fd type from number

[greg@kroah.com (Greg KH)]

On Tue, Aug 19, 2014 at 08:38:24AM -0700, Loris Degioanni wrote:
> (resending making sure this is not part of another thread)
> 
> I'm looking for an efficient way to determine the type of an fd (file, 
> socket...) given its number, from a kernel module.

You don't have a "number" from within the kernel, you have a pointer to
the full file descriptor structure, right?  If so, what is missing from
that structure that you can not find?

> The closest thing I found by looking at the kernel sources is 
> sockfd_lookup(), which works but is limited to telling me if the fd is a 
> socket or not.
> 
> Is there something else I can look at?

What exactly are you trying to do with this?  And what does it matter
what "type" of thing a file descriptor is?  What can you do with that
information?

thanks,

greg k-h