MACH-O(5) File Formats Manual MACH-O(5)
Mach-O - Mach-O assembler and link editor output
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 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 <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 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, 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 segment 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
symbol 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).
as(1), ld(1), nm(1), gdb(1), stab(5), strip(1)
Apple Computer, Inc. October 22, 2001 MACH-O(5)