MACH-O(5)MACH-O(5)NAMEMach-O - Mach-O assembler and link editor output
SYNOPSIS
#include <mach-o/loader.h>
#include <mach-o/nlist.h>
#include <mach-o/stab.h>
#include <mach-o/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) The format
of the object file however is not 4.3BSD a.out format as the name sug‐
gests, but rather Mach-O format. The link editor will make a.out exe‐
cutable 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 <mach-o/loader.h> describes the headers,
<mach-o/nlist.h> describes the symbol table entries with <mach-
o/stab.h> supplementing it, and <mach-o/reloc.h> describes the reloca‐
tion 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 seg‐
ment and all the segment headers are created and the segments them‐
selves 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, the dynamic link editor (if used) and creates the thread(s)
for execution. Any part of the object file that is not part of a seg‐
ment is not mapped in for execution. For executable using the dynamic
link editor the headers and other link edit information is needed to
execute the file. These parts include the relocation entries, the sym‐
bol table and the string table. These parts are 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 down with the -S
option to ld(1) or various options to strip(1).
SEE ALSOas(1), ld(1), nm(1), gdb(1), stab(5), strip(1)Apple Computer, Inc. October 22, 2001 MACH-O(5)
