Difference between revisions of "Mach-O"

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=== __class, __meta_class ===
 
=== __class, __meta_class ===
 +
 +
''the section may omit, if none of modules declare any custom classes''
  
 
Both sections use identical structure objc_class. (objc_class record is declared at objc header)
 
Both sections use identical structure objc_class. (objc_class record is declared at objc header)

Revision as of 22:42, 3 September 2009

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 is no documentation about __OBJC segment and its sections. The following information has been gathered from cctools sources

Structures strings

Some structures in sections contain name pointers. These names are stored in the c-strings section (segment: __TEXT; section: __cstring). The file offset for the string name can be evaluated in the following way:

string_file_offset := cstr_section.offset + (name_addr - cstr_section.addr);

Sections

__image_info

The section contains only the image info information:

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

Flags values:

ImageInfo_F_and_C = $01;
ImageInfo_GC      = $02;
ImageInfo_GC_only = $04;

__module_info

(objc_module record is declared at objc headers).

The number of objc_module structures depends on the number of .m files with objects declarations compiled. _symtab contains the number of classes and categories declared in the module.

objc_module = packed record
  version : culong; // version number = 7 
  size    : culong; // sizeof(objc_module)?
  name    : PChar;  // virtual memory address of the module name
                    // Usually mapped to NULL string
  _symtab : Symtab; // virtual memory address of a proper objc_symtab structure (in __symbols section)
end;

__symbols

the section contains symbol table for a module. (symtable record is declared at objc headers)

 objc_symtab = record
   sel_ref_cnt : culong;  // zero
   refs        : PSEL;    // zero
   cls_def_cnt : cushort; // number of declared classes in the module
   cat_def_cnt : cushort; // number of declared categories
   /// defs: array [0..cls_def_cnt-1] of Pointer; // virtual address of objc class declaration (given at __class section)
 end;

__class, __meta_class

the section may omit, if none of modules declare any custom classes

Both sections use identical structure objc_class. (objc_class record is declared at objc header) Objective-C classes are declared in pair with their meta_classes.

objc_class = record
  isa           : PChar;   // for class declaration: virtual address of meta-class declaration
                           // for meta-class declaration: ...
  super_class   : PChar;   // contains the pointer to super_class name
  name          : PChar;   // pointer to class name 		
  version       : PChat;   // = 0 (for obj-c version 1?)
  info          : culong;  // CLS_CLASS for classes
                           // CLS_META  for meta-classes 
  instance_size : culong;  // size of the instance:
                           // sizeof(objc_class) + VariablesSize 

  ivars         : Pobjc_ivar_list;       // virtual address of objc_ivar_list (stored in "__instance_vars" section)

  methodLists   : PPobjc_method_list;    // pointer to virtual mapped method list
                                         // description for methods (stored in "__inst_meth" section)

  cache         : Pobjc_cache;           // zero
  protocols     : Pobjc_protocol_list;   // todo:
end;

__instance_vars

the section is optional and may omit if none of classes declares instance variables

The section consists of number objc_ivar_list structutres. The number of structures depends upon number of declared classes using instance variables. (objc_ivar_list is declared in objc header)

objc_ivar_list = record
  ivar_count  : cint;   // number of variables in the list
  ivar_list   : array[0..ivar_count-1] of objc_ivar;  variable length structure }
end;
objc_ivar = record
  ivar_name   : PChar; // vm addr of the variable name
  ivar_type   : PChar; // vm addr of obj-c variable encoded type 
  ivar_offset : cint;  // offset from the start of the instance. (the lowest ivar_offset is 40,
                       // because sizeof(objc_class) = 40, and objc_class is also part of the instance)
end;

__protocol

__cat_inst_meth

__cat_cls_meth

Mach-O additional 30Kb size

FPC built mach-o executables are somehow larger, compared to the win32 target, for example

begin
  writeln('hello world');
end.

gives a 30k (stripped) executable for Win, and 60k for Mac OS X.


Jonas Maebe: It's because there was a bug in older versions of the Darwin linker that required adding ".reference" assembler directives for routines that have more than one assembler name (most of the compiler helpers in the rtl have that). This fixed the problem, but as a result they are never smart linked out. It's only a fixed overhead of that 30kb (most programs don't contain any extra routines with multiple assembler names)