C: srand does not influence random number generator

Question

I have the following c code:

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

int main(int argc, char *argv[]) {
  srand(time(NULL));
  printf("%d\n", (int)random());
  return 0;  
}

To my understanding, this should print a different random number every time I execute the program because the random seed is dependent of the system time.

But every time run the program, I get exactly the same output:

1804289383

I still get the same output when I put custom values as argument for srand:

srand(1);

Or

srand(12345);

Does anyone have an Idea why this happens? Maybe it is because of my operating system (Mac OS 10.10.3)? Or the compiler I use (gcc)?

Are there simple alternatives?

Answer 1

Well, your problem here is due to multiple way of making random numbers in C with the standard libraries.

Basically, there is two sets of functions to generate a random number :

From the rand(3) manual:

   #include <stdlib.h>

   int rand(void);
   int rand_r(unsigned int *seedp);
   void srand(unsigned int seed);

From the random(3) manual:

   #include <stdlib.h>

   long int random(void);

   void srandom(unsigned int seed);

   char *initstate(unsigned int seed, char *state, size_t n);
   char *setstate(char *state);

You should just pick the one that suit the best your need. I invite you to further read those manuals for more information ;-)