Given string 'haystack', find if sub-string 'needle' exists.

find_substring.c

char *strstr(const char *haystack, const char *needle)
{
    size_t h_len = strlen(haystack);
    size_t n_len = strlen(needle);
    size_t i, j;
    for (i = 0; i < h_len; i++)
    {
        size_t found = 0;
        for (j = 0; j < n_len; j++)
        {
            if (i + n_len < h_len)
                if (haystack[i + j] == needle[j])
                    found++;
        }
        if (found == n_len)
            return &needle[i];
    }
    return NULL;
}