View source for Embedded-Linux-Device-Driver/C2/Exporting-Symbols-from-a-Module/English


{| style="border-spacing:0;"
| style="border:0.5pt solid #000000;padding:0.176cm;"| '''Visual cue'''
| style="border:0.5pt solid #000000;padding:0.176cm;"| '''Narration '''

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Slide 1:

Welcome slide:
| style="border:0.5pt solid #000000;padding:0.176cm;"| Welcome to the spoken tutorial on '''Exporting Symbols from a Module.'''

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Slide 2:

Learning objectives:
| style="border:0.5pt solid #000000;padding:0.176cm;"| In this tutorial, we will learn how to,

* Export the '''symbols''' from a '''module'''.
* Import the '''symbols '''in a '''module'''.
|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Slide 3:

System Requirements:
| style="border:0.5pt solid #000000;padding:0.176cm;"| To record this tutorial, I am using,

* '''VirtualBox '''version''' 5.2.'''
* '''Ubuntu Linux 18.04 LTS''' operating system.
* '''Linux kernel '''version''' 5.0.0-31 generic.'''
* '''gedit text editor.'''
|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Slide 4:

Prerequisites:
| style="border:0.5pt solid #000000;padding:0.176cm;"| To follow this tutorial, you should be familiar with:

* '''C programming language '''
* Basics of''' Linux kernel'''

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

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Slide 5:

What is '''symbol'''?
| style="border:0.5pt solid #000000;padding:0.176cm;"| '''What is symbol?'''

* '''Symbol''' is either a '''variable''' or a '''function.'''

* '''Symbol '''exported by a '''module''' become a part of the '''kernel symbol table'''. 

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Point to the '''directory''' and''' file''' in '''desktop'''

Point to the''' files.'''
| style="border:0.5pt solid #000000;padding:0.176cm;"| Go to the '''DeviceDriver directory''' in the '''desktop '''which we have created earlier.

Here, I have created a '''directory '''named''' ExportImportModule.'''

In this '''directory''', I have saved three '''module files''' for demonstration.

I have named them as '''export dot c, export dot h, import dot c.'''

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Slide: 

Code files
| style="border:0.5pt solid #000000;padding:0.176cm;"| * The '''files''' used in this tutorial are available in the '''Code Files '''link on this tutorial page.
* Please download and extract them
* Make a copy and then use them while practising

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Open the '''terminal'''

Type >> '''cd Desktop/DeviceDriver/ExportImportModule'''
| style="border:0.5pt solid #000000;padding:0.176cm;"| Open the '''terminal''' by pressing '''ALT+Ctrl+T '''keys simultaneously.

Press the '''Enter''' key after every command.

Go to the directory''' ExportImportModule '''as shown here.

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >> '''gedit export.c'''
| style="border:0.5pt solid #000000;padding:0.176cm;"| Let us open the '''export.c module '''that exports the '''symbols.'''

Type '''gedit space export dot c'''.

Let us understand the '''code''' now.
|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Point to '''header files'''

Open the '''terminal.'''

'''Type >> clear'''
| style="border:0.5pt solid #000000;padding:0.176cm;"| We have to include these '''header files''' at the start of a program.

The''' export.h''' file contains the exported '''symbols''' declaration.

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Open the '''export.h file'''

Type >>''' gedit export.h'''

Highlight '''module_function() '''and '''num'''
| style="border:0.5pt solid #000000;padding:0.176cm;"| Let us open the '''export.h''' file to see the '''source code.'''

Here, we have declared an exported''' function module_function '''and a variable''' num.'''

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Highlight''' EXPORT_SYMBOL(name),'''

'''EXPORT_SYMBOL_GPL(name)'''

Highlight '''EXPORT_SYMBOL_GPL(name)'''

Highlight''' name'''
| style="border:0.5pt solid #000000;padding:0.176cm;"| Let us switch back to '''export.c module'''

'''EXPORT_SYMBOL '''and''' EXPORT_SYMBOL_GPL '''are '''macros.'''

These '''macros''' are used to export the '''symbols''' from a '''module''' to other '''modules.'''

This''' macro '''exports the '''symbols '''to''' GPL_licensed modules '''only.

The '''symbols''' must be exported in the global part of the '''module’s file.'''

It makes them visible to the''' kernel.'''

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Highlight '''EXPORT_SYMBOL(module_function)'''
Highlight '''module_function()'''

Highlight '''printk''' message.
| style="border:0.5pt solid #000000;padding:0.176cm;"| The '''export.c''' module exports '''module_function''' using '''EXPORT_SYMBOL macro'''.

