1<html lang="en"> 2<head> 3<title>Messaging with the GNU Objective-C runtime - Using the GNU Compiler Collection (GCC)</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="Using the GNU Compiler Collection (GCC)"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="Objective_002dC.html#Objective_002dC" title="Objective-C"> 9<link rel="prev" href="Fast-enumeration.html#Fast-enumeration" title="Fast enumeration"> 10<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 11<!-- 12Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 131998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 142010 Free Software Foundation, Inc. 15 16Permission is granted to copy, distribute and/or modify this document 17under the terms of the GNU Free Documentation License, Version 1.3 or 18any later version published by the Free Software Foundation; with the 19Invariant Sections being ``Funding Free Software'', the Front-Cover 20Texts being (a) (see below), and with the Back-Cover Texts being (b) 21(see below). A copy of the license is included in the section entitled 22``GNU Free Documentation License''. 23 24(a) The FSF's Front-Cover Text is: 25 26 A GNU Manual 27 28(b) The FSF's Back-Cover Text is: 29 30 You have freedom to copy and modify this GNU Manual, like GNU 31 software. Copies published by the Free Software Foundation raise 32 funds for GNU development.--> 33<meta http-equiv="Content-Style-Type" content="text/css"> 34<style type="text/css"><!-- 35 pre.display { font-family:inherit } 36 pre.format { font-family:inherit } 37 pre.smalldisplay { font-family:inherit; font-size:smaller } 38 pre.smallformat { font-family:inherit; font-size:smaller } 39 pre.smallexample { font-size:smaller } 40 pre.smalllisp { font-size:smaller } 41 span.sc { font-variant:small-caps } 42 span.roman { font-family:serif; font-weight:normal; } 43 span.sansserif { font-family:sans-serif; font-weight:normal; } 44--></style> 45<link rel="stylesheet" type="text/css" href="../cs.css"> 46</head> 47<body> 48<div class="node"> 49<a name="Messaging-with-the-GNU-Objective-C-runtime"></a> 50<a name="Messaging-with-the-GNU-Objective_002dC-runtime"></a> 51<p> 52Previous: <a rel="previous" accesskey="p" href="Fast-enumeration.html#Fast-enumeration">Fast enumeration</a>, 53Up: <a rel="up" accesskey="u" href="Objective_002dC.html#Objective_002dC">Objective-C</a> 54<hr> 55</div> 56 57<h3 class="section">8.10 Messaging with the GNU Objective-C runtime</h3> 58 59<p>This section is specific for the GNU Objective-C runtime. If you are 60using a different runtime, you can skip it. 61 62 <p>The implementation of messaging in the GNU Objective-C runtime is 63designed to be portable, and so is based on standard C. 64 65 <p>Sending a message in the GNU Objective-C runtime is composed of two 66separate steps. First, there is a call to the lookup function, 67<code>objc_msg_lookup ()</code> (or, in the case of messages to super, 68<code>objc_msg_lookup_super ()</code>). This runtime function takes as 69argument the receiver and the selector of the method to be called; it 70returns the <code>IMP</code>, that is a pointer to the function implementing 71the method. The second step of method invocation consists of casting 72this pointer function to the appropriate function pointer type, and 73calling the function pointed to it with the right arguments. 74 75 <p>For example, when the compiler encounters a method invocation such as 76<code>[object init]</code>, it compiles it into a call to 77<code>objc_msg_lookup (object, @selector(init))</code> followed by a cast 78of the returned value to the appropriate function pointer type, and 79then it calls it. 80 81<ul class="menu"> 82<li><a accesskey="1" href="Dynamically-registering-methods.html#Dynamically-registering-methods">Dynamically registering methods</a> 83<li><a accesskey="2" href="Forwarding-hook.html#Forwarding-hook">Forwarding hook</a> 84</ul> 85 86<!-- ========================================================================= --> 87 </body></html> 88 89