Behavior of lseek() on a fd opened with 'RDONLY' flag, when seeking goes beyond file size.

Behavior of lseek() on a fd opened with 'RDONLY' flag, when seeking goes beyond file size.

[palani saravanan]

Hi,

   [Forgive me if this is not correct alias for this question. 
    (Behavior of lseek() on a fd opened with 'RDONLY' flag, when seeking goes beyond file size.)]

   In linux, I see that it just goes beyond the file size and returns the resulting offset.
   For example, 'rc = lseek(fd, 4L, SEEK_END);' on a file which has 5 byte contents,
   it returns rc as 9.
   I expect that it would return size of the file, i.e.) 5.

   I am not finding any posix documentation for this either.

   May I know the rational behind this behavior?  
   Does the file pointer internally really points to the new location.?

Thanks,
Saravanan



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Re: Behavior of lseek() on a fd opened with 'RDONLY' flag, when seeking goes beyond file size.

[David Newall]

palani saravanan wrote:
>    In linux, I see that it just goes beyond the file size and returns the resulting offset.
>    For example, 'rc = lseek(fd, 4L, SEEK_END);' on a file which has 5 byte contents,
>    it returns rc as 9.
>   

Sounds right.  It's documented that way, too:

       "The lseek() function allows the file offset to be set beyond the
end of  the  file  (but  this
       does  not  change  the  size of the file)."
           -- man 2 lseek
>    I expect that it would return size of the file, i.e.) 5.
>   

Only if you pass (0,SEEK_END).

>    Does the file pointer internally really points to the new location.?
>   

So the documentation promises.

Re: Behavior of lseek() on a fd opened with 'RDONLY' flag, when seeking goes beyond file size.

[palani saravanan]

Thanks David !

I have one more question, Does lseek() permits -ve value as offset, with SEEK_SET mode?
When I execute a simple c program to check that it doesn't return any error.

   errno = 0;
   rd_rc = lseek(rd_fd, -10, SEEK_SET);
returns rd_rc = -10 and errno remains 0.

>From the linux code, I am expecting the return value of -EINVAL,
when SEEK_SET is passed with negative offset (generic_file_llseek).
 http://lxr.linux.no/linux/fs/read_write.c#L34 
What is the expected behavior as per posix and linux?

Thanks,
Saravanan
----- Original Message ----
From: David Newall <davidn@davidnewall.com>
To: palani saravanan <busybeesaravanan0072003@yahoo.co.in>
Cc: linux-kernel@vger.kernel.org
Sent: Tuesday, 17 June, 2008 8:00:09 PM
Subject: Re: Behavior of lseek() on a fd opened with 'RDONLY' flag, when seeking goes beyond file size.

palani saravanan wrote:
>    In linux, I see that it just goes beyond the file size and returns the resulting offset.
>    For example, 'rc = lseek(fd, 4L, SEEK_END);' on a file which has 5 byte contents,
>    it returns rc as 9.
>  

Sounds right.  It's documented that way, too:

       "The lseek() function allows the file offset to be set beyond the
end of  the  file  (but  this
       does  not  change  the  size of the file)."
           -- man 2 lseek
>    I expect that it would return size of the file, i.e.) 5.
>  

Only if you pass (0,SEEK_END).

>    Does the file pointer internally really points to the new location.?
>  

So the documentation promises.



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Re: Behavior of lseek() on a fd opened with 'RDONLY' flag, when seeking goes beyond file size.

[David Newall]

palani saravanan wrote:
> Does lseek() permits -ve value as offset, with SEEK_SET mode? ...
>    errno = 0;
>    rd_rc = lseek(rd_fd, -10, SEEK_SET);
> returns rd_rc = -10 and errno remains 0.
>   

No, lseek does not permit negative file offsets, and should return
EINVAL in your example.  You may have discovered a bug.  For reference,
on my system lseek returns -1 and sets errno to EINVAL, as it should.

If you're running a recent kernel it might be worth looking closer into
this; for older kernels it's probably not worth the effort.

Re: Behavior of lseek() on a fd opened with 'RDONLY' flag, when seeking goes beyond file size.

[palani saravanan]

Thanks David for clarification !

I ran this program in the kernel version, 'Linux 2.6.9-42.7.ELsmp'.
However, In a recent kernel version,  'Linux 2.6.24-19 ', the error EINVAL is returned as you mentioned.

Thanks,
Saravanan
----- Original Message ----
From: David Newall <davidn@davidnewall.com>
To: palani saravanan <busybeesaravanan0072003@yahoo.co.in>
Cc: linux-kernel@vger.kernel.org
Sent: Sunday, 22 June, 2008 7:36:12 AM
Subject: Re: Behavior of lseek() on a fd opened with 'RDONLY' flag, when seeking goes beyond file size.

palani saravanan wrote:
> Does lseek() permits -ve value as offset, with SEEK_SET mode? ...
>    errno = 0;
>    rd_rc = lseek(rd_fd, -10, SEEK_SET);
> returns rd_rc = -10 and errno remains 0.
>  

No, lseek does not permit negative file offsets, and should return
EINVAL in your example.  You may have discovered a bug.  For reference,
on my system lseek returns -1 and sets errno to EINVAL, as it should.

If you're running a recent kernel it might be worth looking closer into
this; for older kernels it's probably not worth the effort.



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