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SQLITE3_MUTEX_ALLOC(3)     Library Functions Manual     SQLITE3_MUTEX_ALLOC(3)

NAME
     sqlite3_mutex_alloc, sqlite3_mutex_free, sqlite3_mutex_enter,
     sqlite3_mutex_try, sqlite3_mutex_leave - Mutexes

SYNOPSIS
     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*);

DESCRIPTION
     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:

        SQLITE_MUTEX_PTHREADS

        SQLITE_MUTEX_W32

        SQLITE_MUTEX_NOOP

     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.  The sqlite3_mutex_alloc() routine returns NULL if it is
     unable to allocate the requested mutex.  The argument to
     sqlite3_mutex_alloc() must one of these integer constants:

        SQLITE_MUTEX_FAST

        SQLITE_MUTEX_RECURSIVE

        SQLITE_MUTEX_STATIC_MASTER

        SQLITE_MUTEX_STATIC_MEM

        SQLITE_MUTEX_STATIC_OPEN

        SQLITE_MUTEX_STATIC_PRNG

        SQLITE_MUTEX_STATIC_LRU

        SQLITE_MUTEX_STATIC_PMEM

        SQLITE_MUTEX_STATIC_APP1

        SQLITE_MUTEX_STATIC_APP2

        SQLITE_MUTEX_STATIC_APP3

        SQLITE_MUTEX_STATIC_VFS1

        SQLITE_MUTEX_STATIC_VFS2

        SQLITE_MUTEX_STATIC_VFS3

     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.  Nine 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.  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.  Attempting to deallocate a static mutex results in
     undefined behavior.

     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 mutex other than an
     SQLITE_MUTEX_RECURSIVE more than once, the behavior is undefined.

     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.

     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.

SEE ALSO
     sqlite3_mutex_held(3), SQLITE_CONFIG_SINGLETHREAD(3), SQLITE_OK(3)

NetBSD 8.99.34                 December 19, 2018                NetBSD 8.99.34