A Linux driver is a Linux module which can be loaded and linked to the kernel at runtime. The driver operates in kernel space and becomes part of the kernel once loaded, the kernel being monolithic. It can then access the symbols exported by the kernel.

When the device driver module is loaded, the driver first registers itself as a driver for a particular device specifying a particular Major number.

It uses the call register_chrdev function for registration. The call takes the Major number, Minor number, device name and an address of a structure of the type file_operations(discussed later) as argument. In our example, we will be using a major number of 89 . The choice of major number is arbitrary but it has to be unique on the system.

The syntax of register_chrdev is

int register_chrdev(unsigned int major,const char *name,struct file_operations *fops)

Driver is unregistered by calling the unregister_chrdev function.

Since device driver is a kernel module, it should implement init_module and cleanup_module functions. The register_chrdev call is done in the init_module function  and unregister_chrdev call is done in the cleanup_module function.

The register_chrdev call returns a non-negative number on success. If we specify the Major number as 0, the kernel returns a Major number unique at that instant which can be used to create a device file.
A device file can be created either before the driver is loaded if we know the major and minor number beforehand or it can be created later after letting the driver specify a major number for us.

Creating a device file

A device file is a special file. It can’t just be created using cat or gedit or shell redirection for that matter. The shell command mknod is usually used to create device file. The syntax of mknod is

mknod path type major minor