Behaviour of setjmp/longjmp and registers

[Andrew Koenig]

In article <7222 at polyslo.CalPoly.EDU>, cquenel at 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 at europa.att.com