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| <title>Using the GNU Compiler Collection (GCC): Messaging with the GNU Objective-C runtime</title> |
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| <h3 class="section">8.10 Messaging with the GNU Objective-C runtime</h3> |
| |
| <p>This section is specific for the GNU Objective-C runtime. If you are |
| using a different runtime, you can skip it. |
| </p> |
| <p>The implementation of messaging in the GNU Objective-C runtime is |
| designed to be portable, and so is based on standard C. |
| </p> |
| <p>Sending a message in the GNU Objective-C runtime is composed of two |
| separate steps. First, there is a call to the lookup function, |
| <code>objc_msg_lookup ()</code> (or, in the case of messages to super, |
| <code>objc_msg_lookup_super ()</code>). This runtime function takes as |
| argument the receiver and the selector of the method to be called; it |
| returns the <code>IMP</code>, that is a pointer to the function implementing |
| the method. The second step of method invocation consists of casting |
| this pointer function to the appropriate function pointer type, and |
| calling the function pointed to it with the right arguments. |
| </p> |
| <p>For example, when the compiler encounters a method invocation such as |
| <code>[object init]</code>, it compiles it into a call to |
| <code>objc_msg_lookup (object, @selector(init))</code> followed by a cast |
| of the returned value to the appropriate function pointer type, and |
| then it calls it. |
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