EPOLL(7)                   Linux Programmer's Manual                  EPOLL(7)

       epoll - I/O event notification facility

       #include <sys/epoll.h>

       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

       *   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

       *  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 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

       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

       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

       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 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 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 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
           filtering 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
           programming 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
           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)).

       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 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

       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 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

       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

       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.

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

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

       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

       The kcmp(2) KCMP_EPOLL_TFD operation can be used to test whether a file
       descriptor is present in an epoll instance.

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

       This page is part of release 5.05 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)