If an error condition occurs, this routine returns an integer
    value indicating the type of error. Possible return values are as
    follows:

    Return      Description

    0           Successful completion.
    [EINVAL]    The value specified by cond or mutex is invalid, or

                Different mutexes are supplied for concurrent
                pthread_cond_wait()  or pthread_cond_timedwait()
                operations on the same condition variable, or

                The mutex was not owned by the calling thread at the
                time of the call.
    [ENOMEM]    The Threads Library cannot acquire memory needed
                to block using a statically initialized condition
                variable.