#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 inter-
       face 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_cre-
          ate1(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 wait-
       ing  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 opera-
       tion done in 4 does not consume the whole  buffer  data,  the  call  to
       epoll_wait(2) done in step 5 might block indefinitely.

       can be used wherever the latter is used since it shares the same seman-

       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 mem-
       ory 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 sys-
              tem.  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 lis-
       ten(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) {

           ev.events = EPOLLIN;
           ev.data.fd = listen_sock;
           if (epoll_ctl(epollfd, EPOLL_CTL_ADD, listen_sock, &ev) == -1) {
               perror("epoll_ctl: listen_sock");
                       if (conn_sock == -1) {
                       ev.events = EPOLLIN | EPOLLET;
                       ev.data.fd = 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  dif-
           ferent 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  pro-
           gramming 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?
           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-ori-
           ented file.)

   Possible pitfalls and ways to avoid them
       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 previ-
       ously 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 3.54 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                             2012-04-17                          EPOLL(7)
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