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

    This routine allows a thread to obtain its own thread identifier.

    This value becomes meaningless when the thread is destroyed.

    Note that the initial thread in a process can "change identity"
    when thread system initialization completes-that is, when the
    multithreading run-time environment is loaded.