// do not list the parent directories
if(strcmp(p_entry->d_name, ".") != 0 && strcmp(p_entry->d_name, "..") != 0)
{
    size_t dir_name_len = strlen(dir_name);
    size_t name_len = strlen(p_entry->d_name);

    char *subname = malloc(dir_name_len + name_len + 2);
    memcpy(subname, dir_name, dir_name_len);
    subname[dir_name_len] = '/';
    memcpy(subname + dir_name_len + 1, p_entry->d_name, name_len+1);
    ls_directory(subname, level + 1);
    free(subname);
}