Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!cernvax!chx400!chx400!bernina!neptune!inf.ethz.ch!brandis
From: brandis@inf.ethz.ch (Marc Brandis)
Newsgroups: comp.unix.aix
Subject: Invalid pointer traps (S/6000)
Summary: No traps on read, traps on write
Keywords: Traps, NIL, NULL, invalid pointers
Message-ID: <24518@neptune.inf.ethz.ch>
Date: 11 Feb 91 09:45:01 GMT
Sender: news@neptune.inf.ethz.ch
Reply-To: brandis@inf.ethz.ch (Marc Brandis)
Organization: Departement Informatik, ETH, Zurich
Lines: 18

I noticed that when a program under AIX on the S/6000 reads through an invalid
pointer, no exception is reported but instead the value 0 is returned. On write,
the exception is reported. This seems to be true for the whole address range
from 0 up to 7fffffff.

As I understand the hardware documentation, the MMU does in fact detect the
illegal access, so the whole thing can only be a matter of the operating system.

Is there a way to turn this off so that all accesses to unmapped pages report
an exception, not only writes?

Thanks for any pointers.


Marc-Michael Brandis
Computer Systems Laboratory, ETH-Zentrum (Swiss Federal Institute of Technology)
CH-8092 Zurich, Switzerland
email: brandis@inf.ethz.ch