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Re: Another Semaphore question.
- To: Linux-IL Mailing list <linux-il(at-nospam)cs.huji.ac.il>
- Subject: Re: Another Semaphore question.
- From: oron(at-nospam)actcom.co.il
- Date: Fri, 04 May 2001 22:24:36 +0300 (IDT)
- In-Reply-To: <Pine.LNX.4.30.0105041326250.6968-100000@frodo.sharat.co.il>
- Sender: oron(at-nospam)actcom.co.il
- Sender: linux-il-bounce(at-nospam)cs.huji.ac.il
On 04-May-2001 Stanislav Malyshev a.k.a Frodo wrote:
>>> This is not a PROBLEM but a FEATURE. All Sys-V IPC (semaphores,
>>> message queues, shared memory) outlive their creators.
>
> Wrong. Shared memory can be pre-deleted, so that it is removed when last
> process is detached. Just like files.
True, but they can only be used by an already attached processes just like
open files that are unlinked (although they appear in ipcs(1), a shmget(2)
fails with 'identifier removed' error).
Actually, this was very interesting remark. Since the API's of the 3 Sys-V
IPC mechanisms is so similar I haven't noticed (until now) that shared
memory has different sematics (because it has shmdt() <==> close(), while
message queues and semaphores don't have an equivalent call).
Thanks for the enlightenment...
BTW: we both agree that for *mutual exclusion* there are better means than
semaphores.
------------------------------------------------
Oron Peled Voice/Fax: +972-4-8228492
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