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

    This routine returns the value currently bound to the specified
    thread-specific data key.

    This routine can be called from a data destructor function.

    When threads are present, the data and keys are thread specific;
    they enable a library to maintain context on a per-thread basis.