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

    This routine obtains and returns the thread sequence number
    for the thread identified by the thread object specified in the
    thread argument.

    The thread sequence number provides a unique identifier for each
    existing thread. A thread's thread sequence number is never
    reused while the thread exists, but can be reused after the
    thread terminates. The debugger interfaces use this sequence
    number to identify each thread in commands and in display output.

    The result of calling this routine is undefined if the thread
    argument does not specify a valid thread object.