1<html lang="en"> 2<head> 3<title>Method signatures - 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="Type-encoding.html#Type-encoding" title="Type encoding"> 9<link rel="prev" href="_0040_0040encode.html#g_t_0040_0040encode" title="@@encode"> 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="Method-signatures"></a> 50<p> 51Previous: <a rel="previous" accesskey="p" href="_0040encode.html#g_t_0040encode">@encode</a>, 52Up: <a rel="up" accesskey="u" href="Type-encoding.html#Type-encoding">Type encoding</a> 53<hr> 54</div> 55 56<h4 class="subsection">8.3.3 Method signatures</h4> 57 58<p>This section documents the encoding of method types, which is rarely 59needed to use Objective-C. You should skip it at a first reading; the 60runtime provides functions that will work on methods and can walk 61through the list of parameters and interpret them for you. These 62functions are part of the public “API” and are the preferred way to 63interact with method signatures from user code. 64 65 <p>But if you need to debug a problem with method signatures and need to 66know how they are implemented (i.e., the “ABI”), read on. 67 68 <p>Methods have their “signature” encoded and made available to the 69runtime. The “signature” encodes all the information required to 70dynamically build invocations of the method at runtime: return type 71and arguments. 72 73 <p>The “signature” is a null-terminated string, composed of the following: 74 75 <ul> 76<li>The return type, including type qualifiers. For example, a method 77returning <code>int</code> would have <code>i</code> here. 78 79 <li>The total size (in bytes) required to pass all the parameters. This 80includes the two hidden parameters (the object <code>self</code> and the 81method selector <code>_cmd</code>). 82 83 <li>Each argument, with the type encoding, followed by the offset (in 84bytes) of the argument in the list of parameters. 85 86 </ul> 87 88 <p>For example, a method with no arguments and returning <code>int</code> would 89have the signature <code>i8@0:4</code> if the size of a pointer is 4. The 90signature is interpreted as follows: the <code>i</code> is the return type 91(an <code>int</code>), the <code>8</code> is the total size of the parameters in 92bytes (two pointers each of size 4), the <code>@0</code> is the first 93parameter (an object at byte offset <code>0</code>) and <code>:4</code> is the 94second parameter (a <code>SEL</code> at byte offset <code>4</code>). 95 96 <p>You can easily find more examples by running the “strings” program 97on an Objective-C object file compiled by GCC. You'll see a lot of 98strings that look very much like <code>i8@0:4</code>. They are signatures 99of Objective-C methods. 100 101 </body></html> 102 103