如何在互联网中断时终止 windows 套接字？ (C++ WinAPI)

Question

我已经建立了一个 Winsock2 连接，但我需要解决互联网中断的情况。这是我的代码；

#include <winsock2.h>
#include <windows.h>
#include <ctime>
int main()
{
    WSADATA w;
    if(WSAStartup(MAKEWORD(2,2),&w)) return 0;
    sockaddr_in sad;
    sad.sin_family=AF_INET;
    sad.sin_addr.s_addr=inet_addr("200.20.186.76");
    sad.sin_port=htons(123);
    sockaddr saddr;
    int saddr_l=sizeof(saddr);
    int s=socket(PF_INET,SOCK_DGRAM,IPPROTO_UDP);
    if(s==INVALID_SOCKET) return 0;
    char msg[48]={8};
    if(sendto(s,msg,sizeof(msg),0,(sockaddr*)&sad,sizeof(sad))==SOCKET_ERROR) return 0;
    if(recvfrom(s,msg,48,0,&saddr,&saddr_l)==SOCKET_ERROR) return 0;
    if(closesocket(s)==SOCKET_ERROR) return 0;
    if(WSACleanup()) return 0;
    return 0;
}

它在这里等待对 return 的调用，正如它所记录的那样。我有两个问题。

我可以像使用 select
还有什么方法可以避免等待并立即 return？文档指出：

When issuing a blocking Winsock call such as sendto, Winsock may need to wait for a network event before the call can complete. Winsock performs an alertable wait in this situation, which can be interrupted by an asynchronous procedure call (APC) scheduled on the same thread.

怎么做？

Answer 1

如果您想发出 recvfrom() 并立即接收 return，请自行决定等待多长时间（我假设 Windows 因为您包含了 winsock2.h), 您可以发出异步 OVERLAPPED 请求，然后通过等待 OVERLAPPED 结构的 hEvent 成员收到信号来随时等待完成。

这是基于您的原始代码的更新示例。

你可以通过 WaitForSingleObject 设置超时时间（下面我等了 6 次 10 秒）
通过传递 OVERLAPPED 指针，表示您将自己等待完成。请注意，OVERLAPPED 结构在发出 hEvent 信号之前不能超出范围。（或释放，如果 OVERLAPPED 是动态分配的）。
在保证 IO 完成之前让 OVERLAPPED 超出范围是一个常见的 Winsock 错误（我在 Winsock 上工作了 10 多年左右 - 我见过这个错误的许多变体）
如下所述，如果您不知道 hEvent 已发出信号，那么在调用 closesocket 之后，您必须等待 hEvent 发出信号后再继续 - closesocket 不保证所有异步 IO 请求在 returning 之前完成。

#define _WINSOCK_DEPRECATED_NO_WARNINGS
#include <winsock2.h>
#include <windows.h>
#include <ctime>
int main()
{
    WSADATA w;
    if (WSAStartup(MAKEWORD(2, 2), &w)) return 0;
    sockaddr_in sad;
    sad.sin_family = AF_INET;
    sad.sin_addr.s_addr = inet_addr("200.20.186.76");
    sad.sin_port = htons(123);
    sockaddr saddr;
    int saddr_l = sizeof(saddr);
    int s = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
    if (s == INVALID_SOCKET) return 0;
    char msg[48] = { 8 };
    if (sendto(s, msg, sizeof(msg), 0, (sockaddr*)&sad, sizeof(sad)) == SOCKET_ERROR) return 0;
    OVERLAPPED ov{};
    ov.hEvent = CreateEvent(nullptr, TRUE, FALSE, nullptr);
    if (ov.hEvent == nullptr) return 0;
    WSABUF wsabuffer{};
    wsabuffer.buf = msg;
    wsabuffer.len = 48;
    DWORD flags = 0;
    if (WSARecvFrom(s, &wsabuffer, 1, nullptr, &flags, &saddr, &saddr_l, &ov, nullptr) == SOCKET_ERROR)
    {
        DWORD gle = WSAGetLastError();
        if (gle != WSA_IO_PENDING) return 0;
    }
    for (DWORD recv_count = 0; recv_count < 6; ++recv_count)
    {
        DWORD wait = WaitForSingleObject(ov.hEvent, 10000);
        if (wait == WAIT_FAILED) return 0;
        if (wait == WAIT_OBJECT_0) break; // WSARecvFrom completed
        if (wait == WAIT_TIMEOUT) continue; // WSARecvFrom is still pended waiting for data
    }
    // assuming WSARecvFrom completed - i.e. ov.hEvent was signaled
    DWORD transferred;
    if (WSAGetOverlappedResult(s, &ov, &transferred, FALSE, &flags))
    {
        // WSARecvFrom completed successfully - 'transferred' shows the # of bytes that were received
    }
    else
    {
        DWORD gle = WSAGetLastError();
        gle;
        // WSARecvFrom failed with the error code in 'gle'
    }
    if (closesocket(s) == SOCKET_ERROR) return 0;
    // with real code, we must guarantee that hEvent is set after calling closesocket
    // e.g. if we get here in an error path
    // closesocket() won't guarantee all async IO has completed before returning
    WaitForSingleObject(ov.hEvent, INFINITE);
    if (WSACleanup()) return 0;
    return 0;
}

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