Path: utzoo!attcan!uunet!husc6!spdcc!ima!cfisun!palladium!nw
From: nw@palladium.UUCP (Neil Webber)
Newsgroups: comp.unix.wizards
Subject: Re: alloca & coroutining
Summary: longjmp botch, core dumped
Message-ID: <528@palladium.UUCP>
Date: 12 Jun 88 16:06:08 GMT
References: <16126@brl-adm.ARPA>
Reply-To: nw@palladium.UUCP (Neil Webber)
Followup-To: comp.lang.c
Organization: Epoch Systems, Marlborough, MA
Lines: 46

In article <16126@brl-adm.ARPA> ted%nmsu.csnet@relay.cs.net writes:
	>
	>But isn't the following idiom for co-routines useful? 
	>
	>  [ sample coroutine implementation deleted ]
	>

Not on a vax running 4.3BSD it isn't:

	Script started on Sun Jun 12 11:20:08 1988

	% cc -o coroutines coroutines.c
	% ./coroutines
	a b c
	longjmp botch
	Illegal instruction (core dumped)
	%

	script done on Sun Jun 12 11:20:28 1988

This happens because, as Chris Torek points out:

	>longjmp is not suitable for coroutines because it is valid
	>for longjmp to attempt to unwind the stack in order to find 
	>the corresponding setjmp, and it is therefore legal for 
	>longjmp to abort if it is attempting to jump the `wrong way' 
	>on the stack.

I don't believe that there is a portable way to *implement* a
coroutining library in C, short of first implementing a processor
emulation and then writing code for that processor :-).  Perhaps
someone can prove otherwise.

The coroutining code posted by ted%nmsu.csnet@relay.cs.net does
work on our Sun 3, *with the Sun compiler*.  It does not work with the
Greenhills compiler unless you take care to throw all the right
compiler switches (force frame pointers, no delayed stack adjustments).

David DiGiacomo (david@sun.uucp) suggests that the C compiler
recognize alloca() as a special case.  This certainly appears to
be the way of the, er, umm, future.  After all, inlining those
strcpy calls gets the Dhrystone numbers way up ;-}

-- 
Neil Webber / Epoch Systems, Marlboro MA / (617) 481-3717
        {harvard!cfisun, linus!alliant}!palladium!nw