man > epoll(7)

epoll(7)                          Miscellaneous Information Manual                          epoll(7)

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  inter‐
          est list.  The ready list is dynamically populated by the kernel as a result of I/O activ‐
          ity 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_cre‐
          ate(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 ex‐
       pecting a response based on the data it already sent.  The reason for this is that edge-trig‐
       gered  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 spec‐
       ified), 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 multi‐
       ple 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 descrip‐
       tor 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) noti‐
       fication 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 scenar‐
       ios.

   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 sys‐
       tem  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  regis‐
              ter  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  se‐
       mantics  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;

           /* 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|EPOLL‐
       OUT).   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 use‐
          ful  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 read‐
          able.

       •  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 in‐
          side 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 re‐
          ferring  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 underly‐
          ing  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  (non‐
          blocking)  read/write yields EAGAIN.  When and how you will use the file descriptor is en‐
          tirely 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
       •  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 as‐
          sociated 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 ignor‐
          ing 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  previ‐
          ous  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 associ‐
          ated 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 direc‐
       tory.  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)

Linux man-pages 6.7                          2023-10-31                                     epoll(7)