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       epoll - I/O event notification facility


       #include <sys/epoll.h>


       epoll  is  a  variant  of  poll(2)  that 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 set up  and  control  an  epoll  set:  epoll_create(2)  (or
       epoll_create1(2)), epoll_ctl(2), epoll_wait(2).

       An   epoll   set   is   connected  to  a  file  descriptor  created  by
       epoll_create(2).   (The  more  recent  epoll_create1(2)   extends   the
       functionality   of   epoll_create(2).)    Interest   for  certain  file
       descriptors is then registered via epoll_ctl(2).  Finally,  the  actual
       wait is started by epoll_wait(2).

   Level-Triggered and Edge-Triggered
       The  epoll event distribution interface is able to behave both as edge-
       triggered (ET) and 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
          added inside the epoll device.

       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 only delivers events 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 indefinitely.

       An  application  that  employs the EPOLLET flag should use non-blocking
       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 non-blocking 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

       Since even  with  the  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.

   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 non-blocking 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 before.

           #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 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 file).

       Q1  What happens if you add the same file descriptor to  an  epoll  set

       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  set.   This  can  be  a useful technique for filtering
           events, if the  duplicate  file  descriptors  are  registered  with
           different events masks.

       Q2  Can  two  epoll sets 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 readable.

       Q4  What happens if the epoll file descriptor is put 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

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

       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()  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 re-read  available

       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  (non-blocking)  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 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
       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

       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
       previously removed and there will be no confusion.


       The epoll API was introduced in Linux  kernel  2.5.44.   Its  interface
       should be finalized in Linux kernel 2.5.66.  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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