This routine unblocks all threads waiting on the specified
    condition variable cond. Calling this routine implies that data
    guarded by the associated mutex has changed, so that it might be
    possible for one or more waiting threads to proceed. The threads
    that are unblocked shall contend for the mutex according to their
    respective scheduling policies (if applicable).

    If only one of the threads waiting on a condition variable may be
    able to proceed, but one of those threads can proceed, then use
    pthread_cond_signal() instead.

    Whether the associated mutex is locked or unlocked, you can still
    call this routine. However, if predictable scheduling behavior is
    required, that mutex should then be locked by the thread calling
    the pthread_cond_broadcast() routine.

    If no threads are waiting on the specified condition variable,
    this routine takes no action. The broadcast does not propagate to
    the next condition variable wait.