Listing 1. example 2.c

#include <linux/kernel.h>
#include <linux/module.h>
#include <sys/syscall.h>

extern void *sys_call_table[];


asmlinkage int (*original_sys_exit)(int);

asmlinkage int our_fake_exit_function(int error_code)
{

        /*print message on console every time we
         *are called*/
        printk("HEY! sys_exit called with error_code=%d\n",error_code);

        /*call the original sys_exit*/
        return original_sys_exit(error_code);
}

/*this function is called when the module is
 *loaded (initialization)*/
int init_module()
{
        /*store reference to the original sys_exit*/
        original_sys_exit=sys_call_table[__NR_exit];

        /*manipulate sys_call_table to call our
         *fake exit function instead
         *of sys_exit*/
        sys_call_table[__NR_exit]=our_fake_exit_function;
}


/*this function is called when the module is
 *unloaded*/
void cleanup_module()
{

        /*make __NR_exit point to the original
         *sys_exit when our module
         *is unloaded*/
        sys_call_table[__NR_exit]=original_sys_exit;

}