c++ - Winsock - 等待调用 accept() 直到客户端真正尝试连接

Question

我正在使用 Winsock API 在 C++ 中创建一个服务器。我想知道是否有任何可能仅在某些连接实际到来时才调用 accept() 函数，因此我不必在 accept() 上阻塞我的线程。换句话说，我想让我的线程等待并仅在客户端尝试连接时调用 accept() 函数。这可能吗？

Answer 1

当您使用 Winsock 时，您可以使用 Microsoft 特定的扩展函数 AcceptEx。这允许您将接受作为“重叠 I/O”执行，这在概念上意味着接受在后台运行，您可以偶尔进入并检查它是否发生，方法是检查 OverlappedResult，或者通过在 OverlappedHandle 上执行 Wait。 AcceptEx 也将选择性地执行第一次接收。

无需编写所有代码并对其进行全面测试，如下所示的代码应该可以工作:

// The following:
//   Has no error checking
//   Assumes sListen is a bound listening socket
//   Some other assumptions I've not listed :)

// Allocate space to store the two sockaddr's that AcceptEx will give you
// 
char lpOutputBuffer[sizeof((sockaddr_in)+16) * 2];

SOCKET sAccept = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
WSAOVERLAPPED olOverlap;

ZeroMemory(&olOverlap, sizeof(WSAOVERLAPPED));
olOverlap.hEvent = WSACreateEvent();

DWORD dwBytes;

BOOL bAcceptRet =
    AcceptEx(sListen, // the listen socket
             sAccept, // the socket to use for the accepted connection
             lpOutputBuffer, // where to store the received information
             0, // don't do a receive, just store the local and remote addresses
             sizeof((sockaddr_in)+16), // size of the local address area
             sizeof((sockaddr_in)+16), // size of the remote address area
             &dwBytes, // set to received bytes if we complete synchronously
             &olOverlap); // our overlapped structure

if (bAcceptRet) {
    // the function completed synchronously.
    // lpOutputBuffer should contain the address information.
    // sAccept should be a connected socket
} else {
    // the function didn't complete synchronously, so is the accept Pending?
    if (ERROR_IO_PENDING == WSAGetLastError()) {
        // in this case, our Accept hasn't happened yet...
        // later in our code we can do the following to check if an accept has occurred:
        // note that the FALSE tells WSAGetOverlappedResult not to wait for the I/O to complete
        // it should return immediately
        ...
        DWORD dwFlags;
        if (WSAGetOverlappedResult(sListen, &olOverlap, &dwBytes, FALSE, &dwFlags)) {
            // the accept has succeeded, so do whatever we need to do with sAccept.
        }
        ...
    }
}

当然，这是一段非常快速的、拼凑而成的代码，可能无法正常工作、不可编译，但它应该让您了解如何做与您想要的类似的事情，以及在哪里查看。

顺便说一句，从技术上讲，设置 WSAOVERLAPPED 结构的 hEvent 参数不是必需的，但这样做可以让您实际上等待要求完成:

if (WAIT_OBJECT_0 == WaitForSingleObject(olOverlap.hEvent, INFINITE)) {
    // The accept occurred, so do something with it
}

我现在将等待有人指出我代码中的巨大明显错误...