[sdiy] C coding

[Cornutt, David K]

I have to post a caveat to something I said earlier, because
if I don't you guys will call me on it. :-)

> Now the C longjmp can definitely take you out of the function,
> but that isn't the same thing and it doesn't work the same way.
> Even in the case of a longjmp, the stack should (if the compiler
> isn't broken) unwind cleanly to the point of the preceding
> setjmp.  

This is only true if the longjmp call takes place in the
same function as the setjmp, or in a function that was
called by the function that called setjmp.  If the function
calls setjmp and then returns, a subsequent longjmp call
is invalid because the stored stack pointer that setjmp
memorized points to a frame that isn't there any more.

(Setjmp/longjmp were put in as a klugey way of doing exception
handling early in on the language's history.  Ritchie has long
since admitted that there are better ways of doing that, such
as the try/catch statements in C++ and Java.)