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From: wesommer@athena.mit.edu (William Sommerfeld)
Newsgroups: comp.lang.c
Subject: Re: Why should "sizeof" be unsigned?
Message-ID: <976@bloom-beacon.MIT.EDU>
Date: Sun, 21-Jun-87 16:12:52 EDT
Article-I.D.: bloom-be.976
Posted: Sun Jun 21 16:12:52 1987
Date-Received: Mon, 22-Jun-87 07:02:15 EDT
References: <1748@plus5.UUCP> <6001@brl-smoke.ARPA>
Sender: daemon@bloom-beacon.MIT.EDU
Reply-To: wesommer@athena.mit.edu
Distribution: world
Organization: Massachusetts Institute of Technology
Lines: 32
Keywords: unsigned ANSI
Summary: unsigned sizeof can screw you in obscure ways.

In article <6001@brl-smoke.ARPA> gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>) writes:
>In article <1748@plus5.UUCP> hokey@plus5.UUCP (Hokey) writes:
>>None of these issues are key - it seems to me having sizeof return an
>>explicitly unsigned value violates "the principle of least astonishment".
>
>(a) AT&T "sizeof" has been this way for many years now.
But Berkeley's hasn't.

>(b) Since sizeof(thing) inherently cannot be negative, an unsigned
>integer value for the sizeof operator seems exactly right.

This can run into problems in existing code..  For example, a couple
of months ago, I spent several hours helping someone track down what
appeared to be a DBM bug on the IBM RT/PC under IBM's BSD port.

I finally tracked it down to the following statement:

	if (i1 <= (i2+3) * sizeof(short))
		return (0);

(in additem(), in lib/libc/gen/ndbm.c).  By examining the assembler
output by the compiler, I determined that it was probably generating
an unsigned compare for that.  (I say "probably" because I can't read
RT assembler very well), and the behavior of the program was as if an
unsigned test was being generated there.

It turns out that the compiler was an ANSI C compiler.  I mistakenly
reported it as a compiler bug, and was corrected on it by someone
who had actually done work on an ANSI C interpreter.

					Bill Sommerfeld
					wesommer@athena.mit.edu