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To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: aeb@veritas.com (Andries Brouwer),
        marcelo@conectiva.com.br (Marcelo Tosatti),
        torvalds@transmeta.com (Linus Torvalds), linux-kernel@vger.kernel.org,
        gilbertd@treblig.org (Dave Gilbert)
Subject: Re: [Patch] shmmin behaviour back to 2.2 behaviour
In-Reply-To: <E14BfJ6-0003qw-00@the-village.bc.nu>
From: Christoph Rohland <cr@sap.com>
Message-ID: <m3hf3ourke.fsf@linux.local>
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Date: 28 Dec 2000 23:13:55 +0100
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Alan Cox <alan@lxorguk.ukuu.org.uk> writes:

> There are fundmental things shm* can do that mmap cannot. Does posix
> shm handle those (leaving segments alive but unattached being the
> obvious one)

Yes:
        shmget           == shm_open (+ ftruncate(fd, size))
        shmat            == mmap (0, size, , , fd, 0)
        shmdt            == munmap (addr, size);
        shmctl(IPC_RMID) == shm_unlink ()
        shmctl(IPC_STAT) == fstat();
        shmctl(IPC_LOCK) == mlock() /*nearly*/
        shmctl(IPC_SET)  == fchown(), fchmod()

You can get the Linux special behaviour to be able to attach to a
removed segment by its shmid by passing the file descriptor for the
posix shm from the attached process to the attaching process.

Did I miss something?
                        Christoph

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