1<html lang="en"> 2<head> 3<title>Function Prototypes - 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="C-Extensions.html#C-Extensions" title="C Extensions"> 9<link rel="prev" href="Attribute-Syntax.html#Attribute-Syntax" title="Attribute Syntax"> 10<link rel="next" href="C_002b_002b-Comments.html#C_002b_002b-Comments" title="C++ Comments"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 141998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 152010 Free Software Foundation, Inc. 16 17Permission is granted to copy, distribute and/or modify this document 18under the terms of the GNU Free Documentation License, Version 1.3 or 19any later version published by the Free Software Foundation; with the 20Invariant Sections being ``Funding Free Software'', the Front-Cover 21Texts being (a) (see below), and with the Back-Cover Texts being (b) 22(see below). A copy of the license is included in the section entitled 23``GNU Free Documentation License''. 24 25(a) The FSF's Front-Cover Text is: 26 27 A GNU Manual 28 29(b) The FSF's Back-Cover Text is: 30 31 You have freedom to copy and modify this GNU Manual, like GNU 32 software. Copies published by the Free Software Foundation raise 33 funds for GNU development.--> 34<meta http-equiv="Content-Style-Type" content="text/css"> 35<style type="text/css"><!-- 36 pre.display { font-family:inherit } 37 pre.format { font-family:inherit } 38 pre.smalldisplay { font-family:inherit; font-size:smaller } 39 pre.smallformat { font-family:inherit; font-size:smaller } 40 pre.smallexample { font-size:smaller } 41 pre.smalllisp { font-size:smaller } 42 span.sc { font-variant:small-caps } 43 span.roman { font-family:serif; font-weight:normal; } 44 span.sansserif { font-family:sans-serif; font-weight:normal; } 45--></style> 46<link rel="stylesheet" type="text/css" href="../cs.css"> 47</head> 48<body> 49<div class="node"> 50<a name="Function-Prototypes"></a> 51<p> 52Next: <a rel="next" accesskey="n" href="C_002b_002b-Comments.html#C_002b_002b-Comments">C++ Comments</a>, 53Previous: <a rel="previous" accesskey="p" href="Attribute-Syntax.html#Attribute-Syntax">Attribute Syntax</a>, 54Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a> 55<hr> 56</div> 57 58<h3 class="section">6.32 Prototypes and Old-Style Function Definitions</h3> 59 60<p><a name="index-function-prototype-declarations-2562"></a><a name="index-old_002dstyle-function-definitions-2563"></a><a name="index-promotion-of-formal-parameters-2564"></a> 61GNU C extends ISO C to allow a function prototype to override a later 62old-style non-prototype definition. Consider the following example: 63 64<pre class="smallexample"> /* <span class="roman">Use prototypes unless the compiler is old-fashioned.</span> */ 65 #ifdef __STDC__ 66 #define P(x) x 67 #else 68 #define P(x) () 69 #endif 70 71 /* <span class="roman">Prototype function declaration.</span> */ 72 int isroot P((uid_t)); 73 74 /* <span class="roman">Old-style function definition.</span> */ 75 int 76 isroot (x) /* <span class="roman">??? lossage here ???</span> */ 77 uid_t x; 78 { 79 return x == 0; 80 } 81</pre> 82 <p>Suppose the type <code>uid_t</code> happens to be <code>short</code>. ISO C does 83not allow this example, because subword arguments in old-style 84non-prototype definitions are promoted. Therefore in this example the 85function definition's argument is really an <code>int</code>, which does not 86match the prototype argument type of <code>short</code>. 87 88 <p>This restriction of ISO C makes it hard to write code that is portable 89to traditional C compilers, because the programmer does not know 90whether the <code>uid_t</code> type is <code>short</code>, <code>int</code>, or 91<code>long</code>. Therefore, in cases like these GNU C allows a prototype 92to override a later old-style definition. More precisely, in GNU C, a 93function prototype argument type overrides the argument type specified 94by a later old-style definition if the former type is the same as the 95latter type before promotion. Thus in GNU C the above example is 96equivalent to the following: 97 98<pre class="smallexample"> int isroot (uid_t); 99 100 int 101 isroot (uid_t x) 102 { 103 return x == 0; 104 } 105</pre> 106 <p class="noindent">GNU C++ does not support old-style function definitions, so this 107extension is irrelevant. 108 109 </body></html> 110 111