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

    This routine sets the thread-specific data value associated with
    the specified key for the current thread. If a value is defined
    for the key in this thread (the current value is not NULL),
    the new value is substituted for it. The key is obtained by a
    previous call to pthread_key_create().

    Different threads can bind different values to the same key.
    These values are typically pointers to blocks of dynamically
    allocated memory that are reserved for use by the calling thread.

    Do not call this routine from a thread-specific data destructor
    function.

    Note that although the type for value (void *) implies that it
    represents an address, the type is being used as a "universal
    scalar type." The Threads Library simply stores value for later
    retrieval.