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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 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 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 (edge-triggered) 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
ii by waiting for an event only after read(2) or write(2)
return EAGAIN.
By contrast, when used as a level-triggered interface, 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) < 0) {
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
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 that two
threads may add the same file descriptor twice. This is a
harmless condition.
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, it is not
recommended.
Q3 Is the epoll file descriptor itself poll/epoll/selectable?
A3 Yes.
Q4 What happens if the epoll file descriptor is put into its own
file descriptor set?
A4 It will fail. However, you can add an epoll file descriptor
inside another epoll file descriptor set.
Q5 Can I send the epoll file descriptor over a unix-socket to
another process?
A5 No.
Q6 Will closing a file descriptor cause it to be removed from all
epoll sets automatically?
A6 Yes.
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 No you don’t. Receiving an event from epoll_wait(2) should
suggest to you that such file descriptor is ready for the
requested I/O operation. You have simply to consider it ready
until you will receive the next EAGAIN. When and how you will
use such file descriptor is entirely up to you. Also, the
condition that the read/write I/O space is exhausted can 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 such file descriptor. The same is true when
writing using write(2).
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 previous 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 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
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 example, FreeBSD has kqueue, and Solaris has /dev/poll.
SEE ALSO
epoll_create(2), epoll_ctl(2), epoll_wait(2)
COLOPHON
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