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+.\"  Copyright (C) 2003  Davide Libenzi
+.\"  Davide Libenzi <davidel@xmailserver.org>
+.\" and Copyright 2009, 2014, 2016, 2018, 2019 Michael Kerrisk <tk.manpages@gmail.com>
+.\"
+.\" SPDX-License-Identifier: GPL-2.0-or-later
+.\"
+.TH epoll_ctl 2 2024-05-02 "Linux man-pages (unreleased)"
+.SH NAME
+epoll_ctl \- control interface for an epoll file descriptor
+.SH LIBRARY
+Standard C library
+.RI ( libc ", " \-lc )
+.SH SYNOPSIS
+.nf
+.B #include <sys/epoll.h>
+.P
+.BI "int epoll_ctl(int " epfd ", int " op ", int " fd ,
+.BI "              struct epoll_event *_Nullable " event );
+.fi
+.SH DESCRIPTION
+This system call is used to add, modify, or remove
+entries in the interest list of the
+.BR epoll (7)
+instance
+referred to by the file descriptor
+.IR epfd .
+It requests that the operation
+.I op
+be performed for the target file descriptor,
+.IR fd .
+.P
+Valid values for the
+.I op
+argument are:
+.TP
+.B EPOLL_CTL_ADD
+Add an entry to the interest list of the epoll file descriptor,
+.IR epfd .
+The entry includes the file descriptor,
+.IR fd ,
+a reference to the corresponding open file description (see
+.BR epoll (7)
+and
+.BR open (2)),
+and the settings specified in
+.IR event .
+.TP
+.B EPOLL_CTL_MOD
+Change the settings associated with
+.I fd
+in the interest list to the new settings specified in
+.IR event .
+.TP
+.B EPOLL_CTL_DEL
+Remove (deregister) the target file descriptor
+.I fd
+from the interest list.
+The
+.I event
+argument is ignored and can be NULL (but see BUGS below).
+.P
+The
+.I event
+argument describes the object linked to the file descriptor
+.IR fd .
+The
+.I struct epoll_event
+is described in
+.BR epoll_event (3type).
+.P
+The
+.I data
+member of the
+.I epoll_event
+structure specifies data that the kernel should save and then return (via
+.BR epoll_wait (2))
+when this file descriptor becomes ready.
+.P
+The
+.I events
+member of the
+.I epoll_event
+structure is a bit mask composed by ORing together zero or more event types,
+returned by
+.BR epoll_wait (2),
+and input flags, which affect its behaviour, but aren't returned.
+The available event types are:
+.TP
+.B EPOLLIN
+The associated file is available for
+.BR read (2)
+operations.
+.TP
+.B EPOLLOUT
+The associated file is available for
+.BR write (2)
+operations.
+.TP
+.BR 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.)
+.TP
+.B EPOLLPRI
+There is an exceptional condition on the file descriptor.
+See the discussion of
+.B POLLPRI
+in
+.BR poll (2).
+.TP
+.B EPOLLERR
+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.
+.IP
+.BR epoll_wait (2)
+will always report for this event; it is not necessary to set it in
+.I events
+when calling
+.BR epoll_ctl ().
+.TP
+.B EPOLLHUP
+Hang up happened on the associated file descriptor.
+.IP
+.BR epoll_wait (2)
+will always wait for this event; it is not necessary to set it in
+.I events
+when calling
+.BR epoll_ctl ().
+.IP
+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.
+.P
+And the available input flags are:
+.TP
+.B EPOLLET
+Requests edge-triggered notification for the associated file descriptor.
+The default behavior for
+.B epoll
+is level-triggered.
+See
+.BR epoll (7)
+for more detailed information about edge-triggered and
+level-triggered notification.
+.TP
+.BR 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
+.BR epoll_wait (2),
+the file descriptor is disabled in the interest list and no other events
+will be reported by the
+.B epoll
+interface.
+The user must call
+.BR epoll_ctl ()
+with
+.B EPOLL_CTL_MOD
+to rearm the file descriptor with a new event mask.
+.TP
+.BR EPOLLWAKEUP " (since Linux 3.5)"
+.\" commit 4d7e30d98939a0340022ccd49325a3d70f7e0238
+If
+.B EPOLLONESHOT
+and
+.B EPOLLET
+are clear and the process has the
+.B 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
+.BR epoll_wait (2)
+until the next call to
+.BR epoll_wait (2)
+on the same
+.BR epoll (7)
+file descriptor,
+the closure of that file descriptor,
+the removal of the event file descriptor with
+.BR EPOLL_CTL_DEL ,
+or the clearing of
+.B EPOLLWAKEUP
+for the event file descriptor with
+.BR EPOLL_CTL_MOD .
+See also BUGS.
+.TP
+.BR EPOLLEXCLUSIVE " (since Linux 4.5)"
+Sets an exclusive wakeup mode for the epoll file descriptor that is being
+attached to the target file descriptor,
+.IR fd .
+When a wakeup event occurs and multiple epoll file descriptors
+are attached to the same target file using
+.BR EPOLLEXCLUSIVE ,
+one or more of the epoll file descriptors will receive an event with
+.BR epoll_wait (2).
+The default in this scenario (when
+.B EPOLLEXCLUSIVE
+is not set) is for all epoll file descriptors to receive an event.
+.B EPOLLEXCLUSIVE
+is thus useful for avoiding thundering herd problems in certain scenarios.
