Newsgroups: comp.lang.c
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Out of range pointers
Message-ID: <1988Oct1.220705.10625@utzoo.uucp>
Organization: U of Toronto Zoology
References: <867@osupyr.mast.ohio-state.edu> <3200@geac.UUCP> <1430@ficc.uu.net> <1988Sep15.145026.20325@ateng.uucp> <16041@ism780c.isc.com <28227@think.UUCP> <8557@smoke.ARPA> <33666@XAIT.XEROX.COM> <8564@smoke.ARPA> <33789@XAIT.XEROX.COM> <1988Sep27.172738.16453@ut <34041@XAIT.XEROX.COM>
Date: Sat, 1 Oct 88 22:07:05 GMT

In article <34041@XAIT.XEROX.COM> g-rh@XAIT.Xerox.COM (Richard Harter) writes:
>>... in general one must draw the line somewhere.  It's almost always
>>possible to add just one more check.  There usually has to be some sort of
>>balance with efficiency.
>
>	It is true that one always has to take into account the tradeoff
>between efficiency and error checking.  However that is not normally the
>tradeoff involved in parameter validation.  Unless the body of a function
>is very short and very fast, parameter validation execution times are a
>nominal part of the total cost of a function invocation.

Ah, but it depends on how much validation you want to do, which was my
point.  I'm passing in a pointer to a complex data structure.  Do I just
check the pointer for being NULL?  Do I confirm that the node it points
at has a magic number in the right place?  Do I inspect the rest of the
node's data for plausibility?  Do I apply the same tests to the node's
neighbors?  Do I apply a graph-tracing algorithm to verify correct
linking of the whole multi-node structure?  I've actually done all of
these things at various times.  It depends on the tradeoffs.

(My normal practice?  I almost always check for NULL, and usually have
magic-number code controlled by #ifdef DEBUG.  And any time my code
vitally depends on some non-obvious property I'll usually throw in an
assert(), partly to document the fact and partly as an error-catcher.
The other things are responses to unusual situations.)
-- 
The meek can have the Earth;    |    Henry Spencer at U of Toronto Zoology
the rest of us have other plans.|uunet!attcan!utzoo!henry henry@zoo.toronto.edu