Linux manual epoll(7)

NAME

epoll - I/O event notification facility

SYNOPSIS

#include <sys/epoll.h>

DESCRIPTION

The epoll API performs a similar task to poll(2): monitoring multiple file descriptors to see if I/O is possible on any of them.
The 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 central concept of the epoll API is the epoll instance, an inkernel data structure which, from a user-space perspective, can be considered as a container for two lists:

  • The interest list (sometimes also called the epoll set): the set of file descriptors that the process has registered an interest in monitoring.

  • The ready list: the set of file descriptors that are “ready” for I/O. The ready list is a subset of (or, more precisely, a set of references to) the file descriptors in the interest list that is dynamically populated by the kernel as a result of I/O activity on those file descriptors.

The following system calls are provided to create and manage an epoll instance:

  • epoll_create(2) creates a new epoll instance and returns a file descriptor referring to that 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), which adds items to the interest list of the epoll instance.
  • epoll_wait(2) waits for I/O events, blocking the calling thread if no events are currently available. (This system call can be thought of as fetching items from the ready list of the epoll instance.)

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; 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 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 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.

If multiple threads (or processes, if child processes have inherited
the epoll file descriptor across fork(2)) are blocked in
epoll_wait(2) waiting on the same the same epoll file descriptor and
a file descriptor in the interest list that is marked for edge-
triggered (EPOLLET) notification becomes ready, just one of the
threads (or processes) is awoken from epoll_wait(2).  This provides a
useful optimization for avoiding "thundering herd" wake-ups in some
scenarios.

Interaction with autosleep
If the system is in autosleep mode via /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 epoll_ctl(2) EPOLLWAKEUP flag.

When the EPOLLWAKEUP flag is set in the events field for a struct
epoll_event, the system will be kept awake from the moment the event
is queued, through the epoll_wait(2) call which returns the event
until the subsequent epoll_wait(2) call.  If the event should keep
the system awake beyond that time, then a separate wake_lock should
be taken before the second epoll_wait(2) call.

/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.

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#define MAX_EVENTS 10
struct epoll_event ev, events[MAX_EVENTS];
int listen_sock, conn_sock, nfds, epollfd;

/* Code to set up listening socket, 'listen_sock',
(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);
}
}
}
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

0.  What is the key used to distinguish the file descriptors regis‐
    tered in an interest list?

    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).

1.  What happens if you register the same file descriptor on an epoll
    instance twice?

    You will probably get EEXIST.  However, it is possible to add a
    duplicate (dup(2), dup2(2), fcntl(2) F_DUPFD) file descriptor to
    the same epoll instance.  This can be a useful technique for fil‐
    tering events, if the duplicate file descriptors are registered
    with different events masks.

2.  Can two epoll instances wait for the same file descriptor?  If
    so, are events reported to both epoll file descriptors?

    Yes, and events would be reported to both.  However, careful pro‐
    gramming may be needed to do this correctly.

3.  Is the epoll file descriptor itself poll/epoll/selectable?

    Yes.  If an epoll file descriptor has events waiting, then it
    will indicate as being readable.

4.  What happens if one attempts to put an epoll file descriptor into
    its own file descriptor set?

    The epoll_ctl(2) call fails (EINVAL).  However, you can add an
    epoll file descriptor inside another epoll file descriptor set.

5.  Can I send an epoll file descriptor over a UNIX domain socket to
    another process?

    Yes, but it does not make sense to do this, since the receiving
    process would not have copies of the file descriptors in the
    interest list.

6.  Will closing a file descriptor cause it to be removed from all
    epoll interest lists?

    Yes, but be aware of the following point.  A file descriptor is a
    reference to an open file description (see open(2)).  Whenever a
    file 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 con‐
    tinues to exist until all file descriptors referring to it have
    been closed.

    A file descriptor is removed from an interest list only after all
    the file descriptors referring to the underlying open file
    description have been closed.  This means that even after a file
    descriptor that is part of an interest list has been closed,
    events may be reported for that file descriptor if other file
    descriptors referring to the same underlying file description
    remain open.  To prevent this happening, the file descriptor must
    be explicitly removed from the interest list (using epoll_ctl(2)
    EPOLL_CTL_DEL) before it is duplicated.  Alternatively, the
    application must ensure that all file descriptors are closed
    (which may be difficult if file descriptors were duplicated
    behind the scenes by library functions that used dup(2) or
    fork(2)).

7.  If more than one event occurs between epoll_wait(2) calls, are
    they combined or reported separately?

    They will be combined.

8.  Does an operation on a file descriptor affect the already col‐
    lected but not yet reported events?

    You can do two operations on an existing file descriptor.  Remove
    would be meaningless for this case.  Modify will reread available
    I/O.

9.  Do I need to continuously read/write a file descriptor until
    EAGAIN when using the EPOLLET flag (edge-triggered behavior)?

    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 can‐
    not 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 try‐
ing to drain it the other files will not get processed causing star‐
vation.  (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 clo‐
sure 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.

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 /proc/[pid]/fdinfo directory. See proc(5) for further details.
The kcmp(2) KCMP_EPOLL_TFD operation can be used to test whether a file descriptor is present in an epoll instance.

SEE ALSO

epoll_create(2), epoll_create1(2), epoll_ctl(2), epoll_wait(2), poll(2), select(2)

COLOPHON

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