(char *)(-1)

[Chris Torek]

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

In article <1063 at tukki.jyu.fi> tarvaine at 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 at 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 at mimsy.umd.edu	Path:	uunet!mimsy!chris