How does srand relate to rand function?

Question

I understand that rand() function generates the same number(s) each you run it if you don't change the seed number. That's where srand() comes in. Time is always changing so I know that you should pass the time(null) parameter to srand. My question is with the code below from a tutorial site.

int main()
{
    int i, n=5;
    time_t t;

    /* Intializes random number generator */
    srand((unsigned) time(&t));

    /* Print 5 random numbers from 0 to 50 */
    for( i = 0 ; i < n ; i++ ) {
        printf("%d\n", rand() % 50);
    }

    return(0);
}

I see no link from the srand

((unsigned) time(&t)); 

and rand.

printf("%d\n", rand() % 50);

Where is the connection between rand and srand? What I mean or expect is I assume rand() will get some parameter from srand() so it knows to generate different numbers each time. I assume it would look something like rand(srand(time(null));

It's like initializing a variable without using it to me. srand is being initialized, but I don't see it being used.

Does rand generate different numbers because srand is called first before rand?

Answer 1

The random number seed is a global static variable. rand and srand both have access to it.

Answer 2

srand() sets the seed which is used by rand to generate "random" numbers (in quotes because they're generally pseudo-random). If you don't call srand before your first call to rand, it's as if you had called srand(1) to set the seed to one.

A lot of code uses the current time as the seed so as to make each program run use a different sequence of random numbers but you can always change that to something like srand(42) during debugging, for the purposes of repeatability. And the call to time() doesn't actually need a variable to place the time in, you can just pass NULL:

srand (time (NULL));

The whole thing could be implemented in a single file with something like the following, the example given in the standard (ISO C99 7.20.2.2 The srand function).

// RAND_MAX assumed to be 32767.
static unsigned long int next = 1;
void srand(unsigned int seed) { next = seed; }
int rand(void) {
    next = next * 1103515245 + 12345;
    return (unsigned int)(next/65536) % 32768;
}

The fact that next is a static variable at the top of the file means that it's invisible to everything outside the file but visible to everything inside (sort of a localised global). That's the communication method between srand() and rand().