This small piece of code was used during the analysis of an 800GB VMWare image after a server crash. notnull.c: 
#include <stdio.h>
#include <stdlib.h>

int main(int argc, const char* argv[]) {
  if (argc != 2) {
    printf("give filenamen");
    exit(1);
  }


  FILE *f = fopen(argv[1], "r");

  const unsigned int BUFSIZE = 4096;
  unsigned char buffer[BUFSIZE];

  unsigned long cnt = 0;
  unsigned long found = 0;
  size_t r;

  while (0 != (r = fread(&buffer, 1, BUFSIZE, f))) {
    cnt += r;
    for (int i = 0; i < r; i++) {
      if (0 != *(buffer + i)) {
        found++;
      }
    }
    if (0 == (cnt % 1000000)) {
      printf("cnt: %ld, found: %ldn", cnt, found);
    }
  }

  printf("finally: cnt: %ld, found: %ldn", cnt, found);
}
Compile it using: 
gcc -std=c99 -o notnull notnull.c