epoll_ctl(2) — Linux manual page


epoll_ctl(2)               System Calls Manual              epoll_ctl(2)

NAME         top

       epoll_ctl - control interface for an epoll file descriptor

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <sys/epoll.h>

       int epoll_ctl(int epfd, int op, int fd,
                     struct epoll_event *_Nullable event);

DESCRIPTION         top

       This system call is used to add, modify, or remove entries in the
       interest list of the epoll(7) instance referred to by the file
       descriptor epfd.  It requests that the operation op be performed
       for the target file descriptor, fd.

       Valid values for the op argument are:

              Add an entry to the interest list of the epoll file
              descriptor, epfd.  The entry includes the file descriptor,
              fd, a reference to the corresponding open file description
              (see epoll(7) and open(2)), and the settings specified in

              Change the settings associated with fd in the interest
              list to the new settings specified in event.

              Remove (deregister) the target file descriptor fd from the
              interest list.  The event argument is ignored and can be
              NULL (but see BUGS below).

       The event argument describes the object linked to the file
       descriptor fd.  The struct epoll_event is described in

       The data member of the epoll_event structure specifies data that
       the kernel should save and then return (via epoll_wait(2)) when
       this file descriptor becomes ready.

       The events member of the epoll_event structure is a bit mask
       composed by ORing together zero or more event types, returned by
       epoll_wait(2), and input flags, which affect its behaviour, but
       aren't returned.  The available event types are:

              The associated file is available for read(2) operations.

              The associated file is available for write(2) operations.

       EPOLLRDHUP (since Linux 2.6.17)
              Stream socket peer closed connection, or shut down writing
              half of connection.  (This flag is especially useful for
              writing simple code to detect peer shutdown when using
              edge-triggered monitoring.)

              There is an exceptional condition on the file descriptor.
              See the discussion of POLLPRI in poll(2).

              Error condition happened on the associated file
              descriptor.  This event is also reported for the write end
              of a pipe when the read end has been closed.

              epoll_wait(2) will always report for this event; it is not
              necessary to set it in events when calling epoll_ctl().

              Hang up happened on the associated file descriptor.

              epoll_wait(2) will always wait for this event; it is not
              necessary to set it in events when calling epoll_ctl().

              Note that when reading from a channel such as a pipe or a
              stream socket, this event merely indicates that the peer
              closed its end of the channel.  Subsequent reads from the
              channel will return 0 (end of file) only after all
              outstanding data in the channel has been consumed.

       And the available input flags are:

              Requests edge-triggered notification for the associated
              file descriptor.  The default behavior for epoll is level-
              triggered.  See epoll(7) for more detailed information
              about edge-triggered and level-triggered notification.

       EPOLLONESHOT (since Linux 2.6.2)
              Requests one-shot notification for the associated file
              descriptor.  This means that after an event notified for
              the file descriptor by epoll_wait(2), the file descriptor
              is disabled in the interest list and no other events will
              be reported by the epoll interface.  The user must call
              epoll_ctl() with EPOLL_CTL_MOD to rearm the file
              descriptor with a new event mask.

       EPOLLWAKEUP (since Linux 3.5)
              If EPOLLONESHOT and EPOLLET are clear and the process has
              the CAP_BLOCK_SUSPEND capability, ensure that the system
              does not enter "suspend" or "hibernate" while this event
              is pending or being processed.  The event is considered as
              being "processed" from the time when it is returned by a
              call to epoll_wait(2) until the next call to epoll_wait(2)
              on the same epoll(7) file descriptor, the closure of that
              file descriptor, the removal of the event file descriptor
              with EPOLL_CTL_DEL, or the clearing of EPOLLWAKEUP for the
              event file descriptor with EPOLL_CTL_MOD.  See also BUGS.

       EPOLLEXCLUSIVE (since Linux 4.5)
              Sets an exclusive wakeup mode for the epoll file
              descriptor that is being attached to the target file
              descriptor, fd.  When a wakeup event occurs and multiple
              epoll file descriptors are attached to the same target
              file using EPOLLEXCLUSIVE, one or more of the epoll file
              descriptors will receive an event with epoll_wait(2).  The
              default in this scenario (when EPOLLEXCLUSIVE is not set)
              is for all epoll file descriptors to receive an event.
              EPOLLEXCLUSIVE is thus useful for avoiding thundering herd
              problems in certain scenarios.

              If the same file descriptor is in multiple epoll
              instances, some with the EPOLLEXCLUSIVE flag, and others
              without, then events will be provided to all epoll
              instances that did not specify EPOLLEXCLUSIVE, and at
              least one of the epoll instances that did specify

              The following values may be specified in conjunction with
              EPOLLET.  EPOLLHUP and EPOLLERR can also be specified, but
              this is not required: as usual, these events are always
              reported if they occur, regardless of whether they are
              specified in events.  Attempts to specify other values in
              events yield the error EINVAL.

