View source for Embedded-Linux-Device-Driver/C2/Simple-Loadable-Module/English


{| style="border-spacing:0;"
| style="border:1pt solid #000000;padding:0.176cm;"| '''Visual cue'''
| style="border:1pt solid #000000;padding:0.176cm;"| '''Narration'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 1:

Welcome slide:
| style="border:1pt solid #000000;padding:0.176cm;"| Welcome to the spoken tutorial on '''Simple Loadable Module.'''
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 2:

Learning objectives:

* Implement a simple '''loadable module'''.
* Create the '''Makefile.'''
* Compile the '''module.'''

| style="border:1pt solid #000000;padding:0.176cm;"| In this tutorial, we will learn how to,

* Implement a simple '''loadable module'''.
* Create a '''Makefile.'''
* Compile a '''module.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| 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.
| style="border:1pt solid #000000;padding:0.176cm;"| 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.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 4:

Prerequisites:

To follow this tutorial, you should be familiar with:

* '''C programming language and'''
* basics of''' Linux kernel''' 

| style="border:1pt solid #000000;padding:0.176cm;"| 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

|-
| style="border:1pt solid #000000;padding:0.176cm;"| 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 [http://spoken-tutorial.org/ https://spoken-tutorial.org] 

| style="border:1pt solid #000000;padding:0.176cm;"| * 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.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 6:

What is a''' kernel module'''?
| style="border:1pt solid #000000;padding:0.176cm;"| * 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. 

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Narration only
| style="border:1pt solid #000000;padding:0.176cm;"| I’ll start with a simple example to explain how to create a '''module.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Point to the '''folder''' and''' file''' in '''desktop'''
| style="border:1pt solid #000000;padding:0.176cm;"| I have saved a file '''simple_module dot c''' in a folder '''DeviceDriver''' in '''Desktop'''.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide:

Code file - sample_module.c

| style="border:1pt solid #000000;padding:0.176cm;"| 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.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type '''gedit simple_module dot c '''and press''' Enter.'''

Text box
| style="border:1pt solid #000000;padding:0.176cm;"| 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 '''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| 
| style="border:1pt solid #000000;padding:0.176cm;"| Let us understand the code now.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Show the '''program'''

Highlight'''<nowiki> <linux/init.h> </nowiki>'''

Highlight'''<nowiki> <linux/module.h></nowiki>'''

Highlight'''<nowiki> <linux/kernel.h></nowiki>'''
| style="border:1pt solid #000000;padding:0.176cm;"| 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.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''MODULE_LICENSE(“GPL”) '''

Highlight '''GPL'''
| style="border:1pt solid #000000;padding:0.176cm;"| 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'''.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''MODULE_AUTHOR'''
| style="border:1pt solid #000000;padding:0.176cm;"| You can give the author name for your '''module '''using this '''macro '''as shown here.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''MODULE_DESCRIPTION'''

| style="border:1pt solid #000000;padding:0.176cm;"| You can simply describe your '''module '''using this macro .

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

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''static int init_func(void)'''

Highlight '''return(0)'''

Highlight '''module_init()'''
| style="border:1pt solid #000000;padding:0.176cm;"| 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 return zero. 

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

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight''' void exit_func (void)'''

Highlight '''module_exit()'''
| style="border:1pt solid #000000;padding:0.176cm;"| 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. '''
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''printk'''

Highlight '''KERN_INFO'''

Press >> '''Ctrl + S'''

Press '''Enter'''

Click on '''close''' option.
| style="border:1pt solid #000000;padding:0.176cm;"| 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.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Switch back to the '''terminal'''.
| style="border:1pt solid #000000;padding:0.176cm;"| Switch back to the '''terminal'''.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| 
| style="border:1pt solid #000000;padding:0.176cm;"| Let us compile the code.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type >> '''gedit Makefile'''

Highlight the makefile

Press '''Enter'''
| style="border:1pt solid #000000;padding:0.176cm;"| 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 .'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Copy paste the makefile content
| style="border:1pt solid #000000;padding:0.176cm;"| Type the '''code '''as shown.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''obj-m'''

Highlight '''simple_module.o'''

Highlight '''all:'''

Highlight '''clean:'''

| style="border:1pt solid #000000;padding:0.176cm;"| 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.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''-C'''

Highlight '''uname -r'''
Highlight''' (PWD)'''
Highlight '''M=$(PWD)'''
Highlight''' modules''' 
| style="border:1pt solid #000000;padding:0.176cm;"| 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.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''clean'''
| style="border:1pt solid #000000;padding:0.176cm;"| The word '''clean''' tells the '''kernel makefile '''to clean the generated '''object files.''' 

