Difference between revisions of "Embedded-Linux-Device-Driver/C2/Exporting-Symbols-from-a-Module/English"

Welcome slide:

Learning objectives:

• Export the symbols from a module.
• Import the symbols in a module.

System Requirements:

• VirtualBox version 5.2.
• Ubuntu Linux 18.04 LTS operating system.
• Linux kernel version 5.0.0-31 generic.
• gedit text editor.

Prerequisites:

• C programming language
• Basics of Linux kernel

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

What is symbol?

• Symbol is either a variable or a function.

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

Point to the files.

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.

Code files

• 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

Type >> cd Desktop/DeviceDriver/ExportImportModule

Press the Enter key after every command.

Go to the directory ExportImportModule as shown here.

Type gedit space export dot c.

Let us understand the code now.

Open the terminal.

Type >> clear

The export.h file contains the exported symbols declaration.

Type >> gedit export.h

Highlight module_function() and num

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

EXPORT_SYMBOL_GPL(name)

Highlight EXPORT_SYMBOL_GPL(name)

Highlight name

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.

Highlight module_function()

Highlight printk message.

This function contains a printk message as shown here.

Highlight int num = 100;

The num variable is defined as shown here.

Highlight exit_func() function

Click on the close.

Switch back to the terminal.

Press Enter

Highlight #include “export.h”

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

Highlight num

Highlight init_func()

Highlight exit_func()

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.

Type >> clear

Press Ctrl + S

Click on the close.

Open the terminal

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.

Type >> password

Type >> make all

Now, type the system password.

Let us compile the modules.

Type make space all

Highlight the output

Type ls .

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

Press Enter

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

Clear the screen.

Press Enter

Type >> clear

Type the command as shown.

Clear the screen.

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

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

Highlight modprobe.

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.

Type lsmod space pipe space head space hyphen 5.

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

Highlight the output

Type dmesg space pipe space tail hyphen 5

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

Type modprobe space hyphen r space import

The modprobe automatically unloads the export dot ko module.

Show the output

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.

Type >> clear

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.

Let us summarize.

Summary:

• Export the symbols from a module.
• Import the symbols in a module.

Assignment:

1. Open the export.c file
2. Change the printk message of module_function.
3. Compile the export and import modules.
4. Load the modules using the modprobe tool.
5. See the output using dmesg command.

About Spoken Tutorial Project:

• The video at the following link summarizes the Spoken Tutorial project.

• Please download and watch it.

Spoken Tutorial workshops :

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Forum questions :

Forum for specific questions :

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

Acknowledgment:

Thank you slide:

This is Mayuri Panchakshari signing off.

Thanks for watching.