strstr

gistfile1.c

char* my_strstr(const char* haystack, const char* needle) {
    if (!*needle)
        return haystack;

    char* h = haystack;
    char* n = needle;
    int found = 0;
    while (*h) {
        char* hp = h;
        while (*hp && *n && *hp == *n) {
            n++;
            hp++;
        }
        if (*n == '\0')
            return h;
        if (*hp == '\0')
            return NULL;
        h++;
    }
    return NULL;
}

char * print_helper(char* str) {
    if (str != NULL)
        return str;
    else
        return "NULL";
}

int main() {
    char *haystack = "abcdefghijklmn";
    printf("%s\n", print_helper(my_strstr(haystack, "abc")));
    printf("%s\n", print_helper(my_strstr(haystack, "gh")));
    printf("%s\n", print_helper(my_strstr(haystack, "n")));
    printf("%s\n", print_helper(my_strstr(haystack, "")));
    printf("%s\n", print_helper(my_strstr(haystack, "zzz")));
    printf("%s\n", print_helper(my_strstr(haystack, "mnop")));
    return 0;
}

Output:

abcdefghijklmn
ghijklmn
n
abcdefghijklmn
NULL
NULL