Hello World Device Driver

hello_world.c:

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

MODULE_LICENSE("GPL");
MODULE_AUTHOR("EMBEDDED GURUJI");

static int hello_init(void)
{
      printk("Hello World\n");
      return 0;

}


static void hello_exit(void)
{
     printk("Bye World\n");

}

module_init(hello_init);
module_exit(hello_exit);

Makefile:

obj-m := hello_world.o
KERNELDIR := /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)

all :
    $(MAKE) -C $(KERNEL) M=$(PWD) modules

clean:

   $(MAKE) -C $(KERNEL) M=$(PWD) clean

Now we will understand the code line by line


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


These header files contains the definition of the module_init() and module_exit() functions.



MODULE_LICENSE("GPL");

MODULE_LICENSE() informs the kernel the license used by the source code,which will affect the functions it can access from  the kernel.MODULE_LICENSE("GPL") can access all the functions.


MODULE_AUTHOR("EMBEDDED GURUJI");

It specifies the author of the code.The importance of the macro is to know the author of the code when it is used by some other people.

static int hello_init(void)
{
      printk("Hello World\n");
      return 0;

}

hello_init() is a function which is registered to module_init and will be called when the driver is loaded.It just prints Hello World to the kernel message buffer.



static void hello_exit(void)
{
     printk("Bye World\n");

}

hello_exit() is a function registered to module_exit and will be called when the driver is unloaded or removed.It prints Bye World to the kernel message buffer.


module_init(hello_init);
module_exit(hello_exit);

These are macros not functions. module_init() macro tells the kernel which function has to be called when the module is loaded.module_exit() macro tells the kernel which function has to be called when the module is unloaded.All the initializations has to be done in the function specified by module_init() and all the unintializations has to be done in the function specified by the module_exit() macro


Makefile contains the instructions for building the module.


obj-m := hello_world.o

obj-m := specifies the list of modules which has to be built.The .o files will be automatically be built from the .c source files.


KERNELDIR := /lib/modules/$(shell uname -r)/build

KERNELDIR is pointing to the location of the kernel source build folder


PWD := $(shell pwd)

PWD is pointing to the location of the current directory.

$(MAKE) -C $(KERNEL) M=$(PWD) modules

This specify to run make with the kernel source directory and  build the modules present only in the PWD directory.

To install the module: 

make

insmod hello_world.ko

To uninstall the module:

rmmod hello_world