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NAME

       epoll - I/O event notification facility

SYNOPSIS

       #include <sys/epoll.h>

DESCRIPTION

       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  central  concept  of  the epoll API is the epoll instance, an in-kernel data structure which, from a
       user-space perspective, can be considered as a container for two lists:

       •  The interest list (sometimes also called the epoll set): the set of file descriptors that the  process
          has registered an interest in monitoring.

       •  The  ready  list: the set of file descriptors that are "ready" for I/O.  The ready list is a subset of
          (or, more precisely, a set of references to) the file descriptors in the  interest  list.   The  ready
          list is dynamically populated by the kernel as a result of I/O activity on those file descriptors.

       The following system calls are provided to create and manage an epoll instance:

       •  epoll_create(2) creates a new 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), which adds items  to  the
          interest list of the epoll instance.

       •  epoll_wait(2)  waits for I/O events, blocking the calling thread if no events are currently available.
          (This system call can be thought of as fetching items from the ready list of the epoll instance.)

   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 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:

       (1)  with nonblocking file descriptors; and

       (2)  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
       semantics.

       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.

       If multiple threads (or processes, if child processes have inherited the  epoll  file  descriptor  across
       fork(2))  are blocked in epoll_wait(2) waiting on the same epoll file descriptor and a file descriptor in
       the interest list that is marked for edge-triggered (EPOLLET) notification becomes ready, just one of the
       threads  (or  processes)  is awoken from epoll_wait(2).  This provides a useful optimization for avoiding
       "thundering herd" wake-ups in some scenarios.

   Interaction with autosleep
       If the system is in autosleep mode via /sys/power/autosleep and an event happens which wakes  the  device
       from  sleep,  the  device driver will keep the device awake only until that event is queued.  To keep the
       device awake until the event has been processed, it is necessary  to  use  the  epoll_ctl(2)  EPOLLWAKEUP
       flag.

       When  the  EPOLLWAKEUP  flag is set in the events field for a struct epoll_event, the system will be kept
       awake from the moment the event is queued, through the epoll_wait(2) call which returns the  event  until
       the  subsequent  epoll_wait(2)  call.  If the event should keep the system awake beyond that time, then a
       separate wake_lock should be taken before the second epoll_wait(2) call.

   /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;

           /* Code to set up listening socket, 'listen_sock',
              (socket(), bind(), listen()) omitted. */

           epollfd = epoll_create1(0);
           if (epollfd == -1) {
               perror("epoll_create1");
               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);
               if (nfds == -1) {
                   perror("epoll_wait");
                   exit(EXIT_FAILURE);
               }

               for (n = 0; n < nfds; ++n) {
                   if (events[n].data.fd == listen_sock) {
                       conn_sock = accept(listen_sock,
                                          (struct sockaddr *) &addr, &addrlen);
                       if (conn_sock == -1) {
                           perror("accept");
                           exit(EXIT_FAILURE);
                       }
                       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
       •  What is the key used to distinguish the file descriptors registered in an interest list?

          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).

       •  What happens if you register the same file descriptor on an epoll instance twice?

          You  will  probably get EEXIST.  However, it is possible to add a duplicate (dup(2), dup2(2), fcntl(2)
          F_DUPFD) file 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.

       •  Can  two  epoll instances wait for the same file descriptor?  If so, are events reported to both epoll
          file descriptors?

          Yes, and events would be reported to both.  However, careful programming may  be  needed  to  do  this
          correctly.

       •  Is the epoll file descriptor itself poll/epoll/selectable?

          Yes.  If an epoll file descriptor has events waiting, then it will indicate as being readable.

       •  What happens if one attempts to put an epoll file descriptor into its own file descriptor set?

          The  epoll_ctl(2)  call  fails (EINVAL).  However, you can add an epoll file descriptor inside another
          epoll file descriptor set.

       •  Can I send an epoll file descriptor over a UNIX domain socket to another process?

          Yes, but it does not make sense to do this, since the receiving process would not have copies  of  the
          file descriptors in the interest list.

       •  Will closing a file descriptor cause it to be removed from all epoll interest lists?

          Yes,  but  be  aware  of  the  following  point.   A  file  descriptor  is a reference to an open file
          description (see open(2)).  Whenever a file 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 interest list only after all the file  descriptors  referring  to
          the  underlying  open file description have been closed.  This means that even after a file descriptor
          that is part of an interest list has been closed, events may be reported for that file  descriptor  if
          other file descriptors referring to the same underlying file description remain open.  To prevent this
          happening, the file descriptor must be explicitly removed from the interest list  (using  epoll_ctl(2)
          EPOLL_CTL_DEL)  before  it  is  duplicated.   Alternatively, the application must ensure that all file
          descriptors are closed (which may be difficult if file descriptors were duplicated behind  the  scenes
          by library functions that used dup(2) or fork(2)).

       •  If more than one event occurs between epoll_wait(2) calls, are they combined or reported separately?

          They will be combined.

       •  Does an operation on a file descriptor affect the already collected but not yet reported events?

          You  can do two operations on an existing file descriptor.  Remove would be meaningless for this case.
          Modify will reread available I/O.

       •  Do I need to continuously read/write a file descriptor until EAGAIN when using the EPOLLET flag (edge-
          triggered behavior)?

          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 themStarvation (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 ready.

       •  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 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 previously removed and there will be no confusion.

VERSIONS

       Some other systems provide  similar  mechanisms;  for  example,  FreeBSD  has  kqueue,  and  Solaris  has
       /dev/poll.

STANDARDS

       Linux.

HISTORY

       Linux 2.5.44.  glibc 2.3.2.

NOTES

       The  set  of  file descriptors that is being monitored via an epoll file descriptor can be viewed via the
       entry for the epoll file descriptor in the process's /proc/pid/fdinfo directory.  See proc(5) for further
       details.

       The kcmp(2) KCMP_EPOLL_TFD operation can be used to test whether a file descriptor is present in an epoll
       instance.

SEE ALSO

       epoll_create(2), epoll_create1(2), epoll_ctl(2), epoll_wait(2), poll(2), select(2)