mirror of https://github.com/mkerrisk/man-pages
320 lines
7.4 KiB
Groff
320 lines
7.4 KiB
Groff
.\" Copyright (C) 2003 Davide Libenzi
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.\" Davide Libenzi <davidel@xmailserver.org>
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.\"
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.\" %%%LICENSE_START(GPLv2+_SW_3_PARA)
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.\" This program is free software; you can redistribute it and/or modify
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.\" it under the terms of the GNU General Public License as published by
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.\" the Free Software Foundation; either version 2 of the License, or
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.\" (at your option) any later version.
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.\"
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.\" This program is distributed in the hope that it will be useful,
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.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
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.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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.\" GNU General Public License for more details.
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.\"
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.\" You should have received a copy of the GNU General Public
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.\" License along with this manual; if not, see
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.\" <http://www.gnu.org/licenses/>.
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.\" %%%LICENSE_END
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.\"
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.TH EPOLL_CTL 2 2015-05-07 "Linux" "Linux Programmer's Manual"
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.SH NAME
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epoll_ctl \- control interface for an epoll file descriptor
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.SH SYNOPSIS
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.B #include <sys/epoll.h>
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.sp
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.BI "int epoll_ctl(int " epfd ", int " op ", int " fd \
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", struct epoll_event *" event );
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.SH DESCRIPTION
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This system call performs control operations on the
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.BR epoll (7)
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instance
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referred to by the file descriptor
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.IR epfd .
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It requests that the operation
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.I op
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be performed for the target file descriptor,
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.IR fd .
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Valid values for the
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.I op
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argument are:
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.TP
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.B EPOLL_CTL_ADD
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Register the target file descriptor
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.I fd
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on the
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.B epoll
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instance referred to by the file descriptor
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.I epfd
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and associate the event
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.I event
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with the internal file linked to
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.IR fd .
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.TP
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.B EPOLL_CTL_MOD
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Change the event
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.I event
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associated with the target file descriptor
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.IR fd .
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.TP
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.B EPOLL_CTL_DEL
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Remove (deregister) the target file descriptor
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.I fd
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from the
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.B epoll
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instance referred to by
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.IR epfd .
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The
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.I event
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is ignored and can be NULL (but see BUGS below).
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.PP
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The
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.I event
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argument describes the object linked to the file descriptor
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.IR fd .
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The
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.I struct epoll_event
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is defined as:
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.sp
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.in +4n
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.nf
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typedef union epoll_data {
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void *ptr;
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int fd;
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uint32_t u32;
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uint64_t u64;
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} epoll_data_t;
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struct epoll_event {
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uint32_t events; /* Epoll events */
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epoll_data_t data; /* User data variable */
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};
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.fi
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.in
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The
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.I events
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member is a bit mask composed using the following available event
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types:
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.TP
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.B EPOLLIN
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The associated file is available for
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.BR read (2)
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operations.
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.TP
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.B EPOLLOUT
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The associated file is available for
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.BR write (2)
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operations.
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.TP
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.BR EPOLLRDHUP " (since Linux 2.6.17)"
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Stream socket peer closed connection,
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or shut down writing half of connection.
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(This flag is especially useful for writing simple code to detect
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peer shutdown when using Edge Triggered monitoring.)
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.TP
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.B EPOLLPRI
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There is urgent data available for
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.BR read (2)
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operations.
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.TP
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.B EPOLLERR
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Error condition happened on the associated file descriptor.
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.BR epoll_wait (2)
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will always wait for this event; it is not necessary to set it in
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.IR events .
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.TP
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.B EPOLLHUP
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Hang up happened on the associated file descriptor.
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.BR epoll_wait (2)
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will always wait for this event; it is not necessary to set it in
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.IR events .
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Note that when reading from a channel such as a pipe or a stream socket,
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this event merely indicates that the peer closed its end of the channel.
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Subsequent reads from the channel will return 0 (end of file)
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only after all outstanding data in the channel has been consumed.
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.TP
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.B EPOLLET
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Sets the Edge Triggered behavior for the associated file descriptor.
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The default behavior for
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.B epoll
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is Level Triggered.
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See
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.BR epoll (7)
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for more detailed information about Edge and Level Triggered event
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distribution architectures.
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.TP
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.BR EPOLLONESHOT " (since Linux 2.6.2)"
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Sets the one-shot behavior for the associated file descriptor.
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This means that after an event is pulled out with
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.BR epoll_wait (2)
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the associated file descriptor is internally disabled and no other events
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will be reported by the
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.B epoll
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interface.
