| <html lang="en"> |
| <head> |
| <title>Null Pointer Constant - 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="Language-Features.html#Language-Features" title="Language Features"> |
| <link rel="prev" href="Variadic-Functions.html#Variadic-Functions" title="Variadic Functions"> |
| <link rel="next" href="Important-Data-Types.html#Important-Data-Types" title="Important Data Types"> |
| <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="Null-Pointer-Constant"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Important-Data-Types.html#Important-Data-Types">Important Data Types</a>, |
| Previous: <a rel="previous" accesskey="p" href="Variadic-Functions.html#Variadic-Functions">Variadic Functions</a>, |
| Up: <a rel="up" accesskey="u" href="Language-Features.html#Language-Features">Language Features</a> |
| <hr> |
| </div> |
| |
| <h3 class="section">A.3 Null Pointer Constant</h3> |
| |
| <p><a name="index-null-pointer-constant-3736"></a> |
| The null pointer constant is guaranteed not to point to any real object. |
| You can assign it to any pointer variable since it has type <code>void |
| *</code>. The preferred way to write a null pointer constant is with |
| <code>NULL</code>. |
| |
| <!-- stddef.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Macro: void * <b>NULL</b><var><a name="index-NULL-3737"></a></var><br> |
| <blockquote><p>This is a null pointer constant. |
| </p></blockquote></div> |
| |
| <p>You can also use <code>0</code> or <code>(void *)0</code> as a null pointer |
| constant, but using <code>NULL</code> is cleaner because it makes the purpose |
| of the constant more evident. |
| |
| <p>If you use the null pointer constant as a function argument, then for |
| complete portability you should make sure that the function has a |
| prototype declaration. Otherwise, if the target machine has two |
| different pointer representations, the compiler won't know which |
| representation to use for that argument. You can avoid the problem by |
| explicitly casting the constant to the proper pointer type, but we |
| recommend instead adding a prototype for the function you are calling. |
| |
| </body></html> |
| |
