.\" Copyright (C) 2003 Davide Libenzi
.\" Davide Libenzi <davidel@xmailserver.org>
+.\" and Copyright 2009, 2014, 2016, 2018, 2019 Michael Kerrisk <tk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(GPLv2+_SW_3_PARA)
.\" This program is free software; you can redistribute it and/or modify
.\" <http://www.gnu.org/licenses/>.
.\" %%%LICENSE_END
.\"
-.TH EPOLL_CTL 2 2015-05-07 "Linux" "Linux Programmer's Manual"
+.TH EPOLL_CTL 2 2020-04-11 "Linux" "Linux Programmer's Manual"
.SH NAME
epoll_ctl \- control interface for an epoll file descriptor
.SH SYNOPSIS
.B #include <sys/epoll.h>
-.sp
+.PP
.BI "int epoll_ctl(int " epfd ", int " op ", int " fd \
", struct epoll_event *" event );
.SH DESCRIPTION
-This system call performs control operations on the
+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
.I op
be performed for the target file descriptor,
.IR fd .
-
+.PP
Valid values for the
.I op
argument are:
.TP
.B EPOLL_CTL_ADD
-Register the target file descriptor
-.I fd
-on the
-.B epoll
-instance referred to by the file descriptor
-.I epfd
-and associate the event
-.I event
-with the internal file linked to
-.IR fd .
+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 event
-.I event
-associated with the target file descriptor
-.IR fd .
+Change the settings associated with
+.IR 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
-.B epoll
-instance referred to by
-.IR epfd .
+from the interest list.
The
.I event
-is ignored and can be NULL (but see BUGS below).
+argument is ignored and can be NULL (but see BUGS below).
.PP
The
.I event
The
.I struct epoll_event
is defined as:
-.sp
+.PP
.in +4n
-.nf
+.EX
typedef union epoll_data {
void *ptr;
int fd;
uint32_t events; /* Epoll events */
epoll_data_t data; /* User data variable */
};
-.fi
+.EE
.in
-
+.PP
+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.
+.PP
The
.I events
-member is a bit mask composed using the following available event
-types:
+member of the
+.I epoll_event
+structure is a bit mask composed by ORing together zero or more of
+the following available event types:
.TP
.B EPOLLIN
The associated file is available for
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.)
+peer shutdown when using edge-triggered monitoring.)
.TP
.B EPOLLPRI
-There is urgent data available for
-.BR read (2)
-operations.
+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 wait for this event; it is not necessary to set it in
-.IR events .
+will always report for this event; it is not necessary to set it in
+.IR 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
-.IR events .
+.IR 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.
.TP
.B EPOLLET
-Sets the Edge Triggered behavior for the associated file descriptor.
+Requests edge-triggered notification for the associated file descriptor.
The default behavior for
.B epoll
-is Level Triggered.
+is level-triggered.
See
.BR epoll (7)
-for more detailed information about Edge and Level Triggered event
-distribution architectures.
+for more detailed information about edge-triggered and
+level-triggered notification.
+.IP
+This flag is an input flag for the
+.I event.events
+field when calling
+.BR epoll_ctl ();
+it is never returned by
+.BR epoll_wait (2).
.TP
.BR EPOLLONESHOT " (since Linux 2.6.2)"
-Sets the one-shot behavior for the associated file descriptor.
-This means that after an event is pulled out with
-.BR epoll_wait (2)
-the associated file descriptor is internally disabled and no other events
+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.
with
.B EPOLL_CTL_MOD
to rearm the file descriptor with a new event mask.
+.IP
+This flag is an input flag for the
+.I event.events
+field when calling
+.BR epoll_ctl ();
+it is never returned by
+.BR epoll_wait (2).
.TP
.BR EPOLLWAKEUP " (since Linux 3.5)"
.\" commit 4d7e30d98939a0340022ccd49325a3d70f7e0238
for the event file descriptor with
.BR EPOLL_CTL_MOD .
See also BUGS.
+.IP
+This flag is an input flag for the
+.I event.events
+field when calling
+.BR epoll_ctl ();
+it is never returned by
+.BR epoll_wait (2).
.TP
.BR EPOLLEXCLUSIVE " (since Linux 4.5)"
Sets an exclusive wakeup mode for the epoll file descriptor that is being
is not set) is for all epoll file descriptors to receive an event.
.BR 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
+some with the
.BR EPOLLEXCLUSIVE
-flag, and others without, then events will provided to all epoll
+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,
+.BR EPOLLWAKEUP ,
and
.BR EPOLLET .
.BR EPOLLHUP
and
.BR EPOLLERR
-can also be specified, but are ignored (as usual).
+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 an error.
+yield the error
+.BR EINVAL .
+.IP
.B EPOLLEXCLUSIVE
may be used only in an
.B EPOLL_CTL_ADD
yield an error.
If
.B EPOLLEXCLUSIVE
-has set using
-.BR epoll_ctl (2),
+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 (2)
+.BR epoll_ctl ()
that specifies
.B EPOLLEXCLUSIVE
in
as an epoll instance will likewise fail.
The error in all of these cases is
.BR EINVAL .
+.IP
+The
+.BR EPOLLEXCLUSIVE
+flag is an input flag for the
+.I event.events
+field when calling
+.BR epoll_ctl ();
+it is never returned by
+.BR epoll_wait (2).
.SH RETURN VALUE
When successful,
.BR epoll_ctl ()
.I op
is not supported by this interface.
.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.
-.TP
.B EINVAL
An invalid event type was specified along with
.B EPOLLEXCLUSIVE
.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.
+.TP
.B ENOENT
.I op
was
Applications that need to be portable to kernels before 2.6.9
should specify a non-null pointer in
.IR event .
-
+.PP
If
.B EPOLLWAKEUP
is specified in
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