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            對(duì)一個(gè)奇怪SOCKET問(wèn)題的研究

               今天測(cè)試網(wǎng)絡(luò)服務(wù)程序時(shí)發(fā)現(xiàn)這樣一個(gè)現(xiàn)象:客戶端登錄到服務(wù)器,服務(wù)器如果驗(yàn)證發(fā)現(xiàn)用戶名不存在,就返回客戶端錯(cuò)誤信息,并斷開(kāi)與客戶端的連接。但是實(shí)際測(cè)試時(shí)卻發(fā)現(xiàn)客戶端并沒(méi)有接收到用戶名不存在的錯(cuò)誤信息,并且明明服務(wù)器端關(guān)閉了連接,甚至停止了服務(wù),但是客戶端仍然顯示是連接狀態(tài)。

               調(diào)試,發(fā)現(xiàn)在斷開(kāi)連接操作之前(即CLOSE SOCKET之前),加斷點(diǎn)或者寫(xiě)LOG或者SLEEP幾毫秒后,客戶端都可接收到錯(cuò)誤信息,并成功斷開(kāi)。于是分析覺(jué)得問(wèn)題可能出在SOCKET的IO處理上,可能SOCKET IO中的數(shù)據(jù)沒(méi)有足夠的時(shí)間完全發(fā)送,SOCKET就被關(guān)閉了。

               仔細(xì)檢查代碼發(fā)現(xiàn)CLOSE SOCKET前做了這樣的操作:

            LINGER lingerStruct;
            lingerStruct.l_onoff  = 1;    
            lingerStruct.l_linger = 0;
            setsockopt( IoSocket, SOL_SOCKET, SO_LINGER,    (char *)&lingerStruct, sizeof(lingerStruct) );
            CancelIo((HANDLE) IoSocket);
            closesocket( IoSocket );
               
               在MSDN中查找setsockeopt關(guān)于LINGER的解釋如下:

            Setting the SO_DONTLINGER option prevents blocking on member function Close while waiting for unsent data to be sent. Setting this option is equivalent to setting SO_LINGER with l_onoff set to 0.

                若設(shè)置了SO_LINGER,并設(shè)置了零超時(shí)間隔,則closesocket()不被阻塞立即執(zhí)行,不論是否有排隊(duì)數(shù)據(jù)未發(fā)送或未被確認(rèn)。這種關(guān)閉方式稱(chēng)為“強(qiáng)制”或“失效”關(guān)閉,因?yàn)樘捉涌诘奶撾娐妨⒓幢粡?fù)位,且丟失了未發(fā)送的數(shù)據(jù)。在遠(yuǎn)端的recv()調(diào)用將以WSAECONNRESET出錯(cuò)。
               若設(shè)置了SO_LINGER并確定了非零的超時(shí)間隔,則closesocket()調(diào)用阻塞進(jìn)程,直到所剩數(shù)據(jù)發(fā)送完畢或超時(shí)。這種關(guān)閉稱(chēng)為“優(yōu)雅的”關(guān)閉。請(qǐng)注意如果套接口置為非阻塞且SO_LINGER設(shè)為非零超時(shí),則closesocket()調(diào)用將以WSAEWOULDBLOCK錯(cuò)誤返回。
               若在一個(gè)流類(lèi)套接口上設(shè)置了SO_DONTLINGER,則closesocket()調(diào)用立即返回。但是,如果可能,排隊(duì)的數(shù)據(jù)將在套接口關(guān)閉前發(fā)送。請(qǐng)注意,在這種情況下WINDOWS套接口實(shí)現(xiàn)將在一段不確定的時(shí)間內(nèi)保留套接口以及其他資源,這對(duì)于想用所以套接口的應(yīng)用程序來(lái)說(shuō)有一定影響。
              簡(jiǎn)言之,setsockeopt函數(shù)使用SO_LINGER規(guī)定了斷開(kāi)SOCKET時(shí)處理未發(fā)送完的數(shù)據(jù)的動(dòng)作。


               查詢UNIX文檔中關(guān)于SO_LINGER參數(shù)的解釋更加詳細(xì):

               SO_LINGER
               此選項(xiàng)指定函數(shù)close對(duì)面向連接的協(xié)議如何操作(如TCP)。缺省close操作是立即返回,如果有數(shù)據(jù)殘留在套接口緩沖區(qū)中則系統(tǒng)將試著將這些數(shù)據(jù)發(fā)送給對(duì)方。

            SO_LINGER選項(xiàng)用來(lái)改變此缺省設(shè)置。使用如下結(jié)構(gòu):
            struct linger {
                 int l_onoff; /* 0 = off, nozero = on */
                 int l_linger; /* linger time */
            };

            有下列三種情況:

