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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 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.

       *  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

       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 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 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(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

       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)


       This page is part of release 3.65 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at

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