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 is not a valid condition
                variable.
    [EBUSY]     The object being referenced by cond is being
                referenced by another thread that is currently
                executing
                pthread_cond_wait()  or pthread_cond_timedwait() on
                the condition variable specified in cond.