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.