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       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

       The central concept of the epoll API is the epoll instance, an  in-kernel  data  structure
       which, from a user-space perspective, 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

       •  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 difference 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 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

       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:

       (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 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

       /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

           #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) {

  = 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) {
              = 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
       •  What  is  the  key  used  to distinguish the file descriptors registered in an interest

          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

       •  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 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.

       •  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

       •  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.

       •  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 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 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  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  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 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-oriented file.)

   Possible pitfalls and ways to avoid themStarvation (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

       •  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 in glibc 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 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)