winapi - 客户端断开连接后如何使命名管道不忙？

Question

我使用命名管道，我想在服务器上重用同一个管道，以便在原始客户端断开连接后连接另一个客户端。我所做的是:

• 服务器使用 CreateNamedPipe 创建管道
• 服务器使用 WriteFile 写入数据，并在返回错误 ERROR_PIPE_LISTENING 时重试(这是在任何客户端连接之前)
• 客户端使用 CreateFile 连接
• 客户端读取数据
• 客户端使用 CloseHandle 关闭管道句柄
• 此时服务器在尝试写入更多数据时收到错误ERROR_NO_DATA
• 服务器使用 DisconnectNamedPipe 断开管道连接，我希望它能再次释放
• 服务器尝试写入数据，收到错误 ERROR_PIPE_NOT_CONNECTED，它会重试，直到没有错误为止
• 但是，当新客户端连接并尝试在管道上CreateFile 时，它会得到ERROR_PIPE_BUSY

因此，我的问题是:我还需要执行哪些其他步骤才能正确断开客户端与管道的连接，以便新客户端可以连接？

Answer 1

问题是您遗漏了 ConnectNamedPipe()，它应该始终在CreateNamedPipe() 或 DisconnectNamedPipe() 之后尝试任何 I/哦。

如果您不想在等待客户端连接时阻塞，您可以在异步 I/O 模式下创建管道，在这种情况下，对 ConnectNamedPipe() 的调用需要一个事件对象，该对象将在客户端连接。或者，您可以设置 PIPE_NOWAIT 并定期调用 ConnectNamedPipe() 直到成功，但这是一项遗留功能，不鼓励使用它。 (在大多数情况下，使用事件对象也比轮询更有效。)

正如您所发现的，Windows 确实允许您在不调用 ConnectNamedPipe() 的情况下逃脱，但由于此行为未记录在案，因此可能应该避免。同样，调用 ConnectNamedPipe() 而不等待它成功重置管道的连接状态这一事实没有记录，不应依赖。

---

根据要求，这里有一些真实世界的代码来演示管道服务器端的使用。这段代码取自一个 GUI 应用程序，因此它使用异步 I/O，但应该注意它一次只与一个客户端对话。 (然而，它可以在多线程中运行，只需稍作修改。)

void wait_for_object(HANDLE object)
{
  DWORD dw;
  MSG msg;

  for (;;) 
  {
    dw = MsgWaitForMultipleObjectsEx(1, &object, INFINITE, QS_ALLINPUT, 0);

    if (dw == WAIT_OBJECT_0) break;
    if (dw == WAIT_OBJECT_0 + 1) 
    {
      while (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE)) DispatchMessage(&msg);
      continue;
    }
    srvfail(L"sleep() messageloop", GetLastError());
  }
}

HANDLE server_pipe;
HANDLE io_event;

void pipe_connection(void)
{
    OVERLAPPED overlapped;
    DWORD dw, err;

    SecureZeroMemory(&overlapped, sizeof(overlapped));
    overlapped.hEvent = io_event;

    if (!ReadFile(server_pipe, input_buffer, sizeof(input_buffer) - 1, NULL, &overlapped))
    {
        err = GetLastError();
        if (err == ERROR_IO_PENDING)
        {
            wait_for_object(io_event);
            if (!GetOverlappedResult(server_pipe, &overlapped, &dw, FALSE)) 
            {
                srvfail(L"Read from pipe failed asynchronously.", GetLastError());
            }
        }
        else
        {
            srvfail(L"Read from pipe failed synchronously.", GetLastError());
        }
    }
    else
    {
        if (!GetOverlappedResult(server_pipe, &overlapped, &dw, FALSE)) 
        {
            srvfail(L"GetOverlappedResult failed reading from pipe.", GetLastError());
        }
    }

    input_buffer[dw] = '\0';

    process_command();

    if (!WriteFile(server_pipe, &output_struct, 
        ((char *)&output_struct.output_string - (char *)&output_struct) + output_struct.string_length, 
        NULL, &overlapped))
    {
        err = GetLastError();
        if (err == ERROR_IO_PENDING)
        {
            wait_for_object(io_event);
            if (!GetOverlappedResult(server_pipe, &overlapped, &dw, FALSE)) 
            {
                srvfail(L"Write to pipe failed asynchronously.", GetLastError());
            }
        }
        else
        {
            srvfail(L"Write to pipe failed synchronously.", GetLastError());
        }
    }
    else
    {
        if (!GetOverlappedResult(server_pipe, &overlapped, &dw, FALSE)) 
        {
            srvfail(L"GetOverlappedResult failed writing to pipe.", GetLastError());
        }
    }

    if (!FlushFileBuffers(server_pipe)) srvfail(L"FlushFileBuffers failed.", GetLastError());
    if (!DisconnectNamedPipe(server_pipe)) srvfail(L"DisconnectNamedPipe failed.", GetLastError());
}

void server(void)
{
    OVERLAPPED overlapped;
    DWORD err, dw; 

    // Create the named pipe

    server_pipe = CreateNamedPipe(pipe_name, PIPE_ACCESS_DUPLEX | FILE_FLAG_FIRST_PIPE_INSTANCE | FILE_FLAG_OVERLAPPED, PIPE_TYPE_MESSAGE | PIPE_READMODE_MESSAGE, 1, buffer_size, buffer_size, 0, NULL);
    if (server_pipe == INVALID_HANDLE_VALUE) srvfail(L"CreateNamedPipe failed.", GetLastError());

    // Wait for connections

    io_event = CreateEvent(NULL, FALSE, FALSE, NULL);
    if (io_event == NULL) srvfail(L"CreateEvent(io_event) failed.", GetLastError());

    for (;;)
    {
        SecureZeroMemory(&overlapped, sizeof(overlapped));
        overlapped.hEvent = io_event;

        if (!ConnectNamedPipe(server_pipe, &overlapped))
        {
            err = GetLastError();
            if (err == ERROR_PIPE_CONNECTED)
            {
                pipe_connection();
            }
            else if (err == ERROR_IO_PENDING)
            {
                wait_for_object(io_event);
                if (!GetOverlappedResult(server_pipe, &overlapped, &dw, FALSE)) 
                {
                    srvfail(L"Pipe connection failed asynchronously.", GetLastError());
                }
                pipe_connection();
            }
            else
            {
                srvfail(L"Pipe connection failed synchronously.", GetLastError());
            }
        }
        else
        {
            if (!GetOverlappedResult(server_pipe, &overlapped, &dw, FALSE)) 
            {
                srvfail(L"GetOverlappedResult failed connecting pipe.", GetLastError());
            }
            pipe_connection();
        }
    }
}

(此代码已从原始代码中删除了多余的逻辑。我没有尝试编译编辑后的版本，因此可能存在一些小问题。)