You cannot invoke the longjmp function from an OpenVMS condition
    handler. However, you may invoke longjmp from a signal handler
    that has been established for any signal supported by the
    Compaq C RTL, subject to the following nesting restrictions:

    o  The longjmp function will not work if you invoke it from
       nested signal handlers. The result of the longjmp function,
       when invoked from a signal handler that has been entered as a
       result of an exception generated in another signal handler, is
       undefined.

    o  Do not invoke the sigsetjmp function from a signal handler
       unless the associated longjmp is to be issued before the
       handling of that signal is completed.

    o  Do not invoke the longjmp function from within an exit handler
       (established with atexit or SYS$DCLEXH). Exit handlers are
       invoked after image tear-down, so the destination address of
       the longjmp no longer exists.

    o  Invoking longjmp from within a signal handler to return to
       the main thread of execution might leave your program in
       an inconsistent state. Possible side effects include the
       inability to perform I/O or to receive any more UNIX signals.
       Use siglongjmp instead.