This routine initializes a mutex object with the Threads Library
    default mutex attributes. A mutex is a synchronization object
    that allows multiple threads to serialize their access to shared
    data.

    The mutex object is initialized and set to the unlocked state.

    Your program can use the PTHREAD_MUTEX_INITIALIZER macro to
    statically initialize a mutex object without calling this
    routine. Static initialization can be used only for a condition
    variable with storage class "extern" or "static" - "automatic"
    (stack local) objects must be initialized by calling tis_mutex_
    init(). Use this macro as follows:

       pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;