epoll(7) — Linux manual page


EPOLL(7)                  Linux Programmer's 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

       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

       • 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

       • 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

       • 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

   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

       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:

       a) with nonblocking file descriptors; and

       b) by waiting for an event only after read(2) or write(2) return

       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

       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

   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.

           #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 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 in‐
           terest 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 de‐
           scription 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 de‐
           scriptors referring to the same underlying file description re‐
           main 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 ap‐
           plication 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

       9.  Do I need to continuously read/write a file descriptor until EA‐
           GAIN 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 de‐
           scriptor 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 ap‐
       plication 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

       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         top

       The epoll API was introduced in Linux kernel 2.5.44.  Support was
       added to glibc in version 2.3.2.

CONFORMING TO         top

       The epoll API is Linux-specific.  Some other systems provide similar
       mechanisms, for example, FreeBSD has kqueue, and Solaris has

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),
       poll(2), select(2)

COLOPHON         top

       This page is part of release 5.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2019-03-06                         EPOLL(7)

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