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From: tom@hcrvx1.UUCP (Tom Kelly)
Newsgroups: comp.lang.c
Subject: Re: Some questions about ANSI C
Message-ID: <1531@hcrvx1.UUCP>
Date: Mon, 17-Aug-87 09:32:54 EDT
Article-I.D.: hcrvx1.1531
Posted: Mon Aug 17 09:32:54 1987
Date-Received: Wed, 19-Aug-87 01:35:53 EDT
References: <497@houxs.UUCP> <165600007@uiucdcsb> <6265@brl-smoke.ARPA>
Reply-To: tom@hcrvx1.UUCP (Tom Kelly)
Organization: HCR Corporation, Toronto
Lines: 70
Summary: 

In article <6265@brl-smoke.ARPA> gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
writes about the latest ANSI C meeting.

I was at the meeting in Paris, so perhaps I can provide some context
to the discussion.

The setjmp() issue arose because of a public comment from Symbolics,
Inc.  On this machine, it is possible to generate the code for
setjmp() inline at the point of call, but it is not possible
for the setjmp() function to determine the necessary information
about its caller.  Thus, setjmp() cannot be implemented as a
function on this machine.

Since setjmp() was a function, it was possible to take the address
of it, and invoke it via a pointer.  This led to an impossible
situation for the commentor.  The suggestion was to make it illegal
to take the address of setjmp(), so that the compiler would always
know when it was being invoked.

The committee felt this was a reasonable request, and that it
was unlikely that very much present or future code would be
seriously harmed by the inability to manipulate setjmp() as
an ordinary function.

As far as the aliasing issue:

A question arose (from various public comments, including one from
Stallman) regarding legal aliasing.  Boiled down to the essentials,
given:

	double *dp; int *ip;

	*ip = 1;
	*dp = 3.0;

Can the compiler deduce after these two statements that *ip has
not been changed, and still contains 1?

The change to the standard is intended to make it so the answer is
yes.  It sets out the cases in which object references can refer
to the same object and the compiler must still get "the right
answer."

Because of various aspects of the C language, and common programming
practice, certain special cases had to be provided in that aliasing
in these cases is permitted.  The struct case arises as follows:

	struct tag {
		int i;
		double d;
		char c;
	} s, t;

	double *dp;

	dp = &(s.d);
	*dp = 3.0;
	s = t;
	*dp ?

Although *dp and s have different types, it was felt that this kind
of aliasing was legal and common.  The language in the proposal is
meant to ensure that a compiler has to get this one right, while
not being required to worry about aliasing in the first example.

Hope this has helped.

Tom Kelly  (416) 922-1937
HCR Corporation, 130 Bloor St. W., Toronto, Ontario, Canada
{utzoo, ihnp4, decvax}!hcr!tom