Description

   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.