Path: utzoo!attcan!uunet!lll-winken!ames!haven!purdue!decwrl!thundr.dec.com!minow
From: minow@thundr.dec.com (Repent! Godot is coming soon! Repent!)
Newsgroups: comp.sys.mac.programmer
Subject: re: serious code generation bug in Lightspeed C
Message-ID: <8901131605.AA09908@decwrl.dec.com>
Date: 13 Jan 89 19:01:00 GMT
Organization: Digital Equipment Corporation
Lines: 29

Tim Maroney (sun!hoptoad!tim) point out that Think C evaluates the left-hand
side of
	(*h)->p = NewPtr(size)
before the right-hand side (and consequently, the dereferenced *h points
to stale data).

This is explicitly permitted by C, both under existing practice and
in the Draft Ansi Standard.  The only exception to this would be
if h is defined as "volatile" (which, it in fact is).  Think C
doesn't appear to support volatile yet (and doesn't claim to be
Ansi compatible).

It would certainly be reasonable for Think to put a note in their
manuals warning programmers of this problem.  Until then, the cautious
programmer would be well advised to write

	{
	    Ptr		temp = NewPtr(size);

	    (*h)->p = temp;
	}

Which should operate correctly.

Thanks, by the way, for pointing this out -- I'm going to go through
a huge pile of code to see if I've been caught by the same problem.

Martin Minow
minow%thundr.dec@decwrl.dec.com