使用 GetQueuedCompletionStatus 和 ERROR_MORE_DATA 的套接字

Question

我正在尝试将 GetQueuedCompletionStatus 与 Winsocks 一起使用，但我似乎无法正确执行。流程如下:

void foo() {
    ...
    SOCKET sck = WSASocket(AF_INET, SOCK_DGRAM, 0, 
        NULL, 0, WSA_FLAG_OVERLAPPED);
    ....
    bind(sck,(struct sockaddr *)&addr,sizeof(struct sockaddr_in));
    HANDLE hPort = CreateIoCompletionPort((HANDLE)sck, NULL, 0, 0 );
    OVERLAPPED pOverlapped = {0,};
    WSARecvFrom(sck,NULL,0,NULL,NULL,(struct sockaddr *)&laddr,&lsize,&pOverlapped,0);
    BOOL bReturn = GetQueuedCompletionStatus(
            hPort,
            &rbytes,
            (LPDWORD)&lpContext,
            &pOutOverlapped,
            INFINITE);
    ...
}

然后，我从外部工具将一些网络数据发送到绑定(bind)端口。 GetQueuedCompletionStatus 返回 FALSE，GetLastError() 返回 ERROR_MORE_DATA，这听起来是正确的，因为我没有在 WSARecvFrom 中提供缓冲区。

问题是如何提供缓冲区来实际从失败的 I/O 操作中获取数据？

我尝试使用原始重叠结构发出 WSARecvFrom，但它只是将另一个读取排队，并且在发送更多网络数据之前对 GetQueuedCompletionStatus 的后续调用不会返回。

在没有重叠结构的情况下调用 WSARecvFrom 会阻止它，并且在发送更多网络数据之前它也不会返回。

那么，如何正确处理 ERROR_MORE_DATA，而不丢失第一次操作的数据？

Answer 1

您必须为 WSARecvFrom() 提供一个缓冲区，就像任何读取操作一样，无论您是否使用 IOCP。您必须确保缓冲区在内存中保持有效，直到 IOCP 操作完成。 IOCP 填充您提供的缓冲区，然后在完成时通知完成端口。

UDP 无法在单个数据报中传输超过 65535 字节，因此您可以将其用作最大缓冲区大小。

在您的示例中，您的代码被编写为同步运行(完全违背了使用 IOCP 的目的)，因此您可以使用本地缓冲区:

void foo() {
    ...
    SOCKET sck = WSASocket(AF_INET, SOCK_DGRAM, 0, NULL, 0, WSA_FLAG_OVERLAPPED);
    if (sck == INVALID_SOCKET)
    {
        // error, do something...
        return;
    }
    ....
    bind(sck,(struct sockaddr *)&addr, sizeof(struct sockaddr_in));
    HANDLE hPort = CreateIoCompletionPort((HANDLE)sck, NULL, 0, 0 );
    if (!hPort)
    {
        // error, do something...
        return;
    }

    WSAOVERLAPPED Overlapped = {0};
    Overlapped.hEvent = WSACreateEvent();

    BYTE buffer[0xFFFF];
    DWORD dwBytesRecvd = 0;
    DWORD dwFlags = 0;
    sockaddr_in fromaddr = {0};
    int fromaddrlen = sizeof(fromaddr);

    WSABUF buf;
    buf.len = sizeof(buffer);
    buf.buf = buffer;

    int iRet = WSARecvFrom(sck, &buf, 1, &dwBytesRecvd, &dwFlags, (sockaddr*)&fromaddr, &fromaddrlen, &Overlapped, NULL);
    if (iRet == SOCKET_ERROR)
    {
        if (WSAGetLastError() != WSA_IO_PENDING)
        {
           // error, do something...
           return;
        }

        DWORD rBytes;
        ULONG_PTR key;
        LPOVERLAPPED pOverlapped = NULL;

        if (!GetQueuedCompletionStatus(hPort, &rbytes, &key, &pOverlapped, INFINITE))
        {
            if (pOverlapped)
            {
                // WSARecvFrom() failed...
            }
            else
            {
                // GetQueuedCompletionStatus() failed...
            }

            // do something...
            return;
        }
    }

    // I/O complete, use buffer, dwBytesRecvd, dwFlags, and fromaddr as needed...
}

但是，这违背了 IOCP 的目的。如果您确实想要同步，则可以使用 recvfrom() 来代替，并让它阻塞调用线程，直到数据到达。当有一个线程池为完成端口提供服务时，IOCP 效果最佳。调用 WSARecvFrom() 并让它在后台工作，不要等待它。让一个单独的线程调用GetQueuedCompletionPort()并在收到数据时处理数据，例如:

struct MyOverlapped
{
    WSAOVERLAPPED overlapped;
    BYTE buffer[0xFFFF];
    DWORD buflen;
    DWORD flags;
    sockaddr_storage fromaddr;
    int fromaddrLen;
};

HANDLE hPort = NULL;

void foo() {
    ...
    SOCKET sck = WSASocket(AF_INET, SOCK_DGRAM, 0, NULL, 0, WSA_FLAG_OVERLAPPED);
    if (sck == INVALID_SOCKET)
    {
        // error, do something...
        return;
    }
    ....
    bind(sck,(struct sockaddr *)&addr, sizeof(struct sockaddr_in));
    hPort = CreateIoCompletionPort((HANDLE)sck, NULL, 0, 0 );
    if (!hPort)
    {
        // error, do something...
        return;
    }

    MyOverlapped *ov = new MyOverlapped;
    ZeroMemory(ov, sizeof(*ov));
    ov->overlapped.hEvent = WSACreateEvent();
    ov->fromaddrlen = sizeof(ov->fromaddr);

    WSABUF buf;
    buf.len = sizeof(ov->buffer);
    buf.buf = ov->buffer;

    int iRet = WSARecvFrom(sck, &buf, 1, &ov->buflen, &ov->flags, (sockaddr*)&ov->fromaddr, &ov->fromaddrlen, (WSAOVERLAPPED*)ov, NULL);
    if (iRet == SOCKET_ERROR)
    {
        if (WSAGetLastError() != WSA_IO_PENDING)
        {
           // error, do something...
           return;
        }

        // WSARecvFrom() is now operating in the background,
        // the IOCP port will be signaled when finished...
    }
    else
    {
        // data is already available,
        // the IOCP port will be signaled immediately...
    }

    ...
}

...

// in another thread...

{
    ...

    DWORD rbytes;
    ULONG_PTR key;
    MyOverlapped *ov = NULL;

    if (!GetQueuedCompletionStatus(hPort, &rbytes, &key, (LPOVERLAPPED*)&ov, INFINITE))
    {
        if (ov)
        {
            // WSARecvFrom() failed...
            // free ov, or reuse it for another operation...
        }
        else
        {
            // GetQueuedCompletionStatus() failed...
        }
    }
    else
    {
        // use ov as needed...
        // free ov, or reuse it for another operation...
    }

    ...
}