Linux Module Stacking Example

What is module stacking?

New modules using the symbols exported by old modules.

Examples of modules stacking in Linux Kernel?

• Msdos filesystem relies on symbols exported by fat module
• Parallel port printer driver (lp) relies on symbols exported by generic parallel port driver (parport)

Let's write a sample device driver code which exports a function myadd performing addition of two numbers:

	#include <linux/kernel.h>
	#include <linux/module.h>
	MODULE_LICENSE("GPL");
	int myadd(int a, int b)
	{
	pr_info("%s: Adding %d with %d\t Result:%d\n",
	__func__, a, b, a+b);
	return a+b;
	}
	EXPORT_SYMBOL(myadd);
	static int module1_init(void)
	{
	pr_info("%s: In init\n", __func__);
	return 0;
	}
	static void module1_exit(void)
	{
	pr_info("%s: In exit\n", __func__);
	}
	module_init(module1_init);
	module_exit(module1_exit);

view raw module1.c hosted with ❤ by GitHub

Code:

Now, we will write another module which uses the exported function:

Code:

	#include <linux/kernel.h>
	#include <linux/module.h>
	MODULE_LICENSE("GPL");
	extern int myadd(int a, int b);
	static int module2_init(void)
	{
	pr_info("%s: In init\n", __func__);
	pr_info("%s: Add:%d\n", __func__, myadd(3, 5));
	return 0;
	}
	static void module2_exit(void)
	{
	pr_info("%s: In exit\n", __func__);
	}
	module_init(module2_init);
	module_exit(module2_exit);

view raw module2.c hosted with ❤ by GitHub

Makefile to Compile two modules:

	obj-m := module1.o
	obj-m += module2.o
	all:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
	clean:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

view raw Makefile hosted with ❤ by GitHub

Build the modules by running "make", Load the first module and then second module using insmod.

Output: