.\"     $NetBSD: eventfd.2,v 1.2 2021/09/23 13:16:13 uwe Exp $
.\"
.\" Copyright (c) 2021 The NetBSD Foundation, Inc.
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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Jason R. Thorpe.
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.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.