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");
}
module_init(hello_2_init);
module_exit(hello_2_exit);
MODULE_LICENSE("GPL");
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
CFLAGS := -O2 -DMODULE -D__KERNEL__ ${WARN} ${INCLUDE}
CC := gcc-3.0
OBJS := ${patsubst %.c, %.o, ${wildcard *.c}}
all: ${OBJS}
.PHONY: clean
clean:
rm -rf *.oAs 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?
