Path: utzoo!attcan!uunet!husc6!uwvax!rutgers!att!ulysses!andante!alice!ark
From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c
Subject: Re: Behaviour of setjmp/longjmp and registers
Message-ID: <8812@alice.UUCP>
Date: 22 Jan 89 16:15:56 GMT
References: <25@torsqnt.UUCP> <8867@bloom-beacon.MIT.EDU> <7222@polyslo.CalPoly.EDU>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 36

In article <7222@polyslo.CalPoly.EDU>, cquenel@polyslo.CalPoly.EDU (96 more school days) writes:

> Should it ever be necessary/desirable to restore any registers
> on a long-jump BESIDES the frame-pointer and/or stack-pointer ?

No, unfortunately.

Once upon a time, the VAX setjmp worked by saving all the
registers in the jmp_buf and longjmp restored it.  That means
that if you say:

	register foo;
	jmp_buf jb;

	foo = 3;
	if (setjmp(jb)) {
		/* exception stuff */
	}
	foo = 7;
	func();

and func calls longjmp(jb), foo will be 3 and not 7 when the
`exception stuff' is executed.

John Reiser figured out a way around it -- a nifty version of
longjmp that unwound the stack, restoring the correct registers
at each iteration -- and that went into at least some VAX C
implementations.

However, when it came to the ANSI committee, they decided it
would be too hard to mandate this kind of implementation.
So they took the least restrictive route possible -- longjmp
doesn't have to work at all unless you beg it.
-- 
				--Andrew Koenig
				  ark@europa.att.com