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+.\"     $NetBSD: eventfd.2,v 1.2 2021/09/23 13:16:13 uwe Exp $
+.\"
+.\" Copyright (c) 2021 The NetBSD Foundation, Inc.
+.\" All rights reserved.
+.\"
+.\" This code is derived from software contributed to The NetBSD Foundation
+.\" by Jason R. Thorpe.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\" notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\" notice, this list of conditions and the following disclaimer in the
+.\" documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
+.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
+.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+.\" POSSIBILITY OF SUCH DAMAGE.
+.\"
+.Dd September 17, 2021
+.Dt EVENTFD 2
+.Os
+.\"
+.\"
+.Sh NAME
+.Nm eventfd ,
+.Nm eventfd_read ,
+.Nm eventfd_write
+.Nd create and interact with a counting event descriptor
+.\"
+.\"
+.Sh SYNOPSIS
+.In sys/eventfd.h
+.Ft int
+.Fn eventfd "unsigned int val" "int flags"
+.Ft int
+.Fn eventfd_read "int efd" "eventfd_t *valp"
+.Ft int
+.Fn eventfd_write "int efd" "eventfd_t val"
+.\"
+.\"
+.Sh DESCRIPTION
+The
+.Nm
+interface presents a simple counting object associated with a file descriptor.
+Writes and reads to this file descriptor increment and decrement the count,
+respectively.
+When the object's value is non-zero, the file descriptor is considered
+.Dq readable ,
+and when the count is less than the maximum value
+.Li UINT64_MAX\^-\^1
+it is considered
+.Dq writable .
+When an
+.Nm
+object is no longer needed, it may be disposed of using
+.Xr close 2 .
+.Pp
+All I/O to an
+.Nm
+object is 8\~bytes in length, which is the space required to store an
+unsigned 64-bit integer.
+Any read or write with a buffer smaller than 8\~bytes will fail with
+.Er EINVAL .
+Only the first 8\~bytes of the buffer will be used.
+.Pp
+The
+.Fn eventfd
+function creates a new counting event object and returns a file descriptor
+representing that object.
+The initial value of the object is specified by the
+.Fa val
+argument.
+The following flags define the behavior of the resulting object:
+.Bl -tag -width Dv
+.It Dv EFD_CLOEXEC
+This is an alias for the
+.Dv O_CLOEXEC
+flag; see
+.Xr open 2
+for more information.
+.It Dv EFD_NONBLOCK
+This is an alias for the
+.Dv O_NONBLOCK
+flag; see
+.Xr open 2
+for more information.
+.It Dv EFD_SEMAPHORE
+Creates a
+.Dq semaphore mode
+object; see below for details.
+.El
+.Pp
+Reads from an
+.Nm
+object return an unsigned 64-bit integer in the caller's buffer.
+The semantics of this value are dependent on whether the
+.Nm
+object was created in
+.Dq semaphore mode :
+.Bl -bullet
+.It
+If the
+.Nm
+object was created in
+.Dq semaphore mode ,
+reads return the value\~1
+and object's counter is decremented by\~1.
+.It
+If the
+.Nm
+object was not created in
+.Dq semaphore mode ,
+reads return the current value of the object's counter
+and reset the counter to\~0.
+.El
+.Pp
+If the value of the
+.Nm
+object's counter is\~0,
+then reads will block, unless the
+.Nm
+object is set for non-blocking I/O.
+.Pp
+Writing to an
+.Nm
+object adds the unsigned 64-bit value provided in the caller's buffer
+to the
+.Nm
+object's counter.
+If adding the specified value would exceed the maximum value, then the
+write will block, unless the
+.Nm
+object is set for non-blocking I/O.
+.Pp
+The convenience functions
+.Fn eventfd_read
+and
+.Fn eventfd_write
+are provided to simplify interacting with
+.Nm
+objects, and are simply wrappers around the
+.Xr read 2
+and
+.Xr write 2
+system calls:
+.Bl -tag -width Fn
+.It Fn eventfd_read efd valp
+Reads the unsigned 64-bit integer value of the
+.Nm
+object and returns it in
+.Fa valp .
+.It Fn eventfd_write efd val
+Writes the unsigned 64-bit integer value
+.Fa val
+to the
+.Nm
+object.
+.El
+.\"
+.\"
+.Sh RETURN VALUES
+The
+.Fn eventfd
+system call returns\~\-1 if an error occurs,
+otherwise the return value is a descriptor representing the
+.Nm
+object.
+.Pp
+.Rv -std eventfd_read eventfd_write
+.\"
+.\"
+.Sh ERRORS
+The
+.Fn eventfd
+system call fails if:
+.Bl -tag -width Er
+.It Bq Er EINVAL
+Flags other than
+.Dv EFD_CLOEXEC ,
+.Dv EFD_NONBLOCK ,
+and
+.Dv EFD_SEMAPHORE
+are set in the
+.Fa flags
+argument.
+.It Bq Er EMFILE
+The per-process descriptor table is full.
+.It Bq Er ENFILE
+The system file table is full.
+.El
+.Pp
+The
+.Fn eventfd_read
+function fails if:
+.Bl -tag -width Er
+.It Bq Er EAGAIN
+The value of the
+.Nm
+object is\~0 and the
+.Nm
+object is set for non-blocking I/O.
+.El
+.Pp
+The
+.Fn eventfd_write
+function fails if:
+.Bl -tag -width Er
+.It Bq Er EAGAIN
+The resulting value of the
+.Nm
+object after adding the value
+.Fa val
+would exceed the maximum value
+.Li UINT64_MAX\^-\^1
+and the
+.Nm
+object is set for non-blocking I/O.
+.It Bq Er EINVAL
+An attempt was made to write a value greater than the maximum value.
+.El
+.Pp
+In addition to the errors returned by
+.Fn eventfd_read
+and
+.Fn eventfd_write ,
+a read from or write to an
+.Nm
+object fails if:
+.Bl -tag -width Er
+.It Bq Er EINVAL
+The size of the buffer is less than 8\~bytes
+.Pq the size required to hold an unsigned 64-bit integer .
+.El
+.\"
+.\"
+.Sh SEE ALSO
+.Xr close 2 ,
+.Xr kevent 2 ,
+.Xr open 2 ,
+.Xr poll 2 ,
+.Xr read 2 ,
+.Xr select 2 ,
+.Xr write 2
+.\"
+.\"
+.Sh HISTORY
+The
+.Nm
+interface first appeared in
+.Nx 10 .
+It is compatible with the
+.Nm
+interface that appeared in Linux 2.6.30.