一个关于 overlapped I/O api 的疑问

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这是一个创建于 2250 天前的主题，其中的信息可能已经有所发展或是发生改变。

在使 win32 overlapped I/O API 的过程中，关于 WSARecv 函数的 msdn 声明如下:

int WSAAPI WSARecv(
  SOCKET                             s,
  LPWSABUF                           lpBuffers,
  DWORD                              dwBufferCount,
  LPDWORD                            lpNumberOfBytesRecvd,
  LPDWORD                            lpFlags,
  LPWSAOVERLAPPED                    lpOverlapped,
  LPWSAOVERLAPPED_COMPLETION_ROUTINE lpCompletionRoutine
);

msdn 上关于这个 API 的解释如下：

The lpOverlapped parameter must be valid for the duration of the overlapped operation. If multiple I/O operations are simultaneously outstanding, each must reference a separate WSAOVERLAPPED structure.

If the lpCompletionRoutine parameter is NULL, the hEvent parameter of lpOverlapped is signaled when the overlapped operation completes if it contains a valid event object handle. An application can use WSAWaitForMultipleEvents or WSAGetOverlappedResult to wait or poll on the event object.

If lpCompletionRoutine is not NULL, the hEvent parameter is ignored and can be used by the application to pass context information to the completion routine. A caller that passes a non-NULL lpCompletionRoutine and later calls WSAGetOverlappedResult for the same overlapped I/O request may not set the fWait parameter for that invocation of WSAGetOverlappedResult to TRUE. In this case the usage of the hEvent parameter is undefined, and attempting to wait on the hEvent parameter would produce unpredictable results.

The completion routine follows the same rules as stipulated for Windows file I/O completion routines. The completion routine will not be invoked until the thread is in an alertable wait state such as can occur when the function WSAWaitForMultipleEvents with the fAlertable parameter set to TRUE is invoked.

按照文档的意思：如果 completionRoutine 不是 null 的话，可以使用 hEvent 参数在回调函数中来传递上下文信息。

那么假如我想传递一个 obj 给 completionRoutine 对应的回调函数，应该怎么传递呢？试过以下的方式：

m_overlap.hEvent = new context_obj;

completionCallback((DWORD dwError,
	DWORD cbTransferred,
	LPWSAOVERLAPPED lpOverlapped,
	DWORD dwFlags){
    
    //does not work, how to pass context info?
    auto* obj = static_cast<context_obj*>(lpOverlapped.hEvent);
    
    }

不知道是不是我理解的有问题，这样传递 context 不行。求教。

8 条回复 • 2018-10-31 09:33:10 +08:00