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.\" Copyright (C) 2003 Davide Libenzi
.\" and Copyright 2008, 2009, 2012 Michael Kerrisk <tk.manpages@gmail.com>
.\" Davide Libenzi <davidel@xmailserver.org>
.\"
.\" SPDX-License-Identifier: GPL-2.0-or-later
.\"
.\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" Modified 2005-04-04 by Marko Kohtala <marko.kohtala@gmail.com>
.\" 2008-10-10, mtk: add description of epoll_create1()
.\"
.TH epoll_create 2 2023-07-16 "Linux man-pages 6.05.01"
.SH NAME
epoll_create, epoll_create1 \- open an epoll file descriptor
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sys/epoll.h>
.PP
.BI "int epoll_create(int " size );
.BI "int epoll_create1(int " flags );
.fi
.SH DESCRIPTION
.BR epoll_create ()
creates a new
.BR epoll (7)
instance.
Since Linux 2.6.8, the
.I size
argument is ignored, but must be greater than zero; see HISTORY.
.PP
.BR epoll_create ()
returns a file descriptor referring to the new epoll instance.
This file descriptor is used for all the subsequent calls to the
.B epoll
interface.
When no longer required, the file descriptor returned by
.BR epoll_create ()
should be closed by using
.BR close (2).
When all file descriptors referring to an epoll instance have been closed,
the kernel destroys the instance
and releases the associated resources for reuse.
.SS epoll_create1()
If
.I flags
is 0, then, other than the fact that the obsolete
.I size
argument is dropped,
.BR epoll_create1 ()
is the same as
.BR epoll_create ().
The following value can be included in
.I flags
to obtain different behavior:
.TP
.B EPOLL_CLOEXEC
Set the close-on-exec
.RB ( FD_CLOEXEC )
flag on the new file descriptor.
See the description of the
.B O_CLOEXEC
flag in
.BR open (2)
for reasons why this may be useful.
.SH RETURN VALUE
On success,
these system calls
return a file descriptor (a nonnegative integer).
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EINVAL
.I size
is not positive.
.TP
.B EINVAL
.RB ( epoll_create1 ())
Invalid value specified in
.IR flags .
.TP
.B EMFILE
The per-process limit on the number of open file descriptors has been reached.
.TP
.B ENFILE
The system-wide limit on the total number of open files has been reached.
.TP
.B ENOMEM
There was insufficient memory to create the kernel object.
.SH STANDARDS
Linux.
.SH HISTORY
.TP
.BR epoll_create ()
Linux 2.6,
glibc 2.3.2.
.\" To be precise: kernel 2.5.44.
.\" The interface should be finalized by Linux kernel 2.5.66.
.TP
.BR epoll_create1 ()
Linux 2.6.27,
glibc 2.9.
.PP
In the initial
.BR epoll_create ()
implementation, the
.I size
argument informed the kernel of the number of file descriptors
that the caller expected to add to the
.B epoll
instance.
The kernel used this information as a hint for the amount of
space to initially allocate in internal data structures describing events.
(If necessary, the kernel would allocate more space
if the caller's usage exceeded the hint given in
.IR size .)
Nowadays,
this hint is no longer required
(the kernel dynamically sizes the required data structures
without needing the hint), but
.I size
must still be greater than zero,
in order to ensure backward compatibility when new
.B epoll
applications are run on older kernels.
.PP
Prior to Linux 2.6.29,
.\" commit 9df04e1f25effde823a600e755b51475d438f56b
a
.I /proc/sys/fs/epoll/max_user_instances
kernel parameter limited live epolls for each real user ID,
and caused
.BR epoll_create ()
to fail with
.B EMFILE
on overrun.
.SH SEE ALSO
.BR close (2),
.BR epoll_ctl (2),
.BR epoll_wait (2),
.BR epoll (7)
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