Hello World (part 2)

As of Linux 2.4, you can rename the init and cleanup functions of your modules; they no longer have to be called init_module() and cleanup_module() respectively. This is done with the module_init() and module_exit() macros. These macros are defined in linux/init.h. The only caveat is that your init and cleanup functions must be defined before calling the macros, otherwise you'll get compilation errors. Here's an example of this technique:

Example 2-3. hello-2.c

/*  hello-2.c - Demonstrating the module_init() and module_exit() macros.  This is the 
 *     preferred over using init_module() and cleanup_module().
 *  Copyright (C) 2001 by Peter Jay Salzman
 *  08/02/2006 - Updated by Rodrigo Rubira Branco <rodrigo@kernelhacking.com>

/* Kernel Programming */
#define MODULE
#define LINUX
#define __KERNEL__

#include <linux/module.h>   // Needed by all modules
#include <linux/kernel.h>   // Needed for KERN_ALERT
#include <linux/init.h>     // Needed for the macros

static int hello_2_init(void)
   printk(KERN_ALERT "Hello, world 2\n");
   return 0;

static void hello_2_exit(void)
   printk(KERN_ALERT "Goodbye, world 2\n");



So now we have two real kernel modules under our belt. With productivity as high as ours, we should have a high powered Makefile. Here's a more advanced Makefile which will compile both our modules at the same time. It's optimized for brevity and scalability. If you don't understand it, I urge you to read the makefile info pages or the GNU Make Manual.

Example 2-4. Makefile for both our modules

WARN    := -W -Wall -Wstrict-prototypes -Wmissing-prototypes
INCLUDE := -isystem /lib/modules/`uname -r`/build/include
CC      := gcc-3.0
OBJS    := ${patsubst %.c, %.o, ${wildcard *.c}}

all: ${OBJS}

.PHONY: clean

    rm -rf *.o

As an exercise to the reader, if we had another module in the same directory, say hello-3.c, how would you modify this Makefile to automatically compile that module?