This '''function''' contains a '''printk''' message as shown here.

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Highlight '''EXPORT_SYMBOL(num)'''

Highlight '''int num = 100;'''
| style="border:0.5pt solid #000000;padding:0.176cm;"| Here, the '''num''' variable is exported using '''EXPORT_SYMBOL''' '''macro'''.

The '''num''' '''variable''' is defined as shown here.

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Highlight''' init_func() function'''

Highlight''' exit_func() function'''
| style="border:0.5pt solid #000000;padding:0.176cm;"| We have defined''' '''the''' init function''' and '''exit function''' with different '''printk''' messages. 

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Press >> '''Ctrl + S'''

Click on the '''close.'''
| style="border:0.5pt solid #000000;padding:0.176cm;"| Now save the '''file '''and close the window.

Switch back to the''' terminal.'''

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >> '''gedit import.c'''

Press''' Enter'''
| style="border:0.5pt solid #000000;padding:0.176cm;"| Let us open''' import dot c module'''.

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Highlight '''header files'''

Highlight '''<nowiki>#include “export.h”</nowiki>'''

| style="border:0.5pt solid #000000;padding:0.176cm;"| We have to include these '''header files.'''

'''export dot h file''' contains the exported symbols declaration used by this''' module.'''

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Highlight '''module_function()'''

Highlight '''num'''

Highlight''' init_func()'''

Highlight''' exit_func()'''
| style="border:0.5pt solid #000000;padding:0.176cm;"| This '''module '''uses the '''function''' and '''variable''' exported by the '''export dot c.'''

'''module_function() '''and''' variable num '''are used in this module as shown.

We have defined '''init function''' and '''exit function '''with different '''printk''' messages.

Now save the''' file '''

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Open the '''terminal.'''

Type >>''' clear '''
| style="border:0.5pt solid #000000;padding:0.176cm;"| Switch back to the '''terminal.'''
|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >> '''gedit Makefile'''

Press '''Ctrl + S'''

Click on the''' close.'''

Open the''' terminal'''
| style="border:0.5pt solid #000000;padding:0.176cm;"| Let us now create a '''Makefile '''to compile the''' modules. '''

Type '''gedit space Makefile'''

Type the '''code '''as shown.

'''Makefile''' is used to '''compile '''the''' export '''and '''import modules.'''

Save''' '''and close the '''file. '''Switch back to the''' terminal.'''

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >>''' sudo su'''

Type >>''' password'''

Type >>''' make all'''

| style="border:0.5pt solid #000000;padding:0.176cm;"| Type '''sudo''' '''space''' '''su '''to be a''' superuser''' 

Now''', '''type the system '''password'''.

Let us compile the '''modules.'''

Type '''make space all'''

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >> '''clear'''

| style="border:0.5pt solid #000000;padding:0.176cm;"| Clear the screen

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >>''' ls'''

Highlight the output
| style="border:0.5pt solid #000000;padding:0.176cm;"| Let us check whether '''modules '''are compiled or not.

Type '''ls '''

As the '''modules '''are compiled, we can see their '''dot ko files '''are created here.

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >> '''clear.'''

| style="border:0.5pt solid #000000;padding:0.176cm;"| Clear the '''screen'''.
|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| 
| style="border:0.5pt solid #000000;padding:0.176cm;"| Next, let us copy the '''dot ko files '''of '''modules''' to the current '''kernel''' '''directory.'''

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >> '''cp export.ko /lib/modules/$(uname -r)'''

Press Enter
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type the '''command '''as shown'''.'''

It makes the '''export dot ko module '''visible to the''' depmod tool.'''

Clear the screen.

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >> '''cp import.ko /lib/modules/$(uname -r)'''

Press Enter

Type >>''' clear'''
| style="border:0.5pt solid #000000;padding:0.176cm;"| Similarly, copy the '''import dot ko file. '''

Type the '''command '''as shown'''.'''

Clear the screen.

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >> '''depmod -a'''

| style="border:0.5pt solid #000000;padding:0.176cm;"| Type''' depmod space hyphen a.'''

This command will''' '''make the '''kernel recognize''' the newly added '''module.'''

Here, the '''depmod tool '''generates a''' module dot dep file. '''

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >> '''modprobe import '''

Highlight '''modprobe.'''
| style="border:0.5pt solid #000000;padding:0.176cm;"| Let us use a modprobe''' tool''' to load the '''modules''' into the '''kernel.'''

