Xref: utzoo comp.lang.c:6401 comp.sys.m68k:667
Path: utzoo!utgpu!water!watmath!clyde!rutgers!sri-spam!ames!lll-lcc!lll-winken!uunet!steinmetz!ge-dab!codas!ateng!chip
From: chip@ateng.UUCP (Chip Salzenberg)
Newsgroups: comp.lang.c,comp.sys.m68k
Subject: Re: C machine
Message-ID: <155@ateng.UUCP>
Date: 19 Jan 88 23:17:03 GMT
References: <461@auvax.UUCP> <9961@mimsy.UUCP> <166@teletron.UUCP> <6936@brl-smoke.ARPA> <147@ateng.UUCP> <39aca826.7f32@apollo.uucp> <7092@brl-smoke.ARPA> <708@umbc3.UMD.EDU> <7109@brl-smoke.ARPA> <712@umbc3.UMD.EDU>
Reply-To: chip@ateng.UUCP (Chip Salzenberg)
Organization: A T Engineering, Tampa, FL
Lines: 29

In the beginning, gwyn@brl.arpa (Doug Gwyn) wrote:
>First, only pointers into the same object can meaningfully be subtracted.
>Second, the result of the subtraction necessarily has implementation-
>defined signed integral type; there is a typedef ptrdiff_t in <stddef.h>.

Then alex@umbc3.UMD.EDU (Alex S. Crain) asks what dpANS says about code like:
>	funcall (ptr1 - ptr2); /* ptr1 - ptr2 > 64k */

The expression "ptr1 - ptr2" evaluates to an expression of the type
ptrdiff_t.  If you did your homework, then you defined the parameter to
funcall as a ptrdiff_t, so your stack will line up neatly.

But Doug did not mention whether ptrdiff_t is a signed or unsigned type.
Or is this not specified in dpANS?  When will I stop asking rhetorical
questions? :-)

>	I would prefer 32bit (or rather, the largest default integer available
>including pointers) ints, because conversion to short int is absolutely 
>painless (mov.l becomes mov.w) while conversion the other way can be a rather
>serious problem.

Conversion from 32 bits to 16 bits is painless, but for many architectures,
manipulation of 32-bit quantities is awkward and/or slow.  Thus X3J11 is
avoiding (whenever possible) giving minimum sizes for standard types.

Repeat after me:  "Not all C programs run on 32-bit architectures."
-- 
Chip Salzenberg                 UUCP: "{codas,uunet}!ateng!chip"
A T Engineering                 My employer's opinions are a trade secret.
       "Anything that works is better than anything that doesn't."