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

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Message-ID: <807740.19494.qm@web8703.mail.in.yahoo.com>
Date:	Sat, 21 Jun 2008 21:18:46 +0530 (IST)
From:	palani saravanan <busybeesaravanan0072003@...oo.co.in>
To:	David Newall <davidn@...idnewall.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Behavior of lseek() on a fd opened with 'RDONLY' flag, when seeking goes beyond file size.

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@...idnewall.com>
To: palani saravanan <busybeesaravanan0072003@...oo.co.in>
Cc: linux-kernel@...r.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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