int indexof(const char *s1, const char *s2) { int length1, length2; const char *tmp = s1; length2 = strlen(s2); if (!length2) { /* code */ printf("The s2 is NULL!.\n"); exit(1); } length1 = strlen(tmp); while(length2 <= length1) { length1--; if (!memcmp(tmp, s2, length2)) { /* code */ return (tmp - s1); } tmp++; } return -1; }