I. Linux kernel articles

2. Module

We can now start. Here, we'll learn how to create a static module (which is build with the kernel).

First, go to your linux source folder. Then create a folder in it, for example:

mkdir my_module

Go in it and let's create our first module. Create a file (my_module.c for example) and put this code in:

#include <linux/init.h>
#include <linux/module.h>
MODULE_LICENSE("Dual BSD/GPL");

static int hello_init(void)
{
    pr_info("my_module: Hello world\n");
    return 0;
}

static void hello_exit(void)
{
    pr_info("my_module: Goodbye, cruel world\n");
}

module_init(hello_init);
module_exit(hello_exit);

Note the two very important calls at the end:

module_init(hello_init);
module_exit(hello_exit);

That's how linux knows what function to call. The init function has to return an integer and the exit one has to return nothing. The two functions don't take arguments. I used pr_info to display messages but you can also use printk.

Now create a Makefile next to your file and put in it (of course, if you created a file called 'hello.c', you will have to set 'hello.o':

obj-y := my_module.o

Now go back to the linux main folder and add your module path to this line:

core-y         += kernel/ mm/ fs/ ipc/ security/ crypto/

Like this:

core-y         += kernel/ mm/ fs/ ipc/ security/ crypto/ block/ my_module/

Now you just have to build your linux (don't worry, it won't take a while this time):

make

Launch qemu and you should see the two messages of your module. If you want to see the kernel messages again, just enter the following command:

dmesg

And if you only want to see your module's messages:

dmesg | grep "my_module"

Here ends this tutorial !