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<html lang="en"><head><title>Using the GNU Compiler Collection (GCC)</title><meta http-equiv="Content-Type" content="text/html"><meta name="description" content="Using the GNU Compiler Collection (GCC)"><meta name="generator" content="makeinfo 4.6"><!--Copyright &copy; 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc.   <p>Permission is granted to copy, distribute and/or modify this documentunder the terms of the GNU Free Documentation License, Version 1.2 orany later version published by the Free Software Foundation; with theInvariant Sections being "GNU General Public License" and "FundingFree Software", the Front-Cover texts being (a) (see below), and withthe Back-Cover Texts being (b) (see below).  A copy of the license isincluded in the section entitled "GNU Free Documentation License".   <p>(a) The FSF's Front-Cover Text is:   <p>A GNU Manual   <p>(b) The FSF's Back-Cover Text is:   <p>You have freedom to copy and modify this GNU Manual, like GNU     software.  Copies published by the Free Software Foundation raise     funds for GNU development.--><meta http-equiv="Content-Style-Type" content="text/css"><style type="text/css"><!--  pre.display { font-family:inherit }  pre.format  { font-family:inherit }  pre.smalldisplay { font-family:inherit; font-size:smaller }  pre.smallformat  { font-family:inherit; font-size:smaller }  pre.smallexample { font-size:smaller }  pre.smalllisp    { font-size:smaller }--></style></head><body><div class="node"><p>Node:&nbsp;<a name="Function%20Prototypes">Function Prototypes</a>,Next:&nbsp;<a rel="next" accesskey="n" href="C---Comments.html#C++%20Comments">C++ Comments</a>,Previous:&nbsp;<a rel="previous" accesskey="p" href="Attribute-Syntax.html#Attribute%20Syntax">Attribute Syntax</a>,Up:&nbsp;<a rel="up" accesskey="u" href="C-Extensions.html#C%20Extensions">C Extensions</a><hr><br></div><h3 class="section">Prototypes and Old-Style Function Definitions</h3><p>GNU C extends ISO C to allow a function prototype to override a laterold-style non-prototype definition.  Consider the following example:<pre class="smallexample">     /* Use prototypes unless the compiler is old-fashioned.  */     #ifdef __STDC__     #define P(x) x     #else     #define P(x) ()     #endif          /* Prototype function declaration.  */     int isroot P((uid_t));          /* Old-style function definition.  */     int     isroot (x)   /* ??? lossage here ??? */          uid_t x;     {       return x == 0;     }     </pre>   <p>Suppose the type <code>uid_t</code> happens to be <code>short</code>.  ISO C doesnot allow this example, because subword arguments in old-stylenon-prototype definitions are promoted.  Therefore in this example thefunction definition's argument is really an <code>int</code>, which does notmatch the prototype argument type of <code>short</code>.   <p>This restriction of ISO C makes it hard to write code that is portableto traditional C compilers, because the programmer does not knowwhether the <code>uid_t</code> type is <code>short</code>, <code>int</code>, or<code>long</code>.  Therefore, in cases like these GNU C allows a prototypeto override a later old-style definition.  More precisely, in GNU C, afunction prototype argument type overrides the argument type specifiedby a later old-style definition if the former type is the same as thelatter type before promotion.  Thus in GNU C the above example isequivalent to the following:<pre class="smallexample">     int isroot (uid_t);          int     isroot (uid_t x)     {       return x == 0;     }     </pre><p>GNU C++ does not support old-style function definitions, so thisextension is irrelevant.   </body></html>
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