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epoll.7: Expand the discussion of the implications of file descriptor duplication 

In particular, note that it may be difficult for an application
to know about the existence of duplicate file descriptors.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2018-06-22 12:17:42 +02:00
parent fc9294cb6f
commit d1d90ea54d
1 changed files with 16 additions and 8 deletions
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@ -475,18 +475,26 @@ a new file descriptor referring to the same open file description is
created.
An open file description continues to exist until all
file descriptors referring to it have been closed.
.IP
A file descriptor is removed from an
.B epoll
set only after all the file descriptors referring to the underlying
open file description have been closed
(or before if the file descriptor is explicitly removed using
.BR epoll_ctl (2)
.BR EPOLL_CTL_DEL ).
interest list only after all the file descriptors referring to the underlying
open file description have been closed.
This means that even after a file descriptor that is part of an
.B epoll
set has been closed,
interest list has been closed,
events may be reported for that file descriptor if other file
descriptors referring to the same underlying file description remain open.
To prevent this happening,
the file descriptor must be explicitly removed from the interest list (using
.BR epoll_ctl (2)
.BR EPOLL_CTL_DEL )
before it is duplicated.
Alternatively,
the application must ensure that all file descriptors are closed
(which may be difficult if file descriptors were duplicated
behind the scenes by library functions that used
.BR dup (2)
or
.BR fork (2)).
.IP 7.
If more than one event occurs between
.BR epoll_wait (2)