The WSAAsyncSelect function requests Windows message-based notification of network events for a socket.

Syntax

int WSAAsyncSelect(
  _In_  SOCKET s,
  _In_  HWND hWnd,
  _In_  unsigned int wMsg,
  _In_  long lEvent
);

Parameters

s [in]

A descriptor that identifies the socket for which event notification is required.

hWnd [in]

A handle that identifies the window that will receive a message when a network event occurs.

wMsg [in]

A message to be received when a network event occurs.

窗口消息循环要接受的消息。

lEvent [in]

A bitmask that specifies a combination of network events in which the application is interested.

Return value

If the WSAAsyncSelect function succeeds, the return value is zero, provided that the application‘s declaration of interest in the network event set was successful. Otherwise, the value SOCKET_ERROR is returned, and a specific error number can be retrieved by calling WSAGetLastError.

成功返0，得到之前声明感兴趣的网络事件

失败返SOCKET_ERROR

Additional error codes can be set when an application window receives a message. This error code is extracted from the lParam in the reply message using theWSAGETSELECTERROR macro. Possible error codes for each network event are listed in the following table.

WSAGETSELECTERROR宏取lParam低字节，看窗口消息内的网络事件的具体错误

Event: FD_CONNECT

Event: FD_CLOSE

Event: FD_ACCEPT

Event: FD_ADDRESS_LIST_CHANGE

Event: FD_GROUP_QOS

Event: FD_OOB

Event: FD_QOS

Event: FD_READ

Event: FD_WRITE

Event: FD_ROUTING_INTERFACE_CHANGE

Remarks

The WSAAsyncSelect function is used to request that WS2_32.DLL should send a message to the window hWnd when it detects any network event specified by thelEvent parameter. The message that should be sent is specified by the wMsg parameter. The socket for which notification is required is identified by the s parameter.

WSAAsyncSelect 要WS2_32.DLL给窗口发消息，当检测到lEvent的时候发，发的是wMsg消息，s套接字被监察

The WSAAsyncSelect function automatically sets socket s to nonblocking mode, regardless of the value of lEvent. To set socket s back to blocking mode, it is first necessary to clear the event record associated with socket s via a call to WSAAsyncSelect with lEvent set to zero. You can then call ioctlsocket or WSAIoctl to set the socket back to blocking mode. For more information about how to set the nonblocking socket back to blocking mode, see the ioctlsocket and WSAIoctlfunctions.

WSAAsyncSelect 把套接字调成非阻塞，而不管你给的lEvent是什么。

要调回阻塞就要用WSAAsyncSelect 把IEvent置零。然后就可以用 ioctlsocket 或者 WSAIoctl 把套接字调成阻塞模式。

The lEvent parameter is constructed by using the bitwise OR operator with any value listed in the following table.

Issuing a WSAAsyncSelect for a socket cancels any previous WSAAsyncSelect or WSAEventSelect for the same socket. For example, to receive notification for both reading and writing, the application must call WSAAsyncSelect with both FD_READ and FD_WRITE, as follows:

关注的事件要一起设置在一个消息里

rc = WSAAsyncSelect(s, hWnd, wMsg, FD_READ|FD_WRITE);

It is not possible to specify different messages for different events. The following code will not work; the second call will cancel the effects of the first, and onlyFD_WRITE events will be reported with message wMsg2:

不要分开去设置在不同消息但又是相同的套接字里。。。

rc = WSAAsyncSelect(s, hWnd, wMsg1, FD_READ);
rc = WSAAsyncSelect(s, hWnd, wMsg2, FD_WRITE);

rc = WSAAsyncSelect(s, hWnd, 0, 0);

这要就不发网络事件了

Although WSAAsyncSelect immediately disables event message posting for the socket in this instance, it is possible that messages could be waiting in the application message queue. Therefore, the application must be prepared to receive network event messages even after cancellation. Closing a socket withclosesocket also cancels WSAAsyncSelect message sending, but the same caveat about messages in the queue still applies.

虽然你这样取消了发送或者说对网络事件的监听，但是之前遗留在Window消息队列里的网络消息还是有效的，也就是随后将根据队列顺序发给窗口，这是就造成了取消之后，窗口仍然会收到网络消息的情况。