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

       *  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 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:

              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
       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.

   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(7) 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 *) &local, &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
       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 set.

       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 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  (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 ready.

       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 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

       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), poll(2), select(2)

COLOPHON

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