strstr

#include <stdio.h>
#include <string.h>
int main(void) {
  FILE *in = fopen("a.txt", "r");
  const char *s = "ab";
  char buf[BUFSIZ];
  int c = 0;
  while (fgets(buf, BUFSIZ - 1, in)) {
    char *p = buf;
    while (1) {
      char *p2 = strstr(p, s);
      if (p2 != NULL) c++;
      else break;
      p = p2 + strlen(s);
    }
  }
  fclose(in);
  printf("%d\n", c);
  return 0;
}