.Dd December 19, 2018
.Dt SQLITE3_MUTEX_METHODS 3
.Os
.Sh NAME
.Nm sqlite3_mutex_methods ,
.Nm sqlite3_mutex_methods
.Nd Mutex Methods Object
.Sh SYNOPSIS
.Vt typedef struct sqlite3_mutex_methods sqlite3_mutex_methods;
.Vt struct sqlite3_mutex_methods ;
.Sh DESCRIPTION
An instance of this structure defines the low-level routines used to
allocate and use mutexes.
.Pp
Usually, the default mutex implementations provided by SQLite are sufficient,
however the application has the option of substituting a custom implementation
for specialized deployments or systems for which SQLite does not provide
a suitable implementation.
In this case, the application creates and populates an instance of
this structure to pass to sqlite3_config() along with the SQLITE_CONFIG_MUTEX
option.
Additionally, an instance of this structure can be used as an output
variable when querying the system for the current mutex implementation,
using the SQLITE_CONFIG_GETMUTEX option.
.Pp
The xMutexInit method defined by this structure is invoked as part
of system initialization by the sqlite3_initialize() function.
The xMutexInit routine is called by SQLite exactly once for each effective
call to sqlite3_initialize().
.Pp
The xMutexEnd method defined by this structure is invoked as part of
system shutdown by the sqlite3_shutdown() function.
The implementation of this method is expected to release all outstanding
resources obtained by the mutex methods implementation, especially
those obtained by the xMutexInit method.
The xMutexEnd() interface is invoked exactly once for each call to
sqlite3_shutdown().
.Pp
The remaining seven methods defined by this structure (xMutexAlloc,
xMutexFree, xMutexEnter, xMutexTry, xMutexLeave, xMutexHeld and xMutexNotheld)
implement the following interfaces (respectively): 
.Bl -bullet
.It
sqlite3_mutex_alloc() 
.It
sqlite3_mutex_free() 
.It
sqlite3_mutex_enter() 
.It
sqlite3_mutex_try() 
.It
sqlite3_mutex_leave() 
.It
sqlite3_mutex_held() 
.It
sqlite3_mutex_notheld() 
.El
.Pp
The only difference is that the public sqlite3_XXX functions enumerated
above silently ignore any invocations that pass a NULL pointer instead
of a valid mutex handle.
The implementations of the methods defined by this structure are not
required to handle this case, the results of passing a NULL pointer
instead of a valid mutex handle are undefined (i.e.
it is acceptable to provide an implementation that segfaults if it
is passed a NULL pointer).
.Pp
The xMutexInit() method must be threadsafe.
It must be harmless to invoke xMutexInit() multiple times within the
same process and without intervening calls to xMutexEnd().
Second and subsequent calls to xMutexInit() must be no-ops.
.Pp
xMutexInit() must not use SQLite memory allocation (sqlite3_malloc()
and its associates).
Similarly, xMutexAlloc() must not use SQLite memory allocation for
a static mutex.
However xMutexAlloc() may use SQLite memory allocation for a fast or
recursive mutex.
.Pp
SQLite will invoke the xMutexEnd() method when sqlite3_shutdown()
is called, but only if the prior call to xMutexInit returned SQLITE_OK.
If xMutexInit fails in any way, it is expected to clean up after itself
prior to returning.
.Sh SEE ALSO
.Xr sqlite3_initialize 3 ,
.Xr sqlite3_malloc 3 ,
.Xr sqlite3_mutex_alloc 3 ,
.Xr sqlite3_mutex_held 3 ,
.Xr sqlite3_mutex_alloc 3 ,
.Xr sqlite3_mutex_held 3 ,
.Xr sqlite3_mutex_alloc 3 ,
.Xr sqlite3_initialize 3 ,
.Xr SQLITE_CONFIG_SINGLETHREAD 3