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From: jfc@athena.mit.edu (John F Carr)
Newsgroups: comp.unix.aix
Subject: Re: Invalid pointer traps (S/6000)
Keywords: Traps, NIL, NULL, invalid pointers
Message-ID: <1991Feb12.033513.27494@athena.mit.edu>
Date: 12 Feb 91 03:35:13 GMT
References: <24518@neptune.inf.ethz.ch>
Sender: news@athena.mit.edu (News system)
Organization: Massachusetts Institute of Technology
Lines: 23

In article <24518@neptune.inf.ethz.ch> brandis@inf.ethz.ch (Marc Brandis) writes:
>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.

The problem is, page zero is mapped and readable.  The MMU detects no
exception.

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

There doesn't appear to be a way in the current OS version to unmap page
zero.  Even using the linker -T and -D flags to move the program text and
data out of page zero doesn't help (it doesn't put anything in page zero,
but it leaves it accessible).

Speculation: IBM found too many things broke when they made NULL pointer
dereferences trap.  The documentation even says that *(int *)0 == 0.  AIX
1.1 made NULL pointer reads trap, and IBM changed this for AIX 1.2 to allow
reads from location 0.  I don't know if the AIX 1 developers talk to the AIX
3 developers or not.

--
    John Carr (jfc@athena.mit.edu)