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

    When you are performing thread-synchronous "wait-form" system
    service calls on OpenVMS such as $QIOW, $ENQW, $GETJPIW, and so
    on, you should use this routine and tis_io_complete() with the
    asynchronous form of the service (that is, without the "W") and
    specify the address of tis_io_complete() as the completion AST
    routine (the AST argument, if any, is ignored). The call must
    also specify an IOSB (or equivalent, such as an LKSB) and if
    possible a unique event flag (see lib$get_ef). Once the library
    code is ready to wait for the I/O, it simply calls tis_sync()
    (just as if it were calling $SYNC).