linux-kernel Linux Hello World Device driver An empty kernel module

Example

#include <linux/init.h>
#include <linux/module.h>

/**
 * This function is called when the module is first loaded.
 */    
static int __init hello_kernel_init(void)
{
    printk("Hello, World!\n");
    return 0;
}

/**
 * This function is called when is called if and when the module is unloaded.
 */
static void __exit hello_kernel_exit(void)
{
    printk("Goodbye, cruel world...\n");
}

/* The names of the init/exit functions are arbitrary, and they are bound using the following macro definitions */
module_init(hello_kernel_init);
module_exit(hello_kernel_exit);

In order to write a Linux device driver (Character-device, Block-device, etc...), it is necessary to create a kernel module that has an entry and exit points.

By itself, the kernel module does nothing; it has no meaningful way to communicate with the userspace. Using the entry point it is possible to create a new character-device, for example, which is then used to communicate with the userspace.