ioctl(2) - Linux manual page (original) (raw)
ioctl(2) System Calls Manual ioctl(2)
NAME top
ioctl - control deviceLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <sys/ioctl.h>**
**int ioctl(int** _fd_**, unsigned long** _op_**, ...);** /* glibc, BSD */
**int ioctl(int** _fd_**, int** _op_**, ...);** /* musl, other UNIX */DESCRIPTION top
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**() operations. The argument _fd_ must be an
open file descriptor.
The second argument is a device-dependent operation 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**() _op_ 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**() _op_ are
located in the file _<sys/ioctl.h>_. See NOTES.RETURN VALUE top
Usually, on success zero is returned. A few **ioctl**() operations
use the return value as an output parameter and return a
nonnegative value on success. On error, -1 is returned, and _[errno](../man3/errno.3.html)_
is set to indicate the error.ERRORS top
**EBADF** _fd_ is not a valid file descriptor.
**EFAULT** _argp_ references an inaccessible memory area.
**EINVAL** _op_ or _argp_ is not valid.
**ENOTTY** _fd_ is not associated with a character special device.
**ENOTTY** The specified operation does not apply to the kind of
object that the file descriptor _fd_ references.VERSIONS top
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).STANDARDS top
None.HISTORY top
Version 7 AT&T UNIX has
**ioctl(int** _fildes_**, int** _op_**, struct sgttyb ***_argp_**);**
(where **struct sgttyb** has historically been used by [stty(2)](../man2/stty.2.html) and
[gtty(2)](../man2/gtty.2.html), and is polymorphic by operation type (like a **void *** would
be, if it had been available)).
SysIII documents _arg_ without a type at all.
4.3BSD has
**ioctl(int** _d_**, unsigned long** _op_**, char ***_argp_**);**
(with **char *** similarly in for **void ***).
SysVr4 has
**int ioctl(int** _fildes_**, int** _op_**, ... /*** _arg_ ***/);**NOTES top
In order to use this call, one needs an open file descriptor.
Often the [open(2)](../man2/open.2.html) call has unwanted side effects, that can be
avoided under Linux by giving it the **O_NONBLOCK** flag.ioctl structure Ioctl op 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.SEE ALSO top
[execve(2)](../man2/execve.2.html), [fcntl(2)](../man2/fcntl.2.html), [ioctl_console(2)](../man2/ioctl%5Fconsole.2.html), [ioctl_fat(2)](../man2/ioctl%5Ffat.2.html), [ioctl_fs(2)](../man2/ioctl%5Ffs.2.html),
[ioctl_fsmap(2)](../man2/ioctl%5Ffsmap.2.html), [ioctl_nsfs(2)](../man2/ioctl%5Fnsfs.2.html), [ioctl_tty(2)](../man2/ioctl%5Ftty.2.html), [ioctl_userfaultfd(2)](../man2/ioctl%5Fuserfaultfd.2.html),
[ioctl_eventpoll(2)](../man2/ioctl%5Feventpoll.2.html), [open(2)](../man2/open.2.html), [sd(4)](../man4/sd.4.html), [tty(4)](../man4/tty.4.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 ioctl(2)
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