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EPOLL(4)                   Linux Programmer's Manual                  EPOLL(4)

       epoll - I/O event notification facility

       #include <&lt;sys/epoll.h>&gt;

       epoll  is a variant of poll(2) that can be used either as Edge or Level
       Triggered interface and scales well to large numbers  of  watched  fds.
       Three  system  calls  are  provided to set up and control an epoll set:
       epoll_create(2), epoll_ctl(2), epoll_wait(2).

       An epoll set is connected to a file descriptor  created  by  epoll_cre-
       ate(2).   Interest  for certain file descriptors is then registered via
       epoll_ctl(2).  Finally, the actual wait is started by epoll_wait(2).

       The epoll event distribution interface is able to behave both  as  Edge
       Triggered  ( ET ) and Level Triggered ( LT ). The difference between ET
       and LT event distribution mechanism can be described as  follows.  Sup-
       pose that this scenario happens :

       1      The  file  descriptor  that represents the read side of a pipe (
              RFD ) is added inside the epoll device.

       2      Pipe writer writes 2Kb of data on the write side of the pipe.

       3      A call to epoll_wait(2) is done that will return  RFD  as  ready
              file descriptor.

       4      The pipe reader reads 1Kb 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 flag, the call to epoll_wait(2) done in step 5 will  proba-
       bly  hang because of the available data still present in the file input
       buffers and the remote peer might be expecting a response based on  the
       data  it already sent. The reason for this is that Edge Triggered event
       distribution delivers events only when events happens on the  monitored
       file.  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 lock indefinitely. The epoll interface, when used with the  EPOL-
       LET flag ( Edge Triggered ) should use non-blocking file descriptors to
       avoid having a blocking read or write starve the task that is  handling
       multiple  file  descriptors.  The suggested way to use epoll as an Edge
       Triggered ( EPOLLET ) interface is  below,  and  possible  pitfalls  to
       avoid follow.

              i      with non-blocking file descriptors

              ii     by  going  to  wait  for  an  event only after read(2) or
                     write(2) return EAGAIN

       On the contrary, when used as a Level Triggered interface, epoll is  by
       all means a faster poll(2), and can be used wherever the latter is used
       since it shares the same semantics. Since even with the Edge  Triggered
       epoll  multiple  events  can  be  generated  up on receival 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
       receival of an event with epoll_wait(2).  When the EPOLLONESHOT flag is
       specified,  it  is  caller  responsibility to rearm the file descriptor
       using epoll_ctl(2) with EPOLL_CTL_MOD.

       While the usage of epoll when employed like a Level Triggered interface
       does  have  the  same  semantics  of  poll(2),  an Edge Triggered usage
       requires more clarifiction to avoid stalls  in  the  application  event
       loop.  In this example, listener is a non-blocking 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.

       struct epoll_event ev, *events;

       for(;;) {
           nfds = epoll_wait(kdpfd, events, maxevents, -1);

           for(n = 0; n < nfds; ++n) {
               if(events[n].data.fd == listener) {
                   client = accept(listener, (struct sockaddr *) &local,
                   if(client < 0){
                   ev.events = EPOLLIN | EPOLLET;
                   ev.data.fd = client;
                   if (epoll_ctl(kdpfd, EPOLL_CTL_ADD, client, &ev) < 0) {
                       fprintf(stderr, "epoll set insertion error: fd=%d0,
                       return -1;

       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  call-
       ing epoll_ctl(2) with EPOLL_CTL_MOD.

QUESTIONS AND ANSWERS (from linux-kernel)
              Q1     What  happens  if  you  add  the  same fd to an epoll_set

              A1     You will probably get EEXIST.  However,  it  is  possible
                     that  two  threads  may  add the same fd twice. This is a
                     harmless condition.

              Q2     Can two epoll sets wait for  the  same  fd?  If  so,  are
                     events reported to both epoll sets fds?

              A2     Yes.  However,  it  is  not  recommended. Yes it would be
                     reported to both.

              Q3     Is the epoll fd itself poll/epoll/selectable?

              A3     Yes.

              Q4     What happens if the epoll fd is put into its own fd set?

              A4     It will fail. However, you can add  an  epoll  fd  inside
                     another epoll fd set.

              Q5     Can  I  send  the  epoll fd over a unix-socket to another

              A5     No.

              Q6     Will the close of an fd cause it to be removed  from  all
                     epoll sets automatically?

              A6     Yes.

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

              A7     They will be combined.

              Q8     Does an operation on an fd affect the  already  collected
                     but not yet reported events?

              A8     You can do two operations on an existing fd. Remove would
                     be meaningless for this case. Modify will re-read  avail-
                     able I/O.

              Q9     Do  I  need to continuously read/write an fd until EAGAIN
                     when using the EPOLLET flag ( Edge Triggered behaviour  )

              A9     No  you  don't.  Receiving  an  event  from epoll_wait(2)
                     should suggest to you that such file descriptor is  ready
                     for  the requested I/O operation. You have simply to con-
                     sider it ready until you will receive  the  next  EAGAIN.
                     When  and  how  you  will  use  such  file  descriptor is
                     entirely  up  to  you.  Also,  the  condition  that   the
                     read/write  I/O  space  is  exhausted  can be detected by
                     checking the amount of data read/write from/to the target
                     file descriptor. For example, if you call read(2) by ask-
                     ing to read a certain amount of data and read(2)  returns
                     a  lower  number  of  bytes,  you  can  be  sure  to have
                     exhausted the read I/O space for  such  file  descriptor.
                     Same is valid when writing using the write(2) function.

              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  caus-
              ing starvation. This 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 pro-
              cessed but still round robin amongst all the ready  files.  This
              also  supports  ignoring  subsequent events you receive for fd's
              that are already ready.

              o If using an event cache...

              If you use an event cache or store all the  fd's  returned  from
              epoll_wait(2),  then make sure to provide a way to mark its clo-
              sure dynamically (ie- caused by a previous event's  processing).
              Suppose you receive 100 events from epoll_wait(2), and in eventi
              #47 a condition causes event #13 to be closed.   If  you  remove
              the  structure and close() the fd for event #13, then your event
              cache might still say there are events waiting for that fd caus-
              ing confusion.

              One solution for this is to call, during the processing of event
              47, epoll_ctl(EPOLL_CTL_DEL) to delete fd 13 and  close(),  then
              mark  its  associated data structure as removed and link it to a
              cleanup list. If you find another event for fd 13 in your  batch
              processing, you will discover the fd had been previously removed
              and there will be no confusion.

       epoll(4) is a new API introduced in Linux kernel 2.5.44.  Its interface
       should be finalized in Linux kernel 2.5.66.

       epoll_create(2), epoll_ctl(2), epoll_wait(2)

Linux                             2002-10-23                          EPOLL(4)