Calculation of the nth term of the Fibonacci sequence in constant time

fibonacci.c

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

uint64_t fibonacci(uint64_t n) {
  static const double golden = 1.61803398874989484820;
  return (uint64_t)floor((pow(golden, (double)n) / sqrt(5.0)) + 0.5);
}


int main(int argc, char * argv[]) {
  if (argc != 2) {
    fprintf(stderr, "Prints the nth number in the fibonacci sequence (0, 1, 1,"
                    " 2, 3, 5,...)\n"
                    "Usage: %s <n>\nwhere:\n\tn is a non-negative integer\n",
                    argv[0]);
    return -1;
  }

  uint64_t n;
  if (sscanf(argv[1], "%" SCNu64, &n) != 1) {
    fprintf(stderr, "Failed to parse integer from '%s'\n", argv[1]);
    return -2;
  }

  printf("%" PRIu64 "\n", fibonacci(n));

  return 0;
}