Question

I need fast way to generate ip numbers that are valid (reserved ips are valid too).
For now i am using this:

unsigned char *p_ip;
unsigned long ul_dst;

p_ip = (unsigned char*) &ul_dst;
for(int i=0;i<sizeof(unsigned long);i++)
  *p_ip++ = rand()%255;
ip.sin_addr.s_addr = ul_dst;

But sometimes it generate non-valid numbers, but this code can generate about 10k of valid ips in a second. Can anyone contribute?
Thank you

Answer 1

calling rand() is probably the slowest part of your code, if you use the implementation of a random function found at http://en.wikipedia.org/wiki/Multiply-with-carry

This is an ultra fast C function for generating random numbers.

storing sizeof(unsigned long) in a registered variable i.e.:

register int size = sizeof(unsigned long)

should also help slightly.

Since you are using 4 chars = 4 x 8 byte memory, you can instead use a 32bit integer which will only require one memory address.
combining the bitshifting, new random method, registered variables, should reduce running times by quite a bit.

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <time.h>

uint32_t ul_dst;
init_rand(time(NULL));
uint32_t random_num = rand_cmwc();

ul_dst = (random_num >> 24 & 0xFF) << 24 | 
         (random_num >> 16 & 0xFF) << 16 | 
         (random_num >> 8 & 0xFF) << 8 | 
         (random_num & 0xFF);

printf("%u\n",ul_dst);
return 0;

Above this code I have the exact copy of the random function from wikipedia.
Hopefully this will run much faster.
We know the size of a 32bit int is 4*8 so no need for the sizeof anymore, and instead of %255 I replaced it with a 255 bit mask