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 /* Modern API. */ 96 struct objc_super 97 { 98 id self; /* The receiver of the message. */ 99 Class super_class; /* The superclass of the receiver. */ 100 }; 101 102 /* This is used by the compiler instead of objc_msg_lookup () when 103 compiling a call to 'super', such as [super method]. This requires 104 sending a message to super->self, but looking up the method as if 105 super->self was in class super->super_class. */ 106 objc_EXPORT IMP objc_msg_lookup_super (struct objc_super *super, SEL sel); 107 108 /* Hooks for method forwarding. They make it easy to substitute the 109 built-in forwarding with one based on a library, such as ffi, that 110 implement closures, thereby avoiding gcc's __builtin_apply 111 problems. __objc_msg_forward2's result will be preferred over that 112 of __objc_msg_forward if both are set and return non-NULL. */ 113 objc_EXPORT IMP (*__objc_msg_forward)(SEL); 114 objc_EXPORT IMP (*__objc_msg_forward2)(id, SEL); 115 116 #ifdef __cplusplus 117 } 118 #endif 119 120 #endif /* not __objc_message_INCLUDE_GNU */ 121