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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