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MACH-O(5)							     MACH-O(5)

NAME
       Mach-O - Mach-O assembler and link editor output

SYNOPSIS
       #include <sys/loader.h>
       #include <nlist.h>
       #include <stab.h>
       #include <reloc.h>

DESCRIPTION
       The object files produced by the assembler and link editor are in Mach-
       O (Mach object) file format.  The file name a.out is the default output
       file  name  of  the  assembler  as(1)  and  the link editor ld(1) as is
       conventional with most UNIX-like compiler systems.  The format  of  the
       object  file  however  is not 4.3BSD a.out format as the name suggests,
       but rather Mach-O format.  The link editor will make  a.out  executable
       if  the resulting format is an executable type and there were no errors
       and no unresolved external references.

       The complete description of a Mach-O file  is  given  in	 a  number  of
       include	 files.	   The	file  <sys/loader.h>  describes	 the  headers,
       <nlist.h>  describes   the   symbol   table   entries   with   <stab.h>
       supplementing it, and <reloc.h> describes the relocation entries.

       The  actual  instructions and data used by the program represented by a
       Mach-O file are the contents of its  sections.	Sections  are  grouped
       together in segments.  Each section carries with it, in its header, the
       information as to which segment it belongs in.  When a file  type  that
       is  executable  is  created  the	 sections  are	placed in their proper
       segment and all the  segment  headers  are  created  and	 the  segments
       themselves  are	padded	out  to	 the  segment alignment (typically the
       target pagesize).  For the object file type produced  by	 an  assembler
       (or by the link editor for further linking) all the sections are placed
       in one segment for compactness.

       When the kernel executes a Mach-O file it maps  in  the	object	file's
       segments,  any  segments	 from  fixed virtual shared libraries that the
       object uses and creates the thread(s) for execution.  Any part  of  the
       object  file  that  is  not  part  of  a	 segment  is not mapped in for
       execution and except for the headers is not needed to execute the file.
       These  parts  include  the relocation entries, the symbol table and the
       string table.  These parts can however be mapped in with the use of the
       link editor's -seglinkedit option which creates a segment that contains
       these parts.  These parts can be stripped with the -s option  to	 ld(1)
       or strip(1).

       For  more  information, see Chapter 8, "Mach Object Files," in the NeXT
       Development Tools manual.

SEE ALSO
       as(1), ld(1), nm(1), gdb(1), stab(5), strip(1)

NeXT Computer, Inc.	       January 10, 1990			     MACH-O(5)
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