Path: utzoo!mnetor!uunet!husc6!rutgers!rochester!cornell!batcomputer!pyramid!prls!mips!hitz
From: hitz@mips.UUCP (David Hitz)
Newsgroups: comp.unix.questions
Subject: Re: Portability Question
Message-ID: <1178@quacky.UUCP>
Date: 18 Dec 87 19:14:19 GMT
References: <122@insyte.uucp> <9803@mimsy.UUCP>
Reply-To: hitz@quacky.UUCP (David Hitz)
Organization: MIPS Computer Systems, Sunnyvale, CA
Lines: 47
Keywords: Portable VAX VMS vs HPUX c
Summary: Does the standard specify integer representation?

In article <9803@mimsy.UUCP> chris@mimsy.UUCP (Chris Torek) writes:
] In article <122@insyte.uucp> jad@insyte.uucp (Jill Diewald) writes:
] ] ... HPUX c ... [and] VMS c ... [give] different answers.  We want to
] ] know which is right (if either) so we can report it as a bug to the
] ] correct source.
]   ...
] ] main() {
] ]    struct { int x : 1; } foo;
] ]
] ]    foo.x = 1;
] ]    printf ("%d\n", foo.x);
] ] }
]
] Whether bitfields are signed is undefined.  I believe the current
] draft says that to get a particular behaviour, you must use either
] of the `signed' or `unsigned' keywords.  In other words, the code
] is wrong, not either of the compilers.

Now I'm curious.  Does ANSII require a hardware implementation to use
2s complement arithmetic for its integer representation?  (Am I allowed
to build a grey code machine?)

If the standard doesn't say, then setting a single bit in a bitfield
could result in an arbitrary integer.

In K&R, the sections on bit fields and bit manipulation operators don't
specify a particular representation.  However, some examples do assume
the following defines each map inidividual bits:

	#define FIELD1	1
	#define FIELD2	2
	#define FIELD3	4

which isn't true in an arbitrarary representation.  And in the section
on unsigned chars they do talk about sign extension which seems to
imply 2s complement arithmetic.

Some other C constructs are affected as well.  For example, does the
standard guarantee that ( (X << 1) == (X * 2))?

Clearly there is too much code that relies on these features for them
to change.  I was just wondering if the standard is explicit.

hitz
-- 
Dave Hitz
UUCP: {decvax,ucbvax,ihnp4}!decwrl!mips!hitz 	DDD: hitz@408-991-0345