c++ - 客户端和服务器无法交换数据，因为都得到 "stuck"

Question

简介:

我研究了 blocking TCP server 的 MSDN 示例和 blocking TCP client .

鉴于修改这些示例以创建简单的聊天应用程序，我想尝试一些简单的事情。

首先，我尝试实现以下内容:

• 从服务器发送消息
• 在客户端接收并显示该消息
• 发送客户端响应
• 接收并显示客户端的响应

相关信息

我提前为冗长的代码道歉，但我坚信提交 SSCCE 与我相关对于客户端和服务器，为了让社区有机会解决问题。

我已尝试使代码尽可能少，但不想省略基本的错误检查。

您可以将两者复制/粘贴到单个 .cpp 文件中，它们应该可以毫无问题地编译和运行:

服务器代码:

#undef UNICODE

#define WIN32_LEAN_AND_MEAN

#include <windows.h>
#include <winsock2.h>
#include <ws2tcpip.h>
#include <string>

#pragma comment (lib, "Ws2_32.lib")

#define DEFAULT_BUFLEN 512
#define DEFAULT_PORT "27015"

int __cdecl main(void) 
{
    WSADATA wsaData;
    SOCKET ListenSocket = INVALID_SOCKET;
    SOCKET ClientSocket = INVALID_SOCKET;

    struct addrinfo *result = NULL;
    struct addrinfo hints;

    int iResult;
    char recvbuf[DEFAULT_BUFLEN] = "";
    int recvbuflen = DEFAULT_BUFLEN;

    // Initialize Winsock
    iResult = WSAStartup(MAKEWORD(2,2), &wsaData);
    if (iResult != 0) {
        printf("WSAStartup failed with error: %d\n", iResult);
        return 1;
    }

    ZeroMemory(&hints, sizeof(hints));
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_protocol = IPPROTO_TCP;
    hints.ai_flags = AI_PASSIVE;

    // Resolve the server address and port
    iResult = getaddrinfo(NULL, DEFAULT_PORT, &hints, &result);
    if ( iResult != 0 ) {
        printf("getaddrinfo failed with error: %d\n", iResult);
        WSACleanup();
        return 1;
    }

    // Create a SOCKET for connecting to server
    ListenSocket = socket(result->ai_family, result->ai_socktype, result->ai_protocol);
    if (ListenSocket == INVALID_SOCKET) {
        printf("socket failed with error: %ld\n", WSAGetLastError());
        freeaddrinfo(result);
        WSACleanup();
        return 1;
    }

    // Setup the TCP listening socket
    iResult = bind( ListenSocket, result->ai_addr, (int)result->ai_addrlen);
    if (iResult == SOCKET_ERROR) {
        printf("bind failed with error: %d\n", WSAGetLastError());
        freeaddrinfo(result);
        closesocket(ListenSocket);
        WSACleanup();
        return 1;
    }

    freeaddrinfo(result);

    iResult = listen(ListenSocket, SOMAXCONN);
    if (iResult == SOCKET_ERROR) {
        printf("listen failed with error: %d\n", WSAGetLastError());
        closesocket(ListenSocket);
        WSACleanup();
        return 1;
    }

    // Accept a client socket
    ClientSocket = accept(ListenSocket, NULL, NULL);
    if (ClientSocket == INVALID_SOCKET) {
        printf("accept failed with error: %d\n", WSAGetLastError());
        closesocket(ListenSocket);
        WSACleanup();
        return 1;
    }

    // No longer need server socket, 
    // because I want to accept only 1 client
    closesocket(ListenSocket);

    // ===================== let us try to send a message...
    std::string message = "Test message from server !!!";
    int total = message.size();
    const int messageLength = message.size();

    while (iResult = send( ClientSocket,
        // send only the missing part of the string, if send failed to deliver entire packet:
        // we move the start of the string forward by messageLength - total
        // while we send remaining number of bytes, which is held in total
        message.substr(messageLength - total, total).c_str(), total, 0),
        iResult > 0)
    {
        total -= iResult;
    }

    if (iResult == SOCKET_ERROR) {
        printf("send failed with error: %d\n", WSAGetLastError());
        closesocket(ClientSocket);
        WSACleanup();
        return 1;
    }

/*  // adding this, seems to solve the problem ???
    iResult = shutdown(ClientSocket, SD_SEND);
    if (iResult == SOCKET_ERROR) {
        printf("shutdown failed with error: %d\n", WSAGetLastError());
        closesocket(ClientSocket);
        WSACleanup();
        return 1;
    }
*/
    // receive response from client...
    while (iResult = recv(ClientSocket, recvbuf, recvbuflen, 0), iResult > 0)
    {
        printf("%s", recvbuf);
        memset(recvbuf, '\0', sizeof(recvbuf));
    }

    if(iResult < 0)
    {
        printf("recv failed with error: %d\n", WSAGetLastError());
        closesocket(ClientSocket);
        WSACleanup();
        return 1;
    }

    // cleanup
    closesocket(ClientSocket);
    WSACleanup();

    getchar();  // so I can stop the console from immediately closing...
    return 0;
}

客户端代码:

#define WIN32_LEAN_AND_MEAN

#include <windows.h>
#include <winsock2.h>
#include <ws2tcpip.h>
#include <string>

#pragma comment (lib, "Ws2_32.lib")


#define DEFAULT_BUFLEN 512
#define DEFAULT_PORT "27015"

int __cdecl main(int argc, char **argv) 
{
    WSADATA wsaData;
    SOCKET ConnectSocket = INVALID_SOCKET;
    struct addrinfo *result = NULL,
                    *ptr = NULL,
                    hints;

    char recvbuf[DEFAULT_BUFLEN] = "";
    int iResult;
    int recvbuflen = DEFAULT_BUFLEN;

