Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!motcsd!xdos!doug
From: doug@xdos.UUCP (Doug Merritt)
Newsgroups: comp.sys.amiga.tech
Subject: Re: M2/C discussion (Re: Can you nest subroutines in C?)
Summary: Answer to "why longjmp/setjmp in C?"
Message-ID: <413@xdos.UUCP>
Date: 5 Jul 89 14:41:12 GMT
References: <4524@crash.cts.com> <18213@usc.edu> <8673@pyr.gatech.EDU>
Reply-To: doug@xdos.UUCP (Doug Merritt)
Organization: Hunter Systems, Mountain View CA (Silicon Valley)
Lines: 36

In article <8673@pyr.gatech.EDU> dsking@pyr.UUCP ( David King) writes:
>	setjmp/longjmp?  Its definately not part of the language definition
>(they actually aren't part of C's definition unless the ANSI committee added 
>it in the last year),

Surprising; I was also surprised when I failed to find them in K&R just
now.

>  *jmp actually are library routines on UNIX that got used in 
>programs so much that they made it into most C systems. 

Well, the fact that they're library routines is irrelevent. There are a
fair number of library routines that are a standard part of the
definition of the C language, including e.g. printf() and strlen().
It's part of the charm of C that i/o and string handling "builtins" are
first class citizens in this way.

>something I've been wondering - what are things like *jmp useful for in an 
>Amiga program?

The cleanest, most defensible use is as a non-local goto to unwind
from deep procedures as part of error recovery. Even the purist Niklaus
Wirth uses (and advocates) non-local goto's for this purpose. Without
setjmp/longjmp, C has no non-local goto at all, and loses this ability.
Makes severe-error handling routines in complex software very messy.
That's why I surprised they're not in the standard.

They're also defined cleanly enough that it's hard to imagine a modern
day general purpose cpu that would have trouble allowing their
implementation, so I wonder why they didn't get included.
Hmmm...perhaps something like a Z8000 with different address spaces for
data and stack? I'm not really awake yet, anybody know?
	Doug
-- 
Doug Merritt		{pyramid,apple}!xdos!doug
Member, Crusaders for a Better Tomorrow		Professional Wildeyed Visionary