Path: utzoo!utgpu!water!watmath!clyde!att!rutgers!cmcl2!nrl-cmf!ames!oliveb!sun!thetone!swilson
From: swilson%thetone@Sun.COM (Scott Wilson)
Newsgroups: comp.sys.mac.programmer
Subject: Pointer Subtraction in THINK C 3.0
Message-ID: <63033@sun.uucp>
Date: 5 Aug 88 16:59:36 GMT
Sender: news@sun.uucp
Reply-To: swilson@sun.UUCP ()
Organization: Sun Microsystems, Mountain View
Lines: 57

I was a little suprised when I ran this program under THINK C 3.0:

	#include <stdio.h>

	printi(i)
	int i;
	{
		printf("i = %d\n", i);
	}

	char buf[10];

	_main()		/* so I can redirect output */
	{
		char *cp1, *cp2;

		cp1 = buf;
		cp2 = buf + 5;
		printi(cp2 - cp1);
		printi((int) (cp2 - cp1));

		printi(&buf[5] - buf);
		printi((int) (&buf[5] - buf));
	}

and got the output:

	i = 0
	i = 5
	i = 0
	i = 5

It looks as though THINK C 3.0 is using long as the resulting type for
pointer subtraction.  (A long gets pushed on the stack instead
of an int, its upper two bytes are zero so printi prints 0.)

According to K&R:

	If two pointers to objects of the same type are subtracted,
	the result is converted ... to an int ...

However, K&R 2nd ed. (and I assume ANSI) says:

	If two pointers to objects of the same type are subtracted, ...
	The type of the result ... is defined as ptrdiff_t in the
	standard header <stddef.h>

Which definiton does THINK C 3.0 claim to use?  Do they specify it?
Is there a stddef.h and is ptrdiff_t defined to be long?

I'm sorry if this a RTFM question, but either I'm at work with the C
manuals or at home with THINK C manuals.

--
Scott Wilson		arpa: swilson@sun.com
Sun Microsystems	uucp: ...!sun!swilson
Mt. View, CA