| <html lang="en"> |
| <head> |
| <title>Malloc Examples - 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="Unconstrained-Allocation.html#Unconstrained-Allocation" title="Unconstrained Allocation"> |
| <link rel="prev" href="Basic-Allocation.html#Basic-Allocation" title="Basic Allocation"> |
| <link rel="next" href="Freeing-after-Malloc.html#Freeing-after-Malloc" title="Freeing after Malloc"> |
| <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="Malloc-Examples"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Freeing-after-Malloc.html#Freeing-after-Malloc">Freeing after Malloc</a>, |
| Previous: <a rel="previous" accesskey="p" href="Basic-Allocation.html#Basic-Allocation">Basic Allocation</a>, |
| Up: <a rel="up" accesskey="u" href="Unconstrained-Allocation.html#Unconstrained-Allocation">Unconstrained Allocation</a> |
| <hr> |
| </div> |
| |
| <h5 class="subsubsection">3.2.2.2 Examples of <code>malloc</code></h5> |
| |
| <p>If no more space is available, <code>malloc</code> returns a null pointer. |
| You should check the value of <em>every</em> call to <code>malloc</code>. It is |
| useful to write a subroutine that calls <code>malloc</code> and reports an |
| error if the value is a null pointer, returning only if the value is |
| nonzero. This function is conventionally called <code>xmalloc</code>. Here |
| it is: |
| |
| <pre class="smallexample"> void * |
| xmalloc (size_t size) |
| { |
| register void *value = malloc (size); |
| if (value == 0) |
| fatal ("virtual memory exhausted"); |
| return value; |
| } |
| </pre> |
| <p>Here is a real example of using <code>malloc</code> (by way of <code>xmalloc</code>). |
| The function <code>savestring</code> will copy a sequence of characters into |
| a newly allocated null-terminated string: |
| |
| <pre class="smallexample"> char * |
| savestring (const char *ptr, size_t len) |
| { |
| register char *value = (char *) xmalloc (len + 1); |
| value[len] = '\0'; |
| return (char *) memcpy (value, ptr, len); |
| } |
| </pre> |
| <p>The block that <code>malloc</code> gives you is guaranteed to be aligned so |
| that it can hold any type of data. In the GNU system, the address is |
| always a multiple of eight on most systems, and a multiple of 16 on |
| 64-bit systems. Only rarely is any higher boundary (such as a page |
| boundary) necessary; for those cases, use <code>memalign</code>, |
| <code>posix_memalign</code> or <code>valloc</code> (see <a href="Aligned-Memory-Blocks.html#Aligned-Memory-Blocks">Aligned Memory Blocks</a>). |
| |
| <p>Note that the memory located after the end of the block is likely to be |
| in use for something else; perhaps a block already allocated by another |
| call to <code>malloc</code>. If you attempt to treat the block as longer than |
| you asked for it to be, you are liable to destroy the data that |
| <code>malloc</code> uses to keep track of its blocks, or you may destroy the |
| contents of another block. If you have already allocated a block and |
| discover you want it to be bigger, use <code>realloc</code> (see <a href="Changing-Block-Size.html#Changing-Block-Size">Changing Block Size</a>). |
| |
| </body></html> |
| |
