Non overlapped Serial Port hangs at CloseHandle

Question

I wrote a serial port class that I developed and for simplicity I used blocking/synchronous/non-overlapped. I went through all MSDN documentations and it was strait forward for me.

I don't have any problem with Opening, Transmitting or Receiving Bytes from the port. All operations are synchronous and there is no-threading complexity.

function TSerialPort.Open: Boolean;
var
  h: THandle;
  port_timeouts: TCommTimeouts;
  dcb: TDCB;
begin
  Result := False;

  if Assigned(FHandleStream) then
  begin
    // already open
    Exit(True);
  end;

  h := CreateFile(PChar('\\?\' + FComPort),
                  GENERIC_WRITE or GENERIC_READ, 0, nil, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0);

  // RaiseLastOSError();
  if h <> INVALID_HANDLE_VALUE then
  begin
    {
      REMARKS at https://docs.microsoft.com/en-us/windows/desktop/api/winbase/ns-winbase-_commtimeouts
      If an application sets ReadIntervalTimeout and ReadTotalTimeoutMultiplier to MAXDWORD and
      sets ReadTotalTimeoutConstant to a value greater than zero and less than MAXDWORD, one
      of the following occurs when the ReadFile function is called:

      * If there are any bytes in the input buffer, ReadFile returns immediately with the bytes in the buffer.
      * If there are no bytes in the input buffer, ReadFile waits until a byte arrives and then returns immediately.
      * If no bytes arrive within the time specified by ReadTotalTimeoutConstant, ReadFile times out.
    }

    FillChar(port_timeouts, Sizeof(port_timeouts), 0);
    port_timeouts.ReadIntervalTimeout := MAXDWORD;
    port_timeouts.ReadTotalTimeoutMultiplier := MAXDWORD;
    port_timeouts.ReadTotalTimeoutConstant := 50; // in ms
    port_timeouts.WriteTotalTimeoutConstant := 2000; // in ms

    if SetCommTimeOuts(h, port_timeouts) then
    begin
      FillChar(dcb, Sizeof(dcb), 0);
      dcb.DCBlength := sizeof(dcb);

      if GetCommState(h, dcb) then
      begin
        dcb.BaudRate := FBaudRate;                            //  baud rate
        dcb.ByteSize := StrToIntDef(FFrameType.Chars[0], 8);  //  data size
        dcb.StopBits := ONESTOPBIT;                           //  1 stop bit
        dcb.Parity   := NOPARITY;
        case FFrameType.ToUpper.Chars[1] of
          'E': dcb.Parity   := EVENPARITY;
          'O': dcb.Parity   := ODDPARITY;
        end;

        dcb.Flags := dcb_Binary or dcb_Parity or dcb_ErrorChar or
                     (DTR_CONTROL_ENABLE shl 4) or (RTS_CONTROL_ENABLE shl 12);

        dcb.ErrorChar := '?'; // parity error will be replaced with this char

        if SetCommState(h, dcb) then
        begin
          FHandleStream := THandleStream.Create(h);
          Result := True;
        end;
      end;
    end;

    if not Result then
    begin
      CloseHandle(h);
    end;
  end;
end;

function TSerialPort.Transmit(const s: TBytes): Boolean;
var
  len: NativeInt;

begin
  Result := False;
  len := Length(s);

  if Assigned(FHandleStream) and (len > 0) then
  begin
    // total timeout to transmit is 2sec!!
    Result := (FHandleStream.Write(s, Length(s)) = len);
  end;
end;

function TSerialPort.Receive(var r: Byte): Boolean;
begin
  Result := False;

  if Assigned(FHandleStream) then
  begin
    // read timeout is 50ms
    Result := (FHandleStream.Read(r, 1) = 1);
  end;
end;

My problem starts at closing the port. After all my communications, when I try to close the serial port, my Application totally hangs at CloseHandle() API. And that happens randomly. Which is meaningless to me since I use synchronous mode, there can not be any pending operations. When I request a close, It must simply close the handle.

I searched the problem on the google and stack-overflow. There are many people who faced the similar problems but most of them are related with .NET serial port driver and their asynchronous mode operations which I don't have.

