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https://github.com/freebsd/freebsd-src
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8269e7673c
Remove core system call implementations and documentation to lib/libsys and lib/libsys/<arch> from lib/libc/sys and lib/libc/<arch>/<sys>. Update paths to allow libc to find them in their new home. Reviewed by: kib, emaste, imp Pull Request: https://github.com/freebsd/freebsd-src/pull/908
207 lines
5.3 KiB
Groff
207 lines
5.3 KiB
Groff
.\" SPDX-License-Identifier: BSD-2-Clause
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.\"
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.\" Copyright (c) 2020 Val Packett <val@packett.cool>
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.Dd October 8, 2020
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.Dt EVENTFD 2
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.Os
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.Sh NAME
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.Nm eventfd
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.Nd create a file descriptor for event notification
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In sys/eventfd.h
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.Ft int
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.Fn eventfd "unsigned int initval" "int flags"
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.Ft int
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.Fn eventfd_read "int fd" "eventfd_t *value"
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.Ft int
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.Fn eventfd_write "int fd" "eventfd_t value"
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.Sh DESCRIPTION
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.Fn eventfd
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creates a special file descriptor with event counter or semaphore semantics,
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designed for interprocess communication.
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The returned file descriptor refers to a kernel object containing an
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unsigned 64-bit integer counter, which is initialized with the value of the
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.Fa initval
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argument.
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.Pp
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The
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.Fa flags
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argument may contain the result of
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.Em or Ns 'ing
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the following values:
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.Pp
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.Bl -tag -width "EFD_SEMAPHORE" -compact
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.It Dv EFD_CLOEXEC
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set FD_CLOEXEC on the file descriptor
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.It Dv EFD_NONBLOCK
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do not block on read/write operations
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.It Dv EFD_SEMAPHORE
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use semaphore semantics
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.El
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.Pp
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File operations have the following semantics:
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.Bl -tag -width EFD_SEMAPHORE
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.It Xr read 2
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If the counter is zero, the call blocks until the counter becomes non-zero, unless
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.Dv EFD_NONBLOCK
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was set, in which case it would fail with
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.Dv EAGAIN
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instead.
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.Pp
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If the counter is non-zero:
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.Bl -bullet
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.It
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If
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.Dv EFD_SEMAPHORE
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is not set, the current value of the counter is returned,
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and the value is reset to zero.
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.It
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If
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.Dv EFD_SEMAPHORE
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is set, the constant 1 is returned, and the value is decremented by 1.
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.El
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.Pp
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The numeric value is encoded as 64-bit (8 bytes) in host byte order.
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The
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.Xr read 2
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call fails with
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.Dv EINVAL
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if there is less than 8 bytes available in the supplied buffer.
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.It Xr write 2
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Adds the given value to the counter.
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The maximum value that can be stored in the counter is the
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maximum unsigned 64-bit integer value minus one (0xfffffffffffffffe).
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.Pp
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If the resulting value exceeds the maximum, the call would block
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until the value is reduced by
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.Xr read 2 ,
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unless
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.Dv EFD_NONBLOCK
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was set, in which case it would fail with
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.Dv EAGAIN
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instead.
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.Pp
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The numeric value is encoded as 64-bit (8 bytes) in host byte order.
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The
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.Xr write 2
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call fails with
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.Dv EINVAL
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if there is less than 8 bytes available in the supplied buffer,
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or if the value 0xffffffffffffffff is given.
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.It Xr poll 2
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When receiving notifications via
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.Xr poll 2 /
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.Xr ppoll 2 /
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.Xr select 2 /
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.Xr pselect 2 /
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.Xr kqueue 2 ,
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the following semantics apply:
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.Bl -bullet
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.It
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The file descriptor is readable when the counter is greater than zero.
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.It
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The file descriptor is writable when the counter is less than the maximum value.
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.El
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.El
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.Pp
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File descriptors created by
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.Fn eventfd
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are passable to other processes via
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.Xr sendmsg 2
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and are preserved across
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.Xr fork 2 ;
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in both cases the descriptors refer to the same counter from both processes.
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Unless
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.Dv O_CLOEXEC
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flag was specified,
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the created file descriptor will remain open across
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.Xr execve 2
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system calls; see
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.Xr close 2 ,
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.Xr fcntl 2
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and
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.Dv O_CLOEXEC
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description.
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.Pp
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.Fn eventfd_read
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and
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.Fn eventfd_write
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are thin wrappers around
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.Xr read 2
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and
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.Xr write 2
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system calls,
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provided for compatibility with glibc.
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.Sh RETURN VALUES
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If successful,
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.Fn eventfd
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returns a non-negative integer, termed a file descriptor.
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It returns \-1 on failure, and sets
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.Va errno
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to indicate the error.
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.Pp
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The
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.Fn eventfd_read
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and
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.Fn eventfd_write
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functions return 0 if the operation succeeded, -1 otherwise.
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.Sh ERRORS
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.Fn eventfd
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may fail with:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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The
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.Fa flags
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argument given to
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.Fn eventfd
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has unknown bits set.
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.It Bq Er EMFILE
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The process has already reached its limit for open
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file descriptors.
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.It Bq Er ENFILE
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The system file table is full.
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.It Bq Er ENOMEM
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No memory was available to create the kernel object.
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.El
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.Sh SEE ALSO
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.Xr close 2 ,
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.Xr kqueue 2 ,
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.Xr poll 2 ,
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.Xr read 2 ,
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.Xr select 2 ,
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.Xr write 2
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.Sh STANDARDS
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The
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.Fn eventfd
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system call is non-standard.
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It is present in Linux.
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.Sh HISTORY
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The
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.Fn eventfd
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system call first appeared in
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.Fx 13.0 .
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