EPOLL(7) Linux Programmers Manual EPOLL(7)
NAME
epoll - I/O event notification facility
SYNOPSIS
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DESCRIPTION
epoll is a variant of poll(2) that can be used either as an edge-trig
gered or a level-triggered interface and scales well to large numbers
of watched file descriptors. Three system calls are provided to set up
and control an epoll set: epoll_create(2), epoll_ctl(2), epoll_wait(2).
An epoll set is connected to a file descriptor created by epoll_cre
ate(2). Interest for certain file descriptors is then registered via
epoll_ctl(2). Finally, the actual wait is started by epoll_wait(2).
Level-Triggered and Edge-Triggered
The epoll event distribution interface is able to behave both as edge-
triggered (ET) and level-triggered (LT). The difference between the
two mechanisms can be described as follows. Suppose that this scenario
happens:
1. The file descriptor that represents the read side of a pipe (rfd) is
added inside the epoll device.
2. A pipe writer writes 2 kB of data on the write side of the pipe.
3. A call to epoll_wait(2) is done that will return rfd as a ready file
descriptor.
4. The pipe reader reads 1 kB of data from rfd.
5. A call to epoll_wait(2) is done.
If the rfd file descriptor has been added to the epoll interface using
the EPOLLET (edge-triggered) flag, the call to epoll_wait(2) done in
step 5 will probably hang despite the available data still present in
the file input buffer; meanwhile the remote peer might be expecting a
response based on the data it already sent. The reason for this is
that edge-triggered mode only delivers events when changes occur on the
monitored file descriptor. So, in step 5 the caller might end up wait
ing for some data that is already present inside the input buffer. In
the above example, an event on rfd will be generated because of the
write done in 2 and the event is consumed in 3. Since the read opera
tion done in 4 does not consume the whole buffer data, the call to
epoll_wait(2) done in step 5 might block indefinitely.
An application that employs the EPOLLET flag should use non-blocking
file descriptors to avoid having a blocking read or write starve a task
that is handling multiple file descriptors. The suggested way to use
epoll as an edge-triggered (EPOLLET) interface is as follows:
i with non-blocking file descriptors; and
ii by waiting for an event only after read(2) or write(2)
return EAGAIN.
By contrast, when used as a level-triggered interface (the default,
when EPOLLET is not specified), epoll is simply a faster poll(2), and
can be used wherever the latter is used since it shares the same seman
tics.
Since even with the edge-triggered epoll multiple events can be gener
ated upon receipt of multiple chunks of data, the caller has the option
to specify the EPOLLONESHOT flag, to tell epoll to disable the associ
ated file descriptor after the receipt of an event with epoll_wait(2).
When the EPOLLONESHOT flag is specified, it is the callers responsi
bility to rearm the file descriptor using epoll_ctl(2) with
EPOLL_CTL_MOD.
Example for Suggested Usage
While the usage of epoll when employed as a level-triggered interface
does have the same semantics as poll(2), the edge-triggered usage
requires more clarification to avoid stalls in the application event
loop. In this example, listener is a non-blocking socket on which lis
ten(2) has been called. The function do_use_fd() uses the new ready
file descriptor until EAGAIN is returned by either read(2) or write(2).
An event-driven state machine application should, after having received
EAGAIN, record its current state so that at the next call to
do_use_fd() it will continue to read(2) or write(2) from where it
stopped before.
struct epoll_event ev, *events;
for (;;) {
nfds = epoll_wait(kdpfd, events, maxevents, -1);
for (n = 0; n < nfds; ++n) {
if (events[n].data.fd == listener) {
client = accept(listener, (struct sockaddr *) &local,
&addrlen);
if (client < 0){
perror("accept");
continue;
}
setnonblocking(client);
ev.events = EPOLLIN | EPOLLET;
ev.data.fd = client;
if (epoll_ctl(kdpfd, EPOLL_CTL_ADD, client, &ev)
== -1) {
fprintf(stderr,
"epoll set insertion error: fd=%d\n",
client);
return -1;
}
} else {
do_use_fd(events[n].data.fd);
}
}
}
When used as an edge-triggered interface, for performance reasons, it
is possible to add the file descriptor inside the epoll interface
(EPOLL_CTL_ADD) once by specifying (EPOLLIN|EPOLLOUT). This allows you
to avoid continuously switching between EPOLLIN and EPOLLOUT calling
epoll_ctl(2) with EPOLL_CTL_MOD.
Questions and Answers
Q0 What is the key used to distinguish the file descriptors in an
epoll set?
A0 The key is the combination of the file descriptor number and the
open file description (also known as an "open file handle", the
kernels internal representation of an open file).
Q1 What happens if you add the same file descriptor to an epoll set
twice?