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Press >> '''Ctrl + S'''

Click on the '''close'''.
| style="border:1pt solid #000000;padding:0.176cm;"| Save the '''file''' and close the '''editor'''.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Open the '''terminal'''

Type >> '''make all'''

Press '''Enter'''
| style="border:1pt solid #000000;padding:0.176cm;"| Switch back to the '''terminal'''.

Let us compile the '''program''',

Type '''make space all.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type''' ls '''and''' '''Press''' Enter'''

Highlight''' ls '''

Show the output

| style="border:1pt solid #000000;padding:0.176cm;"| Now type''' ls.'''

This command lists the '''files''' in the current '''directory.'''

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

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight''' module.symvers'''

Highlight''' mod.c '''
| style="border:1pt solid #000000;padding:0.176cm;"| 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'''.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight''' mod.o'''
| style="border:1pt solid #000000;padding:0.176cm;"| '''Mod.o''' file is '''object file''' resulting from the compilation of the '''mod.c file.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight''' .o'''
| style="border:1pt solid #000000;padding:0.176cm;"| This''' object file''' is created from the compilation of the '''module source file'''.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight''' modules.order'''
| style="border:1pt solid #000000;padding:0.176cm;"| It will list out the order in which the compilation and creation of the '''.ko file '''takes place.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight''' .ko'''
| style="border:1pt solid #000000;padding:0.176cm;"| This is the final '''kernel module''' binary that is loaded into the '''kernel.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type >> '''clear'''

Press''' Enter'''
| style="border:1pt solid #000000;padding:0.176cm;"| Clear the screen

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type '''>> modinfo simple_module.ko'''

Press '''Enter'''

Highlight '''modinfo'''

Highlight '''filename'''

Highlight '''description'''

Highlight '''author, license'''
| style="border:1pt solid #000000;padding:0.176cm;"| 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.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''srcversion'''
| style="border:1pt solid #000000;padding:0.176cm;"| The '''srcversion field '''contains the source code version used to compile a''' module'''.

It is calculated automatically at build time.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''depends'''

| style="border:1pt solid #000000;padding:0.176cm;"| 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.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''retpoline'''

| style="border:1pt solid #000000;padding:0.176cm;"| '''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.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''name'''
| style="border:1pt solid #000000;padding:0.176cm;"| The '''name''' field contains the '''name '''of a''' module.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''vermagic'''
| style="border:1pt solid #000000;padding:0.176cm;"| 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. '''
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type >> '''make clean'''

Press''' Enter'''
| style="border:1pt solid #000000;padding:0.176cm;"| Type '''make space clean.'''

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

'''Type ls.'''

Now there are no '''object files.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| 
| style="border:0.5pt solid #000000;padding:0.176cm;"| With this, we come to the end of this tutorial. Let us summarize.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 6:

Summary:
| style="border:1pt solid #000000;padding:0.176cm;"| In this tutorial, we learned how to

* Implement a '''simple loadable module.'''
* Create a '''Makefile'''. 
* Compile a '''module.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 7:

Assignment:

| style="border:1pt solid #000000;padding:0.176cm;"| As an assignment,

# Open the''' simple_module.c file''' 
# Change the module description''' '''using''' MODULE_DESCRIPTION macro '''
# Run the '''modinfo''' command
# See the output of description.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 8:

About Spoken Tutorial Project:
| style="border:1pt solid #000000;padding:0.176cm;"| The video at the following link summarizes the Spoken Tutorial project.

Please download and watch it.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 9:

Spoken Tutorial workshops:
| style="border:1pt solid #000000;padding:0.176cm;"| The''' Spoken Tutorial Project''' Team:

* conducts workshops and
* gives certificates.

For more details, please write to us.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 10:

Forum questions:
| style="border:1pt solid #000000;padding:0.176cm;"| * Do you have questions in THIS Spoken Tutorial?
* Please visit this site
* Choose the minute and second where you have the question.
* Explain your question briefly.
* The Spoken Tutorial project will ensure an answer
* You will have to register to ask questions
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 11:

Forum for specific questions:

| style="border:1pt solid #000000;padding:0.176cm;"| * 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.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 12:

Forum for specific questions:
| style="border:1pt solid #000000;padding:0.176cm;"| * Do you have general or technical questions on Embedded Linux Device Driver?
* Please visit the FOSSEE forum and post your question.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 13:

Acknowledgment:
| style="border:1pt solid #000000;padding:0.176cm;"| Spoken Tutorial project is funded by MHRD, Government of India.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 14:

Thank you slide:
| style="border:1pt solid #000000;padding:0.176cm;"| This tutorial has been contributed by FOSSEE and Spoken Tutorial Project, IIT Bombay. 

This is Mayuri Panchakshari signing off.

Thanks for watching.

|}