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.\"     @(#)ioctl.2	8.2 (Berkeley) 12/11/93
.\"
.Dd November 6, 2004
.Dt IOCTL 2
.Os
.Sh NAME
.Nm ioctl
.Nd control device
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/ioctl.h
.Ft int
.Fn ioctl "int d" "unsigned long request" "void *argp"
.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.
The argument
.Fa d
must be an open file descriptor.
.Pp
An  ioctl
.Fa request
has encoded in it whether the argument is an
.Dq in
parameter
or
.Dq out
parameter, and the size of the argument
.Fa argp
in bytes.
Macros and defines used in specifying an ioctl
.Fa request
are located in the file
.Ao Pa sys/ioctl.h Ac .
.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 exec 3
is invoked.
.It Dv FIONCLEX
Clear close-on-exec flag.
The file will remain open across
.Xr exec 3 .
.El
.Pp
Some generic ioctls are not implemented for all types of file
descriptors.
These include:
.Bl -tag -width "xxxxxx"
.It Dv FIONREAD "int"
Get the number of bytes that are immediately available for reading.
.It Dv FIONWRITE "int"
Get the number of bytes in the descriptor's send queue.
These bytes are data which has been written to the descriptor but
which are being held by the kernel for further processing.
The nature of the required processing depends on the underlying device.
For tty devices, these bytes are typically queued for delivery
to the tty hardware.
For TCP sockets, these bytes have not yet been acknolwedged by the
other side of the connection.
For files, this operation always returns zero as files do not have
send queues.
.It Dv FIONSPACE "int"
Get the free space in the descriptor's send queue.
This value is the size of the send queue minus the number of bytes
being held in the queue.
Note: while this value represents the number of bytes that may be
added to the queue, other resource limitations may cause a write
not larger than the send queue's space to be blocked.
One such limitation would be a lack of network buffers for a write
to a network connection.
.It Dv FIONBIO "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 "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 "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 argp
is not valid.
.It Bq Er EFAULT
.Fa argp
points outside the process's allocated address space.
.El
.Sh SEE ALSO
.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 .