Path: utzoo!attcan!utgpu!watmath!iuvax!uxc.cso.uiuc.edu!tank!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.std.c
Subject: Re: (char *)(-1)
Keywords: pointers, casts
Message-ID: <18792@mimsy.UUCP>
Date: 28 Jul 89 10:29:03 GMT
References: <GEOFF.89Jun11231924@onyx.cs.warwick.ac.uk> <2619@yunexus.UUCP> <118@psitech.UUCP> <1063@tukki.jyu.fi>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 56

>>... if NULL is returned there was an error getting the next string,
>>if (-1) is returned there were no more symbols.

In article <1063@tukki.jyu.fi> tarvaine@tukki.jyu.fi (Tapani Tarvainen) writes:
>Is it safe to return -1?

No.  (Incidentally, that is not the only problem with the code in
<118@psitech.UUCP>.)

>I mean, isn't it possible that (char *)(-1) is a valid pointer
>in some system and could have been returned by malloc() ... ?

This is entirely possible (although it will be false on most Unix
systems, for SysV and BSD and V7 backwards-compatibility reasons;
this will probably be true for at least another ten years).

It is also possible that (char *)(-1) is a run-time exception, or
even a compile-time exception.

>If -1 isn't safe, is there any other value (besides NULL) that can
>safely be returned from a (any *) function to indicate error?

There is no single value.  One common approach is to return either
the address a dynamically allocated object, NULL, or the address of
one of several statically objects.  For instance:

	#include <stddef.h>		/* defines malloc, etc */
	#include "widget.h"		/* defines widget things */

	struct widget error_widget;

	struct widget *
	make_a_widget(sometype somearg) {
		struct widget *p;

		/* check arguments */
		if (BADARG(somearg))
			return &error_widget;

		/* make a new widget */
		p = malloc(sizeof *p);
		if (p == NULL)
			return NULL;

		/* set it up */
			.
			.
			.
		return p;
	}

Another approach is to return NULL for all exceptions, and provide
an additional status variable or function.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris