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111
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1 /* GNU Objective C Runtime messaging declarations
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145
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2 Copyright (C) 1993-2020 Free Software Foundation, Inc.
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111
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3
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4 This file is part of GCC.
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5
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6 GCC is free software; you can redistribute it and/or modify
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7 it under the terms of the GNU General Public License as published by
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8 the Free Software Foundation; either version 3, or (at your option)
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9 any later version.
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10
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11 GCC is distributed in the hope that it will be useful,
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12 but WITHOUT ANY WARRANTY; without even the implied warranty of
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13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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14 GNU General Public License for more details.
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15
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16 Under Section 7 of GPL version 3, you are granted additional
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17 permissions described in the GCC Runtime Library Exception, version
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18 3.1, as published by the Free Software Foundation.
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19
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20 You should have received a copy of the GNU General Public License and
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21 a copy of the GCC Runtime Library Exception along with this program;
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22 see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
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23 <http://www.gnu.org/licenses/>. */
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24
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25 #ifndef __objc_message_INCLUDE_GNU
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26 #define __objc_message_INCLUDE_GNU
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27
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28 #include "objc.h"
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29 #include "objc-decls.h"
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30
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31 #ifdef __cplusplus
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32 extern "C" {
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33 #endif
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34
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35 /* This file includes declarations of the messaging functions and
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36 types. */
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37
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38 /* Compatibility note: the messaging function is one area where the
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39 GNU runtime and the Apple/NeXT runtime differ significantly. If
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40 you can, it is recommended that you use higher-level facilities
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41 (provided by a Foundation library such as GNUstep Base) to perform
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42 forwarding or other advanced messaging tricks. */
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43
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44 /* This function returns the IMP (C function implementing a method) to
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45 use to invoke the method with selector 'op' of receiver 'receiver'.
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46
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47 This is the function used by the compiler when compiling method
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48 invocations with the GNU runtime. For example, the method call
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49
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50 result = [receiver method];
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51
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52 is compiled by the compiler (with the GNU runtime) into the
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53 equivalent of:
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54
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55 {
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56 IMP function = objc_msg_lookup (receiver, @selector (method));
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57 result = function (receiver, @selector (method));
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58 }
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59
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60 so, a call to objc_msg_lookup() determines the IMP (the C function
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61 implementing the method) to call. Then, the function is called.
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62 If the method takes or returns different arguments, the compiler
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63 will cast 'function' to the right type before invoking it, making
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64 sure arguments and return value are handled correctly.
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65
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66 objc_msg_lookup() must always return a valid function that can be
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67 called with the required method signature (otherwise the
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68 compiler-generated code shown above could segfault). If 'receiver'
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69 is NULL, objc_msg_lookup() returns a C function that does nothing,
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70 ignores all its arguments, and returns NULL (see nil_method.c). If
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71 'receiver' does not respond to the selector 'op', objc_msg_lookup()
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72 will try to call +resolveClassMethod: or resolveInstanceMethod: as
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73 appropriate, and if they return YES, it will try the lookup again
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74 (+resolveClassMethod: and +resolveInstanceMethod: can thus install
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75 dynamically methods as they are requested). If
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76 +resolveClassMethod: or +resolveInstanceMethod: are either not
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77 available, or return NO, or return YES but 'receiver' still doesn't
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78 implement the 'selector' after calling them, the runtime returns a
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79 generic "forwarding" function that can be called with the required
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80 method signature and which can process the method invocation
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81 according to the forwarding API. There are two runtime hooks that
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82 allow Foundation libraries (such as GNUstep-Base) to return their
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83 own forwarding function in preference to the runtime ones. When
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84 that happens, the Foundation library effectively takes complete
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85 control of the forwarding process; any method invocation where the
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86 selector is not implemented by the receiver will end up calling a
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87 forwarding function chosen by the Foundation library. */
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88 objc_EXPORT IMP objc_msg_lookup (id receiver, SEL op);
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89
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90 /* Structure used when a message is send to a class's super class.
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91 The compiler generates one of these structures and passes it to
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92 objc_msg_lookup_super() when a [super method] call is compiled. */
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93
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94 /* Modern API. */
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95 struct objc_super
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96 {
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97 id self; /* The receiver of the message. */
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98 Class super_class; /* The superclass of the receiver. */
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99 };
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100
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101 /* This is used by the compiler instead of objc_msg_lookup () when
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102 compiling a call to 'super', such as [super method]. This requires
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103 sending a message to super->self, but looking up the method as if
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104 super->self was in class super->super_class. */
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105 objc_EXPORT IMP objc_msg_lookup_super (struct objc_super *super, SEL sel);
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106
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107 /* Hooks for method forwarding. They make it easy to substitute the
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108 built-in forwarding with one based on a library, such as ffi, that
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109 implement closures, thereby avoiding gcc's __builtin_apply
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110 problems. __objc_msg_forward2's result will be preferred over that
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111 of __objc_msg_forward if both are set and return non-NULL. */
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112 objc_EXPORT IMP (*__objc_msg_forward)(SEL);
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113 objc_EXPORT IMP (*__objc_msg_forward2)(id, SEL);
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114
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115 #ifdef __cplusplus
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116 }
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117 #endif
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118
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119 #endif /* not __objc_message_INCLUDE_GNU */
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