Winsock“recv”不会在非正常连接终止时返回

Posted 2023-03-15

【中文标题】Winsock“recv”不会在非正常连接终止时返回

【英文标题】：Winsock "recv" doesn't return on a non-graceful connection termination

【发布时间】：2021-11-19 01:34:13

【问题描述】：

我正在运行以下代码：

#undef UNICODE

#define WIN32_LEAN_AND_MEAN

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

// Need to link with Ws2_32.lib
#pragma comment (lib, "Ws2_32.lib")
// #pragma comment (lib, "Mswsock.lib")

#define DEFAULT_BUFLEN 512
#define DEFAULT_PORT "8081"

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;

    // 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;
    

    while (true)
    
        printf("Waiting for connection...\n");
        // 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;
        
        else
        
            printf("Received connection !\n");
        

        // No longer need server socket
        //closesocket(ListenSocket);

        // Receive until the peer shuts down the connection
        do 
            printf("Before recv\n");
            iResult = recv(ClientSocket, recvbuf, recvbuflen, 0);
            printf("After recv\n");
            if (iResult > 0) 
                printf("Bytes received: %d\n", iResult);
                recvbuf[iResult] = 0;
                // Echo the buffer back to the sender
                printf("%s\n", recvbuf);
            
            else if (iResult == 0)
                printf("Connection closing...\n");
            else 
                printf("recv failed with error: %d\n", WSAGetLastError());
                closesocket(ClientSocket);
                WSACleanup();
                return 1;
            

         while (iResult > 0);
        printf("After loop !!\n");
        iResult = shutdown(ClientSocket, SD_SEND);
        if (iResult == SOCKET_ERROR) 
            printf("shutdown failed with error: %d\n", WSAGetLastError());
            closesocket(ClientSocket);
            WSACleanup();
            return 1;
        

        // cleanup
        closesocket(ClientSocket);
        // 
    
    
    // shutdown the connection since we're done
    WSACleanup();

    return 0;

当客户端不正常地终止连接时，即直接关闭而不调用 close() 或任何东西，recv() 函数将阻塞并且不会返回。

我在 recv 函数的文档中读到：

如果套接字是面向连接的，并且远程端已经优雅地关闭了连接，并且所有数据都已接收到，recv 将立即完成，接收到零字节。如果连接已被重置，recv 将失败并返回错误 WSAECONNRESET。

但事实并非如此。就我而言， recv() 只是阻止执行。我想文档指的是在对等方关闭连接后调用 recv() 的情况，但在我的情况下，在调用 recv() 后连接正在关闭。 我该如何处理？

【参考方案1】：

异常连接丢失（网络通信刚刚停止，而不是显式重置）需要时间让操作系统检测到它。 TCP 旨在在网络中断的情况下具有弹性。 recv()最终将失败，但操作系统可能需要几分钟的时间在内部超时并使连接失效。在此之前，不会报告任何错误。

如果您不想等待操作系统使连接失效所需的时间，则必须在自己的代码中启用自己的超时，例如在recv() 之前调用select()，或者通过使用setsockopt(SO_RCVTIMEO)、WSAIoctl(SIO_KEEPALIVE_VALS) 等使recv() 超时退出。然后，如果超时，您可以关闭套接字。