Basic C code to get the nth prime number in a sequence.

nth_prime.c

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

unsigned long nth_prime(unsigned long n) {
	unsigned long p = 2, i=0, c;
	while (i < n) {
		unsigned long n_f = 0; // number of factors
		for (c=(unsigned long)sqrt(p); c>0; c--)
			n_f += (p%c==0);
		i += (n_f==1); // we should only have 1 factor
		p++;
	}
	return --p; // our prime number
}

int main(int argc, char *argv[]) {
	int i=1;
	for (i; i<argc; i++) {
		unsigned long n;
		sscanf(argv[i], "%lu", &n);
		printf("Prime number at interval %lu is %lu.\n", n, nth_prime(n));
	}
	return 0;
}

Updated, since the old version was inaccurate and unnecessarily dependent on booleans.