbol is  currently  defined)  contains  a  series  of
              transfer instructions and is created from "position-independent"
              procedure calls or, if -dp is specified to ld, a list  of  unde-
              fined symbols.

       Symbols  in  object  files  beginning with the letter L are taken to be
       local symbols and unless otherwise specified are purged from ld  output
       files.

ENVIRONMENT
       LD_LIBRARY_PATH
              A  colon-separated  list  of  directories in which to search for
              libraries specified with the -l option.   Similar  to  the  PATH
              environment  variable.   LD_LIBRARY_PATH  also  affects  library
              searching during execution-time loading, causing the  search  to
              use  first  those directories found in the environment variable,
              then any directories specified by -L options,  and  finally  the
              standard  directories  /usr/lib  and /usr/local/lib.  NOTE: when
              running a set-user- or set-group-ID  program,  ld.so  will  only
              search  for  libraries  in  directories  it  "trusts", which are
              /usr/lib, /usr/5lib, /usr/local/lib, and any directories  speci-
              fied  within the executable as a result of -L options given when
              the executable was constructed.

       LD_OPTIONS
              A default set of options to ld.  LD_OPTIONS is interpreted by ld
              just  as  though  its value had been placed on the command line,
              immediately following the name used to invoke ld, as in:

              example% ld $LD_OPTIONS ... other-arguments ...

       Note: Environment variable-names beginning with  the  characters  `LD_'
       are reserved for possible future enhancements to ld.

FILES
       /usr/lib/lib*.a     libraries
       lib*.so.v           shared libraries
       lib*.sa.v           exported, initialized shared library data
       /usr/lib/ld.so      execution-time ld
       /usr/lib/*crt*.o    default program bootstraps
       a.out               output file
       /usr/local/lib

SEE ALSO
       ar(1V), as(1), cc(1V), lorder(1), ranlib(1), strip(1), tsort(1), ldcon-
       fig(8)

BUGS
       Options are being overloaded  and  are  an  inappropriate  vehicle  for
       describing  to ld the wide variety of things it can do.  There needs to
       be a link-editing language which can be used in the more complex situa-
       tions.

       The  -r  option does not properly handle programs assembled with the -k
       (position-independent) flag, invoked from cc with -pic or -PIC.



                                21 January 1990                          LD(1)