mirror of https://github.com/mkerrisk/man-pages
211 lines
6.2 KiB
Groff
211 lines
6.2 KiB
Groff
.\" Copyright (c) 1980, 1991 Regents of the University of California.
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.\" All rights reserved.
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.\"
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.\" %%%LICENSE_START(BSD_4_CLAUSE_UCB)
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\" %%%LICENSE_END
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.\"
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.\" @(#)ioctl.2 6.4 (Berkeley) 3/10/91
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.\"
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.\" Modified 1993-07-23 by Rik Faith <faith@cs.unc.edu>
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.\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>
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.\" Modified 1999-06-25 by Rachael Munns <vashti@dream.org.uk>
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.\" Modified 2000-09-21 by Andries Brouwer <aeb@cwi.nl>
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.\"
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.TH IOCTL 2 2021-03-22 "Linux" "Linux Programmer's Manual"
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.SH NAME
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ioctl \- control device
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.SH SYNOPSIS
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.nf
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.B #include <sys/ioctl.h>
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.PP
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.BI "int ioctl(int " fd ", unsigned long " request ", ...);"
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.\" POSIX says 'request' is int, but glibc has the above
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.\" See https://bugzilla.kernel.org/show_bug.cgi?id=42705
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.fi
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.SH DESCRIPTION
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The
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.BR ioctl ()
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system call manipulates the underlying device parameters of special files.
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In particular, many operating characteristics of character special files
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(e.g., terminals) may be controlled with
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.BR ioctl ()
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requests.
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The argument
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.I fd
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must be an open file descriptor.
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.PP
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The second argument is a device-dependent request code.
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The third argument is an untyped pointer to memory.
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It's traditionally
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.BI "char *" argp
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(from the days before
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.B "void *"
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was valid C), and will be so named for this discussion.
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.PP
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An
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.BR ioctl ()
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.I request
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has encoded in it whether the argument is an
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.I in
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parameter or
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.I out
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parameter, and the size of the argument
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.I argp
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in bytes.
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Macros and defines used in specifying an
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.BR ioctl ()
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.I request
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are located in the file
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.IR <sys/ioctl.h> .
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See NOTES.
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.SH RETURN VALUE
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Usually, on success zero is returned.
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A few
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.BR ioctl ()
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requests use the return value as an output parameter
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and return a nonnegative value on success.
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On error, \-1 is returned, and
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.I errno
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is set to indicate the error.
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.SH ERRORS
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.TP
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.B EBADF
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.I fd
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is not a valid file descriptor.
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.TP
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.B EFAULT
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.I argp
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references an inaccessible memory area.
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.TP
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.B EINVAL
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.I request
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or
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.I argp
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is not valid.
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.TP
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.B ENOTTY
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.I fd
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is not associated with a character special device.
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.TP
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.B ENOTTY
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The specified request does not apply to the kind of object that the
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file descriptor
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.I fd
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references.
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.SH CONFORMING TO
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No single standard.
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Arguments, returns, and semantics of
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.BR ioctl ()
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vary according to the device driver in question (the call is used as a
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catch-all for operations that don't cleanly fit the UNIX stream I/O
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model).
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.PP
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The
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.BR ioctl ()
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system call appeared in Version 7 AT&T UNIX.
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.SH NOTES
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In order to use this call, one needs an open file descriptor.
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Often the
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.BR open (2)
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call has unwanted side effects, that can be avoided under Linux
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by giving it the
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.B O_NONBLOCK
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flag.
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.\"
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.SS ioctl structure
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.\" added two sections - aeb
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Ioctl command values are 32-bit constants.
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In principle these constants are completely arbitrary, but people have
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tried to build some structure into them.
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.PP
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The old Linux situation was that of mostly 16-bit constants, where the
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last byte is a serial number, and the preceding byte(s) give a type
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indicating the driver.
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Sometimes the major number was used: 0x03
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for the
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.B HDIO_*
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ioctls, 0x06 for the
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.B LP*
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ioctls.
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And sometimes
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one or more ASCII letters were used.
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For example,
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.B TCGETS
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has value
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0x00005401, with 0x54 = \(aqT\(aq indicating the terminal driver, and
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.B CYGETTIMEOUT
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has value 0x00435906, with 0x43 0x59 = \(aqC\(aq \(aqY\(aq
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indicating the cyclades driver.
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.PP
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Later (0.98p5) some more information was built into the number.
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One has 2 direction bits
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(00: none, 01: write, 10: read, 11: read/write)
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followed by 14 size bits (giving the size of the argument),
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followed by an 8-bit type (collecting the ioctls in groups
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for a common purpose or a common driver), and an 8-bit
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serial number.
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.PP
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The macros describing this structure live in
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.I <asm/ioctl.h>
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and are
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.B _IO(type,nr)
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and
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.BR "{_IOR,_IOW,_IOWR}(type,nr,size)" .
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They use
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.I sizeof(size)
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so that size is a
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misnomer here: this third argument is a data type.
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.PP
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Note that the size bits are very unreliable: in lots of cases
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they are wrong, either because of buggy macros using
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.IR sizeof(sizeof(struct)) ,
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or because of legacy values.
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.PP
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Thus, it seems that the new structure only gave disadvantages:
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it does not help in checking, but it causes varying values
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for the various architectures.
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.SH SEE ALSO
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.BR execve (2),
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.BR fcntl (2),
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.BR ioctl_console (2),
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.BR ioctl_fat (2),
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.BR ioctl_ficlonerange (2),
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.BR ioctl_fideduperange (2),
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.BR ioctl_fslabel (2),
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.BR ioctl_getfsmap (2),
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.BR ioctl_iflags (2),
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.BR ioctl_ns (2),
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.BR ioctl_tty (2),
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.BR ioctl_userfaultfd (2),
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.BR open (2),
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.\" .BR mt (4),
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.BR sd (4),
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.BR tty (4)
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