176 lines
7.6 KiB
HTML
176 lines
7.6 KiB
HTML
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<title>Memory Allocation Routines</title>
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<!-- keywords: sqlite3_mem_methods -->
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<div class=nosearch>
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<a href="intro.html"><h2>SQLite C Interface</h2></a>
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<h2>Memory Allocation Routines</h2>
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</div>
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<blockquote><pre>
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typedef struct sqlite3_mem_methods sqlite3_mem_methods;
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struct sqlite3_mem_methods {
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void *(*xMalloc)(int); /* Memory allocation function */
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void (*xFree)(void*); /* Free a prior allocation */
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void *(*xRealloc)(void*,int); /* Resize an allocation */
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int (*xSize)(void*); /* Return the size of an allocation */
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int (*xRoundup)(int); /* Round up request size to allocation size */
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int (*xInit)(void*); /* Initialize the memory allocator */
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void (*xShutdown)(void*); /* Deinitialize the memory allocator */
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void *pAppData; /* Argument to xInit() and xShutdown() */
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};
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</pre></blockquote>
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<p>
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An instance of this object defines the interface between SQLite
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and low-level memory allocation routines.</p>
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<p>This object is used in only one place in the SQLite interface.
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A pointer to an instance of this object is the argument to
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<a href="../c3ref/config.html">sqlite3_config()</a> when the configuration option is
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<a href="../c3ref/c_config_covering_index_scan.html#sqliteconfigmalloc">SQLITE_CONFIG_MALLOC</a> or <a href="../c3ref/c_config_covering_index_scan.html#sqliteconfiggetmalloc">SQLITE_CONFIG_GETMALLOC</a>.
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By creating an instance of this object
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and passing it to <a href="../c3ref/config.html">sqlite3_config</a>(<a href="../c3ref/c_config_covering_index_scan.html#sqliteconfigmalloc">SQLITE_CONFIG_MALLOC</a>)
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during configuration, an application can specify an alternative
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memory allocation subsystem for SQLite to use for all of its
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dynamic memory needs.</p>
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<p>Note that SQLite comes with several <a href="../malloc.html#altalloc">built-in memory allocators</a>
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that are perfectly adequate for the overwhelming majority of applications
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and that this object is only useful to a tiny minority of applications
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with specialized memory allocation requirements. This object is
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also used during testing of SQLite in order to specify an alternative
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memory allocator that simulates memory out-of-memory conditions in
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order to verify that SQLite recovers gracefully from such
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conditions.</p>
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<p>The xMalloc, xRealloc, and xFree methods must work like the
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malloc(), realloc() and free() functions from the standard C library.
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SQLite guarantees that the second argument to
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xRealloc is always a value returned by a prior call to xRoundup.</p>
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<p>xSize should return the allocated size of a memory allocation
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previously obtained from xMalloc or xRealloc. The allocated size
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is always at least as big as the requested size but may be larger.</p>
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<p>The xRoundup method returns what would be the allocated size of
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a memory allocation given a particular requested size. Most memory
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allocators round up memory allocations at least to the next multiple
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of 8. Some allocators round up to a larger multiple or to a power of 2.
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Every memory allocation request coming in through <a href="../c3ref/free.html">sqlite3_malloc()</a>
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or <a href="../c3ref/free.html">sqlite3_realloc()</a> first calls xRoundup. If xRoundup returns 0,
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that causes the corresponding memory allocation to fail.</p>
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<p>The xInit method initializes the memory allocator. For example,
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it might allocate any required mutexes or initialize internal data
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structures. The xShutdown method is invoked (indirectly) by
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<a href="../c3ref/initialize.html">sqlite3_shutdown()</a> and should deallocate any resources acquired
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by xInit. The pAppData pointer is used as the only parameter to
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xInit and xShutdown.</p>
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<p>SQLite holds the <a href="../c3ref/c_mutex_fast.html">SQLITE_MUTEX_STATIC_MAIN</a> mutex when it invokes
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the xInit method, so the xInit method need not be threadsafe. The
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xShutdown method is only called from <a href="../c3ref/initialize.html">sqlite3_shutdown()</a> so it does
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not need to be threadsafe either. For all other methods, SQLite
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holds the <a href="../c3ref/c_mutex_fast.html">SQLITE_MUTEX_STATIC_MEM</a> mutex as long as the
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<a href="../c3ref/c_config_covering_index_scan.html#sqliteconfigmemstatus">SQLITE_CONFIG_MEMSTATUS</a> configuration option is turned on (which
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it is by default) and so the methods are automatically serialized.
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However, if <a href="../c3ref/c_config_covering_index_scan.html#sqliteconfigmemstatus">SQLITE_CONFIG_MEMSTATUS</a> is disabled, then the other
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methods must be threadsafe or else make their own arrangements for
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serialization.</p>
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<p>SQLite will never invoke xInit() more than once without an intervening
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call to xShutdown().
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</p><p>See also lists of
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<a href="objlist.html">Objects</a>,
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<a href="constlist.html">Constants</a>, and
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<a href="funclist.html">Functions</a>.</p>
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