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.\"  Davide Libenzi <davidel@xmailserver.org>

.\"

.TH EPOLL 7 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

epoll \- I/O event notification facility

.SH SYNOPSIS

.B #include <sys/epoll.h>

.SH DESCRIPTION

The

.B epoll

API performs a similar task to

.BR poll (2):

monitoring multiple file descriptors to see if I/O is possible on any of them.

The

.B epoll

API 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

.B epoll

instance:

.IP * 3

.BR epoll_create (2)

creates a new

.B epoll

instance and returns a file descriptor referring to that instance.

(The more recent

.BR epoll_create1 (2)

extends the functionality of

.BR epoll_create (2).)

.IP *

Interest in particular file descriptors is then registered via

.BR epoll_ctl (2).

The set of file descriptors currently registered on an

.B epoll

instance is sometimes called an

.I epoll

set.

.IP *

.BR epoll_wait (2)

waits for I/O events,

blocking the calling thread if no events are currently available.

.SS Level-triggered and edge-triggered

The

.B 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:

.IP 1. 3

The file descriptor that represents the read side of a pipe

.RI ( rfd )

is registered on the

.B epoll

instance.

.IP 2.

A pipe writer writes 2\ kB of data on the write side of the pipe.

.IP 3.

A call to

.BR epoll_wait (2)

is done that will return

.I rfd

as a ready file descriptor.

.IP 4.

The pipe reader reads 1\ kB of data from

.IR rfd .

.IP 5.

A call to

.BR epoll_wait (2)

is done.

.PP

If the

.I rfd

file descriptor has been added to the

.B epoll

interface using the

.B EPOLLET

(edge-triggered)

flag, the call to

.BR epoll_wait (2)

done in step

.B 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

delivers events only when changes occur on the monitored file descriptor.

So, in step

.B 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

.I rfd

will be generated because of the write done in

.B 2

and the event is consumed in

.BR 3 .

Since the read operation done in

.B 4

does not consume the whole buffer data, the call to

.BR epoll_wait (2)

done in step

.B 5

might block indefinitely.

.PP

An application that employs the

.B 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

.B epoll

as an edge-triggered

.RB ( EPOLLET )

interface is as follows:

.RS

.TP 4

.B i

with nonblocking file descriptors; and

.TP

.B ii

by waiting for an event only after

.BR read (2)

or

.BR write (2)

return

.BR EAGAIN .

.RE

.PP

By contrast, when used as a level-triggered interface

(the default, when

.B EPOLLET

is not specified),

.B epoll

is simply a faster

.BR poll (2),

and can be used wherever the latter is used since it shares the

same semantics.

.PP

Since even with edge-triggered

.BR epoll ,

multiple events can be generated upon receipt of multiple chunks of data,

the caller has the option to specify the

.B EPOLLONESHOT

flag, to tell

.B epoll

to disable the associated file descriptor after the receipt of an event with

.BR epoll_wait (2).

When the

.B EPOLLONESHOT

flag is specified,

it is the caller's responsibility to rearm the file descriptor using

.BR epoll_ctl (2)

with

.BR EPOLL_CTL_MOD .

.SS Interaction with autosleep

If the system is in

.B autosleep

mode via

.I /sys/power/autosleep

and an event happens which wakes the device from sleep, the device

driver will keep the device awake only until that event is queued.

To keep the device awake until the event has been processed,

it is necessary to use the

.BR epoll_ctl (2)

.B EPOLLWAKEUP

flag.

.PP

When the

.B EPOLLWAKEUP

flag is set in the

.B events

field for a

.IR "struct epoll_event" ,

the system will be kept awake from the moment the event is queued,

through the

.BR epoll_wait (2)

call which returns the event until the subsequent

.BR epoll_wait (2)

call.

If the event should keep the system awake beyond that time,

then a separate

.I wake_lock

should be taken before the second

.BR epoll_wait (2)

call.

