Passing epoll file descriptors across Unix domain sockets

Passing epoll file descriptors across Unix domain sockets

[Michael Kerrisk]

Hi Davide,

After all of the recent discussions about epoll, I've been reviewing
other parts of the epoll.7 man page.  In that page there is the
following:

       Q5  Can  I  send  the  epoll  file descriptor over a Unix
           domain socket to another process?

       A5  No.

This does not seem accurate.  I can understand that it makes no sense
to pass a an epoll file descriptor over a Unix domain socket, because
the receiving process does not have the file descriptors that are in
the epoll set.

However, it is indeed possible (no errors result) to pass an epoll fd
over a Unix doman socket using SCM_RIGHTS.  (I have just now tested
this.)

Can you clarify?  I suspect the text should really read:

       Q5  Can  I  send  an  epoll  file  descriptor over a Unix
           domain socket to another process?

       A5  Yes, but it does not make sense to do this, since the
           receiving  process  would not have copies of the file
           descriptors in the epoll set.

Does that seem okay?

Cheers,

Michael

-- 
Michael Kerrisk
Maintainer of the Linux man-pages project
http://www.kernel.org/doc/man-pages/
Want to report a man-pages bug?  Look here:
http://www.kernel.org/doc/man-pages/reporting_bugs.html

Re: Passing epoll file descriptors across Unix domain sockets

[Davide Libenzi]

On Fri, 29 Feb 2008, Michael Kerrisk wrote:

> Hi Davide,
> 
> After all of the recent discussions about epoll, I've been reviewing
> other parts of the epoll.7 man page.  In that page there is the
> following:
> 
>        Q5  Can  I  send  the  epoll  file descriptor over a Unix
>            domain socket to another process?
> 
>        A5  No.
> 
> This does not seem accurate.  I can understand that it makes no sense
> to pass a an epoll file descriptor over a Unix domain socket, because
> the receiving process does not have the file descriptors that are in
> the epoll set.
> 
> However, it is indeed possible (no errors result) to pass an epoll fd
> over a Unix doman socket using SCM_RIGHTS.  (I have just now tested
> this.)
> 
> Can you clarify?  I suspect the text should really read:
> 
>        Q5  Can  I  send  an  epoll  file  descriptor over a Unix
>            domain socket to another process?
> 
>        A5  Yes, but it does not make sense to do this, since the
>            receiving  process  would not have copies of the file
>            descriptors in the epoll set.
> 
> Does that seem okay?

Looks okay to me.



- Davide