Find the Index of the First Occurrence in a String

int strStr(char * haystack, char * needle){
int haystack_len = strlen(haystack);
    int needle_len = strlen(needle);

    if (needle_len == 0) {
        return 0;
    }

    for (int i = 0; i <= haystack_len - needle_len; i++) {
        int j;
        for (j = 0; j < needle_len; j++) {
            if (haystack[i + j] != needle[j]) {
                break;
            }
        }
        if (j == needle_len) {
            return i;
        }
    }

    return -1;
}