Description

   When setjmp is first called, it returns the value 0. If longjmp
   is then called, naming the same environment as the call to
   setjmp, control is returned to the setjmp call as if it had
   returned normally a second time. The return value of setjmp in
   this second return is the value supplied by you in the longjmp
   call. To preserve the true value of setjmp, the function calling
   setjmp must not be called again until the associated longjmp is
   called.

   The setjmp function preserves the hardware general-purpose
   registers, and the longjmp function restores them. After a
   longjmp, all variables have their values as of the time of the
   longjmp except for local automatic variables not marked volatile.
   These variables have indeterminate values.

   The setjmp and longjmp functions rely on the OpenVMS
   condition-handling facility to effect a nonlocal goto with
   a signal handler. The longjmp function is implemented by
   generating a C RTL specified signal that allows the
   OpenVMS condition-handling facility to unwind back to the desired
   destination.

   The C RTL must be in control of signal handling for any
   VSI C image. For VSI C to be in control of signal handling,
   you must establish all exception handlers through a call to the
   VAXC$ESTABLISH function.

                                  NOTE

      The C RTL provides nonstandard decc$setjmp and decc$fast_
      longjmp functions for Alpha and Integrity server systems. To
      use these nonstandard functions instead of the standard
      ones, a program must be compiled with __FAST_SETJMP or
      __UNIX_SETJMP macros defined.

      Unlike the standard longjmp function, the decc$fast_longjmp
      function does not convert its second argument from 0 to 1.
      After a call to decc$fast_longjmp, a corresponding setjmp
      function returns with the exact value of the second argument
      specified in the decc$fast_longjmp call.