Question

I’am having some trouble to make a Duplex communication between a C# server and a C++ work:

The objective is to create the pipe, read something from the client and write something back.
Everything is working fine if i’m just reading from the client or just writing to the client but I can’t do both one after the other!

Here’s my C# server:

// Create a name pipe
 using (NamedPipeServerStream pipeStream = new NamedPipeServerStream("TestPipe"))
 {
   Console.WriteLine("[Server] Pipe created {0}", pipeStream.GetHashCode());

   // Wait for a connection
   pipeStream.WaitForConnection();
   Console.WriteLine("[Server] Pipe connection established");

   //Reading Part
   using (StreamReader sr = new StreamReader(pipeStream))
   {
      string temp;
      while ((temp = sr.ReadLine()) != null)
      {
         Console.WriteLine("{0}: {1}", DateTime.Now, temp);
       }
     }

     //Writing Part
     using (StreamWriter sw = new StreamWriter(pipeStream))
     {
        sw.AutoFlush = true;
        String st = "send Back\0";
        sw.WriteLine(st);
     }
 }

And Here is the C++ client:

HANDLE hFile;
BOOL flg;
DWORD dwWrite;
char szPipeUpdate[200];
hFile = CreateFile(L"\\\\.\\pipe\\TestPipe", GENERIC_WRITE|GENERIC_READ,
                           0, NULL, OPEN_EXISTING,
                           0, NULL);


 strcpy(szPipeUpdate,"Sending some data from client to server!");
 if(hFile == INVALID_HANDLE_VALUE)
  { 
        DWORD dw = GetLastError();
        printf("CreateFile failed for Named Pipe client\n:" );
  }
  else
  {
      flg = WriteFile(hFile, szPipeUpdate, strlen(szPipeUpdate), &dwWrite, NULL);
      if (FALSE == flg)
      {
         printf("WriteFile failed for Named Pipe client\n");
      }
      else
      {
         printf("WriteFile succeeded for Named Pipe client\n");
      }
  }

  printf("Let's read!\n");
  //Read the datas sent by the server
  BOOL fFinishRead = FALSE;
  do
    {
        char chResponse[200];
        DWORD cbResponse, cbRead;
        cbResponse = sizeof(chResponse);

        fFinishRead = ReadFile(
            hFile,                  // Handle of the pipe
            chResponse,             // Buffer to receive the reply
            cbResponse,             // Size of buffer in bytes
            &cbRead,                // Number of bytes read 
            NULL                    // Not overlapped 
            );

        if (!fFinishRead && ERROR_MORE_DATA != GetLastError())
        {
            DWORD  dwError = GetLastError();
            wprintf(L"ReadFile from pipe failed w/err 0x%08lx\n", dwError);
            break;
        }
        std::cout << chResponse;

    } while (!fFinishRead); // Repeat loop if ERROR_MORE_DATA
 CloseHandle(hFile);

Answer 1

With the Server & Client NamedPipeStreams you need to make sure at least one end of the connection is attempting to read at all times, otherwise writes will fail. I’d guess it’s a timing issue between your client & server around whichever is listening/writing. Here’s a quick blog post explaining this behavior as well.

I’d recomend using two streams on the client & server, both reading/writing from the same pipe.