And also some people forgot to set timeouts properly and they faced blocking issue at ReadFile and WriteFile API that is fully normal. But again this is not my problem, I've set CommTimeouts as it is indicated in MSDN remarks.

function TSerialPort.Close: Boolean;
var
  h: THandle;

begin
  Result := True;

  if Assigned(FHandleStream) then
  begin
    h := FHandleStream.Handle;
    FreeAndNil(FHandleStream);

    if h <> INVALID_HANDLE_VALUE then
    begin
      //PurgeComm(h, PURGE_TXABORT or PURGE_RXABORT or PURGE_TXCLEAR or PURGE_RXCLEAR); // didn't help
      //ClearCommError(h, PDWORD(nil)^, nil);  // didn't help
      //CancelIO(h);  // didn't help
      Result := CloseHandle(h); <------------ hangs here
    end;
  end;
end;

Some people on Microsoft forum, suggest calling CloseHandle() in different thread. I have tried that as well. But that time it hangs while trying to free AnonymousThread that I created. Even I left FreeOnTerminate:=true as default, it hangs and I get memory leakage report by Delphi.

Another bothering problem when it hangs, I have to close Delphi IDE fully and reopen. Otherwise I can't compile the code again since exe is still used.

function TSerialPort.Close: Boolean;
var
  h: THandle;
  t: TThread;
  Event: TEvent;

begin
  Result := True;

  if Assigned(FHandleStream) then
  begin
    h := FHandleStream.Handle;
    FreeAndNil(FHandleStream);

    if h <> INVALID_HANDLE_VALUE then
    begin
      PurgeComm(h, PURGE_TXABORT or PURGE_RXABORT or PURGE_TXCLEAR or PURGE_RXCLEAR);
      Event := TEvent.Create(nil, False, False, 'COM PORT CLOSE');
      t := TThread.CreateAnonymousThread(
        procedure()
        begin
          CloseHandle(h);
          If Assigned(Event) then Event.SetEvent();
        end);

      t.FreeOnTerminate := False;
      t.Start;
      Event.WaitFor(1000);
      FreeAndNil(t);  // <---------- that time it hangs here, why??!!
      FreeAndNil(Event);
    end;
  end;
end;

In my notebook I'm using USB to Serial Port converters from FTDI. Some people said that it is because of FTDI driver. But I'm using all microsoft drivers that is signed by Microsoft Windows Hardware Compatibility Publisher. There is no third party driver in my system. But when I disconnect the USB adapter, CloseHandle API unfreeze itself. Some people reports that, even native Serial Ports that are build in their motherboards have same issue.

So far I couldn't solve the problem. Any help or workaround highly appreciated.

Thanks.

Answer 1

This issue is with the FTDI USB-Serial converter driver. It doesn't handle the hardware flow control properly and on occasion will hang in CloseHandle call.

To get around the issue, implement hardware flow control manually. In C++ (not sure how it would be done in Delphi) set up these DCB structure fields in order to allow manual control of the RTS line:

// Assuming these variables are defined in the header
HANDLE m_hComm; // Comm port handle.
DCB m_dcb;      // DCB comm port settings.

// Put these settings in the DCB structure.
m_dcb.fRtsControl = RTS_CONTROL_ENABLE;
m_dcb.fOutxCtsFlow = TRUE;

Then use

EscapeCommFunction(m_hComm, CLRRTS); // Call this before calling WriteFile.

And

EscapeCommFunction(m_hComm, SETRTS); // Call this after Write is complete.

In your case, because its synchronous - you can just wrap every call to WriteFile with these 2 calls. If using asynchronous (like in my case), call the one with SETRTS after you get the completion event from the ovelapped structure in your WriteFile call.

Used to freeze all the time before we implemented this as we were using 12 serial ports, and only way to unlock the port would be restarting the computer. Now works like a charm with manual control, hasn't frozen once since.

One thing to keep in mind, some USB-Serial devices (or even different versions of FTDI) may invert the RTS line! So if the above doesn't work, try using SETRTS to set the line low and CLRRTS to set it high.

Edit: If you have access to a Windows XP machine, use portmon tool to see what is happening with the RTS line, this way you will know if it is inverted or not or if it is getting the commands at all.