Prevent a process from opening a file more than once

[Venkatram Tummala]

On Tue, Sep 27, 2011 at 9:19 PM, rohan puri <rohan.puri15 at gmail.com> wrote:

>
>
> On Wed, Sep 28, 2011 at 6:17 AM, Venkatram Tummala <venkatram867 at gmail.com
> > wrote:
>
>> 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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>>
>> Hi Venkatram,
>
> I agree with Mulyadi, you maintain a static global variable (int),
>
> in device_open() ->
>
> if(var)
>       return -EBUSY
> var++
>
> &
>
> in device_release() ->
>
> var--
>
>
> I think this should do the job.
>
This will prevent other processes to open the file until a process releases
it. This is not what i need. Only the threads in a process shouldn't be able
to open the file if it is already opened in the process. Other processes
should be able to open it.

Venkat

>
> Regards,
> Rohan Puri
>
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