.SS /proc interfaces

The following interfaces can be used to limit the amount of

kernel memory consumed by epoll:

.\" Following was added in 2.6.28, but them removed in 2.6.29

.\" .TP

.\" .IR /proc/sys/fs/epoll/max_user_instances " (since Linux 2.6.28)"

.\" This specifies an upper limit on the number of epoll instances

.\" that can be created per real user ID.

.TP

.IR /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,

.\" 2.6.29 (in 2.6.28, the default was 1/32 of lowmem)

the default value for

.I max_user_watches

is 1/25 (4%) of the available low memory,

divided by the registration cost in bytes.

.SS Example for suggested usage

While the usage of

.B epoll

when employed as a level-triggered interface does have the same

semantics as

.BR 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

.BR listen (2)

has been called.

The function

.I do_use_fd()

uses the new ready file descriptor until

.B EAGAIN

is returned by either

.BR read (2)

or

.BR write (2).

An event-driven state machine application should, after having received

.BR EAGAIN ,

record its current state so that at the next call to

.I do_use_fd()

it will continue to

.BR read (2)

or

.BR write (2)

from where it stopped before.

.PP

.in +4n

.EX

#define MAX_EVENTS 10

struct epoll_event ev, events[MAX_EVENTS];

int listen_sock, conn_sock, nfds, epollfd;

/* Code to set up listening socket, \(aqlisten_sock\(aq,

   (socket(), bind(), listen()) omitted */

epollfd = epoll_create1(0);

if (epollfd == \-1) {

    perror("epoll_create1");

    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_wait");

        exit(EXIT_FAILURE);

    }

    for (n = 0; n < nfds; ++n) {

        if (events[n].data.fd == listen_sock) {

            conn_sock = accept(listen_sock,

                               (struct sockaddr *) &addr, &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);

        }

    }

}

.EE

.in

.PP

When used as an edge-triggered interface, for performance reasons, it is

possible to add the file descriptor inside the

.B epoll

interface

.RB ( EPOLL_CTL_ADD )

once by specifying

.RB ( EPOLLIN | EPOLLOUT ).

This allows you to avoid

continuously switching between

.B EPOLLIN

and

.B EPOLLOUT

calling

.BR epoll_ctl (2)

with

.BR EPOLL_CTL_MOD .

.SS Questions and answers

.TP 4

.B Q0

What is the key used to distinguish the file descriptors registered in an

.B epoll

set?

.TP

.B 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).

.TP

.B Q1

What happens if you register the same file descriptor on an

.B epoll

instance twice?

.TP

.B A1

You will probably get

.BR EEXIST .

However, it is possible to add a duplicate

.RB ( dup (2),

.BR dup2 (2),

.BR fcntl (2)

.BR F_DUPFD )

file descriptor to the same

.B epoll

instance.

.\" But a file descriptor duplicated by fork(2) can't be added to the

.\" set, because the [file *, fd] pair is already in the epoll set.

.\" That is a somewhat ugly inconsistency.  On the one hand, a child process

.\" cannot add the duplicate file descriptor to the epoll set.  (In every

.\" other case that I can think of, file descriptors duplicated by fork have

.\" similar semantics to file descriptors duplicated by dup() and friends.)  On

.\" the other hand, the very fact that the child has a duplicate of the

.\" file descriptor means that even if the parent closes its file descriptor,

.\" then epoll_wait() in the parent will continue to receive notifications for

.\" that file descriptor because of the duplicated file descriptor in the child.

.\"

.\" See http://thread.gmane.org/gmane.linux.kernel/596462/

.\" "epoll design problems with common fork/exec patterns"

.\"

.\" mtk, Feb 2008

This can be a useful technique for filtering events,

if the duplicate file descriptors are registered with different

.I events

masks.

.TP

.B Q2

Can two

.B epoll

instances wait for the same file descriptor?

If so, are events reported to both

.B epoll

file descriptors?

.TP

.B A2

Yes, and events would be reported to both.

