epoll(7) — Linux manual page

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EPOLL(7)                Linux Programmer's Manual               EPOLL(7)

NAME         top

       epoll - I/O event notification facility

SYNOPSIS         top

       #include <sys/epoll.h>

DESCRIPTION         top

       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:

       a) with nonblocking file descriptors; and

       b) 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
       0.  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).

       1.  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.

       2.  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.

       3.  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.

       4.  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.

       5.  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.

       6.  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)).

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

           They will be combined.

       8.  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.

       9.  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 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         top

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

CONFORMING TO         top

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

NOTES         top

       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         top

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

COLOPHON         top

       This page is part of release 5.13 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22                       EPOLL(7)

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