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.