Mach-O

from Wikipedia:

Mach-O, short for Mach object file format, is a file format for executables, object code, shared libraries, dynamically-loaded code, and core dumps. A derivation of the a.out format, Mach-O offered more extensibility and faster access to information in the symbol table.

File Format Reference can be found here

Following tools are used in Mac OS X to view Mach-O files:

otool - object file displaying tool

nm - display name list (symbol table)

Objective-C segment

There're no documentation found over __OBJC segment and its sections. The following information has been gathered from cctools sources

Sections

__module_info

(objc_module record is declared at objc headers)

objc_module = packed record
  version : culong; // version number  
  size    : culong; // sizeof(objc_module)?
  name    : PChar;
  _symtab : Symtab;
end; 

__image_info

The section contents image info information (8 bytes):

imageInfo = packed record
  version : uint32_t;
  flags   : uint32_t;
end;

Flags values:

F_and_C = $01;
GC      = $02;
GC_only = $04;