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LD(1)											    LD(1)

NAME
       ld - link editor (2BSD)

SYNOPSIS
       ld [ option ] ... file ...

DESCRIPTION
       Ld  combines  several object programs into one, resolves external references, and searches
       libraries.  In the simplest case several object files are given,  and  ld  combines  them,
       producing  an object module which can be either executed or become the input for a further
       ld run.	(In the latter case, the -r option must  be  given  to	preserve  the  relocation
       bits.)  The output of ld is left on a.out.  This file is made executable only if no errors
       occurred during the load.

       The argument routines are concatenated in the order specified.  The  entry  point  of  the
       output is the beginning of the first routine (unless the -e option is specified).

       If  any	argument is a library, it is searched exactly once at the point it is encountered
       in the argument list.  Only those routines defining an unresolved external  reference  are
       loaded.	 If  a	routine from a library references another routine in the library, and the
       library has not been processed by ranlib(1), the referenced routine must appear after  the
       referencing  routine  in  the library.  Thus the order of programs within libraries may be
       important.  The first member of a library should be a file  named  `__.SYMDEF',	which  is
       understood  to be a dictionary for the library as produced by ranlib(1); the dictionary is
       searched iteratively to satisfy as many references as possible.

       The symbols `_etext', `_edata' and `_end' (`etext', `edata' and `end' in C) are	reserved,
       and  if	referred  to, are set to the first location above the program, the first location
       above initialized data, and the first location above all data respectively.  It	is  erro-
       neous to define these symbols.

       Ld understands several options.	Except for -l, they should appear before the file names.

       -D     Take the next argument as a decimal number and pad the data segment with zero bytes
	      to the indicated length.

       -d     Force definition of common storage even if the -r flag is present.

       -e     The following argument is taken to be the name of the entry  point  of  the  loaded
	      program; location 0 is the default.

       -Ldir  Add  dir	to the list of directories in which libraries are searched for.  Directo-
	      ries specified with -L are searched before the standard directories.

       -lx    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.

       -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 and shared among all users  execut-
	      ing the file.  This involves moving the data areas up to the first possible 8K byte
	      boundary following the end of the text.  This option creates  a  `pure  executable'
	      format.

       -i     When the output file is executed, the program text and data areas will live in sep-
	      arate address spaces.  The only difference between this option and -n is that  here
	      the  text  and data segments are in separate address spaces and both start at loca-
	      tion 0.  This option creates a `separate executable' format.

       -z     This option is a synonym for the -i option.  On other systems (4.3BSD for  example)
	      the  -z option causes a demand paged executable to be built.  This option was added
	      to 2.11BSD because some systems (those which use gcc) do not safely ignore (with	a
	      warning)	the  -i  option.   Adding the -z option to 2.11BSD allows makefiles to be
	      copied freely between multiple platforms once again.

       -O     This is a text replacement overlay file; only the text segment will be replaced  by
	      execve(2).   Shared data must have the same size as the program overlaid, otherwise
	      the execve(2) will fail.	The entry point to the overlay may be defined with the -e
	      option.  This option allows the creation of a `replacement executable' format.

       -o     The  name  argument  after -o is used as the name of the ld output file, instead of
	      a.out.

       -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 com-
	      mon symbols, and suppresses the `undefined symbol' diagnostics.	(Note  that  this
	      option cannot be used with overlays (-Z) since they cannot be reloaded.)

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

       -q     ("quiet")  Suppress  the	reporting  of undefined symbols.  Normally only used when
	      building networked kernels - the large number of undefined symbols is  normal  (due
	      to the three phase link proceedure) but can be distracting none the less.

       -t     ("trace")  Print the name of each file as it is processed.

       -u     Take the following argument as a symbol and enter it as undefined in the symbol ta-
	      ble.  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.

       -v     ("verbose")  Print the VM statistics.  Printing out the number of pages swapped  to
	      and  from the VM tmp file is now optional and only used when a problem is suspected
	      (or if you are voyeuristic).

       -X     Save local symbols except for those whose names begin with  `L'.	 This  option  is
	      used  by cc(1) to discard internally-generated labels while retaining symbols local
	      to routines.

