179 lines
7.2 KiB
HTML
179 lines
7.2 KiB
HTML
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<a href="intro.html"><h2>SQLite C Interface</h2></a>
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<h2>Mutex Methods Object</h2>
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</div>
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<blockquote><pre>
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typedef struct sqlite3_mutex_methods sqlite3_mutex_methods;
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struct sqlite3_mutex_methods {
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int (*xMutexInit)(void);
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int (*xMutexEnd)(void);
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sqlite3_mutex *(*xMutexAlloc)(int);
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void (*xMutexFree)(sqlite3_mutex *);
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void (*xMutexEnter)(sqlite3_mutex *);
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int (*xMutexTry)(sqlite3_mutex *);
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void (*xMutexLeave)(sqlite3_mutex *);
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int (*xMutexHeld)(sqlite3_mutex *);
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int (*xMutexNotheld)(sqlite3_mutex *);
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};
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</pre></blockquote>
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<p>
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An instance of this structure defines the low-level routines
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used to allocate and use mutexes.</p>
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<p>Usually, the default mutex implementations provided by SQLite are
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sufficient, however the application has the option of substituting a custom
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implementation for specialized deployments or systems for which SQLite
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does not provide a suitable implementation. In this case, the application
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creates and populates an instance of this structure to pass
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to sqlite3_config() along with the <a href="../c3ref/c_config_covering_index_scan.html#sqliteconfigmutex">SQLITE_CONFIG_MUTEX</a> option.
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Additionally, an instance of this structure can be used as an
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output variable when querying the system for the current mutex
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implementation, using the <a href="../c3ref/c_config_covering_index_scan.html#sqliteconfiggetmutex">SQLITE_CONFIG_GETMUTEX</a> option.</p>
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<p>The xMutexInit method defined by this structure is invoked as
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part of system initialization by the sqlite3_initialize() function.
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The xMutexInit routine is called by SQLite exactly once for each
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effective call to <a href="../c3ref/initialize.html">sqlite3_initialize()</a>.</p>
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<p>The xMutexEnd method defined by this structure is invoked as
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part of system shutdown by the sqlite3_shutdown() function. The
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implementation of this method is expected to release all outstanding
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resources obtained by the mutex methods implementation, especially
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those obtained by the xMutexInit method. The xMutexEnd()
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interface is invoked exactly once for each call to <a href="../c3ref/initialize.html">sqlite3_shutdown()</a>.</p>
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<p>The remaining seven methods defined by this structure (xMutexAlloc,
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xMutexFree, xMutexEnter, xMutexTry, xMutexLeave, xMutexHeld and
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xMutexNotheld) implement the following interfaces (respectively):</p>
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<p><ul>
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<li> <a href="../c3ref/mutex_alloc.html">sqlite3_mutex_alloc()</a> </li>
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<li> <a href="../c3ref/mutex_alloc.html">sqlite3_mutex_free()</a> </li>
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<li> <a href="../c3ref/mutex_alloc.html">sqlite3_mutex_enter()</a> </li>
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<li> <a href="../c3ref/mutex_alloc.html">sqlite3_mutex_try()</a> </li>
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<li> <a href="../c3ref/mutex_alloc.html">sqlite3_mutex_leave()</a> </li>
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<li> <a href="../c3ref/mutex_held.html">sqlite3_mutex_held()</a> </li>
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<li> <a href="../c3ref/mutex_held.html">sqlite3_mutex_notheld()</a> </li>
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</ul></p>
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<p>The only difference is that the public sqlite3_XXX functions enumerated
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above silently ignore any invocations that pass a NULL pointer instead
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of a valid mutex handle. The implementations of the methods defined
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by this structure are not required to handle this case. The results
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of passing a NULL pointer instead of a valid mutex handle are undefined
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(i.e. it is acceptable to provide an implementation that segfaults if
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it is passed a NULL pointer).</p>
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<p>The xMutexInit() method must be threadsafe. It must be harmless to
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invoke xMutexInit() multiple times within the same process and without
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intervening calls to xMutexEnd(). Second and subsequent calls to
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xMutexInit() must be no-ops.</p>
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<p>xMutexInit() must not use SQLite memory allocation (<a href="../c3ref/free.html">sqlite3_malloc()</a>
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and its associates). Similarly, xMutexAlloc() must not use SQLite memory
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allocation for a static mutex. However xMutexAlloc() may use SQLite
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memory allocation for a fast or recursive mutex.</p>
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<p>SQLite will invoke the xMutexEnd() method when <a href="../c3ref/initialize.html">sqlite3_shutdown()</a> is
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called, but only if the prior call to xMutexInit returned SQLITE_OK.
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If xMutexInit fails in any way, it is expected to clean up after itself
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prior to returning.
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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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