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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:

       a) with nonblocking file descriptors; and

       b) 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
       0.  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

       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  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.

       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 programming 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

       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 interest 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 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)).

       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 I/O.

       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-oriented

   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

       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 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 confusion.

       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 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)


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