客户端与服务器多消息发送问题：连接过早关闭需维持长连接

嘿，我明白你的问题了——现在你的客户端和服务器只能完成一次消息收发就断开，想要改成能保持连接、连续发送多条消息对吧？这问题其实出在单次交互后就直接关闭套接字的逻辑上，咱们调整一下循环结构就能搞定。

核心修改思路 #

不管是服务器还是客户端，都需要在成功建立连接后进入一个持续的交互循环，直到主动触发断开（比如用户输入退出指令）或者出现网络错误才关闭连接。

修改后的服务器代码 #

#undef UNICODE
#define WIN32_LEAN_AND_MEAN

#include <windows.h>
#include <winsock2.h>
#include <ws2tcpip.h>
#include <stdlib.h>
#include <stdio.h>

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

#define DEFAULT_BUFLEN 512
#define DEFAULT_PORT "27015"

int __cdecl main(void) {
    WSADATA wsaData;
    int iResult;

    SOCKET ListenSocket = INVALID_SOCKET;
    SOCKET ClientSocket = INVALID_SOCKET;

    struct addrinfo *result = NULL;
    struct addrinfo hints;

    int iSendResult;
    char recvbuf[DEFAULT_BUFLEN];
    int recvbuflen = DEFAULT_BUFLEN;

    // 初始化Winsock
    iResult = WSAStartup(MAKEWORD(2,2), &wsaData);
    if (iResult != 0) {
        printf("WSAStartup failed: %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;

    // 解析服务器地址和端口
    iResult = getaddrinfo(NULL, DEFAULT_PORT, &hints, &result);
    if (iResult != 0) {
        printf("getaddrinfo failed: %d\n", iResult);
        WSACleanup();
        return 1;
    }

    // 创建监听套接字
    ListenSocket = socket(result->ai_family, result->ai_socktype, result->ai_protocol);
    if (ListenSocket == INVALID_SOCKET) {
        printf("socket failed: %ld\n", WSAGetLastError());
        freeaddrinfo(result);
        WSACleanup();
        return 1;
    }

    // 绑定套接字
    iResult = bind( ListenSocket, result->ai_addr, (int)result->ai_addrlen);
    if (iResult == SOCKET_ERROR) {
        printf("bind failed: %d\n", WSAGetLastError());
        freeaddrinfo(result);
        closesocket(ListenSocket);
        WSACleanup();
        return 1;
    }

    freeaddrinfo(result);

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

    // 接受客户端连接
    ClientSocket = accept(ListenSocket, NULL, NULL);
    if (ClientSocket == INVALID_SOCKET) {
        printf("accept failed: %d\n", WSAGetLastError());
        closesocket(ListenSocket);
        WSACleanup();
        return 1;
    }

    // 关闭监听套接字，因为我们只处理一个客户端连接
    closesocket(ListenSocket);

    // ===== 关键修改：进入持续接收循环 =====
    do {
        // 接收客户端消息
        iResult = recv(ClientSocket, recvbuf, recvbuflen, 0);
        if (iResult > 0) {
            printf("Received %d bytes: %s\n", iResult, recvbuf);

            // 回显消息给客户端
            iSendResult = send( ClientSocket, recvbuf, iResult, 0 );
            if (iSendResult == SOCKET_ERROR) {
                printf("send failed: %d\n", WSAGetLastError());
                closesocket(ClientSocket);
                WSACleanup();
                return 1;
            }
            printf("Sent %d bytes back to client\n", iSendResult);

            // 清空缓冲区，避免残留数据
            ZeroMemory(recvbuf, recvbuflen);
        }
        else if (iResult == 0) {
            printf("Client closing connection\n");
        }
        else {
            printf("recv failed: %d\n", WSAGetLastError());
            closesocket(ClientSocket);
            WSACleanup();
            return 1;
        }

