libobjc/objc/message.h

   1 /* GNU Objective C Runtime messaging declarations
   2    Copyright (C) 1993-2023 Free Software Foundation, Inc.
   3
   4 This file is part of GCC.
   5
   6 GCC is free software; you can redistribute it and/or modify
   7 it under the terms of the GNU General Public License as published by
   8 the Free Software Foundation; either version 3, or (at your option)
   9 any later version.
  10
  11 GCC is distributed in the hope that it will be useful,
  12 but WITHOUT ANY WARRANTY; without even the implied warranty of
  13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14 GNU General Public License for more details.
  15
  16 Under Section 7 of GPL version 3, you are granted additional
  17 permissions described in the GCC Runtime Library Exception, version
  18 3.1, as published by the Free Software Foundation.
  19
  20 You should have received a copy of the GNU General Public License and
  21 a copy of the GCC Runtime Library Exception along with this program;
  22 see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
  23 <http://www.gnu.org/licenses/>.  */
  24
  25 #ifndef __objc_message_INCLUDE_GNU
  26 #define __objc_message_INCLUDE_GNU
  27
  28 #include "objc.h"
  29 #include "objc-decls.h"
  30
  31 #ifdef __cplusplus
  32 extern "C" {
  33 #endif
  34
  35 /* This file includes declarations of the messaging functions and
  36    types.  */
  37
  38 /* Compatibility note: the messaging function is one area where the
  39    GNU runtime and the Apple/NeXT runtime differ significantly.  If
  40    you can, it is recommended that you use higher-level facilities
  41    (provided by a Foundation library such as GNUstep Base) to perform
  42    forwarding or other advanced messaging tricks.  */
  43
  44 /* This function returns the IMP (C function implementing a method) to
  45    use to invoke the method with selector 'op' of receiver 'receiver'.
  46
  47    This is the function used by the compiler when compiling method
  48    invocations with the GNU runtime.  For example, the method call
  49
  50      result = [receiver method];
  51
  52    is compiled by the compiler (with the GNU runtime) into the
  53    equivalent of:
  54
  55    {
  56      IMP function = objc_msg_lookup (receiver, @selector (method));
  57      result = function (receiver, @selector (method));
  58    }
  59
  60    so, a call to objc_msg_lookup() determines the IMP (the C function
  61    implementing the method) to call.  Then, the function is called.
  62    If the method takes or returns different arguments, the compiler
  63    will cast 'function' to the right type before invoking it, making
  64    sure arguments and return value are handled correctly.
  65
  66    objc_msg_lookup() must always return a valid function that can be
  67    called with the required method signature (otherwise the
  68    compiler-generated code shown above could segfault).  If 'receiver'
  69    is NULL, objc_msg_lookup() returns a C function that does nothing,
  70    ignores all its arguments, and returns NULL (see nil_method.c).  If
  71    'receiver' does not respond to the selector 'op', objc_msg_lookup()
  72    will try to call +resolveClassMethod: or resolveInstanceMethod: as
  73    appropriate, and if they return YES, it will try the lookup again
  74    (+resolveClassMethod: and +resolveInstanceMethod: can thus install
  75    dynamically methods as they are requested).  If
  76    +resolveClassMethod: or +resolveInstanceMethod: are either not
  77    available, or return NO, or return YES but 'receiver' still doesn't
  78    implement the 'selector' after calling them, the runtime returns a
  79    generic "forwarding" function that can be called with the required
  80    method signature and which can process the method invocation
  81    according to the forwarding API.  There are two runtime hooks that
  82    allow Foundation libraries (such as GNUstep-Base) to return their
  83    own forwarding function in preference to the runtime ones.  When
  84    that happens, the Foundation library effectively takes complete
  85    control of the forwarding process; any method invocation where the
  86    selector is not implemented by the receiver will end up calling a
  87    forwarding function chosen by the Foundation library.  */
  88 objc_EXPORT IMP objc_msg_lookup (id receiver, SEL op);
  89
  90 /* Structure used when a message is send to a class's super class.
  91    The compiler generates one of these structures and passes it to
  92    objc_msg_lookup_super() when a [super method] call is compiled.  */
  93
  94 /* Modern API.  */
  95 struct objc_super
  96 {
  97   id    self;        /* The receiver of the message.  */
  98   Class super_class; /* The superclass of the receiver.  */
  99 };
 100
 101 /* This is used by the compiler instead of objc_msg_lookup () when
 102    compiling a call to 'super', such as [super method].  This requires
 103    sending a message to super->self, but looking up the method as if
 104    super->self was in class super->super_class.  */
 105 objc_EXPORT IMP objc_msg_lookup_super (struct objc_super *super, SEL sel);
 106
 107 /* Hooks for method forwarding.  They make it easy to substitute the
 108    built-in forwarding with one based on a library, such as ffi, that
 109    implement closures, thereby avoiding gcc's __builtin_apply
 110    problems.  __objc_msg_forward2's result will be preferred over that
 111    of __objc_msg_forward if both are set and return non-NULL.  */
 112 objc_EXPORT IMP (*__objc_msg_forward)(SEL);
 113 objc_EXPORT IMP (*__objc_msg_forward2)(id, SEL);
 114
 115 #ifdef __cplusplus
 116 }
 117 #endif
 118
 119 #endif /* not __objc_message_INCLUDE_GNU */