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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
       descriptors.  The following system calls are  provided  to  create  and  manage  an  epoll

       *  epoll_create(2)  creates  an  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).  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 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:

              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 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.

   /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;

           /* Set up listening socket, 'listen_sock' (socket(),
              bind(), listen()) */

           epollfd = epoll_create(10);
           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 *) &local, &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
       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

       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) 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.

       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 programming 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

       Q4  What  happens  if  one  attempts  to  put  an  epoll file descriptor into its own file
           descriptor set?

       A4  The epoll_ctl(2) call will  fail  (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 set.

       Q6  Will  closing  a  file  descriptor  cause  it  to  be  removed  from  all  epoll  sets

       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  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 epoll set only after all the file descriptors referring to the underlying open
           file  description  have been closed (or before if the 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 I/O.

       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-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.


       epoll_create(2), epoll_create1(2), epoll_ctl(2), epoll_wait(2)


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