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To: Dave Gilbert <gilbertd@treblig.org>
Cc: linux-kernel@vger.kernel.org
Subject: Re: shmat returning NULL with 0 sized segment
In-Reply-To: <Pine.LNX.4.10.10012250109450.666-100000@tardis.home.dave>
From: Christoph Rohland <cr@sap.com>
In-Reply-To: <Pine.LNX.4.10.10012250109450.666-100000@tardis.home.dave>
Message-ID: <m3vgs8b9u5.fsf@linux.local>
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Date: 26 Dec 2000 10:31:37 +0100
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Dave Gilbert <gilbertd@treblig.org> writes:

>   I'm trying to debug a weird problem with Xine - its screwing up its use
> of shared memory for regions I haven't sussed yet.  One odd consequence is
> that it has apparently successfully managed to allocate a 0 byte chunk of
> shared memory; shmat is then called with shmaddr=0 and shmflg=0; the
> result of shmat is 0
> 
>   Is this what shmat is supposed to do in this (admittedly odd)
> circumstance? The error behaviour is defined in the man page as returning
> -1 on error.

Yes, this should be competely legal and wanted. Some programs use
shmget (..,0,..) to test if the segment is there. Apparently Xine does
this while setting the IPC_CREATE flag. This is legal on 2.4 (wasn't
in 2.2) and gives you a 0 byte segment.

shmat will give you then the legal address 0 like mmap would.

Greetings
                Christoph

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