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From: operator@IRIS.KTH.DK (Martin Liversage)
Newsgroups: comp.sys.sgi
Subject: Re: news_server dies on VGX dropping core
Message-ID: <9102060912.AA18739@iris.kth.dk>
Date: 6 Feb 91 09:12:23 GMT
References: <4291@ns-mx.uiowa.edu>
Sender: usenet@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 43

In article <4291@ns-mx.uiowa.edu> randy frank
<uunet.uu.net!ns-mx!tessa.iaf.uiowa.edu> writes:

> Running a program which seems to run fine on  GTX and PI 4Ds.  Program
> uses four shared memory segments (Two are 2K, one is 64K, and the fourth
> is ~16M).  Program is actually three programs.  Each is spawned via system().
> Each has its own graphics window(s). A total of 5 windows are used when the
> programs run.  There is a launcher program which sits in the background
> until one of the programs stop.  Then it waits for all others to detach
> from shared memory and it takes control.  Problem: news_server drops core
> when the programs try to quit.

I just want to point out that the NeWS server uses a shared memory
segment (at least on my machine: 4D/20, IRIX 3.2). It has a shared
memory id of 0 and has permission 0555 (so anyone can write to it).
Programming in C it is very easy to forget to initialize a variable,
and if it has storage class static or extern it will be initialized to 0.

Maybe your program by mistake attaches to segment 0 and writes to
it. This will cause the NeWS server to bomb, but you will probably
discover that the behaviour in general will be erratic. There is
probably also a difference between different hardware platforms in how
the shared memory segment is used causing you program to behave
differently on different machins.

I once made this mistake and my machine went on crashing in strange
ways all day long until I solved the problem. I think that this shared
memory segment writable by anyone is pretty dangerous to programmers
fiddling with shared memory (but giving it an id of 0 is inviting for
real trouble).

Hope this helps.

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