View source for Embedded-Linux-Device-Driver/C3/Creating-a-New-Character-Device/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 '''Creating a new character device.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 2:

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

* Create a''' class of a device''' in the '''sysfs.'''
* Create a''' device file''' in '''sysfs '''and''' dev directory. '''

|-
| style="border:1pt solid #000000;padding-top:0cm;padding-bottom:0cm;padding-left:0.206cm;padding-right:0.191cm;"| Slide 3:

System Requirements:
| style="border:1pt 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 '''and
* '''gedit text editor'''

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

Prerequisites:
| 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:

'''Class and its device :'''
| style="border:1pt solid #000000;padding:0.176cm;"| * A''' class '''specifies the type of '''device''' that has a similar behavior.
* For example, '''audio devices'''. 

* '''Classes''' provide a grouping of '''devices''' based on '''functionality'''.

* Mostly you can associate a '''device''' with a specific '''class'''.

* For example, '''block devices''' such as '''hard drives''' are associated with the '''block class'''.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Point to the '''folder''' and''' file''' in '''Desktop'''

Point to the '''files.'''
| style="border:1pt solid #000000;padding:0.176cm;"| Go to the '''DeviceDriver '''folder on the '''Desktop'''.

Here I have created a new '''directory '''as '''CreateNewDevice.'''

In this '''directory''', I have saved a '''simple_driver dot c driver file '''and''' Makefile.'''

I’ll use these files for demonstration.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide:5

'''Code files''':
| style="border:1pt 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:1pt solid #000000;padding:0.176cm;"| Open the''' terminal''' 
| style="border:1pt solid #000000;padding:0.176cm;"| Open the''' terminal''' by pressing '''Alt+Ctrl+T''' keys simultaneously.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type >> '''cd Desktop/DeviceDriver/CreateNewDevice'''

| style="border:1pt solid #000000;padding:0.176cm;"| Go to the directory '''CreateNewDevice''' as shown here.

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

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

open '''simple_driver.c'''

| style="border:1pt solid #000000;padding:0.176cm;"| Type''' gedit space simple_driver dot c''' to open the '''driver file.'''

I have used the same file ''' simple_driver dot c '''which we used earlier.

In this file, I have added required''' kernels functions'''.

These '''kernel functions''' are used to create a new '''character device '''and its''' class.'''

Let me explain the code.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight'''<nowiki> #include<device.h></nowiki>'''
| style="border:1pt solid #000000;padding:0.176cm;"| You have to include this '''header file.'''

The''' kernel functions''' that are required to create a new '''device '''are declared in it.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''static struct class *cl'''
| style="border:1pt solid #000000;padding:0.176cm;"| First we have to declare a '''class structure''' for a new '''device''' as shown here.

A '''class''' is defined in the '''kernel''' with the '''struct class structure.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| 
| style="border:1pt solid #000000;padding:0.176cm;"| Let us see how to create a '''class''' using the '''kernels function''' in the '''sysfs.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''struct class *class_create()'''

Highlight '''owner'''

Highlight '''name'''
| style="border:1pt solid #000000;padding:0.176cm;"| The''' class_create function''' is used to create a '''class''' in the '''sysfs filesystem.'''

This '''function''' returns the '''struct class pointer'''.

'''owner '''specifies the''' pointer''' to the '''module''' in which this '''struct class '''is defined.

'''name '''specifies the '''pointer '''to a '''string''' for the name of this '''class.'''

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

| style="border:1pt solid #000000;padding:0.176cm;"| Here,''' class_create() function''' will create '''my_class''' in the '''sysfs.'''

The '''sysfs''' exports the system information from the '''kernel space''' to '''user space'''.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight''' unregister_chrdev_region'''

Highlight '''printk '''

| style="border:1pt solid #000000;padding:0.176cm;"| On failure, it will unregister the '''device '''and will exit with the failure message.

