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181 lines
8.8 KiB
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181 lines
8.8 KiB
HTML
<!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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<!-- This manual describes how to install and use the GNU multiple precision
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arithmetic library, version 6.1.0.
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Copyright 1991, 1993-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document under
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the terms of the GNU Free Documentation License, Version 1.3 or any later
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<head>
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<title>Custom Allocation (GNU MP 6.1.0)</title>
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<meta name="description" content="How to install and use the GNU multiple precision arithmetic library, version 6.1.0.">
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<meta name="keywords" content="Custom Allocation (GNU MP 6.1.0)">
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</head>
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<body lang="en">
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<a name="Custom-Allocation"></a>
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<div class="header">
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<p>
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Next: <a href="Language-Bindings.html#Language-Bindings" accesskey="n" rel="next">Language Bindings</a>, Previous: <a href="C_002b_002b-Class-Interface.html#C_002b_002b-Class-Interface" accesskey="p" rel="prev">C++ Class Interface</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Custom-Allocation-1"></a>
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<h2 class="chapter">13 Custom Allocation</h2>
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<a name="index-Custom-allocation"></a>
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<a name="index-Memory-allocation"></a>
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<a name="index-Allocation-of-memory"></a>
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<p>By default GMP uses <code>malloc</code>, <code>realloc</code> and <code>free</code> for memory
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allocation, and if they fail GMP prints a message to the standard error output
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and terminates the program.
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</p>
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<p>Alternate functions can be specified, to allocate memory in a different way or
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to have a different error action on running out of memory.
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</p>
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<dl>
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<dt><a name="index-mp_005fset_005fmemory_005ffunctions"></a>Function: <em>void</em> <strong>mp_set_memory_functions</strong> <em>(<br> void *(*<var>alloc_func_ptr</var>) (size_t), <br> void *(*<var>realloc_func_ptr</var>) (void *, size_t, size_t), <br> void (*<var>free_func_ptr</var>) (void *, size_t))</em></dt>
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<dd><p>Replace the current allocation functions from the arguments. If an argument
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is <code>NULL</code>, the corresponding default function is used.
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</p>
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<p>These functions will be used for all memory allocation done by GMP, apart from
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temporary space from <code>alloca</code> if that function is available and GMP is
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configured to use it (see <a href="Build-Options.html#Build-Options">Build Options</a>).
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</p>
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<p><strong>Be sure to call <code>mp_set_memory_functions</code> only when there are no
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active GMP objects allocated using the previous memory functions! Usually
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that means calling it before any other GMP function.</strong>
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</p></dd></dl>
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<p>The functions supplied should fit the following declarations:
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</p>
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<dl>
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<dt><a name="index-allocate_005ffunction"></a>Function: <em>void *</em> <strong>allocate_function</strong> <em>(size_t <var>alloc_size</var>)</em></dt>
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<dd><p>Return a pointer to newly allocated space with at least <var>alloc_size</var>
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bytes.
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</p></dd></dl>
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<dl>
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<dt><a name="index-reallocate_005ffunction"></a>Function: <em>void *</em> <strong>reallocate_function</strong> <em>(void *<var>ptr</var>, size_t <var>old_size</var>, size_t <var>new_size</var>)</em></dt>
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<dd><p>Resize a previously allocated block <var>ptr</var> of <var>old_size</var> bytes to be
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<var>new_size</var> bytes.
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</p>
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<p>The block may be moved if necessary or if desired, and in that case the
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smaller of <var>old_size</var> and <var>new_size</var> bytes must be copied to the new
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location. The return value is a pointer to the resized block, that being the
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new location if moved or just <var>ptr</var> if not.
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</p>
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<p><var>ptr</var> is never <code>NULL</code>, it’s always a previously allocated block.
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<var>new_size</var> may be bigger or smaller than <var>old_size</var>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-free_005ffunction"></a>Function: <em>void</em> <strong>free_function</strong> <em>(void *<var>ptr</var>, size_t <var>size</var>)</em></dt>
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<dd><p>De-allocate the space pointed to by <var>ptr</var>.
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</p>
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<p><var>ptr</var> is never <code>NULL</code>, it’s always a previously allocated block of
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<var>size</var> bytes.
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</p></dd></dl>
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<p>A <em>byte</em> here means the unit used by the <code>sizeof</code> operator.
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</p>
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<p>The <var>reallocate_function</var> parameter <var>old_size</var> and the
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<var>free_function</var> parameter <var>size</var> are passed for convenience, but of
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course they can be ignored if not needed by an implementation. The default
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functions using <code>malloc</code> and friends for instance don’t use them.
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</p>
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<p>No error return is allowed from any of these functions, if they return then
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they must have performed the specified operation. In particular note that
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<var>allocate_function</var> or <var>reallocate_function</var> mustn’t return
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<code>NULL</code>.
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</p>
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<p>Getting a different fatal error action is a good use for custom allocation
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functions, for example giving a graphical dialog rather than the default print
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to <code>stderr</code>. How much is possible when genuinely out of memory is
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another question though.
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</p>
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<p>There’s currently no defined way for the allocation functions to recover from
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an error such as out of memory, they must terminate program execution. A
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<code>longjmp</code> or throwing a C++ exception will have undefined results. This
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may change in the future.
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</p>
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<p>GMP may use allocated blocks to hold pointers to other allocated blocks. This
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will limit the assumptions a conservative garbage collection scheme can make.
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</p>
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<p>Since the default GMP allocation uses <code>malloc</code> and friends, those
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functions will be linked in even if the first thing a program does is an
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<code>mp_set_memory_functions</code>. It’s necessary to change the GMP sources if
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this is a problem.
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</p>
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<br>
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<dl>
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<dt><a name="index-mp_005fget_005fmemory_005ffunctions"></a>Function: <em>void</em> <strong>mp_get_memory_functions</strong> <em>(<br> void *(**<var>alloc_func_ptr</var>) (size_t), <br> void *(**<var>realloc_func_ptr</var>) (void *, size_t, size_t), <br> void (**<var>free_func_ptr</var>) (void *, size_t))</em></dt>
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<dd><p>Get the current allocation functions, storing function pointers to the
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locations given by the arguments. If an argument is <code>NULL</code>, that
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function pointer is not stored.
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</p>
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<p>For example, to get just the current free function,
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</p>
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<div class="example">
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<pre class="example">void (*freefunc) (void *, size_t);
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mp_get_memory_functions (NULL, NULL, &freefunc);
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</pre></div>
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</dd></dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Language-Bindings.html#Language-Bindings" accesskey="n" rel="next">Language Bindings</a>, Previous: <a href="C_002b_002b-Class-Interface.html#C_002b_002b-Class-Interface" accesskey="p" rel="prev">C++ Class Interface</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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