A simple linear congruence random number generator

/* Generating random number sequences using the formula (linear congruence)
x[k+1] = (a*x[k] + c)mod m
where a, c, and m are parameters set by the user and passed as command line 
parameters together with a seed i.e. x[0]
As a simple example try  a=7, c=1, m=13, and seed=5
A more sophisticated selection would be a=69069, c=0, 
m=2^32=4294967296, and seed=31
It will print out, in a sort of random order, up to m-1 distinct values. 
Then it loops.
*/

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

static long seed = 13;
static long a;
static long c;
static long m;

void random_init(long s) {
if (s != 0) seed = s;
}

long random() {
seed = (a*seed + c)%m;
return seed;
}

int main(int argc, char * argv[]) {
if (argc != 5) {
printf("usage: %s a, c, m, seed\n", argv[0]);
return 1;
}
a = atoi(argv[1]);
c = atoi(argv[2]);
m = atoi(argv[3]);
long s = atoi(argv[4]);
random_init(s);
int k;
for (k = 0; k < m-1; k++) { 
printf("%8ld", random());
if (k % 8 == 7) { // after 8 elements go to a new line
printf("\n");
sleep(1); // sleep for a second
} 
}
printf("\n");
return 0;
}