Otherwise, '''printk '''will print the message that the '''class''' is created successfully.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| 
| style="border:1pt solid #000000;padding:0.176cm;"| Let us see the''' kernel function''' used to create a new '''device file.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''struct device *device_create()'''
| style="border:1pt solid #000000;padding:0.176cm;"| '''device_create() function''' is used to create the '''device file''' in the''' dev directory.'''

It will also create the '''device file''' in the '''sysfs'''.

It '''returns''' the''' struct device pointer.'''

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

Highlight '''parent'''

Highlight '''devt'''

Highlight '''drvdata '''

Highlight '''fmt '''
| style="border:1pt solid #000000;padding:0.176cm;"| We will see the '''parameters''' that are passed to '''device_create() function'''.

'''class '''specifies the '''device class'''.

'''parent '''specifies the parent '''device''' of this '''device'''. 

If it is not specified, it is referred to as '''NULL.'''

'''devt '''specifies the''' dev_t variable''' which holds the '''device number.'''

'''drvdata '''is a '''pointer''' to the '''driver data'''.

'''fmt '''is a '''string''' used for the '''device name.'''

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

Highlight''' new_device'''

Highlight''' NULL'''
| style="border:1pt solid #000000;padding:0.176cm;"| Here, I have given a new '''character device''' name as '''new_device'''.

'''cl '''specifies the '''pointer''' to the '''class structure''' of this new '''device'''.

As the '''device''' does not have a parent '''device''', I have passed the''' NULL pointer'''.

'''device_num''' holds the major and minor numbers of the '''new_device'''.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| 
| style="border:1pt solid #000000;padding:0.176cm;"| Let us see the''' kernel function '''used to remove the '''class of device '''to free its''' memory.'''
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''void class_destroy(struct class *class)'''

Highlight '''cl'''
| style="border:1pt solid #000000;padding:0.176cm;"| '''class_destroy function '''destroys a '''struct class structure.'''

'''class '''specifies the '''pointer''' to the '''struct class''' that has to be destroyed.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''class_destroy(cl), unregister_chrdev_region()'''

Highlight '''class_destroy(cl)'''
| style="border:1pt solid #000000;padding:0.176cm;"| These''' functions''' are executed when the''' device file''' creation fails.

Here, '''class_destroy()''' will remove the''' class '''of '''device''' from the '''sysfs.'''
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight''' printk messages'''
| style="border:1pt solid #000000;padding:0.176cm;"| Depending upon its success or failure, the corresponding messages will be printed.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| 
| style="border:1pt solid #000000;padding:0.176cm;"| Now let us see the''' kernel function '''used to remove the '''device '''from the '''kernel'''.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''void device_destroy'''

Highlight '''class'''

Highlight '''devt'''
| style="border:1pt solid #000000;padding:0.176cm;"| '''device_destroy function '''removes a''' device''' that was created with '''device_create().'''

This '''function''' is used in the '''exit function''' to free the''' kernel's memory'''. 

'''Class''' is a''' pointer '''to the '''struct class''' that this '''device''' was '''registered''' with.

'''devt '''specifies the '''dev_t''' of the '''device''' that was previously '''registered'''.

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

Highlight '''printk'''

| style="border:1pt solid #000000;padding:0.176cm;"| In the''' exit function''', we have to '''return''' all the resources to the''' system.'''
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''class_destroy() '''and '''device_destroy()'''

Highlight '''printk messages.'''
| style="border:1pt solid #000000;padding:0.176cm;"| Here, the '''class''' and '''device''' will be removed by their corresponding''' functions.'''

'''Printk''' message indicates the successful removal of the '''class '''and''' device.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''unregister_chrdev_region() '''and '''printk'''
| style="border:1pt solid #000000;padding:0.176cm;"| Finally, we have to '''unregister''' the '''device''' from the '''kernel''' as shown here.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Save the''' file'''

| style="border:1pt solid #000000;padding:0.176cm;"| Save and close the '''file'''.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type >> '''gedit Makefile '''

Save the''' Makefile '''
| style="border:1pt solid #000000;padding:0.176cm;"| Let us create a '''Makefile''' to '''compile''' the '''driver.'''