Type''' modprobe space import '''

Here, the '''import module '''depends on '''export module.'''

The''' modprobe''' tool also loads the''' export dot ko module.'''

Clear the screen.

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >> '''lsmod | head -5'''

| style="border:0.5pt solid #000000;padding:0.176cm;"| Let’s check whether our '''modules '''are loaded into the '''kernel '''or not.

Type''' lsmod space pipe space head space hyphen 5.'''

We can see their names appear here as they are loaded into the '''kernel.'''

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >> '''clear'''

| style="border:0.5pt solid #000000;padding:0.176cm;"| Clear the''' screen.'''
|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >>''' dmesg | tail -5'''

Highlight the '''output'''
| style="border:0.5pt solid #000000;padding:0.176cm;"| Let us see the “loaded” '''printk''' messages of '''modules'''. 

Type''' dmesg space pipe space tail hyphen 5 '''

It shows the''' printk '''messages of '''export '''and''' import module’s init function.'''

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >> '''clear'''

| style="border:0.5pt solid #000000;padding:0.176cm;"| Clear the '''screen'''.
|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >> '''modprobe -r import'''

| style="border:0.5pt solid #000000;padding:0.176cm;"| Let us unload the '''modules''' from the '''kernel.'''

Type''' modprobe space hyphen r space import'''

The''' modprobe''' automatically unloads the '''export dot ko '''module.

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >> '''lsmod | head -5'''

Show the output
| style="border:0.5pt solid #000000;padding:0.176cm;"| Let us check whether our '''modules '''are unloaded from the '''kernel''' or not.

Type''' lsmod space pipe space head space hyphen 5. '''

Here, both the '''module’s '''names are not in this list.

It indicates that they are unloaded from the '''kernel '''successfully.

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >> '''clear'''
| style="border:0.5pt solid #000000;padding:0.176cm;"| Clear the '''screen'''.

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >>''' dmesg | tail -3 '''

Type >>''' clear'''
| style="border:0.5pt solid #000000;padding:0.176cm;"| Let us see “unloaded” '''printk''' messages of '''modules.'''

Type '''dmesg space pipe space tail space hyphen 3. '''

Now, we can see the messages from the '''exit function '''of both''' modules.'''

Clear the screen.

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >> '''make clean'''

| style="border:0.5pt solid #000000;padding:0.176cm;"| Type '''make space clean '''command to remove all '''files '''created by '''compilation.'''

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Type >> '''exit'''
| style="border:0.5pt solid #000000;padding:0.176cm;"| To go back to''' '''the''' regular user, '''type '''exit.'''

|-
| style="border:0.5pt 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:0.5pt solid #000000;padding:0.176cm;"| Slide 6:

Summary:
| style="border:0.5pt solid #000000;padding:0.176cm;"| In this tutorial, we learnt how to

* Export the '''symbols''' from a '''module'''.
* Import the '''symbols '''in a '''module'''.

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Slide 7:

Assignment:
| style="border:0.5pt solid #000000;padding:0.176cm;"| As an assignment,

# Open the '''export.c file'''
# Change the '''printk message''' of '''module_function.'''
# Compile the '''export '''and''' import modules.'''
# Load the modules using the modprobe''' tool.'''
# See the output using '''dmesg command.'''

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Slide 8:

About Spoken Tutorial Project:
| style="border:0.5pt solid #000000;padding:0.176cm;"| * The video at the following link summarizes the Spoken Tutorial project.

* Please download and watch it.
|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Slide 9:

Spoken Tutorial workshops :
| style="border:0.5pt solid #000000;padding:0.176cm;"| The''' Spoken Tutorial Project''' Team:

* conducts workshops and
* gives certificates.

For more details, please write to us.
|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Slide 10:

Forum questions :
| style="border:0.5pt solid #000000;padding:0.176cm;"| Please post your timed queries in this forum.

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Slide 11:

Forum for specific questions :

| style="border:0.5pt solid #000000;padding:0.176cm;"| * Do you have any general or technical questions on Embedded Linux Device Driver?
* Please visit the FOSSEE forum and post your question.
|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Slide 12:

Acknowledgment:
| style="border:0.5pt solid #000000;padding:0.176cm;"| The Spoken Tutorial Project is funded by MHRD, Government of India.

|-
| style="border:0.5pt solid #000000;padding:0.176cm;"| Slide 13:

Thank you slide:
| style="border:0.5pt 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.

|}