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+.\" Copyright (c) 1980, 1991, 1993
+.\"	The Regents of the University of California.  All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 4. Neither the name of the University nor the names of its contributors
+.\"    may be used to endorse or promote products derived from this software
+.\"    without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\"     @(#)ioctl.2	8.2 (Berkeley) 12/11/93
+.\"
+.\" $FreeBSD$
+.\"
+.Dd May 11, 2010
+.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" ...
+.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 d
+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 d
+argument
+is not a valid descriptor.
+.It Bq Er ENOTTY
+The
+.Fa d
+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 d
+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 .