This routine deletes the thread-specific data key specified by
    the key argument, which must have been previously returned by
    pthread_key_create().

    The thread-specific data values associated with key need not be
    NULL at the time this routine is called. The application must
    free any application storage or perform any cleanup actions for
    data structures related to the deleted key or associated thread-
    specific data in any threads. This cleanup can be done either
    before or after this routine is called.

    Attempting to use the key after calling this routine results in
    unpredictable behavior.

    No destructor functions are invoked by this routine. Any
    destructor functions that may have been associated with key shall
    no longer be called upon thread exit. pthread_key_delete() can be
    called from within destructor functions.