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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
#include <sqlite3.h>
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_MAIN
⊕ 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(),
sqlite3_mutex_leave(), or sqlite3_mutex_free() is a NULL pointer, then
any of the four routines behaves as a no-op.
IMPLEMENTATION NOTES
These declarations were extracted from the interface documentation at
line 7836.
SQLITE_API sqlite3_mutex *sqlite3_mutex_alloc(int);
SQLITE_API void sqlite3_mutex_free(sqlite3_mutex*);
SQLITE_API void sqlite3_mutex_enter(sqlite3_mutex*);
SQLITE_API int sqlite3_mutex_try(sqlite3_mutex*);
SQLITE_API void sqlite3_mutex_leave(sqlite3_mutex*);
SEE ALSO
sqlite3_mutex_held(3), SQLITE_CONFIG_SINGLETHREAD(3), SQLITE_OK(3)
NetBSD 10.99 August 24, 2023 NetBSD 10.99