Type''' gedit space Makefile.'''

Type the''' code '''as shown or you can make use of the downloaded''' Makefile'''.

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

| style="border:1pt solid #000000;padding:0.176cm;"| Let us '''compile''' the '''driver.'''

Type '''make space all.'''

Clear the screen.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type >>''' insmod simple_driver.ko'''

Type''' sudo su'''
| style="border:1pt solid #000000;padding:0.176cm;"| Now let us load the '''driver '''into the''' kernel.'''

Type''' sudo space su '''and''' password''' to be a '''superuser'''.

For that, type '''insmod space simple_driver.ko'''
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type >> '''clear'''
| style="border:1pt solid #000000;padding:0.176cm;"| Clear the screen.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type >> '''dmesg | grep simple_driver'''
Highlight '''respective printk messages'''

| style="border:1pt solid #000000;padding:0.176cm;"| Let us see the loaded '''printk '''messages from the '''initialisation function.'''

Type '''dmesg space pipe space grep space simple_driver.'''

It shows that the '''class_create() function''' is executed successfully.

Similarly, it shows that the '''device_create() function '''is executed successfully.

These messages indicate that our '''device''' and its '''class''' is created successfully.

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

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type >>''' ls -l /sys/class/ | grep my_class'''

Highlight the output

Type >>''' ls -l /sys/class/my_class/'''

Highlight the output
| style="border:1pt solid #000000;padding:0.176cm;"| Now let’s check whether the '''class''' is created in the '''sysfs '''or not.

Type '''ls space hyphen l space slash sys slash class slash pipeline grep my underscore class.'''

Here, you can see the new '''my_class''' is registered successfully.

Similarly check whether a '''device''' is created in the '''sysfs '''or not.

Type '''ls space hyphen l space slash sys slash class slash my class.'''

This shows that the '''new_device''' is created in the '''sysfs'''.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type '''ls -l /dev/new_device'''

Highlight the output
| style="border:1pt solid #000000;padding:0.176cm;"| Let us check if the '''device file''' is created under the '''dev directory '''or not.

Type''' ls space hyphen l space slash dev slash new_device.'''

You can see the '''new_device file''' is created.

The '''user''' can access this''' file''' to transfer the '''data''' to the '''new_device''' using its''' driver.'''
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type >> '''clear'''
| style="border:1pt solid #000000;padding:0.176cm;"| Clear the screen.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type''' rmmod simple_driver .ko'''

Type '''dmesg | grep simple_driver'''

Type >> clear
| style="border:1pt solid #000000;padding:0.176cm;"| Now let us unload the '''driver.'''

Type '''rmmod space simple_driver dot ko'''

Clear the screen.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight the '''output '''
| style="border:1pt solid #000000;padding:0.176cm;"| To see the unloaded '''printk '''messages type the '''dmesg command''' as shown.

The output shows that the '''device file''' and its '''class''' are removed from the '''kernel.'''

It indicates that the '''device''' is also '''unregistered''' here. 

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

Type >> '''exit'''
| style="border:1pt solid #000000;padding:0.176cm;"| Let us remove the '''object files''' created after the '''compilation.'''

Type '''make clean.'''

To be a regular '''user''' , type''' exit.'''

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

* Create a''' class''' of a '''device''' in the '''sysfs.'''
* Create a''' device file''' in '''sysfs '''and''' dev directory. '''
|-
| 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_driver.c file.'''
# Create a''' device '''with a different name.
# Compile and load the '''simple_driver dot c.'''
# See the newly created '''device file''' under the '''sysfs.'''
|-
| 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;"| Please post your timed queries in this forum.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 11:

Forum for specific questions:
| style="border:1pt 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:1pt solid #000000;padding:0.176cm;"| Slide 12:

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

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 13:

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.

|}