Description

   This routine initializes a mutex with the attributes specified
   by the mutex attributes object specified in the attr argument. A
   mutex is a synchronization object that allows multiple threads to
   serialize their access to shared data.

   The mutex is initialized and set to the unlocked state. If attr
   is set to NULL, the default mutex attributes are used. The
   pthread_mutexattr_settype() routine can be used to specify the
   type of mutex that is created (normal, recursive, or errorcheck).

   Use the PTHREAD_MUTEX_INITIALIZER macro to statically initialize
   a mutex without calling this routine. Statically initialized
   mutexes need not be destroyed using pthread_mutex_destroy(). Use
   this macro as follows:

      pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER

   Only normal mutexes can be statically initialized.

   A mutex is a resource of the process, not part of any particular
   thread. A mutex is neither destroyed nor unlocked automatically
   when any thread exits. If a mutex is allocated on a stack, static
   initializers cannot be used on the mutex.