Although monolithic kernel faster than a micro-kernel  but it has disadvantage of lack modularity and extendsibility.

On modern monolithic kernels, this problem has been resolved  by using kernel modules. A linux kernel module is an object file that contains code that can extend the kernel functionality at run-time. Most of the device drivers are used in the form of linux kernel modules.

An example of a linux kernel module

We will create a simple C program as below with name is helloworld.c. When loading into the kernel, it will generate the message "Hello world". When unloading the kernel module, the "Bye bye"message will be generated.

#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/module.h>

MODULE_DESCRIPTION("Simple kernel module");
MODULE_AUTHOR("SanDan");
MODULE_LICENSE("GPL");

static int __init simple_init(void)
{
        pr_debug("Hello, world!\n");
        return 0;
}

static void __exit simple_exit(void)
{
        pr_debug("Bye bye!\n");
}

module_init(simple_init);
module_exit(simple_exit);

Compiling linux kernel module

In order to compile a linux kernel module, we will create a make file

obj-m += helloworld.o
all:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

When we run “make”, it should compile your module successfully. The resulting file is “helloworld.ko”

Loading a linux kernel module

Now we can insert the module to test it. To do this, run:

sudo insmod helloworld.ko

If all goes well, you won’t see a thing. The printk function doesn’t output to the console but rather the kernel log. To see that, we’ll need to run:

sudo dmesg

You should see the “Hello, World!” line prefixed by a timestamp. This means our linux  kernel module loaded and successfully printed to the kernel log. We can also check to see if the module is still loaded:

lsmod | grep “helloworld”

To remove the module, run:

sudo rmmod helloworld

If you run dmesg again, you’ll see “Bye bye!” in the logs. You can also use lsmod again to confirm it was unloaded.

Conclusion

I hope you’ve enjoyed our romp through kernel land. Though the examples I’ve provided are basic, you can use this structure to construct your own module that does very complex tasks.