              EPOLLEXCLUSIVE may be used only in an EPOLL_CTL_ADD
              operation; attempts to employ it with EPOLL_CTL_MOD yield
              an error.  If EPOLLEXCLUSIVE has been set using
              epoll_ctl(), then a subsequent EPOLL_CTL_MOD on the same
              epfd, fd pair yields an error.  A call to epoll_ctl() that
              specifies EPOLLEXCLUSIVE in events and specifies the
              target file descriptor fd as an epoll instance will
              likewise fail.  The error in all of these cases is EINVAL.

RETURN VALUE         top

       When successful, epoll_ctl() returns zero.  When an error occurs,
       epoll_ctl() returns -1 and errno is set to indicate the error.

ERRORS         top

       EBADF  epfd or fd is not a valid file descriptor.

       EEXIST op was EPOLL_CTL_ADD, and the supplied file descriptor fd
              is already registered with this epoll instance.

       EINVAL epfd is not an epoll file descriptor, or fd is the same as
              epfd, or the requested operation op is not supported by
              this interface.

       EINVAL An invalid event type was specified along with
              EPOLLEXCLUSIVE in events.

       EINVAL op was EPOLL_CTL_MOD and events included EPOLLEXCLUSIVE.

       EINVAL op was EPOLL_CTL_MOD and the EPOLLEXCLUSIVE flag has
              previously been applied to this epfd, fd pair.

       EINVAL EPOLLEXCLUSIVE was specified in event and fd refers to an
              epoll instance.

       ELOOP  fd refers to an epoll instance and this EPOLL_CTL_ADD
              operation would result in a circular loop of epoll
              instances monitoring one another or a nesting depth of
              epoll instances greater than 5.

       ENOENT op was EPOLL_CTL_MOD or EPOLL_CTL_DEL, and fd is not
              registered with this epoll instance.

       ENOMEM There was insufficient memory to handle the requested op
              control operation.

       ENOSPC The limit imposed by /proc/sys/fs/epoll/max_user_watches
              was encountered while trying to register (EPOLL_CTL_ADD) a
              new file descriptor on an epoll instance.  See epoll(7)
              for further details.

       EPERM  The target file fd does not support epoll.  This error can
              occur if fd refers to, for example, a regular file or a

STANDARDS         top


HISTORY         top

       Linux 2.6, glibc 2.3.2.

NOTES         top

       The epoll interface supports all file descriptors that support

BUGS         top

       Before Linux 2.6.9, the EPOLL_CTL_DEL operation required a non-
       null pointer in event, even though this argument is ignored.
       Since Linux 2.6.9, event can be specified as NULL when using
       EPOLL_CTL_DEL.  Applications that need to be portable to kernels
       before Linux 2.6.9 should specify a non-null pointer in event.

       If EPOLLWAKEUP is specified in flags, but the caller does not
       have the CAP_BLOCK_SUSPEND capability, then the EPOLLWAKEUP flag
       is silently ignored.  This unfortunate behavior is necessary
       because no validity checks were performed on the flags argument
       in the original implementation, and the addition of the
       EPOLLWAKEUP with a check that caused the call to fail if the
       caller did not have the CAP_BLOCK_SUSPEND capability caused a
       breakage in at least one existing user-space application that
       happened to randomly (and uselessly) specify this bit.  A robust
       application should therefore double check that it has the
       CAP_BLOCK_SUSPEND capability if attempting to use the EPOLLWAKEUP

SEE ALSO         top

       epoll_create(2), epoll_wait(2), ioctl_eventpoll(2), poll(2),

COLOPHON         top

       This page is part of the man-pages (Linux kernel and C library
       user-space interface documentation) project.  Information about
       the project can be found at 
       ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report
       for this manual page, see
       This page was obtained from the tarball man-pages-6.9.1.tar.gz
       fetched from
       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
       2024-06-26.  If you discover any rendering problems in this HTML
       version of the page, or you believe there is a better or more up-
       to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not
       part of the original manual page), send a mail to

Linux man-pages 6.9.1          2024-06-12                   epoll_ctl(2)

Pages that refer to this page: epoll_create(2)epoll_wait(2)io_uring_enter2(2)io_uring_enter(2)signalfd(2)syscalls(2)epoll_event(3type)sd_event_add_io(3)sd_event_get_fd(3)sd_notify(3)proc_pid_fdinfo(5)epoll(7)