Find all Mersenne Primes in C given enough memory and time

mersenne.c

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

int
isPrime(uintmax_t n) {
  if (n < 2) {
    return 0;
  } else if (n == 2 || n == 3) {
    return 1;
  } else if (n % 2 == 0 || n % 3 == 0) {
    return 0;
  } else {
    double max_divisor = sqrt(n);
    int divisor = 5;
    while (divisor <= max_divisor) {
      if (n % divisor == 0 || n % (divisor + 2) == 0) {
        return 0;
      }
      divisor += 6;
    }
    return 1;
  }
}

int
isMersennePrime(uintmax_t n) {
  return (isPrime(n) && (fmod(log2(n+1), 1) == 0.0));
}

int main(int argc, char** argv) {
  uintmax_t num = 1;
  while (1) {
    if(isMersennePrime(num)) {
      printf("%lu is a mersenne prime\n", num);
    }
    num+=2; // only odd numbers can be mersenne primes
  }
}