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.\"     @(#)ioctl.2	8.2 (Berkeley) 12/11/93
.\"
.Dd $Mdocdate: August 11 2019 $
.Dt IOCTL 2
.Os
.Sh NAME
.Nm ioctl
.Nd control device
.Sh SYNOPSIS
.In sys/ioctl.h
.Ft int
.Fn ioctl "int d" "unsigned long request" "..."
.Sh DESCRIPTION
The
.Fn ioctl
function manipulates the underlying device parameters of special files.
In particular, many operating
characteristics of character special files (e.g., terminals)
may be controlled with
.Fn ioctl
requests.
.Pp
The argument
.Fa d
must be an open file descriptor.
The third argument is called
.Fa arg
and contains additional information needed by this device
to perform the requested function.
.Fa arg
is either an
.Li int
or a pointer to a device-specific data structure, depending upon
the given
.Fa request .
.Pp
An
.Nm
.Fa request
has encoded in it whether the argument is an
.Dq in
parameter
or
.Dq out
parameter, and the size of the third argument
.Pq Fa arg
in bytes.
Macros and defines used in specifying an ioctl
.Fa request
are located in the file
.In sys/ioctl.h .
.Sh GENERIC IOCTLS
Some ioctls are applicable to any file descriptor.
These include:
.Bl -tag -width "xxxxxx"
.It Dv FIOCLEX
Set close-on-exec flag.
The file will be closed when
.Xr execve 2
is invoked.
.It Dv FIONCLEX
Clear close-on-exec flag.
The file will remain open across
.Xr execve 2 .
.El
.Pp
Some generic ioctls are not implemented for all types of file
descriptors.
These include:
.Bl -tag -width "xxxxxx"
.It Dv FIONREAD Fa "int *"
Get the number of bytes that are immediately available for reading.
.It Dv FIONBIO Fa "int *"
Set non-blocking I/O mode if the argument is non-zero.
In non-blocking mode,
.Xr read 2
or
.Xr write 2
calls return \-1 and set
.Va errno
to
.Er EAGAIN
immediately when no data is available.
.It Dv FIOASYNC Fa "int *"
Set asynchronous I/O mode if the argument is non-zero.
In asynchronous mode, the process or process group specified by
.Dv FIOSETOWN
will start receiving
.Dv SIGIO
signals when data is available.
The
.Dv SIGIO
signal will be delivered when data is available on the file
descriptor.
.It Dv FIOSETOWN, FIOGETOWN Fa "int *"
Set/get the process or the process group (if negative) that should receive
.Dv SIGIO
signals when data is available.
.El
.Sh RETURN VALUES
If an error has occurred, a value of \-1 is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn ioctl
will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
.Fa d
is not a valid descriptor.
.It Bq Er ENOTTY
.Fa d
is not associated with a character
special device.
.It Bq Er ENOTTY
The specified request does not apply to the kind
of object that the descriptor
.Fa d
references.
.It Bq Er EINVAL
.Fa request
or
.Fa arg
is not valid.
.It Bq Er EFAULT
.Fa arg
points outside the process's allocated address space.
.El
.Sh SEE ALSO
.Xr cdio 1 ,
.Xr chio 1 ,
.Xr mt 1 ,
.Xr execve 2 ,
.Xr fcntl 2 ,
.Xr intro 4 ,
.Xr tty 4
.Sh HISTORY
An
.Fn ioctl
function call appeared in
.At v7 .