C - Library functions - Using srand() and rand()

Today a tiny tutorial to see how using srand() and rand() from the libc.

We will generate a number in a range between 0 and 2.
For that, we will use the functions:

• time()
• srand()
• rand()
• sleep()

The time() function because we want a random number that changes every second.
We will use it with the sleep() function.

We put a seed, generated by the time() function), into the srand() one.
Then we display this number with the rand() function.
And as we want a number between 0 and 2, we use the modulo operator (%).

Let’s see it in details with this srand() / rand() example:

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

int main (int argc, char *argv[])
{
  int seed;
  int i;

  i = 0;
  seed = time(NULL);
  srand(seed);
  while (i < 5)
  {
    sleep(1);
    printf("rand[%d]= %d\n", i, rand() % 3);
    ++i;
  }
  return 0;
}

Result:

Every second, we can see something like that:

rand[0]= 2

You can now generate all random numbers you want. Well done.