// gcc -std=c99
#include <stdbool.h>
#include <stdio.h>
#include <string.h>

static int
count_substr(const char *str, const char* substr, bool overlap) {
  if (strlen(substr) == 0) return -1;

  int count = 0;
  int increment = overlap ? 1 : strlen(substr);
  for (char* s = (char*)str; (s = strstr(s, substr)); s += increment)
    ++count;
  return count;
}

int main() {
  char *substrs[] = {"a", "aa", "aaa", "b", "", NULL };

  for (char** s = substrs; *s != NULL; ++s)
    printf("'%s' ->  %d, no overlap: %d\n", *s, count_substr("aaaaa", *s, true),
	   count_substr("aaaaa", *s, false));
}