221 lines
8.4 KiB
HTML
221 lines
8.4 KiB
HTML
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<!-- keywords: sqlite3_mutex_alloc sqlite3_mutex_enter sqlite3_mutex_free sqlite3_mutex_leave sqlite3_mutex_try -->
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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>Mutexes</h2>
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</div>
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<blockquote><pre>
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sqlite3_mutex *sqlite3_mutex_alloc(int);
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void sqlite3_mutex_free(sqlite3_mutex*);
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void sqlite3_mutex_enter(sqlite3_mutex*);
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int sqlite3_mutex_try(sqlite3_mutex*);
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void sqlite3_mutex_leave(sqlite3_mutex*);
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</pre></blockquote>
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<p>
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The SQLite core uses these routines for thread
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synchronization. Though they are intended for internal
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use by SQLite, code that links against SQLite is
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permitted to use any of these routines.</p>
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<p>The SQLite source code contains multiple implementations
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of these mutex routines. An appropriate implementation
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is selected automatically at compile-time. The following
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implementations are available in the SQLite core:</p>
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<p><ul>
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<li> SQLITE_MUTEX_PTHREADS
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<li> SQLITE_MUTEX_W32
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<li> SQLITE_MUTEX_NOOP
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</ul></p>
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<p>The SQLITE_MUTEX_NOOP implementation is a set of routines
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that does no real locking and is appropriate for use in
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a single-threaded application. The SQLITE_MUTEX_PTHREADS and
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SQLITE_MUTEX_W32 implementations are appropriate for use on Unix
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and Windows.</p>
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<p>If SQLite is compiled with the SQLITE_MUTEX_APPDEF preprocessor
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macro defined (with "-DSQLITE_MUTEX_APPDEF=1"), then no mutex
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implementation is included with the library. In this case the
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application must supply a custom mutex implementation using the
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<a href="../c3ref/c_config_covering_index_scan.html#sqliteconfigmutex">SQLITE_CONFIG_MUTEX</a> option of the sqlite3_config() function
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before calling sqlite3_initialize() or any other public sqlite3_
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function that calls sqlite3_initialize().</p>
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<p>The sqlite3_mutex_alloc() routine allocates a new
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mutex and returns a pointer to it. The sqlite3_mutex_alloc()
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routine returns NULL if it is unable to allocate the requested
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mutex. The argument to sqlite3_mutex_alloc() must one of these
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integer constants:</p>
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<p><ul>
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<li> SQLITE_MUTEX_FAST
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<li> SQLITE_MUTEX_RECURSIVE
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<li> SQLITE_MUTEX_STATIC_MAIN
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<li> SQLITE_MUTEX_STATIC_MEM
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<li> SQLITE_MUTEX_STATIC_OPEN
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<li> SQLITE_MUTEX_STATIC_PRNG
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<li> SQLITE_MUTEX_STATIC_LRU
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<li> SQLITE_MUTEX_STATIC_PMEM
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<li> SQLITE_MUTEX_STATIC_APP1
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<li> SQLITE_MUTEX_STATIC_APP2
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<li> SQLITE_MUTEX_STATIC_APP3
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<li> SQLITE_MUTEX_STATIC_VFS1
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<li> SQLITE_MUTEX_STATIC_VFS2
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<li> SQLITE_MUTEX_STATIC_VFS3
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</ul></p>
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<p>The first two constants (SQLITE_MUTEX_FAST and SQLITE_MUTEX_RECURSIVE)
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cause sqlite3_mutex_alloc() to create
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a new mutex. The new mutex is recursive when SQLITE_MUTEX_RECURSIVE
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is used but not necessarily so when SQLITE_MUTEX_FAST is used.
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The mutex implementation does not need to make a distinction
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between SQLITE_MUTEX_RECURSIVE and SQLITE_MUTEX_FAST if it does
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not want to. SQLite will only request a recursive mutex in
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cases where it really needs one. If a faster non-recursive mutex
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implementation is available on the host platform, the mutex subsystem
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might return such a mutex in response to SQLITE_MUTEX_FAST.</p>
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<p>The other allowed parameters to sqlite3_mutex_alloc() (anything other
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than SQLITE_MUTEX_FAST and SQLITE_MUTEX_RECURSIVE) each return
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a pointer to a static preexisting mutex. Nine static mutexes are
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used by the current version of SQLite. Future versions of SQLite
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may add additional static mutexes. Static mutexes are for internal
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use by SQLite only. Applications that use SQLite mutexes should
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use only the dynamic mutexes returned by SQLITE_MUTEX_FAST or
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SQLITE_MUTEX_RECURSIVE.</p>
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<p>Note that if one of the dynamic mutex parameters (SQLITE_MUTEX_FAST
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or SQLITE_MUTEX_RECURSIVE) is used then sqlite3_mutex_alloc()
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returns a different mutex on every call. For the static
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mutex types, the same mutex is returned on every call that has
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the same type number.</p>
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<p>The sqlite3_mutex_free() routine deallocates a previously
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allocated dynamic mutex. Attempting to deallocate a static
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mutex results in undefined behavior.</p>
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<p>The sqlite3_mutex_enter() and sqlite3_mutex_try() routines attempt
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to enter a mutex. If another thread is already within the mutex,
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sqlite3_mutex_enter() will block and sqlite3_mutex_try() will return
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SQLITE_BUSY. The sqlite3_mutex_try() interface returns <a href="../rescode.html#ok">SQLITE_OK</a>
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upon successful entry. Mutexes created using
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SQLITE_MUTEX_RECURSIVE can be entered multiple times by the same thread.
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In such cases, the
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mutex must be exited an equal number of times before another thread
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can enter. If the same thread tries to enter any mutex other
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than an SQLITE_MUTEX_RECURSIVE more than once, the behavior is undefined.</p>
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<p>Some systems (for example, Windows 95) do not support the operation
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implemented by sqlite3_mutex_try(). On those systems, sqlite3_mutex_try()
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will always return SQLITE_BUSY. The SQLite core only ever uses
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sqlite3_mutex_try() as an optimization so this is acceptable
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behavior.</p>
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<p>The sqlite3_mutex_leave() routine exits a mutex that was
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previously entered by the same thread. The behavior
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is undefined if the mutex is not currently entered by the
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calling thread or is not currently allocated.</p>
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<p>If the argument to sqlite3_mutex_enter(), sqlite3_mutex_try(), or
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sqlite3_mutex_leave() is a NULL pointer, then all three routines
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behave as no-ops.</p>
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<p>See also: <a href="../c3ref/mutex_held.html">sqlite3_mutex_held()</a> and <a href="../c3ref/mutex_held.html">sqlite3_mutex_notheld()</a>.
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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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