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Added Q0/A0, making explicit that the key for items in an epoll 

set is [file descriptor, open file description]
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Michael Kerrisk 2008-02-28 13:24:52 +00:00
parent cf3eb59854
commit 7fb5cf0fb6
1 changed files with 26 additions and 3 deletions
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man7/epoll.7
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@ -18,7 +18,7 @@
.\"
.\" Davide Libenzi <davidel@xmailserver.org>
.\"
.TH EPOLL 7 2008-02-26 "Linux" "Linux Programmer's Manual"
.TH EPOLL 7 2008-02-28 "Linux" "Linux Programmer's Manual"
.SH NAME
epoll \- I/O event notification facility
.SH SYNOPSIS
@ -233,6 +233,15 @@ with
.BR EPOLL_CTL_MOD .
.SS Questions and Answers
.TP 4
.B Q0
What is the key used to distinguish the file descriptors in an epoll set?
.TP
.B A0
The key is the combination of the file descriptor number and
the open file description
(also known as "open file handle",
the kernel's internal representation of an open file).
.TP
.B Q1
What happens if you add the same file descriptor to an epoll set twice?
.TP
@ -243,11 +252,25 @@ However, it is possible to add a duplicate
.RB ( dup (2),
.BR dup2 (2),
.BR fcntl (2)
.BR F_DUPFD ,
.BR fork (2))
.BR F_DUPFD )
descriptor to the same
.B epoll
set.
.\" But a descriptor duplicated by fork(2) can't be added to the
.\" set, which is an ugly inconsistency. On the one hand, a child process
.\" cannot add the duplicate file descriptor to the epoll set. (In every
.\" other case that I can think of, descriptors duplicated by fork have
.\" similar semantics to descriptors duplicated by dup() and friends.) On
.\" the other hand, the very fact that the child has a duplicate of the
.\" descriptor means that even if the parent closes its descriptor, then
.\" epoll_wait() in the parent will continue to receive notifications for
.\" that descriptor because of the duplicated descriptor in the child.
.\"
.\" The choice of [file *, fd] as the key for epoll sets really does seem
.\" unfortunate. Keying on [pid, fd] would probably have given saner
.\" semantics. Obviously it can't be changed now though.
.\"
.\" mtk, Feb 2008
This can be a useful technique for filtering events,
if the duplicate file descriptors are registered with different
.I events