5.2.2. A first hello_world module

Code:

/*                                                                                                                                               
 * A first simple hello_world kernel module                                                                                                      
 */

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

static int hello_init(void)
{
        printk(KERN_ALERT "Hello world!\n");
        return 0;
}

static void hello_exit(void)
{
        printk(KERN_ALERT "Goodbye, cruel world\n");
}

module_init(hello_init);
module_exit(hello_exit);

Makefile:

obj-m := hello.o

Build it:

make -C ~/git/linux-denx/ M=`pwd` modules ARCH=powerpc

loading

• insmod
• modprobe

Extentions:

• __init, __initdata, __exit, __exitdata (include/linux/init.h)

• __devinit / __devexit (normal function if !CONFIG_HOTPLUG, else equal __init / __exit

• MODULE_AUTHOR, MODULE_LICENSE, MODULE_DESCRIPTION

• Kernel symbol visibility: static, EXPORT_SYMBOL[_GPL[_FUTURE]](a)

• check out kernel symbols: /proc/kallsyms (nm(1))