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From: farren@hoptoad.uucp (Mike Farren)
Newsgroups: comp.sys.ibm.pc
Subject: Re: MSC 4.0 and the Twilight Zone
Message-ID: <2088@hoptoad.uucp>
Date: Mon, 4-May-87 11:05:29 EDT
Article-I.D.: hoptoad.2088
Posted: Mon May  4 11:05:29 1987
Date-Received: Tue, 5-May-87 01:57:52 EDT
References: <2454@uw-june.UUCP>
Reply-To: farren@hoptoad.UUCP (Mike Farren)
Organization: Nebula Consultants in San Francisco
Lines: 34

In article <2454@uw-june.UUCP> roper@uw-june.UUCP (Michael Roper) writes:
>Can anyone please explain the behavior of the following few lines of code?
[which doesn't work]
>
>main (argc, argv)
>int argc;
>char *argv[];
>{
> char  *s;
>
> s = strdup (argv[1]);
> strcat (s, ".U");
> do_nothing (s);
>}

Note that you are defining s (and, later, do_nothing_str) as a pointer to
char, but you are never creating an array that the pointers will point to.
As a result, the strcat and strdup routines, which expect that the pointers
they receive WILL point to such arrays, go ahead and blindly copy the strings
they have received as arguments to the place that the pointers point to,
which is the address represented by whatever happened to be in the pointers
when the routine was entered.  It's amazing that the routine works as well as
it does - give it "fred" as an argument, and it even works correctly - but
this is just chance.

Remember - pointers point to things, arrays hold things, and never the
twain should meet!


-- 
----------------
                 "... if the church put in half the time on covetousness
Mike Farren      that it does on lust, this would be a better world ..."
hoptoad!farren       Garrison Keillor, "Lake Wobegon Days"