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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. The following system calls are provided to create and manage an epoll
instance:
* An epoll instance created by epoll_create(2), which returns a file descriptor referring to the epoll instance. (The
more recent epoll_create1(2) extends the functionality of epoll_create(2).)
* Interest in particular file descriptors is then registered via epoll_ctl(2). The set of file descriptors currently
registered on an epoll instance is sometimes called an epoll set.
* 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 as 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 registered on the epoll instance.
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; mean-
while 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 nonblocking 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 nonblocking 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 edge-triggered epoll, multiple events can be generated upon receipt of multiple chunks of data, the call-
er 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.
/proc interfaces
The following interfaces can be used to limit the amount of kernel memory consumed by epoll:
/proc/sys/fs/epoll/max_user_watches (since Linux 2.6.28)
This specifies a limit on the total number of file descriptors that a user can register across all epoll instances
on the system. The limit is per real user ID. Each registered file descriptor costs roughly 90 bytes on a 32-bit
kernel, and roughly 160 bytes on a 64-bit kernel. Currently, the default value for max_user_watches is 1/25 (4%)
of the available low memory, divided by the registration cost in bytes.
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 nonblocking 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.
#define MAX_EVENTS 10
struct epoll_event ev, events[MAX_EVENTS];
int listen_sock, conn_sock, nfds, epollfd;
/* Set up listening socket, 'listen_sock' (socket(),
bind(), listen()) */
epollfd = epoll_create(10);
if (epollfd == -1) {
perror("epoll_create");
exit(EXIT_FAILURE);
}
ev.events = EPOLLIN;
ev.data.fd = listen_sock;
if (epoll_ctl(epollfd, EPOLL_CTL_ADD, listen_sock, &ev) == -1) {
perror("epoll_ctl: listen_sock");
exit(EXIT_FAILURE);
}
for (;;) {
nfds = epoll_wait(epollfd, events, MAX_EVENTS, -1);
if (nfds == -1) {
perror("epoll_pwait");
exit(EXIT_FAILURE);
}
for (n = 0; n < nfds; ++n) {
if (events[n].data.fd == listen_sock) {
conn_sock = accept(listen_sock,
(struct sockaddr *) &local, &addrlen);
if (conn_sock == -1) {
perror("accept");
exit(EXIT_FAILURE);
}
setnonblocking(conn_sock);
ev.events = EPOLLIN | EPOLLET;
ev.data.fd = conn_sock;
if (epoll_ctl(epollfd, EPOLL_CTL_ADD, conn_sock,
&ev) == -1) {
perror("epoll_ctl: conn_sock");
exit(EXIT_FAILURE);
}
} 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 registered 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 register the same file descriptor on an epoll instance 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 instance. This can be a useful technique for filtering events, if the duplicate file
descriptors are registered with different events masks.
Q2 Can two epoll instances wait for the same file descriptor? If so, are events reported to both epoll file descrip-
tors?
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 one attempts to put an epoll file descriptor 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 descrip-
tor 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
reread 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 (nonblocking) 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-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 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. Support was added to glibc in version 2.3.2.
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_create1(2), epoll_ctl(2), epoll_wait(2)
COLOPHON
This page is part of release 3.25 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 2009-02-01 EPOLL(7)
