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  following  system calls are provided to create and manage an epoll

   *  epoll_create(2)  creates  an  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).   The  set of file descriptors currently registered on
      an epoll instance is sometimes called an epoll set.

   *  epoll_wait(2) waits for I/O events, blocking the calling  thread  if
      no events are currently available.

   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.

   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
   Q0  What is the key used to distinguish the file descriptors registered
       in an epoll set?

   A0  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).

   Q1  What  happens  if you register the same file descriptor on an epoll
       instance twice?

   A1  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.

   Q2  Can two epoll instances wait for the same file descriptor?  If  so,
       are events reported to both epoll file descriptors?

   A2  Yes,  and  events  would  be  reported  to  both.  However, careful
       programming may be needed to do this correctly.

   Q3  Is the epoll file descriptor itself poll/epoll/selectable?

   A3  Yes.  If an epoll file descriptor has events waiting, then it  will
       indicate as being readable.

   Q4  What  happens  if one attempts to put an epoll file descriptor into
       its own file descriptor set?

   A4  The epoll_ctl(2) call will fail (EINVAL).  However, you can add  an
       epoll file descriptor inside another epoll file descriptor set.

   Q5  Can  I  send  an epoll file descriptor over a UNIX domain socket to
       another process?

   A5  Yes, but it does not make sense to do  this,  since  the  receiving
       process  would not have copies of the file descriptors in the epoll

   Q6  Will closing a file descriptor cause it  to  be  removed  from  all
       epoll sets automatically?

   A6  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 epoll set only
       after all the file descriptors referring  to  the  underlying  open
       file description have been closed (or before if the file descriptor
       is explicitly  removed  using  epoll_ctl(2)  EPOLL_CTL_DEL).   This
       means  that  even  after a file descriptor that is part of an epoll
       set  has  been  closed,  events  may  be  reported  for  that  file
       descriptor   if  other  file  descriptors  referring  to  the  same
       underlying file description remain open.

   Q7  If more than one event occurs between epoll_wait(2) calls, are they
       combined or reported separately?

   A7  They will be combined.

   Q8  Does an operation on a file descriptor affect the already collected
       but not yet reported events?

   A8  You can do two operations on an existing file  descriptor.   Remove
       would  be  meaningless for this case.  Modify will reread available

   Q9  Do I need to continuously read/write a file descriptor until EAGAIN
       when using the EPOLLET flag (edge-triggered behavior) ?

   A9  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


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


   This  page  is  part of release 4.09 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

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