RE: Where does kernel store per task file position?

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> -----Original Message-----
> From: Rajat Sharma [mailto:fs.rajat@xxxxxxxxx]
> Sent: Wednesday, January 30, 2013 11:16 AM
> To: Pranay Kumar Srivastava
> Cc: kernelnewbies@xxxxxxxxxxxxxxxxx
> Subject: Re: Where does kernel store per task file position?
> 
> > I'm still not able to figure out where exactly is the position of file stored per
> task_struct.
> struct file * itself is per process (task_struct) so file->f_pos is file position per
> process, if thats what you are looking for. I hope you haven't assumed that
> struct file itself is unique for a file, i.e. per inode? Then that assumption is
> wrong.
> -Rajat

[Pranay Kumar Srivastava] That really was a stupid question, it says right there get_empty_filp() in do_sys_open. For forks the inherited file have common struct file [Correct?] but for the files opened after fork in child/parent will not have shared struct file[Correct?].  So the same dentry can be pointed to by multiple struct file[Correct?] that's why there's an increment of dentry while doing lookup[Correct?].

Thanks a lot!
> 
> On Tue, Jan 29, 2013 at 6:38 PM, Pranay Kumar Srivastava
> <Pranay.Shrivastava@xxxxxxx> wrote:
> Hi Everyone,
> 
> I was trying to find out where does Linux store per process file position?
> Since struct file is allocated once when the file is first opened
> (get_empty_filp() via do_sys_open) .I looked at these,
> 
> Copy_process--->copy_files-->dup_fd  it seemed to allocate only (struct
> file*)
> 
> struct files_struct , but I couldn't find any field that is actually being used to
> store the file position.
> 
> 
> I'm still not able to figure out where exactly is the position of file stored per
> task_struct. Secondly even if this was being saved does the kernel changes
> f_pos of struct file whenever a (read/write) is done? I don't that happens
> [Correct?].
> 
> Regards,
> Pranay Kumar Srivastava
> 
> 
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