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    SubjectRe: epoll design problems with common fork/exec patterns
    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

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