epoll(7)                                          Miscellaneous Information 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 perspec‐
       tive, 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 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  dif‐
       ference 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  wait‐
       ing  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  (EPOL‐
       LET) 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  de‐
       vice  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  ker‐
              nel, 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  be‐
       tween EPOLLIN and EPOLLOUT calling epoll_ctl(2) with EPOLL_CTL_MOD.

   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 han‐
          dle", the kernel's internal representation of an open file).

       •  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  de‐
          scriptor to the same epoll instance.  This can be a useful technique for filtering events, if the duplicate file descrip‐
          tors 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 refer‐
          ring to the same open file description is created.  An open file description continues to exist until all  file  descrip‐
          tors 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  inter‐
          est 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  behav‐

          Receiving  an event from epoll_wait(2) should suggest to you that such file descriptor is ready for the requested I/O op‐
          eration.  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
       •  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 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.

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

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

       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)

Linux man-pages 6.03                                         2023-02-05                                                    epoll(7)