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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 create and manage an epoll instance:

       *  An epoll instance created by epoll_create(2), which returns  a  file
          descriptor  referring  to  the  epoll  instance.   (The  more recent
          epoll_create1(2) extends the functionality of epoll_create(2).)

       *  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.

       *  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 as 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
          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

       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  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

       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 epoll:

       /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 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 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 file).

       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 readable.

       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

       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 reread 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 (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 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.  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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