libobjc/objc/message.h

   1 /* GNU Objective C Runtime messaging declarations
   2    Copyright (C) 1993, 1995, 1996, 2004, 2009,
   3    2010 Free Software Foundation, Inc.
   4
   5 This file is part of GCC.
   6
   7 GCC is free software; you can redistribute it and/or modify
   8 it under the terms of the GNU General Public License as published by
   9 the Free Software Foundation; either version 3, or (at your option)
  10 any later version.
  11
  12 GCC is distributed in the hope that it will be useful,
  13 but WITHOUT ANY WARRANTY; without even the implied warranty of
  14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15 GNU General Public License for more details.
  16
  17 Under Section 7 of GPL version 3, you are granted additional
  18 permissions described in the GCC Runtime Library Exception, version
  19 3.1, as published by the Free Software Foundation.
  20
  21 You should have received a copy of the GNU General Public License and
  22 a copy of the GCC Runtime Library Exception along with this program;
  23 see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
  24 <http://www.gnu.org/licenses/>.  */
  25
  26 #ifndef __objc_message_INCLUDE_GNU
  27 #define __objc_message_INCLUDE_GNU
  28
  29 #include "objc.h"
  30 #include "objc-decls.h"
  31
  32 #ifdef __cplusplus
  33 extern "C" {
  34 #endif
  35
  36 /* This file includes declarations of the messaging functions and
  37    types.  */
  38
  39 /* Compatibility note: the messaging function is one area where the
  40    GNU runtime and the Apple/NeXT runtime differ significantly.  If
  41    you can, it is recommended that you use higher-level facilities
  42    (provided by a Foundation library such as GNUstep Base) to perform
  43    forwarding or other advanced messaging tricks.  */
  44
  45 /* This function returns the IMP (C function implementing a method) to
  46    use to invoke the method with selector 'op' of receiver 'receiver'.
  47
  48    This is the function used by the compiler when compiling method
  49    invocations with the GNU runtime.  For example, the method call
  50
  51      result = [receiver method];
  52
  53    is compiled by the compiler (with the GNU runtime) into the
  54    equivalent of:
  55
  56    {
  57      IMP function = objc_msg_lookup (receiver, @selector (method));
  58      result = function (receiver, @selector (method));
  59    }
  60
  61    so, a call to objc_msg_lookup() determines the IMP (the C function
  62    implementing the method) to call.  Then, the function is called.
  63    If the method takes or returns different arguments, the compiler
  64    will cast 'function' to the right type before invoking it, making
  65    sure arguments and return value are handled correctly.
  66
  67    objc_msg_lookup() must always return a valid function that can be
  68    called with the required method signature (otherwise the
  69    compiler-generated code shown above could segfault).  If 'receiver'
  70    is NULL, objc_msg_lookup() returns a C function that does nothing,
  71    ignores all its arguments, and returns NULL (see nil_method.c).  If
  72    'receiver' does not respond to the selector 'op', objc_msg_lookup()
  73    will try to call +resolveClassMethod: or resolveInstanceMethod: as
  74    appropriate, and if they return YES, it will try the lookup again
  75    (+resolveClassMethod: and +resolveInstanceMethod: can thus install
  76    dynamically methods as they are requested).  If
  77    +resolveClassMethod: or +resolveInstanceMethod: are either not
  78    available, or return NO, or return YES but 'receiver' still doesn't
  79    implement the 'selector' after calling them, the runtime returns a
  80    generic "forwarding" function that can be called with the required
  81    method signature and which can process the method invocation
  82    according to the forwarding API.  There are two runtime hooks that
  83    allow Foundation libraries (such as GNUstep-Base) to return their
  84    own forwarding function in preference to the runtime ones.  When
  85    that happens, the Foundation library effectively takes complete
  86    control of the forwarding process; any method invocation where the
  87    selector is not implemented by the receiver will end up calling a
  88    forwarding function chosen by the Foundation library.  */
  89 objc_EXPORT IMP objc_msg_lookup (id receiver, SEL op);
  90
  91 /* Structure used when a message is send to a class's super class.
  92    The compiler generates one of these structures and passes it to
  93    objc_msg_lookup_super() when a [super method] call is compiled.  */
  94
  95 /* In the traditional API, the super class field is called 'class' in
  96    Objective-C and 'super_class' in Objective-C++.  In the new API
  97    (objc/runtime.h) it is always called 'super_class'.  We detect the
  98    "traditional API" by the fact that the objc/objc-api.h header
  99    include guards are defined, which means objc/objc-api.h has been
 100    included.  This works because objc/message.h does not exist in the
 101    Traditional API and is only read because objc-api.h itself includes
 102    it.  */
 103 #ifdef __objc_api_INCLUDE_GNU
 104 /* Traditional API.  */
 105 typedef struct objc_super
 106 {
 107   id    self;       /* Id of the object sending the message. */
 108 #ifdef __cplusplus
 109   Class super_class;
 110 #else
 111   Class class;        /* Object's super class. */
 112 #endif
 113 } Super, *Super_t;
 114 #else
 115 /* Modern API.  */
 116 struct objc_super
 117 {
 118   id    self;        /* The receiver of the message.  */
 119   Class super_class; /* The superclass of the receiver.  */
 120 };
 121 #endif
 122
 123 /* This is used by the compiler instead of objc_msg_lookup () when
 124    compiling a call to 'super', such as [super method].  This requires
 125    sending a message to super->self, but looking up the method as if
 126    super->self was in class super->super_class.  */
 127 objc_EXPORT IMP objc_msg_lookup_super (struct objc_super *super, SEL sel);
 128
 129 /* Hooks for method forwarding.  They make it easy to substitute the
 130    built-in forwarding with one based on a library, such as ffi, that
 131    implement closures, thereby avoiding gcc's __builtin_apply
 132    problems.  __objc_msg_forward2's result will be preferred over that
 133    of __objc_msg_forward if both are set and return non-NULL.  */
 134 objc_EXPORT IMP (*__objc_msg_forward)(SEL);
 135 objc_EXPORT IMP (*__objc_msg_forward2)(id, SEL);
 136
 137 #ifdef __cplusplus
 138 }
 139 #endif
 140
 141 #endif /* not __objc_message_INCLUDE_GNU */