Re: negative seek offsets in VFS

[Andi Kleen <ak@muc.de>]

On Fri, May 27, 2005 at 11:39:40AM -0700, Bryan Henderson wrote:
> OK, I remember that now.  I probably forgot it because I couldn't think of 
> any use for that special range of values more negative than -4095.  Can 
> you give an example of a system call that returns integers less than 
> -4095?

Various ioctls for example can return arbitary values.

> 
> And separately, I'm still confused about how you expect /dev/kmem file 
> offsets to work.  While I was mistaken about the existence of negative 
> addresses, I know there are no negative file offsets in POSIX.  So how 
> does one look at a negative address via the POSIX file interface and 
> /dev/kmem?

pread,pwrite,lseek(...,SEEK_SET) should all work.

/dev/kmem is firmly outside POSIX, so this is fine.
For other devices the behaviour will not change.

> 
> Even if you add the concept of negative file offsets as a glibc/Linux 
> extension of POSIX, you have an ambiguity problem at least with offset -1, 
> right?

The kernel doesn't use this range as address, so in practice
the problem does not occur.

-Andi