sqlite3_mutex *sqlite3_mutex_alloc(int); void sqlite3_mutex_free(sqlite3_mutex*); void sqlite3_mutex_enter(sqlite3_mutex*); int sqlite3_mutex_try(sqlite3_mutex*); void sqlite3_mutex_leave(sqlite3_mutex*);
The SQLite core uses these routines for thread synchronization. Though they are intended for internal use by SQLite, code that links against SQLite is permitted to use any of these routines.
The SQLite source code contains multiple implementations of these mutex routines. An appropriate implementation is selected automatically at compile-time. The following implementations are available in the SQLite core:
The SQLITE_MUTEX_NOOP implementation is a set of routines that does no real locking and is appropriate for use in a single-threaded application. The SQLITE_MUTEX_PTHREADS and SQLITE_MUTEX_W32 implementations are appropriate for use on Unix and Windows.
If SQLite is compiled with the SQLITE_MUTEX_APPDEF preprocessor macro defined (with "-DSQLITE_MUTEX_APPDEF=1"), then no mutex implementation is included with the library. In this case the application must supply a custom mutex implementation using the SQLITE_CONFIG_MUTEX option of the sqlite3_config() function before calling sqlite3_initialize() or any other public sqlite3_ function that calls sqlite3_initialize().
The sqlite3_mutex_alloc() routine allocates a new mutex and returns a pointer to it. If it returns NULL that means that a mutex could not be allocated. SQLite will unwind its stack and return an error. The argument to sqlite3_mutex_alloc() is one of these integer constants:
The first two constants (SQLITE_MUTEX_FAST and SQLITE_MUTEX_RECURSIVE) cause sqlite3_mutex_alloc() to create a new mutex. The new mutex is recursive when SQLITE_MUTEX_RECURSIVE is used but not necessarily so when SQLITE_MUTEX_FAST is used. The mutex implementation does not need to make a distinction between SQLITE_MUTEX_RECURSIVE and SQLITE_MUTEX_FAST if it does not want to. SQLite will only request a recursive mutex in cases where it really needs one. If a faster non-recursive mutex implementation is available on the host platform, the mutex subsystem might return such a mutex in response to SQLITE_MUTEX_FAST.
The other allowed parameters to sqlite3_mutex_alloc() (anything other than SQLITE_MUTEX_FAST and SQLITE_MUTEX_RECURSIVE) each return a pointer to a static preexisting mutex. Six static mutexes are used by the current version of SQLite. Future versions of SQLite may add additional static mutexes. Static mutexes are for internal use by SQLite only. Applications that use SQLite mutexes should use only the dynamic mutexes returned by SQLITE_MUTEX_FAST or SQLITE_MUTEX_RECURSIVE.
Note that if one of the dynamic mutex parameters (SQLITE_MUTEX_FAST or SQLITE_MUTEX_RECURSIVE) is used then sqlite3_mutex_alloc() returns a different mutex on every call. But for the static mutex types, the same mutex is returned on every call that has the same type number.
The sqlite3_mutex_free() routine deallocates a previously allocated dynamic mutex. SQLite is careful to deallocate every dynamic mutex that it allocates. The dynamic mutexes must not be in use when they are deallocated. Attempting to deallocate a static mutex results in undefined behavior. SQLite never deallocates a static mutex.
The sqlite3_mutex_enter() and sqlite3_mutex_try() routines attempt to enter a mutex. If another thread is already within the mutex, sqlite3_mutex_enter() will block and sqlite3_mutex_try() will return SQLITE_BUSY. The sqlite3_mutex_try() interface returns SQLITE_OK upon successful entry. Mutexes created using SQLITE_MUTEX_RECURSIVE can be entered multiple times by the same thread. In such cases the, mutex must be exited an equal number of times before another thread can enter. If the same thread tries to enter any other kind of mutex more than once, the behavior is undefined. SQLite will never exhibit such behavior in its own use of mutexes.
Some systems (for example, Windows 95) do not support the operation implemented by sqlite3_mutex_try(). On those systems, sqlite3_mutex_try() will always return SQLITE_BUSY. The SQLite core only ever uses sqlite3_mutex_try() as an optimization so this is acceptable behavior.
The sqlite3_mutex_leave() routine exits a mutex that was previously entered by the same thread. The behavior is undefined if the mutex is not currently entered by the calling thread or is not currently allocated. SQLite will never do either.
If the argument to sqlite3_mutex_enter(), sqlite3_mutex_try(), or sqlite3_mutex_leave() is a NULL pointer, then all three routines behave as no-ops.