1 /* GNU Objective C Runtime messaging declarations 2 Copyright (C) 1993-2016 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 */ 120