            1. l_onoff為0,則該選項(xiàng)關(guān)閉,l_linger的值被忽略,等于缺省情況,close立即返回;
            2. l_onoff為非0,l_linger為0,則套接口關(guān)閉時(shí)TCP夭折連接,TCP將丟棄保留在套接口發(fā)送緩沖區(qū)中的任何數(shù)據(jù)并發(fā)送一個(gè)RST給對(duì)方,而不是通常的四分組終止序列,這避免了TIME_WAIT狀態(tài);
            3. l_onoff 為非0,l_linger為非0,當(dāng)套接口關(guān)閉時(shí)內(nèi)核將拖延一段時(shí)間(由l_linger決定)。如果套接口緩沖區(qū)中仍殘留數(shù)據(jù),進(jìn)程將處于睡眠狀態(tài),直 到(a)所有數(shù)據(jù)發(fā)送完且被對(duì)方確認(rèn),之后進(jìn)行正常的終止序列(描述字訪問(wèn)計(jì)數(shù)為0)或(b)延遲時(shí)間到。此種情況下,應(yīng)用程序檢查close的返回值是 非常重要的,如果在數(shù)據(jù)發(fā)送完并被確認(rèn)前時(shí)間到,close將返回EWOULDBLOCK錯(cuò)誤且套接口發(fā)送緩沖區(qū)中的任何數(shù)據(jù)都丟失。close的成功返 回僅告訴我們發(fā)送的數(shù)據(jù)(和FIN)已由對(duì)方TCP確認(rèn),它并不能告訴我們對(duì)方應(yīng)用進(jìn)程是否已讀了數(shù)據(jù)。如果套接口設(shè)為非阻塞的,它將不等待close完 成。
            l_linger的單位依賴(lài)于實(shí)現(xiàn),4.4BSD假設(shè)其單位是時(shí)鐘滴答(百分之一秒),但Posix.1g規(guī)定單位為秒。

               在了解了原理之后,將代碼中的lingerStruct.l_linger 設(shè)置為非零值,問(wèn)題立即被解決。

               這里把這個(gè)問(wèn)題寫(xiě)出來(lái),希望能夠給大家?guī)?lái)點(diǎn)啟示。

            posted on 2007-11-14 11:45 迷宮の未來(lái) 閱讀(3140) 評(píng)論(6)  編輯 收藏 引用

            評(píng)論

            # re: 關(guān)閉SOCKET時(shí)需注意的問(wèn)題[未登錄](méi) 2007-11-14 15:45 heroboy

            shutdown(...) first.  回復(fù)  更多評(píng)論   

            # re: 關(guān)閉SOCKET時(shí)需注意的問(wèn)題 2007-11-14 16:12 追夢(mèng)時(shí)代

            @heroboy
            謝謝,剛測(cè)試了shutdown也可以解決問(wèn)題  回復(fù)  更多評(píng)論   

            # re: 關(guān)閉SOCKET時(shí)需注意的問(wèn)題 2007-11-14 16:21 追夢(mèng)時(shí)代

            這里貼上MSDN對(duì)于shutdown的注意事項(xiàng),shutdown不管SO_LINGER如何設(shè)置都不會(huì)堵塞。

            The shutdown function is used on all types of sockets to disable reception, transmission, or both.

            If the how parameter is SD_RECEIVE, subsequent calls to the recv function on the socket will be disallowed. This has no effect on the lower protocol layers. For TCP sockets, if there is still data queued on the socket waiting to be received, or data arrives subsequently, the connection is reset, since the data cannot be delivered to the user. For UDP sockets, incoming datagrams are accepted and queued. In no case will an ICMP error packet be generated.

            If the how parameter is SD_SEND, subsequent calls to the send function are disallowed. For TCP sockets, a FIN will be sent after all data is sent and acknowledged by the receiver.

            Setting how to SD_BOTH disables both sends and receives as described above.

            The shutdown function does not close the socket. Any resources attached to the socket will not be freed until closesocket is invoked.

            To assure that all data is sent and received on a connected socket before it is closed, an application should use shutdown to close connection before calling closesocket. For example, to initiate a graceful disconnect:
            1. Call WSAAsyncSelect to register for FD_CLOSE notification.
            2. Call shutdown with how=SD_SEND.
            When FD_CLOSE received, call recv until zero returned, or SOCKET_ERROR.
            3. Call closesocket.

            Note The shutdown function does not block regardless of the SO_LINGER setting on the socket.

            An application should not rely on being able to reuse a socket after it has been shut down. In particular, a Windows Sockets provider is not required to support the use of connect on a socket that has been shut down.
              回復(fù)  更多評(píng)論   

            # re: 關(guān)閉SOCKET時(shí)需注意的問(wèn)題 2007-11-14 16:34 追夢(mèng)時(shí)代

            http://hi.baidu.com/developer_chen/blog/item/53208b4594f4bf25cefca322.html
            這篇文章描述了如何安全的關(guān)閉SOCKET  回復(fù)  更多評(píng)論   

            # re: 對(duì)一個(gè)奇怪SOCKET問(wèn)題的研究 2007-12-23 17:21 秦歌

            shutdown管用  回復(fù)  更多評(píng)論   

            # re: 對(duì)一個(gè)奇怪SOCKET問(wèn)題的研究 2008-01-31 22:00 abettor

            默認(rèn)情況下,linger是保持TIME_WAIT狀態(tài)的。
            如果設(shè)置了linger選項(xiàng),closesocket的時(shí)候就會(huì)以粗暴的形式實(shí)現(xiàn)。也就是,在斷開(kāi)連接的三次握手時(shí),一旦接受到了對(duì)方的ACK后,發(fā)送一個(gè)RST就立即清理資源,而并不等待TCP/IP協(xié)議棧真的將全部數(shù)據(jù)發(fā)出,更不會(huì)等上兩個(gè)MSL的TIME_WAIT狀態(tài)。這樣的話,也許RST根本就還沒(méi)有發(fā)送出去,所以對(duì)等端意識(shí)不到連接已經(jīng)中斷。  回復(fù)  更多評(píng)論   


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