Description

   This routine initializes the mutex attributes object specified
   by the attr argument with a set of default values. A mutex
   attributes object is used to specify the attributes of one or
   more mutexes when they are created. The attributes object created
   by this routine is used only in calls to the pthread_mutex_init()
   routine.

   When a mutex attributes object is used to create a mutex, the
   values of the individual attributes determine the characteristics
   of the new mutex. Thus, attributes objects act as additional
   arguments to mutex creation. Changing individual attributes
   in an attributes object does not affect any mutexes that were
   previously created using that attributes object.

   You can use the same mutex attributes object in successive calls
   to pthread_mutex_init(), from any thread. If multiple threads
   can change attributes in a shared mutex attributes object,
   your program must use a mutex to protect the integrity of the
   attributes object's contents.

   Results are undefined if this routine is called and the attr
   argument specifies a mutex attributes object that is already
   initialized.