This routine unblocks at least one thread waiting on the
    specified condition variable cond. Calling this routine implies
    that data guarded by the associated mutex has changed, thus it
    might be possible for one of the waiting threads to proceed. In
    general, only one thread will be released.

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

    This routine should be called when any thread waiting on the
    specified condition variable might find its predicate true,
    but only one thread should proceed. If more than one thread can
    proceed, or if any of the threads would not be able to proceed,
    then you must use pthread_cond_broadcast().

    The scheduling policy determines which thread is awakened. For
    policies SCHED_FIFO and SCHED_RR, a blocked thread is chosen
    in priority order, using first-in/first-out (FIFO) within
    priorities.

    If the calling thread holds the lock to the target condition
    variable's associated mutex while setting the variable's wait
    predicate, that thread can call pthread_cond_signal() to signal
    the variable even after releasing the lock on that mutex.
    However, for more predictable scheduling behavior, call pthread_
    cond_signal() before releasing the target condition variable's
    associated mutex.