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) 1988-2013 Free Software Foundation, Inc. 13 14Permission is granted to copy, distribute and/or modify this document 15under the terms of the GNU Free Documentation License, Version 1.3 or 16any later version published by the Free Software Foundation; with the 17Invariant Sections being ``Funding Free Software'', the Front-Cover 18Texts being (a) (see below), and with the Back-Cover Texts being (b) 19(see below). A copy of the license is included in the section entitled 20``GNU Free Documentation License''. 21 22(a) The FSF's Front-Cover Text is: 23 24 A GNU Manual 25 26(b) The FSF's Back-Cover Text is: 27 28 You have freedom to copy and modify this GNU Manual, like GNU 29 software. Copies published by the Free Software Foundation raise 30 funds for GNU development.--> 31<meta http-equiv="Content-Style-Type" content="text/css"> 32<style type="text/css"><!-- 33 pre.display { font-family:inherit } 34 pre.format { font-family:inherit } 35 pre.smalldisplay { font-family:inherit; font-size:smaller } 36 pre.smallformat { font-family:inherit; font-size:smaller } 37 pre.smallexample { font-size:smaller } 38 pre.smalllisp { font-size:smaller } 39 span.sc { font-variant:small-caps } 40 span.roman { font-family:serif; font-weight:normal; } 41 span.sansserif { font-family:sans-serif; font-weight:normal; } 42--></style> 43<link rel="stylesheet" type="text/css" href="../cs.css"> 44</head> 45<body> 46<div class="node"> 47<a name="Method-signatures"></a> 48<p> 49Previous: <a rel="previous" accesskey="p" href="_0040encode.html#g_t_0040encode">@encode</a>, 50Up: <a rel="up" accesskey="u" href="Type-encoding.html#Type-encoding">Type encoding</a> 51<hr> 52</div> 53 54<h4 class="subsection">8.3.3 Method signatures</h4> 55 56<p>This section documents the encoding of method types, which is rarely 57needed to use Objective-C. You should skip it at a first reading; the 58runtime provides functions that will work on methods and can walk 59through the list of parameters and interpret them for you. These 60functions are part of the public “API” and are the preferred way to 61interact with method signatures from user code. 62 63 <p>But if you need to debug a problem with method signatures and need to 64know how they are implemented (i.e., the “ABI”), read on. 65 66 <p>Methods have their “signature” encoded and made available to the 67runtime. The “signature” encodes all the information required to 68dynamically build invocations of the method at runtime: return type 69and arguments. 70 71 <p>The “signature” is a null-terminated string, composed of the following: 72 73 <ul> 74<li>The return type, including type qualifiers. For example, a method 75returning <code>int</code> would have <code>i</code> here. 76 77 <li>The total size (in bytes) required to pass all the parameters. This 78includes the two hidden parameters (the object <code>self</code> and the 79method selector <code>_cmd</code>). 80 81 <li>Each argument, with the type encoding, followed by the offset (in 82bytes) of the argument in the list of parameters. 83 84 </ul> 85 86 <p>For example, a method with no arguments and returning <code>int</code> would 87have the signature <code>i8@0:4</code> if the size of a pointer is 4. The 88signature is interpreted as follows: the <code>i</code> is the return type 89(an <code>int</code>), the <code>8</code> is the total size of the parameters in 90bytes (two pointers each of size 4), the <code>@0</code> is the first 91parameter (an object at byte offset <code>0</code>) and <code>:4</code> is the 92second parameter (a <code>SEL</code> at byte offset <code>4</code>). 93 94 <p>You can easily find more examples by running the “strings” program 95on an Objective-C object file compiled by GCC. You'll see a lot of 96strings that look very much like <code>i8@0:4</code>. They are signatures 97of Objective-C methods. 98 99 </body></html> 100 101