Path: utzoo!utgpu!utstat!jarvis.csri.toronto.edu!mailrus!ames!amdcad!crackle!tim
From: tim@crackle.amd.com (Tim Olson)
Newsgroups: comp.arch
Subject: Re: Endian wars
Message-ID: <24384@amdcad.AMD.COM>
Date: 8 Feb 89 19:44:14 GMT
References: <6133@columbia.edu> <3300050@m.cs.uiuc.edu> <8480@aw.sei.cmu.edu>
Sender: news@amdcad.AMD.COM
Reply-To: tim@amd.com (Tim Olson)
Organization: Advanced Micro Devices, Inc. Sunnyvale CA
Lines: 26
Summary:
Expires:
Sender:
Followup-To:

In article <8480@aw.sei.cmu.edu> firth@bd.sei.cmu.edu (Robert Firth) writes:
| >while (*p++ = *q++);
| 
| This is a neat and beautiful idiom in PDP-11 Assembler.  There is,
| however, one problem with the equivalent C code: it is incorrect.
| After termination of the loop, the variables p and q, though declared
| of type 'pointer-to-char', will hold values that do not point to
| declared or allocated objects of type 'char'.  Should you ever have
| the misfortune to port this code to a machine with hardware segmentation,
| and automatic segment bounds checking as part of the address arithmetic,
| (or be a consultant involved in such a port), you will face this
| problem.
| 
| Could someone inform me whether the current C standard has fixed this?
| The simplest answer I guess is to rule that the address of array[upb+1]
| must always be legal; in practice this means the implementation has to
| leave dead space at the end of each memory segment.

That is exactly what is done in the current proposed ANSI C standard;
the address is legal to compute, although dereferencing the address is
undefined.  Only a single byte of "dead space" is required for this.


	-- Tim Olson
	Advanced Micro Devices
	(tim@crackle.amd.com)