void ls_directory(char *dir_name, int level)
{
    DIR *p_dir = NULL; // pointer to a directory
    struct dirent *p_entry; // pointer to a directory entry
    if(!(p_dir = opendir(dir_name)))
    {
        fprintf(stderr, "Error while opening directory %s: %s\n", dir_name, strerror(errno));
        return;
    }

    size_t dir_name_len = strlen(dir_name);
    printf("%s\n", dir_name);
    while((p_entry = readdir(p_dir)))
    {
        if(is_dir(p_entry->d_name))
        {
            // do not list the parent directories
            if(strcmp(p_entry->d_name, ".") != 0 && strcmp(p_entry->d_name, "..") != 0)
            {
                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);
            }
        }
        else
        {
            printf("%s\n", p_entry->d_name);
        }
    }
    closedir(p_dir);
}