Prevent a process from opening a file more than once

[Venkatram Tummala]

On Tue, Sep 27, 2011 at 5:40 PM, Jeff Haran <jharan at bytemobile.com> wrote:

>
>
> From: kernelnewbies-bounces at kernelnewbies.org
> [mailto:kernelnewbies-bounces at kernelnewbies.org] On Behalf Of Venkatram
> Tummala
> Sent: Tuesday, September 27, 2011 5:31 PM
> To: Mulyadi Santosa
> Cc: kernelnewbies
> Subject: Re: Prevent a process from opening a file more than once
>
> On Tue, Sep 27, 2011 at 5:22 PM, Mulyadi Santosa
> <mulyadi.santosa at gmail.com> wrote:
> Hi :)
>
> On Wed, Sep 28, 2011 at 06:56, Venkatram Tummala
> <venkatram867 at gmail.com> wrote:
> > Hi All,
> > I have a simple device driver which creates a /dev/XYZ file. I need to
> > prevent a process from opening the file more than once. However,
> multiple
> > processes can open the file simultaneously. Is there any any elegant
> way to
> > do this other than checking all opened files in the process ?
> Uhm, keep a reference count and increment it on every file open in
> your module? How does that sound?
> Well, which refcount should i use? I can't use the refcount in the file
> object as the file objects passed to me are different each time the file
> is opened in the process.
>
> When you say "I need to prevent a process from opening the file more
> than once.", do you mean a single process opening the file, closing it
> and then opening it again would be disallowed?

No. If the file is already opened in the process, the process shouldn't  be
allowed to open the file again. It is fine if the process opens, closes &
then opens the file again.

> Or do you mean that a
> single process opening the file, keeping it open and then opening it
> again under another fd would be disallowed?
>
Yes, this is what i am looking for.

>
> How about multiple threads within the same process? Are they treated as
> the same process by these rules?
>
Yes. Threads are treated as the same process. So, if one thread has the file
already opened, another thread in the same process shouldn't be able to open
it.

Venkat
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