SQLite

/*
** Static variables used for thread synchronization.
**
** inMutex      the nesting depth of the recursive mutex.  The thread
**              holding mutexMain can read this variable at any time.
**              But is must hold mutexAux to change this variable.  Other
**              threads must hold mutexAux to read the variable and can
**              never write.
**
** mutexOwner   The thread id of the thread holding mutexMain.  Same
**              access rules as for inMutex.
**
** mutexOwnerValid   True if the value in mutexOwner is valid.  The same
**                   access rules apply as for inMutex.
**
** mutexMain    The main mutex.  Hold this mutex in order to get exclusive
**              access to SQLite data structures.
**
** mutexAux     An auxiliary mutex needed to access variables defined above.
**
** Mutexes are always acquired in this order: mutexMain mutexAux.   It
** is not necessary to acquire mutexMain in order to get mutexAux - just
** do not attempt to acquire them in the reverse order: mutexAux mutexMain.
** Either get the mutexes with mutexMain first or get mutexAux only.
**
** When running on a platform where the three variables inMutex, mutexOwner,
** and mutexOwnerValid can be set atomically, the mutexAux is not required.
** On many systems, all three are 32-bit integers and writing to a 32-bit
** integer is atomic.  I think.  But there are no guarantees.  So it seems
** safer to protect them using mutexAux.
*/
static int inMutex = 0;
#ifdef SQLITE_UNIX_THREADS
static pthread_t mutexOwner;          /* Thread holding mutexMain */
static int mutexOwnerValid = 0;       /* True if mutexOwner is valid */
static pthread_mutex_t mutexMain = PTHREAD_MUTEX_INITIALIZER; /* The mutex */
static pthread_mutex_t mutexAux = PTHREAD_MUTEX_INITIALIZER;  /* Aux mutex */
#endif

/*
** The following pair of routine implement mutual exclusion for