A1 You will probably get EEXIST. However, it is possible to add a
duplicate (dup(2), dup2(2), fcntl(2) F_DUPFD) descriptor to the
same epoll set. This can be a useful technique for filtering
events, if the duplicate file descriptors are registered with dif
ferent events masks.
Q2 Can two epoll sets wait for the same file descriptor? If so, are
events reported to both epoll file descriptors?
A2 Yes, and events would be reported to both. However, careful pro
gramming may be needed to do this correctly.
Q3 Is the epoll file descriptor itself poll/epoll/selectable?
A3 Yes. If an epoll file descriptor has events waiting then it will
indicate as being readable.
Q4 What happens if the epoll file descriptor is put into its own file
descriptor set?
A4 The epoll_ctl(2) call will fail (EINVAL). However, you can add an
epoll file descriptor inside another epoll file descriptor set.
Q5 Can I send an epoll file descriptor over a Unix domain socket to
another process?
A5 Yes, but it does not make sense to do this, since the receiving
process would not have copies of the file descriptors in the epoll
set.
Q6 Will closing a file descriptor cause it to be removed from all
epoll sets automatically?
A6 Yes, but be aware of the following point. A file descriptor is a
reference to an open file description (see open(2)). Whenever a
descriptor is duplicated via dup(2), dup2(2), fcntl(2) F_DUPFD, or
fork(2), 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.
A file descriptor is removed from an epoll set only after all the
file descriptors referring to the underlying open file description
have been closed (or before if the descriptor is explicitly removed
using epoll_ctl() EPOLL_CTL_DEL). This means that even after a
file descriptor that is part of an epoll set has been closed,
events may be reported for that file descriptor if other file
descriptors referring to the same underlying file description
remain open.
Q7 If more than one event occurs between epoll_wait(2) calls, are they
combined or reported separately?
A7 They will be combined.
Q8 Does an operation on a file descriptor affect the already collected
but not yet reported events?
A8 You can do two operations on an existing file descriptor. Remove
would be meaningless for this case. Modify will re-read available
I/O.
Q9 Do I need to continuously read/write a file descriptor until EAGAIN
when using the EPOLLET flag (edge-triggered behavior) ?
A9 Receiving an event from epoll_wait(2) should suggest to you that
such file descriptor is ready for the requested I/O operation. You
must consider it ready until the next (non-blocking) read/write
yields EAGAIN. When and how you will use the file descriptor is
entirely up to you.
For packet/token-oriented files (e.g., datagram socket, terminal in
canonical mode), the only way to detect the end of the read/write
I/O space is to continue to read/write until EAGAIN.
For stream-oriented files (e.g., pipe, FIFO, stream socket), the
condition that the read/write I/O space is exhausted can also be
detected by checking the amount of data read from / written to the
target file descriptor. For example, if you call read(2) by asking
to read a certain amount of data and read(2) returns a lower number
of bytes, you can be sure of having exhausted the read I/O space
for the file descriptor. The same is true when writing using
write(2). (Avoid this latter technique if you cannot guarantee
that the monitored file descriptor always refers to a stream-ori
ented file.)
Possible Pitfalls and Ways to Avoid Them
o Starvation (edge-triggered)
If there is a large amount of I/O space, it is possible that by trying
to drain it the other files will not get processed causing starvation.
(This problem is not specific to epoll.)
The solution is to maintain a ready list and mark the file descriptor
as ready in its associated data structure, thereby allowing the appli
cation to remember which files need to be processed but still round
robin amongst all the ready files. This also supports ignoring subse
quent events you receive for file descriptors that are already ready.
o If using an event cache...
If you use an event cache or store all the file descriptors returned
from epoll_wait(2), then make sure to provide a way to mark its closure
dynamically (i.e., caused by a previous events processing). Suppose
you receive 100 events from epoll_wait(2), and in event #47 a condition
causes event #13 to be closed. If you remove the structure and
close(2) the file descriptor for event #13, then your event cache might
still say there are events waiting for that file descriptor causing
confusion.
One solution for this is to call, during the processing of event 47,
epoll_ctl(EPOLL_CTL_DEL) to delete file descriptor 13 and close(2),
then mark its associated data structure as removed and link it to a
cleanup list. If you find another event for file descriptor 13 in your
batch processing, you will discover the file descriptor had been previ
ously removed and there will be no confusion.
VERSIONS
The epoll API was introduced in Linux kernel 2.5.44. Its interface
should be finalized in Linux kernel 2.5.66.
CONFORMING TO
The epoll API is Linux-specific. Some other systems provide similar
mechanisms, for example, FreeBSD has kqueue, and Solaris has /dev/poll.
SEE ALSO
epoll_create(2), epoll_ctl(2), epoll_wait(2)
COLOPHON
This page is part of release 3.05 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2008-02-28 EPOLL(7)
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