epoll(7) — Linux manual page


epoll(7)            Miscellaneous Information Manual            epoll(7)

NAME         top

       epoll - I/O event notification facility

SYNOPSIS         top

       #include <sys/epoll.h>

DESCRIPTION         top

       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
       in-kernel 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.  The ready list 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

       •  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

       (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

       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:

       (1)  with nonblocking file descriptors; and

       (2)  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 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

       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.

           #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) {

           ev.events = EPOLLIN;
           ev.data.fd = listen_sock;
           if (epoll_ctl(epollfd, EPOLL_CTL_ADD, listen_sock, &ev) == -1) {
               perror("epoll_ctl: listen_sock");

           for (;;) {
               nfds = epoll_wait(epollfd, events, MAX_EVENTS, -1);
               if (nfds == -1) {

               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) {
                       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");
                   } else {

       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

   Questions and answers
       •  What is the key used to distinguish the file descriptors
          registered 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

       •  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 filtering events, if the duplicate file
          descriptors are registered with different events masks.

       •  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
          programming may be needed to do this correctly.

       •  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.

       •  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

       •  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.

       •  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 continues 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)).

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

          They will be combined.

       •  Does an operation on a file descriptor affect the already
          collected 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.

       •  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

          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 themStarvation (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.

       •  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         top

       Some other systems provide similar mechanisms; for example,
       FreeBSD has kqueue, and Solaris has /dev/poll.

STANDARDS         top


HISTORY         top

       Linux 2.5.44.  glibc 2.3.2.

NOTES         top

       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         top

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

COLOPHON         top

       This page is part of the man-pages (Linux kernel and C library
       user-space interface documentation) project.  Information about
       the project can be found at 
       ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report
       for this manual page, see
       This page was obtained from the tarball man-pages-6.9.1.tar.gz
       fetched from
       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
       2024-06-26.  If you discover any rendering problems in this HTML
       version of the page, or you believe there is a better or more up-
       to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not
       part of the original manual page), send a mail to

Linux man-pages 6.9.1          2024-06-12                       epoll(7)

Pages that refer to this page: accept(2)epoll_create(2)epoll_ctl(2)epoll_wait(2)eventfd(2)futex(2)ioctl_eventpoll(2)io_uring_enter2(2)io_uring_enter(2)kcmp(2)open(2)perf_event_open(2)perfmonctl(2)pidfd_open(2)poll(2)recv(2)seccomp_unotify(2)select(2)select_tut(2)signalfd(2)timerfd_create(2)userfaultfd(2)sd-event(3)sd_event_add_io(3)sd_event_get_fd(3)proc_pid_fdinfo(5)proc_sys_fs(5)systemd.exec(5)capabilities(7)fanotify(7)inotify(7)mq_overview(7)pipe(7)socket(7)udp(7)