    } while (iResult > 0); // 直到客户端断开（iResult=0）或出错才退出循环

    // 关闭客户端套接字
    iResult = shutdown(ClientSocket, SD_SEND);
    if (iResult == SOCKET_ERROR) {
        printf("shutdown failed: %d\n", WSAGetLastError());
        closesocket(ClientSocket);
        WSACleanup();
        return 1;
    }

    closesocket(ClientSocket);
    WSACleanup();

    return 0;
}

修改后的客户端代码 #

#undef UNICODE
#define WIN32_LEAN_AND_MEAN

#include <windows.h>
#include <winsock2.h>
#include <ws2tcpip.h>
#include <stdlib.h>
#include <stdio.h>

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

#define DEFAULT_BUFLEN 512
#define DEFAULT_PORT "27015"
#define DEFAULT_SERVER "127.0.0.1"

int __cdecl main(void) {
    WSADATA wsaData;
    int iResult;

    SOCKET ConnectSocket = INVALID_SOCKET;
    struct addrinfo *result = NULL,
                    *ptr = NULL,
                    hints;

    char sendbuf[DEFAULT_BUFLEN];
    char recvbuf[DEFAULT_BUFLEN];
    int recvbuflen = DEFAULT_BUFLEN;

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

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

    // 解析服务器地址和端口
    iResult = getaddrinfo(DEFAULT_SERVER, DEFAULT_PORT, &hints, &result);
    if (iResult != 0) {
        printf("getaddrinfo failed: %d\n", iResult);
        WSACleanup();
        return 1;
    }

    // 尝试连接到服务器地址
    for(ptr=result; ptr != NULL ;ptr=ptr->ai_next) {
        ConnectSocket = socket(ptr->ai_family, ptr->ai_socktype, ptr->ai_protocol);
        if (ConnectSocket == INVALID_SOCKET) {
            printf("socket failed: %ld\n", WSAGetLastError());
            WSACleanup();
            return 1;
        }

        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;
    }

    // ===== 关键修改：进入持续发送循环 =====
    printf("Enter messages to send (type 'exit' to quit):\n");
    while(1) {
        // 获取用户输入
        fgets(sendbuf, DEFAULT_BUFLEN, stdin);
        
        // 去掉fgets带来的换行符
        sendbuf[strcspn(sendbuf, "\n")] = '\0';

        // 判断是否退出
        if(strcmp(sendbuf, "exit") == 0) {
            printf("Closing connection...\n");
            break;
        }

        // 发送消息到服务器
        iResult = send( ConnectSocket, sendbuf, (int)strlen(sendbuf), 0 );
        if (iResult == SOCKET_ERROR) {
            printf("send failed: %d\n", WSAGetLastError());
            closesocket(ConnectSocket);
            WSACleanup();
            return 1;
        }

        printf("Sent %d bytes: %s\n", iResult, sendbuf);

        // 接收服务器的回显
        iResult = recv(ConnectSocket, recvbuf, recvbuflen, 0);
        if (iResult > 0) {
            printf("Received %d bytes: %s\n", iResult, recvbuf);
            ZeroMemory(recvbuf, recvbuflen);
        }
        else if (iResult == 0) {
            printf("Server closed connection\n");
            break;
        }
        else {
            printf("recv failed: %d\n", WSAGetLastError());
            break;
        }
    }

    // 关闭套接字
    iResult = shutdown(ConnectSocket, SD_SEND);
    if (iResult == SOCKET_ERROR) {
        printf("shutdown failed: %d\n", WSAGetLastError());
        closesocket(ConnectSocket);
        WSACleanup();
        return 1;
    }

    closesocket(ConnectSocket);
    WSACleanup();

    return 0;
}

关键修改点说明 #

1. 服务器端：

   • 在accept()成功获取客户端套接字后，添加了一个do-while循环，持续调用recv()接收消息，直到客户端断开（iResult == 0）或者接收出错。
   • 每次接收完消息后处理（比如回显），然后继续等待下一条消息，而不是直接关闭套接字。

2. 客户端：

   • 建立连接后进入一个while(1)无限循环，让用户可以反复输入消息发送。
   • 添加了退出条件：当用户输入exit时，主动断开连接。
   • 每次发送消息后接收服务器的响应，然后回到输入状态，保持连接不关闭。

这样修改后，客户端和服务器就能保持长连接，你可以连续发送多条消息，直到主动退出或者出现网络问题。

内容的提问来源于stack exchange，提问作者Jon