Problems with current file descriptor

Problems with current file descriptor

[Sergio Paracuellos]

Hi all,

I have problems to know whats is the current file descriptor. 

"task_struct" have a files_struct structure which contains all files for
a process in "fd". If I want to get the complete path for one of them,
How can I know what is the correct one in fd_array?

I suppose is something like:

path = d_path(task->files->fd[x]->f_dentry,
task->files->fd[x]->f_vfsmnt, buf, PAGE_SIZE);

But I don't know how to know this x for my correct file (for example the
last file I used).

Thanks in advance...

Cheers,
        Sergio

Re: Problems with current file descriptor

[Jörn Engel]

On Wed, 26 October 2005 17:43:12 +0200, Sergio Paracuellos wrote:
> 
> I have problems to know whats is the current file descriptor. 
> 
> "task_struct" have a files_struct structure which contains all files for
> a process in "fd". If I want to get the complete path for one of them,

A file descriptor doesn't have an associated patch.  Think about
sockets.  Think about unlinked files.  etc.

Jörn

-- 
When in doubt, punt.  When somebody actually complains, go back and fix it...
The 90% solution is a good thing.
-- Rob Landley
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Re: Problems with current file descriptor

[winter]

2005/10/26, Sergio Paracuellos <sparacuellos@lock-linux.com>:
> Hi all,
>
> I have problems to know whats is the current file descriptor.
>
> "task_struct" have a files_struct structure which contains all files for
> a process in "fd". If I want to get the complete path for one of them,
> How can I know what is the correct one in fd_array?
>
> I suppose is something like:
>
> path = d_path(task->files->fd[x]->f_dentry,
> task->files->fd[x]->f_vfsmnt, buf, PAGE_SIZE);
>
> But I don't know how to know this x for my correct file (for example the
> last file I used).
current file ?what is meaning?
for exmaple ,you opened 3 file, file1,file2, file3. which is current file?

Re: Problems with current file descriptor

[Sergio Paracuellos]

El jue, 27-10-2005 a las 10:34 +0800, winter escribió:
> 2005/10/26, Sergio Paracuellos <sparacuellos@lock-linux.com>:
> > Hi all,
> >
> > I have problems to know whats is the current file descriptor.
> >
> > "task_struct" have a files_struct structure which contains all files for
> > a process in "fd". If I want to get the complete path for one of them,
> > How can I know what is the correct one in fd_array?
> >
> > I suppose is something like:
> >
> > path = d_path(task->files->fd[x]->f_dentry,
> > task->files->fd[x]->f_vfsmnt, buf, PAGE_SIZE);
> >
> > But I don't know how to know this x for my correct file (for example the
> > last file I used).
> current file ?what is meaning?
> for exmaple ,you opened 3 file, file1,file2, file3. which is current file?

Sorry, I want the last file I used. For example if I just have done "cat
foobar" the file I want is this.

Thanks in advance,  

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Re: Problems with current file descriptor

[winter]

2005/10/27, Sergio Paracuellos <sparacuellos@lock-linux.com>:
> > current file ?what is meaning?
> > for exmaple ,you opened 3 file, file1,file2, file3. which is current file?
>
> Sorry, I want the last file I used. For example if I just have done "cat
> foobar" the file I want is this.
>
if you used the command "cat foobar", the linux kernel's do the
following 3 phase for you:
  1. open() --> sys_open()  // in this phase, the filp struct is
create and save at
                                        // current->files->fd[x]. the x=fd.
  2. read() --> sys_read()    // in this phase, read file's data, the
fd is not changed.
  3. close() --> sys_close() // in this phase, free the filp struct.
the result is
                                       // current->files->fd[fd] = 0.
so it is, kernel can not save the last file which you opend.
which  phase you want get the path at?

Re: Problems with current file descriptor

[Sergio Paracuellos]

El jue, 27-10-2005 a las 16:30 +0800, winter escribió:
> 2005/10/27, Sergio Paracuellos <sparacuellos@lock-linux.com>:
> > > current file ?what is meaning?
> > > for exmaple ,you opened 3 file, file1,file2, file3. which is current file?
> >
> > Sorry, I want the last file I used. For example if I just have done "cat
> > foobar" the file I want is this.
> >
> if you used the command "cat foobar", the linux kernel's do the
> following 3 phase for you:
>   1. open() --> sys_open()  // in this phase, the filp struct is
> create and save at
>                                         // current->files->fd[x]. the x=fd.
>   2. read() --> sys_read()    // in this phase, read file's data, the
> fd is not changed.
>   3. close() --> sys_close() // in this phase, free the filp struct.
> the result is
>                                        // current->files->fd[fd] = 0.
> so it is, kernel can not save the last file which you opend.
> which  phase you want get the path at?

The problem is I need to get complete path in lookup(). Lookup is the
first thig kernel execute and in lookup I can't get complete path
because I have a dentry structure...

cat foobar was an example I need to get the path in other operations too
like "write..." but always in lookup function.

Any ideas about it?

Thanks in advance,

	Sergio
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Re: Problems with current file descriptor

[Sergio Paracuellos]

El jue, 27-10-2005 a las 11:06 +0200, Sergio Paracuellos escribió:
> El jue, 27-10-2005 a las 16:30 +0800, winter escribió:
> > 2005/10/27, Sergio Paracuellos <sparacuellos@lock-linux.com>:
> > > > current file ?what is meaning?
> > > > for exmaple ,you opened 3 file, file1,file2, file3. which is current file?
> > >
> > > Sorry, I want the last file I used. For example if I just have done "cat
> > > foobar" the file I want is this.
> > >
> > if you used the command "cat foobar", the linux kernel's do the
> > following 3 phase for you:
> >   1. open() --> sys_open()  // in this phase, the filp struct is
> > create and save at
> >                                         // current->files->fd[x]. the x=fd.
> >   2. read() --> sys_read()    // in this phase, read file's data, the
> > fd is not changed.
> >   3. close() --> sys_close() // in this phase, free the filp struct.
> > the result is
> >                                        // current->files->fd[fd] = 0.
> > so it is, kernel can not save the last file which you opend.
> > which  phase you want get the path at?
> 
> The problem is I need to get complete path in lookup(). Lookup is the
> first thig kernel execute and in lookup I can't get complete path
> because I have a dentry structure...
> 
> cat foobar was an example I need to get the path in other operations too
> like "write..." but always in lookup function.
> 
> Any ideas about it?
> 
> Thanks in advance,
> 
> 	Sergio

I solve my problem. I was wrong in my code. I was using like dentry
current->fs->pwd but I had to use the dentry in use in lookup function.

Thanks to all. 

Cheers,
        Sergio

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