# 2019q1 Homework9 (daemon)
contributed by < `zodf0055980` >
###### tags: `Linux 核心設計`
## 修改
首先先嘗試傳輸一些簡單的字串

[  336.832786] fastecho: socket create ok....
[  336.832788] fastecho: setsockopt ok....
[  336.832791] fastecho: socket bind ok....
[  336.832793] fastecho: socket listen ok....
[  352.349299] fastecho: start get response
[  392.883371] fastecho: get request = aaa
               3d\xe0f\xea\xfd\xb5U
[  392.883373] fastecho: start send request.
[  392.883402] fastecho: send request = aaa
               3d\xe0f\xea\xfd\xb5U
[  392.883403] fastecho: start get response
[  396.968222] fastecho: get request = bbb
               3d\xe0f\xea\xfd\xb5U
[  396.968224] fastecho: start send request.
[  396.968258] fastecho: send request = bbb
               3d\xe0f\xea\xfd\xb5U
[  396.968259] fastecho: start get response
[  400.748929] fastecho: get request = bbb
               3d\xe0f\xea\xfd\xb5U
[  400.748932] fastecho: start get response
[  400.748935] fastecho: get request = bbb
               3d\xe0f\xea\xfd\xb5U
[  400.748936] fastecho: start send request.
[  400.749006] fastecho: send request = bbb
               3d\xe0f\xea\xfd\xb5U

發現最後傳輸的字串會多執行好幾次 get request 和 send request，以及傳送字串有產生亂碼，並且亂碼每次重新執行的結果皆不同。因此觀看程式碼：

static int fastecho_init_module(void)
{
    int error = open_listen(&listen_sock);
    if (error < 0) {
        printk(KERN_ERR MODULE_NAME ": listen socket open error\n");
        return error;
    }

    param.listen_sock = listen_sock;

    echo_server = kthread_run(echo_server_daemon, &param, MODULE_NAME);
    if (IS_ERR(echo_server)) {
        printk(KERN_ERR MODULE_NAME ": cannot start server daemon\n");
        close_listen(listen_sock);
    }

    return 0;
}

可以看到在 init_module 的函式中利用 kthread_run 去在背景程式中執行一個 server 端的程序。

    while (!kthread_should_stop()) {
        res = get_request(sock, buf, BUF_SIZE - 1);
        if (res <= 0) {
            if (res) {
                printk(KERN_ERR MODULE_NAME ": get request error = %d\n", res);
            }
            break;
        }

        res = send_request(sock, buf, strlen(buf));
        if (res < 0) {
            printk(KERN_ERR MODULE_NAME ": send request error = %d\n", res);
            break;
        }
    }

    res = get_request(sock, buf, BUF_SIZE - 1);
    res = send_request(sock, buf, strlen(buf));

而程序中透過 while (!kthread_should_stop()) 去確保可以一直執行到我們把它關閉。而關閉後則會在傳送和接收字串一次，因此導致 dmesg 的訊息會重複。我首先將它註解調並在執行 get_request 前先透過 memset 把 buffer 給清空。

while (!kthread_should_stop()) {
    memset(buf,0,BUF_SIZE);
    res = get_request(sock, buf, BUF_SIZE - 1);
    if (res <= 0) {
        if (res) {
            printk(KERN_ERR MODULE_NAME ": get request error = %d\n", res);
        }
        break;
    }

    res = send_request(sock, buf, strlen(buf));
    if (res < 0) {
        printk(KERN_ERR MODULE_NAME ": send request error = %d\n", res);
        break;
    }
}

// res = get_request(sock, buf, BUF_SIZE - 1);
// res = send_request(sock, buf, strlen(buf));

[ 2829.146997] fastecho: socket listen ok....
[ 2831.793319] fastecho: start get response
[ 2833.345299] fastecho: get request = aaa

[ 2833.345301] fastecho: start send request.
[ 2833.345357] fastecho: send request = aaa

[ 2833.345359] fastecho: start get response
[ 2835.341717] fastecho: get request = bbb

[ 2835.341720] fastecho: start send request.
[ 2835.341764] fastecho: send request = bbb

[ 2835.341766] fastecho: start get response
[ 2838.395686] fastecho: get request = 

發現雖然沒有亂碼了，但顯示的訊息仍然可有換行字元的存在，並且最後還會在傳送一個空的字串。

[kernel-recvmsg](https://www.kernel.org/doc/htmldocs/networking/API-kernel-recvmsg.html) 內有說到： The returned value is the total number of bytes received, or an error.
嘗試去把接收到的字串長度顯示出來：

[ 3254.286760] fastecho: start get response
[ 3255.487711] fastecho: length = 4
[ 3255.487713] fastecho: get request = aa

[ 3255.487714] fastecho: start send request.
[ 3255.487772] fastecho: length = 3
[ 3255.487773] fastecho: send request = aa

發現仍然有多餘的字元存在，並且程式中沒有考慮到接收失敗的情況。