Restrictions

   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
   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.