Device files are supposed to represent physical devices. Most physical devices are used for output as well as input, so
there has to be some mechanism for device drivers in the kernel to get the output to send to the device from processes. This
is done by opening the device file for output and writing to it, just like writing to a file. In the following example, this
is implemented by device_write.
This is not always enough. Imagine you had a serial port connected to a modem (even if you have an internal modem, it is still implemented from the CPU's perspective as a serial port connected to a modem, so you don't have to tax your imagination too hard). The natural thing to do would be to use the device file to write things to the modem (either modem commands or data to be sent through the phone line) and read things from the modem (either responses for commands or the data received through the phone line). However, this leaves open the question of what to do when you need to talk to the serial port itself, for example to send the rate at which data is sent and received.
The answer in Unix is to use a special function called ioctl (short for Input Output ConTroL).
Every device can have its own ioctl commands, which can be read ioctl's (to send
information from a process to the kernel), write ioctl's (to return information to a process),
[1] both or neither. The ioctl function is called with three parameters: the file
descriptor of the appropriate device file, the ioctl number, and a parameter, which is of type long so you can use a cast to
use it to pass anything. [2]
The ioctl number encodes the major device number, the type of the ioctl, the command, and the type of the parameter.
This ioctl number is usually created by a macro call (_IO, _IOR, _IOW
or _IOWR --- depending on the type) in a header file. This header file should then be included both by the
programs which will use ioctl (so they can generate the appropriate ioctl's) and by
the kernel module (so it can understand it). In the example below, the header file is chardev.h and the program which uses it is ioctl.c.
If you want to use ioctls in your own kernel modules, it is best to receive an official
ioctl assignment, so if you accidentally get somebody else's ioctls, or if they get
yours, you'll know something is wrong. For more information, consult the kernel source tree at
Documentation/ioctl-number.txt.
| [1] | Notice that here the roles of read and write are reversed again, so in
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| [2] | This isn't exact. You won't be able to pass a structure, for example, through an ioctl --- but you will be able to pass a pointer to the structure. |