PIPE_NOWAIT 断开连接

Question

我有一段代表简单管道服务器的代码。当我设置 PIPE_NOWAIT 属性然后尝试使用 CreateFile 函数从客户端连接到它时 - 没有任何反应，我们仍在第二个块中。当我删除 PIPE_NOWAIT 标志时 - 我们挂在 ConnectNamedPipe 函数中，在从客户端调用 CreateFile 之后，我们进入了第一个块，这意味着我们成功了。为什么会这样？

HANDLE hPipe;
std::string msg;
DWORD dwRead;

hPipe = CreateNamedPipe(L"\\.\pipe\Pipe100500",
    PIPE_ACCESS_DUPLEX,   // FILE_FLAG_FIRST_PIPE_INSTANCE is not needed but forces CreateNamedPipe(..) to fail if the pipe already exists...
    PIPE_TYPE_MESSAGE | PIPE_READMODE_MESSAGE | PIPE_NOWAIT,
    1,
    1024 * 16,
    1024 * 16,
    NMPWAIT_USE_DEFAULT_WAIT,
    NULL);

while (true)
{
    if (ConnectNamedPipe(hPipe, NULL) != FALSE)
    {
        // Waiting in the ConnectNamedPipe and then we are here
    }
    else
    {
        // always here when PIPE_NOWAIT
    }
}

Answer 1

根据 CreateNamedPipe() 文档：

PIPE_NOWAIT
0x00000001

Nonblocking mode is enabled. In this mode, ReadFile, WriteFile, and ConnectNamedPipe always return immediately.

根据 ConnectNamedPipe() 文档：

If the specified pipe handle is in nonblocking mode, ConnectNamedPipe always returns immediately. In nonblocking mode, ConnectNamedPipe returns a nonzero value the first time it is called for a pipe instance that is disconnected from a previous client. This indicates that the pipe is now available to be connected to a new client process. In all other situations when the pipe handle is in nonblocking mode, ConnectNamedPipe returns zero. In these situations, GetLastError returns ERROR_PIPE_LISTENING if no client is connected, ERROR_PIPE_CONNECTED if a client is connected, and ERROR_NO_DATA if a previous client has closed its pipe handle but the server has not disconnected. Note that a good connection between client and server exists only after the ERROR_PIPE_CONNECTED error is received.

Note Nonblocking mode is supported for compatibility with Microsoft LAN Manager version 2.0, and it should not be used to achieve asynchronous input and output (I/O) with named pipes.

因此，要正确使用 PIPE_NOWAIT（即使您真的根本不应该使用它！），您还需要这样的东西：

HANDLE hPipe;
std::string msg;
DWORD dwRead;

hPipe = CreateNamedPipe(L"\\.\pipe\Pipe100500",
    PIPE_ACCESS_DUPLEX,   // FILE_FLAG_FIRST_PIPE_INSTANCE is not needed but forces CreateNamedPipe(..) to fail if the pipe already exists...
    PIPE_TYPE_MESSAGE | PIPE_READMODE_MESSAGE | PIPE_NOWAIT,
    1,
    1024 * 16,
    1024 * 16,
    NMPWAIT_USE_DEFAULT_WAIT,
    NULL);

if (hPipe != INVALID_HANDLE_VALUE)
{
    while (true)
    {
        if (ConnectNamedPipe(hPipe, NULL))
        {
            // pipe has been disconnected and is now available to be connected to again...
        }
        else
        {
            switch (GetLastError())
            {
                case ERROR_PIPE_LISTENING:
                    // no client is connected...
                    break;

                case ERROR_PIPE_CONNECTED:
                   // a client is now connected...
                   break;

                case ERROR_NO_DATA:
                    // a previous client has closed its pipe handle but the server has not disconnected yet...
                    DisconnectNamedPipe(hPipe);
                    break;

                default:
                    // fatal error...
                    break;
            }
        }
    }
}

否则，按照文档中的说明进行操作，根本不要使用 PIPE_NOWAIT。如果您不想 ConnectNamedPipe() 阻塞，请改用 Overlapped I/O。 MSDN 提供了这样的示例：

Named Pipe Server Using Overlapped I/O

Named Pipe Server Using Completion Routines

PIPE_NOWAIT 断开连接

PIPE_NOWAIT breaks connetion

c++

winapi

pipe

named-pipes