epoll


SYNOPSIS
       #include <sys/epoll.h>

DESCRIPTION
       epoll  is a variant of poll(2) that can be used either as an edge-trig-
       gered 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
       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 only delivers events 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.

       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
       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 non-blocking 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) {
               perror("epoll_create");
               exit(EXIT_FAILURE);
           }

           ev.events = EPOLLIN;
           ev.data.fd = listen_sock;
           if (epoll_ctl(epollfd, EPOLL_CTL_ADD, listen_sock, &ev) == -1) {
               perror("epoll_ctl: listen_sock");
               exit(EXIT_FAILURE);
           }

           for (;;) {
               nfds = epoll_wait(epollfd, events, MAX_EVENTS, -1);
                       }
                       setnonblocking(conn_sock);
                       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");
                           exit(EXIT_FAILURE);
                       }
                   } else {
                       do_use_fd(events[n].data.fd);
                   }
               }
           }

       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?

       A4  The epoll_ctl(2) call will fail (EINVAL).  However, you can add  an
           epoll file descriptor inside another epoll file descriptor set.
           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
           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 (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-ori-
           ented 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
       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 previ-
       ously removed and there will be no confusion.

VERSIONS
       The epoll API was introduced in Linux kernel 2.5.44.  Support was added
       to glibc in version 2.3.2.

CONFORMING TO
       The  epoll  API  is Linux-specific.  Some other systems provide similar
       mechanisms, for example, FreeBSD has kqueue, and Solaris has /dev/poll.

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

COLOPHON
       This page is part of release 3.23 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                             2009-02-01                          EPOLL(7)
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