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.Dd August 10, 2021
.Dt EVENTFD 3C
.Os
.Sh NAME
.Nm eventfd ,
.Nm eventfd_read ,
.Nm eventfd_write
.Nd create a file descriptor for event notification
.Sh SYNOPSIS
.In sys/eventfd.h
.Ft int
.Fo eventfd
.Fa "unsigned int initval"
.Fa "int flags"
.Fc
.Ft int
.Fo eventfd_read
.Fa "int fd"
.Fa "eventfd_t *valp"
.Fc
.Ft int
.Fo eventfd_write
.Fa "int fd"
.Fa "eventfd_t val"
.Fc
.Sh DESCRIPTION
The
.Fn eventfd
function creates an
.Xr eventfd 5
instance that has an associated 64-bit unsigned counter.
It returns a file descriptor that can be operated upon via
.Xr read 2 ,
.Xr write 2
and the facilities that notify of file descriptor activity
.Po e.g. ,
.Xr poll 2 ,
.Xr port_get 3C ,
.Xr epoll_wait 3C Ns
.Pc .
To dispose of the instance,
.Xr close 2
should be called on the file descriptor.
.Pp
The
.Fa initval
argument specifies the initial value of the 64-bit counter associated with the
instance.
.Po
Note that this limits the initial value to be a 32-bit quantity despite the
fact that the underlying counter is 64-bit
.Pc .
.Pp
The
.Fa flags
argument specifies additional parameters for the instance, and can have any of
the following values:
.Bl -tag -width Ds
.It Dv EFD_CLOEXEC
Instance will be closed upon an
.Xr exec 2 ;
see
.Xr open 2 Ns 's
description of
.Dv O_CLOEXEC .
.It Dv EFD_NONBLOCK
Instance will be set to be non-blocking.
A
.Xr read 2
on an
.Nm eventfd
instance that has been initialized with
.Dv EFD_NONBLOCK
will return
.Er EAGAIN
in lieu of blocking if the count associated with the instance is zero.
.It Dv EFD_SEMAPHORE
Provide counting semaphore semantics whereby a
.Xr read 2
will atomically decrement rather than atomically clear the count when it
becomes non-zero.
See below for details on
.Xr read 2
semantics.
.El
.Pp
The following operations can be performed upon an
.Nm eventfd
instance:
.Bl -tag -width Ds
.It Xr read 2
Atomically reads and modifies the value of the 64-bit counter associated
with the instance.
The precise semantics of
.Xr read 2
depend on the disposition of
.Dv EFD_SEMAPHORE
with
respect to the instance: if
.Dv EFD_SEMAPHORE
was set when the instance was created,
.Xr read 2
will
.Em atomically decrement
the counter if
.Pq and when
it is non-zero, copying the value 1 to the eight byte buffer passed to the
system call; if
.Dv EFD_SEMAPHORE
was not set,
.Xr read 2
will
.Em atomically clear
the counter if
.Pq and when
it is non-zero, copying the former value of the counter to the eight byte
buffer passed to the system call.
In either case,
.Xr read 2
will block if the counter is
zero
.Po or return
.Er EAGAIN
if the instance was created with
.Dv EFD_NONBLOCK
.Pc .
If the buffer specified to
.Xr read 2
is less than
eight bytes in length,
.Er EINVAL
will be returned.
.It Xr write 2
Atomically adds the 64-bit value pointed to by the buffer to the 64-bit
counter associated with the instance.
If the resulting value would overflow, the
.Xr write 2
will block until the value would not overflow
.Po or return
.Er EAGAIN
if the instance was created with
.Dv EFD_NONBLOCK
.Pc .
If the buffer specified to
.Xr write 2
is less than eight bytes in length,
.Er EINVAL
will be returned.
.It Xo
.Xr poll 2 ,
.Xr port_get 3C ,
.Xr  epoll_wait 3C
.Xc
Provide notification when the 64-bit counter associated
with the instance is ready for reading or writing, as specified.
If the 64-bit value associated with the instance is non-zero,
.Dv POLLIN
and
.Dv POLLRDNORM
will be set; if the value 1 can be added the value
without blocking,
.Dv POLLOUT
and
.Dv POLLWRNORM
will be set.
.El
.Pp
.Fn eventfd_read
and
.Fn eventfd_write
are provided for compatibility with
.Sy glibc
and are wrappers around
.Xr read 2
and
.Xr write 2 ,
respectively.
They return
.Sy 0
if the correct number of bytes was transferred and
.Sy -1
otherwise.
These functions may return
.Sy -1
without setting
.Va errno .
.Sh RETURN VALUES
Upon successful completion,
.Fn eventfd
returns a file descriptor associated with the instance.
Otherwise,
.Sy -1
is returned and
.Va errno
is set to indicate the error.
.Pp
Upon successful completion,
.Fn eventfd_read
and
.Fn eventfd_write
return
.Sy 0 .
Otherwise,
.Sy -1
is returned.
.Sh ERRORS
The
.Fn eventfd
function will fail if:
.Bl -tag -width Er
.It Er EINVAL
The
.Fa flags
are invalid.
.It Er EMFILE
There are currently
.Pf { Sy OPEN_MAX Ns }
file descriptors open in the calling process.
.El
.Sh SEE ALSO
.Xr poll 2 ,
.Xr read 2 ,
.Xr write 2 ,
.Xr epoll_wait 3C ,
.Xr port_get 3C ,
.Xr eventfd 5