Winsock "recv" 不会 return 非正常连接终止

Question

我是运行以下代码：

#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，即没有调用 close() 或任何东西就关闭，recv() 函数正在阻塞并且不会 return.

我在 recv 函数的文档中读到：

If the socket is connection oriented and the remote side has shut down the connection gracefully, and all data has been received, a recv will complete immediately with zero bytes received. If the connection has been reset, a recv will fail with the error WSAECONNRESET.

但事实并非如此。在我的例子中，recv() 只是阻止执行。我猜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？

Answer 1

异常连接丢失（网络通信刚刚停止，与显式重置相比）需要时间 OS 才能检测到。 TCP 被设计为在网络中断的情况下具有弹性。 recv() 将最终失败，但 OS 可能需要几分钟才能在内部超时并使连接无效。在此之前，不会报告任何错误。

如果您不想等待 OS 必要的时间使连接无效，您将必须在您自己的代码中启用您自己的超时，例如通过调用 select() 在 recv() 之前，或者通过使用 setsockopt(SO_RCVTIMEO)、WSAIoctl(SIO_KEEPALIVE_VALS) 等使 recv() 超时退出。然后，如果超时结束，您可以关闭套接字。

Winsock "recv" 不会 return 非正常连接终止

Winsock "recv" doesn't return on a non-graceful connection termination

sockets

windows

tcp