       -x     Do not preserve local (non-.globl) symbols in the output symbol table;  only  enter
	      external symbols.  This option saves some space in the output file.  It also allows
	      temporary labels	to be discarded to prevent redefinition in sucessive ld's.  Warn-
	      ing:  adb uses these local symbols, especially when debugging overlaid programs, so
	      some debugging information is necessarily lost if this option is used.

       -Z     Indicate the creation of an automatic-overlay format.  In addition a -i or -n  must
	      be  present  as overlays only work with shared text objects.  Repeated instances of
	      -Z bracket the modules that will be loaded into a given  overlay.   Modules  before
	      the  first  -Z or after the concluding -Y will be loaded into the non-overlaid text
	      (base) area.  Note that there may be a maximum of  NOVL  (currently  15)	overlays.
	      This option produces the `overlaid pure executable' and the `overlaid separate exe-
	      cutable' formats.  The loader creates a small entry interface in the  base  segment
	      for  each subroutine in an overlay.  This interface ("thunk") arranges for the cor-
	      rect overlay to be present before the actual routine is entered.

       -Y     Terminate text overlays.	This allows any remaining  modules  or	libraries  to  be
	      loaded  into  the  base area.  Note that the -Y option used to be -L, but had to be
	      changed when the loader was brought up to date with the 4.3BSD loader which uses -L
	      to indicate a directory to be searched for library references.

AUTOMATIC TEXT OVERLAY EXAMPLE
       To set up an automatic text overlay object with the loader, use a command of the form:

	      ld -n -X /lib/crt0.o base.o base2.o
	      -Z ov1a.o ov1b.o ...
	      -Z ov2a.o ov2b.o ...
	      -Y base3.o ... -lovc

       Assembly  source  code  must be compiled using the assembler overlay flags: "as -V prog.s"
       which causes the assembler to leave certain symbols unresolved so that  ld  may	rearrange
       them.  The various system compilers automatically use this option.

       When arranging modules into overlays, the following rules control the maximum sizes for an
       executable file.  The magic numbers are due to the granularity of PDP-11 segmentation reg-
       isters  (there  are  8  registers, each controlling an 8192-byte segment).  The program is
       made up of four areas: base text, overlay text, data + bss, and stack sections.	The  size
       of  the	overlay  section  is controlled by the size of the largest of the overlays.  Each
       section starts at an address that is a multiple of 8Kb, thus the size of each  section  is
       rounded up to a multiple of 8Kb.

       In  the	case  of separate overlaid executable files, the text and overlays share one 64Kb
       byte address space; and the data + bss and stack share the other.  Thus, the total of  the
       base  text  size  (rounded up to an 8Kb boundary) plus the maximum overlay size (similarly
       rounded) must be less than or equal to 64Kb.  Or, put another way, since there are only	8
       segmentation registers available, the number of segmentation registers needed for an over-
       laid object must be less than or equal to 8.  As an example, if the base text segment  has
       36800  bytes  and the largest overlay takes 14144, the base will fit in 5 segments and the
       overlays in 2 segments; leaving one to spare.  The data and bss together  must  fit  in	7
       segments  (56K  bytes),	leaving  one 8Kb segment for the stack.  All of the limits can be
       checked by using checkobj(1).

       For pure overlaid programs, the rules are similar except that all four sections share  one
       64K-byte  address space.  The number of segments required by the text, overlay, data + bss
       and stack are calculated in the same way.  The sum of the segments required, including one
       for the stack, must be less than or equal to 8.	Example: a program has 8128 bytes of base
       text, the largest overlay is 16248 bytes, and the data and  bss	total  19500.	The  text
       requires 1 8Kb segment, the overlays 2, and data and bss use 4, leaving one for the stack.

FILES
       /lib/lib*.a	      libraries
       /usr/lib/lib*.a	      more libraries
       /usr/local/lib/lib*.a  still more libraries
       a.out		      output file

SEE ALSO
       adb(1), ar(1), as(1), cc(1), checkobj(1), f77(1), file(1), ranlib(1), size(1), a.out(5)

BUGS
       The  text  overlay  scheme presented is unique to the PDP-11 and 2BSD.  The -i, -P, -Z, -Y
       options are specific to 2BSD.  The -q and -v options are new with 2.11BSD.

3rd Berkeley Distribution		   May 08, 1995 				    LD(1)
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