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From
SubjectRe: File Descriptors & Linux 2.0
Date
Followup to:  <4qs1aq$4qv@picard.cistron.nl>
By author: miquels@cistron.nl (Miquel van Smoorenburg)
In newsgroup: linux.dev.kernel
>
> I just checked the Linux-2.0.0 kernel and the sys_select function never
> writes further in your fd_set than the nr_fds argument to select(). This
> means that if NR_OPEN is increased, and the size of fd_set increases with it,
> it doesn't have consequences for older applications. The only thing is that
> an old application can't handle a fd > 255 in a select() if one is passed
> through fork()/exec() but I know of no applications that do this. So I think
> you can do this without trouble.
>
> If it's the other way around, and you try to pass a nr_fds > NR_OPEN to
> select() the kernel sys_select function will round it down to NR_OPEN
> quietly. So that doesn't cause trouble either.
>
> Somehow I get the feeling that sys_select() was coded with this already
> in mind..
>

How many applications do you know that actually check if the file
descriptor they get from the kernel is in the range 0 <= fd < NR_OPEN?
Most apps just use FD_SET() without any further checking.

-hpa
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