What is the purpose of ioctl set of functions in linux?

Question

In Linux/freeBSD kernel whenever we have to make a driver module for a device, we make a file in the /dev/ folder and use it to communicate with the other processes.

If that is so, what is the purpose of the ioctl set of functions ? Whatever information, we want to convey with the device driver can be written to/read from this file.

Can anyone please explain it ?

I have tried reading about it on tldp.org but couldn't really understand it.

Answer 1

ioctl is used for I/O control, and as an example lets take a serial port. You open the serial port device, and can read from the serial port, and write to the serial port, but how do you set e.g. the baud-rate? Or other control options? It's not possible using read or write as those are for reading and writing data from/to the serial port, you need another function for this, which is where the ioctl function comes in.

Answer 2

It would be possible to just create another device file for "control" tasks and use plain reads/writes on that, instead of ioctl. This way ioctl wouldn't be needed at all. Plan 9 operating system did stuff this way.

Thing is - Unix systems just do it in another way. They always did, and since there isn't anything really wrong with it, and lots of software uses ioctl, why bother changing it?