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194 lines
8.8 KiB
HTML
194 lines
8.8 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>malloc (The Red Hat newlib C Library)</title>
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<meta name="description" content="malloc (The Red Hat newlib C Library)">
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<meta name="keywords" content="malloc (The Red Hat newlib C Library)">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Document-Index.html#Document-Index" rel="index" title="Document Index">
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<link href="Document-Index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Stdlib.html#Stdlib" rel="up" title="Stdlib">
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<link href="mallinfo.html#mallinfo" rel="next" title="mallinfo">
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</head>
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<body lang="en">
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<a name="malloc"></a>
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<div class="header">
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<p>
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Next: <a href="mallinfo.html#mallinfo" accesskey="n" rel="next">mallinfo</a>, Previous: <a href="lldiv.html#lldiv" accesskey="p" rel="prev">lldiv</a>, Up: <a href="Stdlib.html#Stdlib" accesskey="u" rel="up">Stdlib</a> [<a href="Document-Index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Document-Index.html#Document-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="malloc_002c-realloc_002c-free_002d_002d_002dmanage-memory"></a>
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<h3 class="section">2.24 <code>malloc</code>, <code>realloc</code>, <code>free</code>—manage memory</h3>
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<a name="index-malloc"></a>
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<a name="index-realloc"></a>
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<a name="index-reallocf"></a>
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<a name="index-free"></a>
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<a name="index-memalign"></a>
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<a name="index-malloc_005fusable_005fsize"></a>
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<a name="index-_005fmalloc_005fr"></a>
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<a name="index-_005frealloc_005fr"></a>
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<a name="index-_005freallocf_005fr"></a>
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<a name="index-_005ffree_005fr"></a>
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<a name="index-_005fmemalign_005fr"></a>
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<a name="index-_005fmalloc_005fusable_005fsize_005fr"></a>
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<p><strong>Synopsis</strong>
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</p><div class="example">
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<pre class="example">#include <stdlib.h>
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void *malloc(size_t <var>nbytes</var>);
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void *realloc(void *<var>aptr</var>, size_t <var>nbytes</var>);
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void *reallocf(void *<var>aptr</var>, size_t <var>nbytes</var>);
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void free(void *<var>aptr</var>);
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void *memalign(size_t <var>align</var>, size_t <var>nbytes</var>);
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size_t malloc_usable_size(void *<var>aptr</var>);
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void *_malloc_r(void *<var>reent</var>, size_t <var>nbytes</var>);
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void *_realloc_r(void *<var>reent</var>,
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void *<var>aptr</var>, size_t <var>nbytes</var>);
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void *_reallocf_r(void *<var>reent</var>,
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void *<var>aptr</var>, size_t <var>nbytes</var>);
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void _free_r(void *<var>reent</var>, void *<var>aptr</var>);
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void *_memalign_r(void *<var>reent</var>,
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size_t <var>align</var>, size_t <var>nbytes</var>);
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size_t _malloc_usable_size_r(void *<var>reent</var>, void *<var>aptr</var>);
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</pre></div>
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<p><strong>Description</strong><br>
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These functions manage a pool of system memory.
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</p>
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<p>Use <code>malloc</code> to request allocation of an object with at least
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<var>nbytes</var> bytes of storage available. If the space is available,
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<code>malloc</code> returns a pointer to a newly allocated block as its result.
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</p>
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<p>If you already have a block of storage allocated by <code>malloc</code>, but
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you no longer need all the space allocated to it, you can make it
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smaller by calling <code>realloc</code> with both the object pointer and the
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new desired size as arguments. <code>realloc</code> guarantees that the
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contents of the smaller object match the beginning of the original object.
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</p>
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<p>Similarly, if you need more space for an object, use <code>realloc</code> to
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request the larger size; again, <code>realloc</code> guarantees that the
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beginning of the new, larger object matches the contents of the
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original object.
