VMS Help POSIX Threads, TIS routines, tis_getspecific, Return Values  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    No errors are returned. This routine returns the data value
    associated with the specified thread-specific data key key. If
    no data value is associated with key, or if key is not defined,
    then a NULL value is returned.