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.