However, careful programming may be needed to do this correctly.

.TP

.B Q3

Is the

.B epoll

file descriptor itself poll/epoll/selectable?

.TP

.B A3

Yes.

If an

.B epoll

file descriptor has events waiting, then it will

indicate as being readable.

.TP

.B Q4

What happens if one attempts to put an

.B epoll

file descriptor into its own file descriptor set?

.TP

.B A4

The

.BR epoll_ctl (2)

call fails

.RB ( EINVAL ).

However, you can add an

.B epoll

file descriptor inside another

.B epoll

file descriptor set.

.TP

.B Q5

Can I send an

.B epoll

file descriptor over a UNIX domain socket to another process?

.TP

.B 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

.B epoll

set.

.TP

.B Q6

Will closing a file descriptor cause it to be removed from all

.B epoll

sets automatically?

.TP

.B A6

Yes, but be aware of the following point.

A file descriptor is a reference to an open file description (see

.BR open (2)).

Whenever a file descriptor is duplicated via

.BR dup (2),

.BR dup2 (2),

.BR fcntl (2)

.BR F_DUPFD ,

or

.BR 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

.B epoll

set only after all the file descriptors referring to the underlying

open file description have been closed

(or before if the file descriptor is explicitly removed using

.BR epoll_ctl (2)

.BR EPOLL_CTL_DEL ).

This means that even after a file descriptor that is part of an

.B 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.

.TP

.B Q7

If more than one event occurs between

.BR epoll_wait (2)

calls, are they combined or reported separately?

.TP

.B A7

They will be combined.

.TP

.B Q8

Does an operation on a file descriptor affect the

already collected but not yet reported events?

.TP

.B A8

You can do two operations on an existing file descriptor.

Remove would be meaningless for

this case.

Modify will reread available I/O.

.TP

.B Q9

Do I need to continuously read/write a file descriptor

until

.B EAGAIN

when using the

.B EPOLLET

flag (edge-triggered behavior) ?

.TP

.B A9

Receiving an event from

.BR 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

.BR EAGAIN .

When and how you will use the file descriptor is entirely up to you.

.IP

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

.BR EAGAIN .

.IP

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

.BR read (2)

by asking to read a certain amount of data and

.BR 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

.BR write (2).

(Avoid this latter technique if you cannot guarantee that

the monitored file descriptor always refers to a stream-oriented file.)

.SS Possible pitfalls and ways to avoid them

.TP

.B o Starvation (edge-triggered)

.PP

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

.BR epoll .)

.PP

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.

.TP

.B o If using an event cache...

.PP

If you use an event cache or store all the file descriptors returned from

.BR 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

.BR epoll_wait (2),

and in event #47 a condition causes event #13 to be closed.

If you remove the structure and

.BR 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.

.PP

One solution for this is to call, during the processing of event 47,

.BR epoll_ctl ( EPOLL_CTL_DEL )

to delete file descriptor 13 and

.BR 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.

.SH VERSIONS

The

.B epoll

API was introduced in Linux kernel 2.5.44.

.\" Its interface should be finalized in Linux kernel 2.5.66.

Support was added to glibc in version 2.3.2.

.SH CONFORMING TO

The

.B epoll

API is Linux-specific.

Some other systems provide similar

mechanisms, for example, FreeBSD has

.IR kqueue ,

and Solaris has

.IR /dev/poll .

.SH NOTES

The set of file descriptors that is being monitored via

an epoll file descriptor can be viewed via the entry for

the epoll file descriptor in the process's

.IR /proc/[pid]/fdinfo

directory.

See

.BR proc (5)

for further details.

.PP

The

.BR kcmp (2)

.B KCMP_EPOLL_TFD

operation can be used to test whether a file descriptor

is present in an epoll instance.

.SH SEE ALSO

.BR epoll_create (2),

.BR epoll_create1 (2),

.BR epoll_ctl (2),

.BR epoll_wait (2),

.BR poll (2),

.BR select (2)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.