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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 inter-
     face 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 a new epoll instance and returns a file

descriptor referring to that instance. (The more recent epoll_cre-

        ate1(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 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 opera-
     tion 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 seman-
     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 mem-
     ory 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  sys-
            tem.   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  lis-
     ten(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) {
             exit(EXIT_FAILURE); }    =    EPOLLIN;    =    listen_sock;    if
         (epoll_ctl(epollfd, EPOLL_CTL_ADD, listen_sock, &ev) == -1) {
             perror("epoll_ctl: listen_sock");
             exit(EXIT_FAILURE); }
         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,
                     if (conn_sock == -1) {
            = EPOLLIN | EPOLLET;
            = 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 fil-
         tering 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  pro-
         gramming 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  fails  (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 contin-
         ues  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-ori-
         ented 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  appli-
     cation  to  remember  which  files need to be processed but still round
     robin amongst all the ready files.  This also supports ignoring  subse-
     quent events you receive for file descriptors that are already ready.
     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 previ-
     ously 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 4.16 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 2017-09-15 EPOLL(7)

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