Path: utzoo!attcan!uunet!snorkelwacker!spdcc!ima!haddock!karl@haddock.ima.isc.com
From: karl@haddock.ima.isc.com (Karl Heuer)
Newsgroups: comp.std.c
Subject: Re: all bits zero?
Message-ID: <16245@haddock.ima.isc.com>
Date: 21 Mar 90 18:49:17 GMT
References: <2108@gmdzi.UUCP>
Sender: karl@haddock.ima.isc.com
Reply-To: karl@haddock.ima.isc.com (Karl Heuer)
Organization: Interactive Systems, Cambridge, MA 02138-5302
Lines: 18
Supersedes: <16237@haddock.ima.isc.com>

In article <2108@gmdzi.UUCP> wittig@gmdzi.UUCP (Georg Wittig) writes:
>Does ANSI C require that in the internal representation of the integer
>constant ``0'' all bits are zero?

Assuming it's not in a pointer context--in which case it would have been a
null pointer constant, not an integer constant, despite having the same
spelling--yes.  Non-negative integers, signed or unsigned, whether formed by
constants of any radix or by run-time expressions, are required to be
represented in a "pure binary system".  Without such a guarantee, the bitwise
operators would be much less useful.

Much less is said about negative integers.  The value -1 looks like 1111 in
two's complement, 1110 in one's complement, and 1001 in sign-magnitude.  All
of these (and presumably even stranger things) are legal; in the latter two
cases a conversion from signed int to unsigned int actually entails a change
in the bit-pattern (since (unsigned)-1 == UINT_MAX which looks like 1111).

Karl W. Z. Heuer (karl@ima.ima.isc.com or harvard!ima!karl), The Walking Lint