[kernelreadfans] about add system call.

  • From: mingz <mingz@xxxxxxxxxxx>
  • To: "kernelreadfans@xxxxxxxxxxxxx" <kernelreadfans@xxxxxxxxxxxxx>
  • Date: Thu, 29 May 2003 10:41:40 -0400

I test the add system call today and I believe it is possible to add system 
call in module. i think the reason to modify the kernel code is to persist 
the system call in kernel only. if u only need dynamically use it. this is a 
way. Wait for comments:

compile the module using a simple makefile, see my post in bbs;
compile the user space code using gcc -o call call.c is enough

------------------kernel module ------------------------------------
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/slab.h>

#ifndef __NR_addtotal
#define __NR_addtotal 222

int (*old_sys) (struct pt_regs regs);
extern void *sys_call_table[];

asmlinkage int
sys_myfunc (int myinput)
  printk ("1. jjww printk from kernel %d\n", myinput);
  return myinput;

hello_init (void)
  old_sys = sys_call_table[__NR_addtotal];
  sys_call_table[__NR_addtotal] = sys_myfunc;
  return 0;

hello_clean (void)
  if (sys_call_table[__NR_addtotal] != sys_myfunc)
      printk ("!!WARNING!! Another module hook\n");
  sys_call_table[__NR_addtotal] = old_sys;

module_init (hello_init);
module_exit (hello_clean);

----------------------------user space code---------------------------------
#include <stdio.h>
#include <stdlib.h>
#include <linux/unistd.h>

#ifndef __NR_addtotal
#define __NR_addtotal   222

_syscall1 (int, addtotal, int, num)
     int main ()
  int i, j;

    printf ("Please input a number\n");
  while (scanf ("%d", &i) == EOF);
  if ((j = addtotal (i)) == -1)
      printf ("Error occurred in syscall-addtotal();\n");
      exit (1);
  printf ("return %d \n", j);


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