GNU Info
(libc.info)IOCTLs
Generic I/O Control operations
==============================
The GNU system can handle most input/output operations on many
different devices and objects in terms of a few file primitives -
`read', `write' and `lseek'. However, most devices also have a few
peculiar operations which do not fit into this model. Such as:
* Changing the character font used on a terminal.
* Telling a magnetic tape system to rewind or fast forward. (Since
they cannot move in byte increments, `lseek' is inapplicable).
* Ejecting a disk from a drive.
* Playing an audio track from a CD-ROM drive.
* Maintaining routing tables for a network.
Although some such objects such as sockets and terminals (1) have
special functions of their own, it would not be practical to create
functions for all these cases.
Instead these minor operations, known as "IOCTL"s, are assigned code
numbers and multiplexed through the `ioctl' function, defined in
`sys/ioctl.h'. The code numbers themselves are defined in many
different headers.
- Function: int ioctl (int FILEDES, int COMMAND, ...)
The `ioctl' function performs the generic I/O operation COMMAND on
FILEDES.
A third argument is usually present, either a single number or a
pointer to a structure. The meaning of this argument, the
returned value, and any error codes depends upon the command used.
Often -1 is returned for a failure.
On some systems, IOCTLs used by different devices share the same
numbers. Thus, although use of an inappropriate IOCTL _usually_ only
produces an error, you should not attempt to use device-specific IOCTLs
on an unknown device.
Most IOCTLs are OS-specific and/or only used in special system
utilities, and are thus beyond the scope of this document. For an
example of the use of an IOCTL, see Note: Out-of-Band Data.
---------- Footnotes ----------
(1) Actually, the terminal-specific functions are implemented with
IOCTLs on many platforms.
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