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The user must call
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.BR epoll_ctl ()
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with
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.B EPOLL_CTL_MOD
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to rearm the file descriptor with a new event mask.
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.TP
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.BR EPOLLWAKEUP " (since Linux 3.5)"
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.\" commit 4d7e30d98939a0340022ccd49325a3d70f7e0238
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If
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.B EPOLLONESHOT
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and
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.B EPOLLET
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are clear and the process has the
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.B CAP_BLOCK_SUSPEND
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capability,
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ensure that the system does not enter "suspend" or
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"hibernate" while this event is pending or being processed.
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The event is considered as being "processed" from the time
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when it is returned by a call to
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.BR epoll_wait (2)
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until the next call to
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.BR epoll_wait (2)
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on the same
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.BR epoll (7)
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file descriptor,
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the closure of that file descriptor,
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the removal of the event file descriptor with
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.BR EPOLL_CTL_DEL ,
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or the clearing of
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.B EPOLLWAKEUP
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for the event file descriptor with
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.BR EPOLL_CTL_MOD .
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See also BUGS.
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.SH RETURN VALUE
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When successful,
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.BR epoll_ctl ()
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returns zero.
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When an error occurs,
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.BR epoll_ctl ()
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returns \-1 and
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.I errno
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is set appropriately.
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.SH ERRORS
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.TP
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.B EBADF
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.I epfd
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or
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.I fd
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is not a valid file descriptor.
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.TP
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.B EEXIST
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.I op
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was
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.BR EPOLL_CTL_ADD ,
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and the supplied file descriptor
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.I fd
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is already registered with this epoll instance.
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.TP
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.B EINVAL
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.I epfd
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is not an
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.B epoll
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file descriptor,
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or
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.I fd
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is the same as
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.IR epfd ,
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or the requested operation
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.I op
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is not supported by this interface.
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.TP
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.B ENOENT
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.I op
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was
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.B EPOLL_CTL_MOD
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or
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.BR EPOLL_CTL_DEL ,
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and
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.I fd
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is not registered with this epoll instance.
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.TP
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.B ENOMEM
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There was insufficient memory to handle the requested
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.I op
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control operation.
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.TP
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.B ENOSPC
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The limit imposed by
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.I /proc/sys/fs/epoll/max_user_watches
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was encountered while trying to register
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.RB ( EPOLL_CTL_ADD )
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a new file descriptor on an epoll instance.
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See
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.BR epoll (7)
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for further details.
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.TP
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.B EPERM
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The target file
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.I fd
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does not support
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.BR epoll .
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This error can occur if
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.I fd
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refers to, for example, a regular file or a directory.
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.SH VERSIONS
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.BR epoll_ctl ()
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was added to the kernel in version 2.6.
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.\" To be precise: kernel 2.5.44.
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.\" The interface should be finalized by Linux kernel 2.5.66.
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.SH CONFORMING TO
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.BR epoll_ctl ()
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is Linux-specific.
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Library support is provided in glibc starting with version 2.3.2.
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.SH NOTES
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The
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.B epoll
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interface supports all file descriptors that support
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.BR poll (2).
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.SH BUGS
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In kernel versions before 2.6.9, the
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.B EPOLL_CTL_DEL
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operation required a non-null pointer in
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.IR event ,
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even though this argument is ignored.
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Since Linux 2.6.9,
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.I event
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can be specified as NULL
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when using
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.BR EPOLL_CTL_DEL .
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Applications that need to be portable to kernels before 2.6.9
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should specify a non-null pointer in
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.IR event .
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If
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.B EPOLLWAKEUP
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is specified in
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.IR flags ,
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but the caller does not have the
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.BR CAP_BLOCK_SUSPEND
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capability, then the
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.B EPOLLWAKEUP
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flag is
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.IR "silently ignored" .
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This unfortunate behavior is necessary because no validity
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checks were performed on the
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.IR flags
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argument in the original implementation, and the addition of the
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.B EPOLLWAKEUP
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with a check that caused the call to fail if the caller did not have the
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.B CAP_BLOCK_SUSPEND
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capability caused a breakage in at least one existing user-space
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application that happened to randomly (and uselessly) specify this bit.
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.\" commit a8159414d7e3af7233e7a5a82d1c5d85379bd75c (behavior change)
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.\" https://lwn.net/Articles/520198/
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A robust application should therefore double check that it has the
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.B CAP_BLOCK_SUSPEND
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capability if attempting to use the
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.B EPOLLWAKEUP
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flag.
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.SH SEE ALSO
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.BR epoll_create (2),
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.BR epoll_wait (2),
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.BR poll (2),
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.BR epoll (7)
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