ioctl — control device
#include <sys/ioctl.h>
int
ioctl( |
int fd, |
| unsigned long request, | |
...); |
The 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 ioctl() requests. The
argument fd must be
an open file descriptor.
The second argument is a device-dependent request code.
The third argument is an untyped pointer to memory. It's
traditionally char *argp (from the days before
void * was valid C),
and will be so named for this discussion.
An ioctl() request has encoded in it
whether the argument is an in parameter or out parameter, and the size
of the argument argp in bytes. Macros and
defines used in specifying an ioctl() request are located in the file
<sys/ioctl.h> See NOTES.
Usually, on success zero is returned. A few ioctl() requests use the return value as an
output parameter and return a nonnegative value on success.
On error, −1 is returned, and errno is set to indicate the error.
fd is not a
valid file descriptor.
argp
references an inaccessible memory area.
request or
argp is not
valid.
fd is not
associated with a character special device.
The specified request does not apply to the kind of
object that the file descriptor fd references.
No single standard. Arguments, returns, and semantics of
ioctl() vary according to the
device driver in question (the call is used as a catch-all
for operations that don't cleanly fit the UNIX stream I/O
model).
The ioctl() system call
appeared in Version 7 AT&T UNIX.
In order to use this call, one needs an open file
descriptor. Often the open(2) call has unwanted
side effects, that can be avoided under Linux by giving it
the O_NONBLOCK flag.
Ioctl command values are 32-bit constants. In principle these constants are completely arbitrary, but people have tried to build some structure into them.
The old Linux situation was that of mostly 16-bit
constants, where the last byte is a serial number, and the
preceding byte(s) give a type indicating the driver.
Sometimes the major number was used: 0x03 for the
HDIO_* ioctls,
0x06 for the LP*
ioctls. And sometimes one or more ASCII letters were used.
For example, TCGETS has value
0x00005401, with 0x54 = 'T' indicating the terminal driver,
and CYGETTIMEOUT has value
0x00435906, with 0x43 0x59 = 'C' 'Y' indicating the
cyclades driver.
Later (0.98p5) some more information was built into the number. One has 2 direction bits (00: none, 01: write, 10: read, 11: read/write) followed by 14 size bits (giving the size of the argument), followed by an 8-bit type (collecting the ioctls in groups for a common purpose or a common driver), and an 8-bit serial number.
The macros describing this structure live in
<asm/ioctl.h> and are _IO(type,nr) and
{_IOR,_IOW,_IOWR}(type, nr, size). They use sizeof(size) so that size
is a misnomer here: this third argument is a data type.
Note that the size bits are very unreliable: in lots of
cases they are wrong, either because of buggy macros using
sizeof(sizeof(struct)), or
because of legacy values.
Thus, it seems that the new structure only gave disadvantages: it does not help in checking, but it causes varying values for the various architectures.
execve(2), fcntl(2), ioctl_console(2), ioctl_fat(2), ioctl_ficlonerange(2), ioctl_fideduperange(2), ioctl_fslabel(2), ioctl_getfsmap(2), ioctl_iflags(2), ioctl_ns(2), ioctl_tty(2), ioctl_userfaultfd(2), open(2), sd(4), tty(4)
This page is part of release 5.11 of the Linux man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
|
Copyright (c) 1980, 1991 Regents of the University of California. All rights reserved. %%%LICENSE_START(BSD_4_CLAUSE_UCB) 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. 3. All advertising materials mentioning features or use of this software must display the following acknowledgement: This product includes software developed by the University of California, Berkeley and its contributors. 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. %%%LICENSE_END (#)ioctl.2 6.4 (Berkeley) 3/10/91 Modified 1993-07-23 by Rik Faith <faithcs.unc.edu> Modified 1996-10-22 by Eric S. Raymond <esrthyrsus.com> Modified 1999-06-25 by Rachael Munns <vashtidream.org.uk> Modified 2000-09-21 by Andries Brouwer <aebcwi.nl> |