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I have given the description as “First Linux kernel module”.
Visual cue Narration
Slide 1:

Welcome slide:

Welcome to the spoken tutorial on Simple Loadable Module.
Slide 2:

Learning objectives:

  • Implement a simple loadable module.
  • Create the Makefile.
  • Compile the module.
In this tutorial, we will learn how to,
  • Implement a simple loadable module.
  • Create a Makefile.
  • Compile a module.
Slide 3:

System Requirements:

  • VirtualBox 5.2
  • Ubuntu Linux 18.04 LTS operating system.
  • Linux kernel version 5.0.0-31 generic.
  • gedit Text Editor.
To record this tutorial, I am using,
  • VirtualBox 5.2
  • Ubuntu Linux 18.04 LTS operating system.
  • Linux kernel version 5.0.0-31 generic
  • gedit Text Editor.
Slide 4:


To follow this tutorial, you should be familiar with:

  • C programming language and
  • basics of Linux kernel
To follow this tutorial, you should be familiar with:
  • C programming language and
  • basics of Linux kernel

If not, then go through the C/C++ and Linux spoken tutorials on this website

Slide 5:

Gedit Installation:

  • To proceed further, make sure that gedit is installed on your system.
  • If not, to do the installation, follow the instructions in gedit series on this https://spoken-tutorial.org
  • To proceed further, make sure that Gedit is installed on your system.
  • If not, to do the installation, follow the instructions in Gedit series on this website.
Slide 6:

What is a kernel module?

  • Code that is added to the kernel at runtime is called a module.
  • It extends the functionality of the kernel without the need to reboot the system.
  • It communicates with the base kernel to complete their functions.
Narration only I’ll start with a simple example to explain how to create a module.
Point to the folder and file in desktop I have saved a file simple_module dot c in a folder DeviceDriver in Desktop.

Code file - sample_module.c

Please download simple_module dot c from the Code files link of this tutorial.

Make use of this file while practising.

Remember the file path.

Type gedit simple_module dot c and press Enter.

Text box

Let us open this file.

Open the terminal by pressing Ctrl+Alt+T keys simultaneously.

Press the Enter key after every command.

Go to the folder where simple_module dot c is saved.

Type gedit simple_module dot c

Let us understand the code now.
Show the program

Highlight <linux/init.h>

Highlight <linux/module.h>

Highlight <linux/kernel.h>

At the start of the program, there are some header files.

Init dot h file is used to start the init process.

The module dot h file contains functions related to the modules.

The kernel dot h file contains functions such as printk.


Highlight GPL

Next line of code is about GPL.

GPL means GNU Public License.

It is otherwise called as GNU, which is a free and open-source license.

Highlight MODULE_AUTHOR You can give the author name for your module using this macro as shown here.
Highlight MODULE_DESCRIPTION You can simply describe your module using this macro.

I have given the description as “First Linux kernel module”.

Highlight static int init_func(void)

Highlight return(0)

Highlight module_init()

The init function will execute when a module is loaded into the kernel.

So it is called as an initialization function of a module.

If this function fails then the error will be returned.

It will return 0 to the kernel on successful execution.

Initialization function prints a simple message and returns zero.

The module init macro is used to register the init function.

Highlight void exit_func (void)

Highlight module_exit()

The exit function will execute when the module is unloaded from the kernel.

It is mostly used to cleanup the resources acquired by the module

The exit function will undo whatever the init function did.

The module underscore exit macro is used to register the exit function.

Highlight printk

Highlight KERN_INFO

Press >> Ctrl + S

Press Enter

Click on close option.

In kernel programming, printk is used as a printing function.

It is similar to printf function in C programming.

Using this function, string data will get loaded in the kernel log buffer.

printk has an optional prefix string, Loglevel such as KERN_INFO.

Loglevel specifies the type of message being sent to the kernel message log.

KERN underscore INFO is used for informational messages.

For more log levels, go through the Additional reading material link of this tutorial.

Press Ctrl + S keys to save the program .

Close the file.

Switch back to the terminal. Switch back to the terminal.
Let us compile the code.
Type >> gedit Makefile

Highlight the makefile

Press Enter

For that, we have to create a Makefile.

Type gedit Makefile

The Makefile is a special file containing shell commands.

Press Enter.

Create a Makefile where you have saved the simple_module dot c .

