Re: epoll design problems with common fork/exec patterns

Davide Libenzi
2008-02-27T19:35:28Z

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On Tue, 26 Feb 2008, Chris "ã~B¯" Heath wrote:

> On Tue, 2008-02-26 at 10:51 -0800, Davide Libenzi wrote:
> >
> > Yes, you can't add the same fd twice. Think about a DB where "file*,fd" is
> > the key.
>
> To clarify, the key appears to be file* plus the user-space integer that
> represents the fd.

Yes, that's what I said.

> > > c) It is possible to add duplicated file descriptors referring to the same
> > > underlying open file description ("file *"). As you note, this can be a
> > > useful filtering technique, if the two file descriptors specify different
> > > masks.
> > >
> > > Assuming that is all correct, for man-pages-2.79, I've reworked the text
> > > for Q1/A1 as follows:
> > >
> > > Q1 What happens if you add the same file descriptor
> > > to an epoll set twice?
> > >
> > > A1 You will probably get EEXIST. However, it is pos-
> > > sible to add a duplicate (dup(2), dup2(2),
> > > fcntl(2) F_DUPFD, fork(2)) descriptor to the same
> > > epoll set. This can be a useful technique for
> > > filtering events, if the duplicate file descrip-
> > > tors are registered with different events masks.
> > >
> > > Seem okay Davide?
> >
> > Looks sane to me.
>
> I think fork(2) should not be in the above list. fork(2) duplicates the
> kernel's fd, but the user-space integer that represents the fd remains
> the same, so you will get EEXIST if you try to add the fd that was
> duplicated by fork.

Good catch, fork(2) should not be there.

- Davide