mirror of https://github.com/mkerrisk/man-pages
217 lines
5.3 KiB
Groff
217 lines
5.3 KiB
Groff
.\"
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.\" epoll by Davide Libenzi ( efficient event notification retrieval )
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.\" Copyright (C) 2003 Davide Libenzi
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.\"
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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 License
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.\" along with this program; if not, write to the Free Software
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.\" Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
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.\"
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.\" Davide Libenzi <davidel@xmailserver.org>
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.\"
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.\" 2007-04-30: mtk, Added description of epoll_pwait()
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.\"
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.TH EPOLL_WAIT 2 2012-08-16 "Linux" "Linux Programmer's Manual"
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.SH NAME
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epoll_wait, epoll_pwait \- wait for an I/O event on an epoll file descriptor
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.SH SYNOPSIS
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.nf
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.B #include <sys/epoll.h>
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.sp
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.BI "int epoll_wait(int " epfd ", struct epoll_event *" events ,
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.BI " int " maxevents ", int " timeout );
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.BI "int epoll_pwait(int " epfd ", struct epoll_event *" events ,
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.BI " int " maxevents ", int " timeout ,
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.BI " const sigset_t *" sigmask );
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.fi
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.SH DESCRIPTION
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The
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.BR epoll_wait ()
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system call waits for events on the
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.BR epoll (7)
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instance referred to by the file descriptor
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.IR epfd .
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The memory area pointed to by
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.I events
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will contain the events that will be available for the caller.
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Up to
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.I maxevents
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are returned by
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.BR epoll_wait ().
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The
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.I maxevents
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argument must be greater than zero.
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The
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.I timeout
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argument specifies the minimum number of milliseconds that
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.BR epoll_wait ()
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will block.
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(This interval will be rounded up to the system clock granularity,
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and kernel scheduling delays mean that the blocking interval
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may overrun by a small amount.)
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Specifying a
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.I timeout
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of \-1 causes
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.BR epoll_wait ()
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to block indefinitely, while specifying a
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.I timeout
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equal to zero cause
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.BR epoll_wait ()
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to return immediately, even if no events are available.
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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 data
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of each returned structure will contain the same data the user set with an
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.BR epoll_ctl (2)
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.RB ( EPOLL_CTL_ADD , EPOLL_CTL_MOD )
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while the
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.I events
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member will contain the returned event bit field.
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.SS epoll_pwait()
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The relationship between
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.BR epoll_wait ()
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and
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.BR epoll_pwait ()
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is analogous to the relationship between
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.BR select (2)
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and
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.BR pselect (2):
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like
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.BR pselect (2),
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.BR epoll_pwait ()
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allows an application to safely wait until either a file descriptor
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becomes ready or until a signal is caught.
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The following
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.BR epoll_pwait ()
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call:
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.nf
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ready = epoll_pwait(epfd, &events, maxevents, timeout, &sigmask);
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.fi
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is equivalent to
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.I atomically
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executing the following calls:
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.nf
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sigset_t origmask;
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sigprocmask(SIG_SETMASK, &sigmask, &origmask);
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ready = epoll_wait(epfd, &events, maxevents, timeout);
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sigprocmask(SIG_SETMASK, &origmask, NULL);
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.fi
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.PP
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The
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.I sigmask
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argument may be specified as NULL, in which case
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.BR epoll_pwait ()
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is equivalent to
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.BR epoll_wait ().
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.SH "RETURN VALUE"
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When successful,
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.BR epoll_wait ()
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returns the number of file descriptors ready for the requested I/O, or zero
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if no file descriptor became ready during the requested
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.I timeout
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milliseconds.
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When an error occurs,
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.BR epoll_wait ()
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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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is not a valid file descriptor.
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.TP
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.B EFAULT
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The memory area pointed to by
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.I events
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is not accessible with write permissions.
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.TP
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.B EINTR
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The call was interrupted by a signal handler before either any of the
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requested events occurred or the
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.I timeout
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expired; see
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.BR signal (7).
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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, or
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.I maxevents
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is less than or equal to zero.
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.SH VERSIONS
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.BR epoll_wait ()
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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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Library support is provided in glibc starting with version 2.3.2.
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.BR epoll_pwait ()
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was added to Linux in kernel 2.6.19.
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Library support is provided in glibc starting with version 2.6.
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.SH CONFORMING TO
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.BR epoll_wait ()
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is Linux-specific.
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.SH NOTES
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While one thread is blocked in a call to
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.BR epoll_pwait (),
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it is possible for another thread to add a file descriptor to the waited-upon
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.B epoll
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instance.
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If the new file descriptor becomes ready,
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it will cause the
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.BR epoll_wait ()
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call to unblock.
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.SH BUGS
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In kernels before 2.6.37, a
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.I timeout
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value larger than approximately
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.I LONG_MAX / HZ
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milliseconds is treated as \-1 (i.e., infinity).
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Thus, for example, on a system where the
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.I sizeof(long)
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is 4 and the kernel
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.I HZ
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value is 1000,
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this means that timeouts greater than 35.79 minutes are treated as infinity.
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.SH "SEE ALSO"
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.BR epoll_create (2),
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.BR epoll_ctl (2),
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.BR epoll (7)
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