+.IP
+If the same file descriptor is in multiple epoll instances,
+some with the
+.B EPOLLEXCLUSIVE
+flag, and others without, then events will be provided to all epoll
+instances that did not specify
+.BR EPOLLEXCLUSIVE ,
+and at least one of the epoll instances that did specify
+.BR EPOLLEXCLUSIVE .
+.IP
+The following values may be specified in conjunction with
+.BR EPOLLEXCLUSIVE :
+.BR EPOLLIN ,
+.BR EPOLLOUT ,
+.BR EPOLLWAKEUP ,
+and
+.BR EPOLLET .
+.B EPOLLHUP
+and
+.B 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
+.IR events .
+Attempts to specify other values in
+.I events
+yield the error
+.BR EINVAL .
+.IP
+.B EPOLLEXCLUSIVE
+may be used only in an
+.B EPOLL_CTL_ADD
+operation; attempts to employ it with
+.B EPOLL_CTL_MOD
+yield an error.
+If
+.B EPOLLEXCLUSIVE
+has been set using
+.BR epoll_ctl (),
+then a subsequent
+.B EPOLL_CTL_MOD
+on the same
+.IR epfd ,\~ fd
+pair yields an error.
+A call to
+.BR epoll_ctl ()
+that specifies
+.B EPOLLEXCLUSIVE
+in
+.I events
+and specifies the target file descriptor
+.I fd
+as an epoll instance will likewise fail.
+The error in all of these cases is
+.BR EINVAL .
+.SH RETURN VALUE
+When successful,
+.BR epoll_ctl ()
+returns zero.
+When an error occurs,
+.BR epoll_ctl ()
+returns \-1 and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+.TP
+.B EBADF
+.I epfd
+or
+.I fd
+is not a valid file descriptor.
+.TP
+.B EEXIST
+.I op
+was
+.BR EPOLL_CTL_ADD ,
+and the supplied file descriptor
+.I fd
+is already registered with this epoll instance.
+.TP
+.B EINVAL
+.I epfd
+is not an
+.B epoll
+file descriptor,
+or
+.I fd
+is the same as
+.IR epfd ,
+or the requested operation
+.I op
+is not supported by this interface.
+.TP
+.B EINVAL
+An invalid event type was specified along with
+.B EPOLLEXCLUSIVE
+in
+.IR events .
+.TP
+.B EINVAL
+.I op
+was
+.B EPOLL_CTL_MOD
+and
+.I events
+included
+.BR EPOLLEXCLUSIVE .
+.TP
+.B EINVAL
+.I op
+was
+.B EPOLL_CTL_MOD
+and the
+.B EPOLLEXCLUSIVE
+flag has previously been applied to this
+.IR epfd ,\~ fd
+pair.
+.TP
+.B EINVAL
+.B EPOLLEXCLUSIVE
+was specified in
+.I event
+and
+.I fd
+refers to an epoll instance.
+.TP
+.B ELOOP
+.I fd
+refers to an epoll instance and this
+.B 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.
+.TP
+.B ENOENT
+.I op
+was
+.B EPOLL_CTL_MOD
+or
+.BR EPOLL_CTL_DEL ,
+and
+.I fd
+is not registered with this epoll instance.
+.TP
+.B ENOMEM
+There was insufficient memory to handle the requested
+.I op
+control operation.
+.TP
+.B ENOSPC
+The limit imposed by
+.I /proc/sys/fs/epoll/max_user_watches
+was encountered while trying to register
+.RB ( EPOLL_CTL_ADD )
+a new file descriptor on an epoll instance.
+See
+.BR epoll (7)
+for further details.
+.TP
+.B EPERM
+The target file
+.I fd
+does not support
+.BR epoll .
+This error can occur if
+.I fd
+refers to, for example, a regular file or a directory.
+.SH STANDARDS
+Linux.
+.SH HISTORY
+Linux 2.6,
+.\" To be precise: Linux 2.5.44.
+.\" The interface should be finalized by Linux 2.5.66.
+glibc 2.3.2.
+.SH NOTES
+The
+.B epoll
+interface supports all file descriptors that support
+.BR poll (2).
+.SH BUGS
+Before Linux 2.6.9, the
+.B EPOLL_CTL_DEL
+operation required a non-null pointer in
+.IR event ,
+even though this argument is ignored.
+Since Linux 2.6.9,
+.I event
+can be specified as NULL
+when using
+.BR EPOLL_CTL_DEL .
+Applications that need to be portable to kernels before Linux 2.6.9
+should specify a non-null pointer in
+.IR event .
+.P
+If
+.B EPOLLWAKEUP
+is specified in
+.IR flags ,
+but the caller does not have the
+.B CAP_BLOCK_SUSPEND
+capability, then the
+.B EPOLLWAKEUP
+flag is
+.IR "silently ignored" .
+This unfortunate behavior is necessary because no validity
+checks were performed on the
+.I flags
+argument in the original implementation, and the addition of the
+.B EPOLLWAKEUP
+with a check that caused the call to fail if the caller did not have the
+.B CAP_BLOCK_SUSPEND
+capability caused a breakage in at least one existing user-space
+application that happened to randomly (and uselessly) specify this bit.
+.\" commit a8159414d7e3af7233e7a5a82d1c5d85379bd75c (behavior change)
+.\" https://lwn.net/Articles/520198/
+A robust application should therefore double check that it has the
+.B CAP_BLOCK_SUSPEND
+capability if attempting to use the
+.B EPOLLWAKEUP
+flag.
+.SH SEE ALSO
+.BR epoll_create (2),
+.BR epoll_wait (2),
+.BR poll (2),
+.BR epoll (7)