linux CPU loading calculate

http://hi.baidu.com/lionpanther/item/e908c37abdaf1c3f71442380

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>  
struct occupy       
{
   char name[20];      
   unsigned int user;  
   unsigned int nice;  
   unsigned int system;
   unsigned int idle; 
};
float g_cpu_used;          
int cpu_num;               
void cal_occupy(struct occupy *, struct occupy *); 
void get_occupy(struct occupy *);

int main()                  
{
   struct occupy ocpu[10];   
   struct occupy ncpu[10];   
   int i;

cpu_num = sysconf(_SC_NPROCESSORS_ONLN);
   for(;;)                            
   {  
      sleep(1);                       
      get_occupy(ocpu);                
      sleep(1);                       
      get_occupy(ncpu);                
      for (i=0; i<cpu_num; i++)         
      {
         cal_occupy(&ocpu[i], &ncpu[i]); 
         printf("%f \n", g_cpu_used);    
      }
   }
}
void cal_occupy (struct occupy *o, struct occupy *n)
{
   double od, nd;   
   double id, sd;   
   double scale;    
   od = (double) (o->user + o->nice + o->system +o->idle);//第一次(用户+优先级+系统+空闲)的时间
   nd = (double) (n->user + n->nice + n->system +n->idle);//第二次(用户+优先级+系统+空闲)的时间
   scale = 100.0 / (float)(nd-od);      
   id = (double) (n->user - o->user);   
   sd = (double) (n->system - o->system);
   g_cpu_used = ((sd+id)*100.0)/(nd-od);
}
void get_occupy (struct occupy *o)
{
   FILE *fd;        
   int n;           
   char buff[1024];  
   fd = fopen ("/proc/stat", "r"); 
   fgets (buff, sizeof(buff), fd); 
   for(n=0;n<cpu_num;n++)          
   {
      fgets (buff, sizeof(buff),fd);
      sscanf (buff, "%s %u %u %u %u", &o[n].name, &o[n].user, &o[n].nice,&o[n].system, &o[n].idle);
      fprintf (stderr, "%s %u %u %u %u\n", o[n].name, o[n].user, o[n].nice,o[n].system, o[n].idle);
   }
   fclose(fd);    
}