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.\"     @(#)ioctl.2	8.2 (Berkeley) 12/11/93
.\"
.Dd September 11, 2013
.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 fd" "unsigned long request" ...
.Sh DESCRIPTION
The
.Fn ioctl
system call 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 fd
must be an open file descriptor.
.Pp
The third argument to
.Fn ioctl
is traditionally named
.Va "char *argp" .
Most uses of
.Fn ioctl ,
however, require the third argument to be a
.Vt caddr_t
or an
.Vt int .
.Pp
An
.Fn ioctl
.Fa request
has encoded in it whether the argument is an
.Dq in
argument
or
.Dq out
argument, 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
.In sys/ioctl.h .
.Sh GENERIC IOCTLS
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 TCP sockets, these bytes have not yet been acknowledged by the
other side of the connection.
.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.
.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
The
.Fn ioctl
system call
will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa fd
argument
is not a valid descriptor.
.It Bq Er ENOTTY
The
.Fa fd
argument
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 fd
references.
.It Bq Er EINVAL
The
.Fa request
or
.Fa argp
argument
is not valid.
.It Bq Er EFAULT
The
.Fa argp
argument
points outside the process's allocated address space.
.El
.Sh SEE ALSO
.Xr execve 2 ,
.Xr fcntl 2 ,
.Xr intro 4 ,
.Xr tty 4
.Sh HISTORY
The
.Fn ioctl
function appeared in
.At v7 .