Copy paste the makefile content Type the code as shown.
Highlight obj-m

Highlight simple_module.o

Highlight all:

Highlight clean:

The obj hyphen m variable tells the kernel Makefile that this module needs to be compiled.

When you run the make all command, then commands under all section will be executed.

If you run the make clean command, it executes commands under the clean section.

Highlight -C

Highlight uname -r Highlight (PWD) Highlight M=$(PWD) Highlight modules

Using hyphen C, we are running make command inside the kernel source directory.

As you know, uname hyphen r finds the current Linux kernel version of a system.

PWD means the present working directory.

This option tells the kernel makefile that the source code for the driver is in PWD.

The word modules tell the kernel makefile to build the modules.

But it will not build the complete kernel source code.

Highlight clean The word clean tells the kernel makefile to clean the generated object files.
Press >> Ctrl + S

Click on the close.

Save the file and close the editor.
Open the terminal

Type >> make all

Press Enter

Switch back to the terminal.

Let us compile the program.

Type make space all.

Type ls and press Enter

Highlight ls

Show the output

Now type ls.

This command lists the files in the current directory.

Here, you can see the number of files created in this directory.

Highlight module.symvers

Highlight mod.c

The module.symvers file contains a list of all symbols

These symbols are exported from the kernel build.

The mod.c file contains information about the module.

Highlight mod.o Mod.o file is object file resulting from the compilation of the mod.c file.
Highlight .o This object file is created from the compilation of the module source file.
Highlight modules.order It will list out the order in which the compilation and creation of the .ko file takes place.
Highlight .ko This is the final kernel module binary that is loaded into the kernel.
Type >> clear

Press Enter

Clear the screen.
Type >> modinfo simple_module.ko

Press Enter

Highlight modinfo

Highlight filename

Highlight description

Highlight author, license

Let us see the details of a module.

Type modinfo space simple_module dot ko.

This command displays information about the Linux kernel module.

The filename field contains an absolute path to the .ko kernel object file.

This field displays a short description of a module.

Author field shows author and License field shows the license of the module.

Highlight srcversion The srcversion field contains the source code version used to compile a module.

It is calculated automatically at build time.

Highlight depends The depends field contains all modules on which this module depends.

It is shown empty here as this module does not depend on any other module.

Highlight retpoline Retpoline field indicates that the module is compiled with retpoline method.

Modules must also be compiled with a retpoline-aware compiler.

Otherwise the kernel can be vulnerable.

Retpoline technique protects the kernel from the hacker’s attacks.

Highlight name The name field contains the name of a module.
Highlight vermagic This field shows the version magic number of a module.

This version magic number is used while loading the kernel module.

The version magic number of module and current kernel version should be the same.

Otherwise it will fail to load in the current kernel.

Type >> make clean

Press Enter

Type make space clean.

This command removes all object files that are created after the compilation.

Type ls.

Now there are no object files.

With this, we come to the end of this tutorial. Let us summarize.
Slide 6:


In this tutorial, we learned how to
  • Implement a simple loadable module.
  • Create a Makefile.
  • Compile a module.
Slide 7:


As an assignment,
  1. Open the simple_module.c file
  2. Change the module description using MODULE_DESCRIPTION macro
  3. Run the modinfo command
  4. See the output of description.
Slide 8:

About Spoken Tutorial Project:

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Please download and watch it.

Slide 9:

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For more details, please write to us.

Slide 10:

Forum questions:

* Do you have questions in THIS Spoken Tutorial?
  • Please visit this site
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Slide 11:

Forum for specific questions:

* The Spoken Tutorial forum is for specific questions on this tutorial.
  • Please do not post unrelated and general questions on them.
  • This will help reduce the clutter.
  • With less clutter, we can use these discussions as instructional material.
Slide 12:

Forum for specific questions:

* Do you have general or technical questions on Embedded Linux Device Driver?
  • Please visit the FOSSEE forum and post your question.
Slide 13:


Spoken Tutorial project is funded by MHRD, Government of India.
Slide 14:

Thank you slide:

This tutorial has been contributed by FOSSEE and Spoken Tutorial Project, IIT Bombay.

This is Mayuri Panchakshari signing off.

Thanks for watching.

Contributors and Content Editors

Nancyvarkey, Nirmala Venkat