Description

   This routine generates a unique, thread-specific data key that
   is visible to all threads in the process. The variable key
   provided by this routine is an opaque object used to locate
   thread-specific data. Although the same key value can be used
   by different threads, the values bound to the key by pthread_
   setspecific() are maintained on a per-thread basis and persist
   for the life of the calling thread. The initial value of the key
   in all threads is NULL.

   The Threads Library imposes a maximum number of thread-specific
   data keys, equal to the symbolic constant PTHREAD_KEYS_MAX.

   Thread-specific data allows client software to associate "static"
   information with the current thread. For example, where a routine
   declares a variable static in a single-threaded program, a
   multithreaded version of the program might create a thread-
   specific data key to store the same variable.

   This routine generates and returns a new key value. The key
   reserves a cell within each thread. Each call to this routine
   creates a new cell that is unique within an application
   invocation. Keys must be generated from initialization code that
   is guaranteed to be called only once within each process. (See
   the pthread_once() description for more information.)

   When a thread terminates, its thread-specific data is
   automatically destroyed; however, the key remains unless
   destroyed by a call to pthread_key_delete(). An optional
   destructor function can be associated with each key. At thread
   exit, if a key has a non-NULL destructor pointer, and the thread
   has a non-NULL value associated with that key, the destructor
   function is called with the current associated value as its sole
   a