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EPOLL(7) Linux Programmer's Manual EPOLL(7)
NAME
epoll - I/O event notification facility
SYNOPSIS
#include <sys/epoll.h>
DESCRIPTION
epoll is a variant of poll(2) that can be used either as an edge-triggered 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_create(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 fol-
lows. 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 waiting 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 operation 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 semantics.
Since even with the edge-triggered epoll multiple events can be generated upon receipt of
multiple chunks of data, the caller has the option to specify the EPOLLONESHOT flag, to
tell epoll to disable the associated file descriptor after the receipt of an event with
epoll_wait(2). When the EPOLLONESHOT flag is specified, it is the caller's responsibility
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 listen(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 kernel's 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
different 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 programming 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 descrip-
tor 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 automati-
cally?
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 descrip-
tor 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 descrip-
tor 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-oriented
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 application to remember which files need
to be processed but still round robin amongst all the ready files. This also supports
ignoring subsequent 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 previ-
ous event's 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 associ-
ated 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 previously 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 exam-
ple, 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.ker-
nel.org/doc/man-pages/.
Linux 2008-02-28 EPOLL(7)
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