| <html lang="en"> |
| <head> |
| <title>Macro Definitions - The GNU C Library</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="The GNU C Library"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="Using-the-Library.html#Using-the-Library" title="Using the Library"> |
| <link rel="prev" href="Header-Files.html#Header-Files" title="Header Files"> |
| <link rel="next" href="Reserved-Names.html#Reserved-Names" title="Reserved Names"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| This file documents the GNU C library. |
| |
| This is Edition 0.12, last updated 2007-10-27, |
| of `The GNU C Library Reference Manual', for version |
| 2.8 (Sourcery G++ Lite 2011.03-41). |
| |
| Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002, |
| 2003, 2007, 2008, 2010 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being ``Free Software Needs Free Documentation'' |
| and ``GNU Lesser General Public License'', the Front-Cover texts being |
| ``A GNU Manual'', and with the Back-Cover Texts as in (a) below. A |
| copy of the license is included in the section entitled "GNU Free |
| Documentation License". |
| |
| (a) The FSF's Back-Cover Text is: ``You have the freedom to |
| copy and modify this GNU manual. Buying copies from the FSF |
| supports it in developing GNU and promoting software freedom.''--> |
| <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 } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="Macro-Definitions"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Reserved-Names.html#Reserved-Names">Reserved Names</a>, |
| Previous: <a rel="previous" accesskey="p" href="Header-Files.html#Header-Files">Header Files</a>, |
| Up: <a rel="up" accesskey="u" href="Using-the-Library.html#Using-the-Library">Using the Library</a> |
| <hr> |
| </div> |
| |
| <h4 class="subsection">1.3.2 Macro Definitions of Functions</h4> |
| |
| <p><a name="index-shadowing-functions-with-macros-23"></a><a name="index-removing-macros-that-shadow-functions-24"></a><a name="index-undefining-macros-that-shadow-functions-25"></a> |
| If we describe something as a function in this manual, it may have a |
| macro definition as well. This normally has no effect on how your |
| program runs—the macro definition does the same thing as the function |
| would. In particular, macro equivalents for library functions evaluate |
| arguments exactly once, in the same way that a function call would. The |
| main reason for these macro definitions is that sometimes they can |
| produce an inline expansion that is considerably faster than an actual |
| function call. |
| |
| <p>Taking the address of a library function works even if it is also |
| defined as a macro. This is because, in this context, the name of the |
| function isn't followed by the left parenthesis that is syntactically |
| necessary to recognize a macro call. |
| |
| <p>You might occasionally want to avoid using the macro definition of a |
| function—perhaps to make your program easier to debug. There are |
| two ways you can do this: |
| |
| <ul> |
| <li>You can avoid a macro definition in a specific use by enclosing the name |
| of the function in parentheses. This works because the name of the |
| function doesn't appear in a syntactic context where it is recognizable |
| as a macro call. |
| |
| <li>You can suppress any macro definition for a whole source file by using |
| the ‘<samp><span class="samp">#undef</span></samp>’ preprocessor directive, unless otherwise stated |
| explicitly in the description of that facility. |
| </ul> |
| |
| <p>For example, suppose the header file <samp><span class="file">stdlib.h</span></samp> declares a function |
| named <code>abs</code> with |
| |
| <pre class="smallexample"> extern int abs (int); |
| </pre> |
| <p class="noindent">and also provides a macro definition for <code>abs</code>. Then, in: |
| |
| <pre class="smallexample"> #include <stdlib.h> |
| int f (int *i) { return abs (++*i); } |
| </pre> |
| <p class="noindent">the reference to <code>abs</code> might refer to either a macro or a function. |
| On the other hand, in each of the following examples the reference is |
| to a function and not a macro. |
| |
| <pre class="smallexample"> #include <stdlib.h> |
| int g (int *i) { return (abs) (++*i); } |
| |
| #undef abs |
| int h (int *i) { return abs (++*i); } |
| </pre> |
| <p>Since macro definitions that double for a function behave in |
| exactly the same way as the actual function version, there is usually no |
| need for any of these methods. In fact, removing macro definitions usually |
| just makes your program slower. |
| |
| </body></html> |
| |
