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 central concept of the epoll API is the epoll instance, an  in-ker-
       nel data structure which, from a user-space perspective, can be consid-
       ered as a container for two lists:

       o The interest list (sometimes also called the epoll set): the  set  of
         file descriptors that the process has registered an interest in moni-

       o 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 refer-
         ences 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

       o epoll_create(2)  creates  a new epoll instance and returns a file de-
         scriptor referring to that instance.   (The  more  recent  epoll_cre-
         ate1(2) extends the functionality of epoll_create(2).)

       o Interest  in  particular  file  descriptors  is  then  registered via
         epoll_ctl(2), which adds items to the interest list of the epoll  in-

       o 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 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:

       a) with nonblocking file descriptors; and

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

       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.

       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 "thun-
       dering 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 de-
       vice 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 be-
       fore 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 de-
              scriptor 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,  di-
              vided 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  re-
       quires  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) {

           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 du-
           plicate  (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 dif-
           ferent 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 inter-
           est 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 contin-
           ues to exist until all file descriptors referring to it  have  been

           A  file  descriptor is removed from an interest list only after all
           the file descriptors referring to the underlying open file descrip-
           tion  have been closed.  This means that even after a file descrip-
           tor that is part of an interest list has been closed, events may be
           reported  for that file descriptor if other file descriptors refer-
           ring to the same underlying file description remain open.  To  pre-
           vent 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  de-
           scriptors  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-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 5.07 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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