    // Initialize Winsock
    iResult = WSAStartup(MAKEWORD(2,2), &wsaData);
    if (iResult != 0) 
    {
        printf("WSAStartup failed with error: %d\n", iResult);
        return 1;
    }

    ZeroMemory( &hints, sizeof(hints) );
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_protocol = IPPROTO_TCP;

    // Resolve the server address and port
    iResult = getaddrinfo("127.0.0.1", DEFAULT_PORT, &hints, &result);
    if ( iResult != 0 ) 
    {
        printf("getaddrinfo failed with error: %d\n", iResult);
        WSACleanup();
        return 1;
    }

    // Attempt to connect to an address until one succeeds
    for(ptr=result; ptr != NULL ;ptr=ptr->ai_next) 
    {
        // Create a SOCKET for connecting to server
        ConnectSocket = socket(ptr->ai_family, ptr->ai_socktype, ptr->ai_protocol);
        if (ConnectSocket == INVALID_SOCKET) 
        {
            printf("socket failed with error: %ld\n", WSAGetLastError());
            WSACleanup();
            return 1;
        }

        // Connect to server.
        iResult = connect( ConnectSocket, ptr->ai_addr, (int)ptr->ai_addrlen);
        if (iResult == SOCKET_ERROR) 
        {
            closesocket(ConnectSocket);
            ConnectSocket = INVALID_SOCKET;
            continue;
        }
        break;
    }

    freeaddrinfo(result);

    if (ConnectSocket == INVALID_SOCKET) 
    {
        printf("Unable to connect to server!\n");
        WSACleanup();
        return 1;
    }

    // receive message from server...
    while (iResult = recv(ConnectSocket, recvbuf, recvbuflen, 0), iResult > 0)
    {
        printf("%s", recvbuf);
        memset(recvbuf, '\0', sizeof(recvbuf));
    }

    if(iResult < 0)
    {
        printf("recv failed with error: %d\n", WSAGetLastError());
        closesocket(ConnectSocket);
        WSACleanup();
        return 1;
    }

    // ===================== let us try to send a message...
    std::string message = "Client response...";
    int total = message.size();
    const int messageLength = message.size();

    while (iResult = send( ConnectSocket,
        // send only the missing part of the string, if send failed to deliver entire packet:
        // we move the start of the string forward by messageLength - total
        // while we send remaining number of bytes, which is held in total
        message.substr(messageLength - total, total).c_str(), total, 0),
        iResult > 0)
    {
        total -= iResult;
    }

    if (iResult == SOCKET_ERROR) {
        printf("send failed with error: %d\n", WSAGetLastError());
        closesocket(ConnectSocket);
        WSACleanup();
        return 1;
    }

    // shutdown the connection since no more data will be sent
    iResult = shutdown(ConnectSocket, SD_SEND);
    if (iResult == SOCKET_ERROR) {
        printf("shutdown failed with error: %d\n", WSAGetLastError());
        closesocket(ConnectSocket);
        WSACleanup();
        return 1;
    }

    // cleanup
    closesocket(ConnectSocket);
    WSACleanup();

    getchar();  // so I can stop the console from immediately closing...
    return 0;
}

问题:

我已经实现了解决方案，但没有得到预期的结果。

服务器发送消息，客户端成功接收并显示它，但随后客户端无限卡住，而不是将其响应发送给服务器，这也无限地阻塞了服务器。

我为解决这个问题所做的努力:

第一次尝试:

使用调试器，我在客户端的接收 block 之后放置了断点，只是为了确定客户端在收到第一条消息后永远不会到达那里。

我相信 while 循环应该再次调用 recv，它应该返回 0，从而强制循环结束。

在我点击 Continue 后，调试器甚至没有继续显示客户端接收缓冲区的内容，而是表现出我目前无法描述的行为，因为我不是以英语为母语的人。

第二次尝试:

我也曾尝试使用 CreateThread 将服务器的接收循环放入线程中，但这也无济于事。

我也曾尝试将来自客户端的接收循环放入线程中，但也失败了。

我曾尝试将客户端和服务器接收循环放入线程中，但也失败了。

第三次尝试:

最后，我在服务器代码中添加了对shutdown(ClientSocket, SD_SEND)的调用，你会在代码的下半部分找到它，它被注释掉了。

这似乎解决了问题，但我不确定这是否是正确的解决方案，因为我刚刚开始使用 Winsock。

问题:

• 如何绕过在 send 之后添加 shutdown 的“解决方案”？
• 如果以上是唯一的方法，我应该在 recv 之后做同样的事情吗？我应该发送什么参数(SD_SEND、SD_RECEIVE 或SD_BOTH) ?

再次为冗长的帖子道歉，但作为新手，我已尝试提供尽可能多的信息以使您的任务更轻松。

Answer 1

在简要浏览了您的代码后，我敢猜测客户端中的以下代码块:

while (iResult = recv(ConnectSocket, recvbuf, recvbuflen, 0), iResult > 0)
{
    printf("%s", recvbuf);
    memset(recvbuf, '\0', sizeof(recvbuf));
}

实际上是你的问题。您提到关闭服务器端的套接字可以解决问题。对于套接字，只要套接字处于事件状态或直到数据通过，recv 调用就会阻塞，但是当套接字关闭时，您将获得 0 的 recv。

如果你只想接收一个消息，而不是在recv上循环，你应该在处理完第一个recv后循环回recv调用，或者你应该在套接字上轮询以查看是否有实际上数据首先可用。