First Kernel Programming

In this blog i use 'kernal' instead of 'kernel' intentionally to identify my files.

In computing, the kernel is the main component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources (the communication between hardware and software components). Usually as a basic component of an operating system, a kernel can provide the lowest-level abstraction layer for the resources (especially processors and I/O devices) that application software must control to perform its function. It typically makes these facilities available to application processes through inter-process communication mechanisms and system calls. (source : Wikipedia)

There are numerous materials available on linux kernel.

In general we insert and remove the module from a kernel in order to avoid recompiling the whole kernel from the scratch. Here in this blog we will get to know how to write a first linux kernel programming module from the basic. 

  Step 1 : Creating a folder kernalprogramming

                     

    Step 2 : Create a file called firstkernal.c

Step 3 : Write the first kernel program

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

// To start the module

static int hello_init(void) {
    printk(KERN_ALERT "TEST : Hello world. this is jithu");  
    return 0;
}

// To end the module

static void hello_exit(void) {
    printk(KERN_ALERT "TEST : Hello world. good bye");  
}

// To call the start and end modules

module_init(hello_init);
module_exit(hello_exit);

In the above program, linux/init.h and linux/module.h are the header files located in /usr/src/

This location is important to provide in the Makefile which will be discussed soon.

Next hello_init is a function which prints "TEST : Hello world. this is jithu"

 

Kernel programs do not call conventional c libraries such as "printf" "scanf" etc.. 

So we are using "printk" which will output into the screen

Another function is hello_exit, is a function which prints "TEST : Hello world. good bye"

module_init(hello_init); 

This line is the starting point. The parameter is the function and the function is called when we run/ insert the program into the kernel. (like a main in conventional programming)

module_exit(hello_exit);

This line is the ending point. The parameter is the function and the function is called when we run/ remove the program from the kernel. 

Step 4 : Creating a Makefile

#obj-m += filename.o

obj-m += firstkernal.o

#linux header files
#we can check using the command 'uname -r'

KDIR = /usr/src/linux-headers-3.2.0-23-generic

all:
    $(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules

clean:
    rm -rf *.o *.ko *.mod.* *.symvers *.order

first line says obj-m += firstkernal.o

It means build driver calling firstkernal.o

obj = object  m = module 

KDIR is the directory where the linux headers are located

we can find the version using the command uname -r

Step 5 : Executing a make file

Just type 'make'

compiled files are created

Step 6 : Inserting the module into the kernel

sudo insmod firstkernal.ko 

and type

dmesg  : 

dmesg (for "display message" or "driver message" ) is a command on most Linux and Unix based operating systems that prints the message buffer of the kernel. 

 The output is 

Here I already I ran 3 times. The screen shot is for 4th time running "TEST : Hello world. this is jithu" is the output for the inserting a module

 Step 7 and final step : Removing the module from the kernel

sudo rmmod firstkernal.ko 

and type dmesg

 "TEST : Hello world. good bye" is the output for the removing a module 

That is the end of first basic kernel programming to print hello !! 

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