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From: jeffs@soul.esd.sgi.com (Jeff Smith)
Newsgroups: comp.unix.aix
Subject: Re: Invalid pointer traps (S/6000)
Keywords: Traps, NIL, NULL, invalid pointers
Message-ID: <1991Feb13.223557.3901@odin.corp.sgi.com>
Date: 13 Feb 91 22:35:57 GMT
References: <24518@neptune.inf.ethz.ch> <1991Feb12.033513.27494@athena.mit.edu>
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Reply-To: jeffs@sgi.com
Organization: Silicon Graphics, Mt View, Ca
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In article <1991Feb12.033513.27494@athena.mit.edu>, jfc@athena.mit.edu
(John F Carr) writes:
|> 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.

Lots of things do break when *(char *)0 != 0.  On early AIX/ps 1, *(char *)0
was really 'L'.  The coff header was mapped in at address 0, and the first
byte of the magic number corresponded 'L' I belive.  This broke lots of
utilities from the RT tree (parts of PS/2 and RS/6000 AIX started here).

I wasn't around when they made *(char *)0 trap, but, I know it wasn't a
easy time.  Surprised they changed it back in 1.2 though.  I used 1.2 for
8 months or so, but never noticed it.

And no, the AIX 1 developers (PS/2) and the AIX 3 (RS/6000) developers
don't talk much.

jeffs@sgi.com