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| author | Andrew Clayton <andrew@digital-domain.net> | 2019-09-21 00:42:11 +0100 |
|---|---|---|
| committer | Michael Kerrisk <mtk.manpages@gmail.com> | 2019-09-23 15:57:21 +0200 |
| commit | e95f6bf482d1683e903cfd4089a473ef4b10395e (patch) | |
| tree | 58c43093a290502b9b5de2770db8c05f94ddc539 | |
| parent | 6c578de29d40bff6e5a5ce93398e0a59fc7221f9 (diff) | |
| download | man-pages-e95f6bf482d1683e903cfd4089a473ef4b10395e.tar.gz | |
signalfd.2: Note about interactions with epoll & fork
Using signalfd(2) with epoll(7) and fork(2) can lead to some head
scratching.
It seems that when a signalfd file descriptor is added to epoll
you will only get notifications for signals sent to the process
that added the file descriptor to epoll.
So if you have a signalfd fd registered with epoll and then call
fork(2), perhaps by way of daemon(3) for example. Then you will
find that you no longer get notifications for signals sent to the
newly forked process.
User kentonv on ycombinator[0] explained it thus
"One place where the inconsistency gets weird is when you
use signalfd with epoll. The epoll will flag events on the
signalfd based on the process where the signalfd was
registered with epoll, not the process where the epoll is
being used. One case where this can be surprising is if you
set up a signalfd and an epoll and then fork() for the
purpose of daemonizing -- now you will find that your epoll
mysteriously doesn't deliver any events for the signalfd
despite the signalfd otherwise appearing to function as
expected."
And another post from the same person[1].
And then there is this snippet from this kernel commit message[2]
"If you share epoll fd which contains our sigfd with another
process you should blame yourself. signalfd is "really
special"."
So add a note to the man page that points this out where people
will hopefully find it sooner rather than later!
[0]: https://news.ycombinator.com/item?id=9564975
[1]: https://stackoverflow.com/questions/26701159/sending-signalfd-to-another-process/29751604#29751604
[2]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d80e731ecab420ddcb79ee9d0ac427acbc187b4b
Signed-off-by: Andrew Clayton <andrew@digital-domain.net>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
| -rw-r--r-- | man2/signalfd.2 | 17 |
1 files changed, 17 insertions, 0 deletions
diff --git a/man2/signalfd.2 b/man2/signalfd.2 index 497ee4cbde..a96ff6441c 100644 --- a/man2/signalfd.2 +++ b/man2/signalfd.2 @@ -261,6 +261,23 @@ itself and the signals that are directed to the process (i.e., the entire thread group). (A thread will not be able to read signals that are directed to other threads in the process.) +.SS epoll(7) semantics +If you add a signalfd file descriptor to +.BR epoll(7) +then +.BR epoll_wait(2) +will only return events for signals received by the process that did +the +.BR epoll_ctl(2). +If you then +.BR fork(2), +say by calling +.BR daemon(3), +then you will find that you don't get any notifications for sent +signals. For this to work, you need to add the signalfd file +descriptor to +.BR epoll(7) +after forking. .SH RETURN VALUE On success, .BR signalfd () |