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From: mouse@thunder.mcrcim.mcgill.edu (der Mouse)
Newsgroups: comp.lang.c
Subject: Re: What does ANSI C say about short circuit evaluation?
Message-ID: <1991May25.130344.20921@thunder.mcrcim.mcgill.edu>
Date: 25 May 91 13:03:44 GMT
References: <1991May22.092404.25297@ucthpx.uct.ac.za>
Organization: McGill Research Centre for Intelligent Machines
Lines: 37

In article <1991May22.092404.25297@ucthpx.uct.ac.za>, gram@uctcs.uucp (Graham Wheeler) writes:

> i) Does ANSI C say that compilers can rearrange the order of
>    expression evaluation?

In general, yes.  There are a few exceptions; notably, the &&, ||, ?:,
and , operators promise some things about the order in which their
operands are evaluated (and in some cases, about whether certain
operands are evaluated at all).

> ii) Does it say that Boolean expressions must be evaluated with
>     short-circuit evaluation?

When using the short-circuit operators && and ||, yes.

> We think the answers to both of these are yes, but we aren't sure.

Mostly.  The second one is yes; the first one is a qualified yes.

> This would mean that a statement like:

>       if (ptr && ptr->next) ...

> could potentially cause a segmentation violation on a system with
> meory protection if the compiler was stupid enough to try to evaluate
> the second (more complex) subexpression first - ie, one cannot rely
> on short-circuit evaluation to prevent this from happening.

No, this is not a danger.  A compiler producing code for that example
that evaluates ptr->next when ptr == 0 (ie, when ptr is a null pointer
of its type) is simply broken.

                                        der Mouse

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