VMS Help POSIX Threads, PTHREAD routines, pthread_setspecific, Arguments  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 key

    Thread-specific key that identifies the thread-specific data to
    receive value. This key value must be obtained from pthread_key_
    create().

 value

    New thread-specific data value to associate with the specified
    key for the calling thread.