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</p>
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<p>When you no longer need an object originally allocated by <code>malloc</code>
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or <code>realloc</code> (or the related function <code>calloc</code>), return it to the
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memory storage pool by calling <code>free</code> with the address of the object
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as the argument. You can also use <code>realloc</code> for this purpose by
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calling it with <code>0</code> as the <var>nbytes</var> argument.
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</p>
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<p>The <code>reallocf</code> function behaves just like <code>realloc</code> except if the
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function is required to allocate new storage and this fails. In this
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case <code>reallocf</code> will free the original object passed in whereas
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<code>realloc</code> will not.
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</p>
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<p>The <code>memalign</code> function returns a block of size <var>nbytes</var> aligned
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to a <var>align</var> boundary. The <var>align</var> argument must be a power of
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two.
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</p>
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<p>The <code>malloc_usable_size</code> function takes a pointer to a block
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allocated by <code>malloc</code>. It returns the amount of space that is
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available in the block. This may or may not be more than the size
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requested from <code>malloc</code>, due to alignment or minimum size
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constraints.
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</p>
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<p>The alternate functions <code>_malloc_r</code>, <code>_realloc_r</code>, <code>_reallocf_r</code>,
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<code>_free_r</code>, <code>_memalign_r</code>, and <code>_malloc_usable_size_r</code> are reentrant
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versions. The extra argument <var>reent</var> is a pointer to a reentrancy structure.
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</p>
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<p>If you have multiple threads of execution which may call any of these
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routines, or if any of these routines may be called reentrantly, then
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you must provide implementations of the <code>__malloc_lock</code> and
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<code>__malloc_unlock</code> functions for your system. See the documentation
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for those functions.
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</p>
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<p>These functions operate by calling the function <code>_sbrk_r</code> or
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<code>sbrk</code>, which allocates space. You may need to provide one of these
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functions for your system. <code>_sbrk_r</code> is called with a positive
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value to allocate more space, and with a negative value to release
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previously allocated space if it is no longer required.
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See <a href="Stubs.html#Stubs">Stubs</a>.
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</p>
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<br>
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<p><strong>Returns</strong><br>
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<code>malloc</code> returns a pointer to the newly allocated space, if
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successful; otherwise it returns <code>NULL</code>. If your application needs
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to generate empty objects, you may use <code>malloc(0)</code> for this purpose.
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</p>
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<p><code>realloc</code> returns a pointer to the new block of memory, or <code>NULL</code>
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if a new block could not be allocated. <code>NULL</code> is also the result
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when you use ‘<code>realloc(<var>aptr</var>,0)</code>’ (which has the same effect as
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‘<code>free(<var>aptr</var>)</code>’). You should always check the result of
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<code>realloc</code>; successful reallocation is not guaranteed even when
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you request a smaller object.
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</p>
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<p><code>free</code> does not return a result.
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</p>
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<p><code>memalign</code> returns a pointer to the newly allocated space.
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</p>
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<p><code>malloc_usable_size</code> returns the usable size.
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</p>
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<br>
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<p><strong>Portability</strong><br>
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<code>malloc</code>, <code>realloc</code>, and <code>free</code> are specified by the ANSI C
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standard, but other conforming implementations of <code>malloc</code> may
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behave differently when <var>nbytes</var> is zero.
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</p>
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<p><code>memalign</code> is part of SVR4.
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</p>
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<p><code>malloc_usable_size</code> is not portable.
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</p>
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<p>Supporting OS subroutines required: <code>sbrk</code>.
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</p><br>
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<hr>
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<div class="header">
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<p>
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Next: <a href="mallinfo.html#mallinfo" accesskey="n" rel="next">mallinfo</a>, Previous: <a href="lldiv.html#lldiv" accesskey="p" rel="prev">lldiv</a>, Up: <a href="Stdlib.html#Stdlib" accesskey="u" rel="up">Stdlib</a> [<a href="Document-Index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Document-Index.html#Document-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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