Question

I have a question regarding calling methods from different threads. Well I am using WinUSB driver to communicate with USB device. I have separate thread to read data from device. Commands to device are set within main thread. Actually I am using WinUSB_WritePipe and WinUSB_ReadPipe methods to do such operations. In thread where data is read I use asynchronus method of reading with overlapped structure and WaitForMultipleObject. My device has some features that I need to set and this is done via GUI in main thread.

I am observing some strange behaviour. My question is do I need to lock calls (eg. with mutex) to this methods so just one thread at time is accessing or calling method.

OLD WAY:

type TMyThread = TThread
   protected
      procedure Execute; override;
end;

procedure TMyThread.Execute;
begin
  while not Terminated do
  begin   
     WinUsb_ReadPipe(Pipe, Amount, Overlapped)
     ErrNo := GetLastError;                   
     if ErrNo = ERROR_IO_PENDING then 
     begin 
       wRes = WaitForMultipleObjects(2, @HndEvt, false);
       if wRes = WAIT_OBJECT_0 then
       begin 
         ResetEvent(Overlapped.hEvent);
         WinUSB_GetOVerlappedResult
         DoSomethingWithData; // Do something
       end;
     end;  
  end;
end;

MainThread:
begin
  // Set device sample rate
  WinUSB_WritePipe (Pipe, Amount, Data, ...)
end;

NEW WAY:

type TMyThread = TThread
   protected
      procedure Execute; override;

   public
     procedure Lock;
     procedure Unlock;

     constructor Create(ASuspended: boolean); override;
     destructor  Destroy; override;
end;

constructor TMyThread.Create(ASuspended: boolean); 
begin
  hMtx := CreateMutex(nil, false, nil);
end;

destructor  TMyThread.Destroy(ASuspended: boolean); 
begin
  CloseHandle(hMtx);
end;

procedure TMyThread.Lock; 
begin
  WaitForSingleObject(hMtx, 10000);
end;

procedure TMyThread.Unlock; 
begin
  ReleaseMutex(hMtx);
end;

procedure TMyThread.Execute;
begin
  while not Terminated do
  begin   

     Lock;
     WinUsb_ReadPipe(Pipe, Amount, Overlapped)
     Unlock;

     ErrNo := GetLastError;                   
     if ErrNo = ERROR_IO_PENDING then 
     begin 
       wRes = WaitForMultipleObjects(2, @HndEvt, false);
       if wRes = WAIT_OBJECT_0 then
       begin 
         ResetEvent(Overlapped.hEvent);
         Lock;
         WinUSB_GetOVerlappedResult
         Unlock;

         DoSomethingWithData; // Do something
       end;
     end;  
  end;
end;

MainThread:
begin
  // Set device sample rate
  Lock; // same mutex as in TMYThread
  WinUSB_WritePipe (Pipe, Amount, Data, ...)
  Unlock; // same mutex as in TMYThread 
end;

This is very simplified code which intension is only for description of my problem and do not reflect my programming skills. 🙂 Of course with same mutex I lock then calling of same method in main thread.

I hope I described my problem as simple as possible… so again: Do I need to lock calls to those methods in different threads?

Thank you for your time and answers in advance. I really apppreciate it!

Br, Nix

Answer 1

if you download and read the winUSB white paper you will find that there is one big drawback of winUSB : it does not support multiple concurrent applications for the same USB device. IMHO this means you can’t have concurrent calls for reading.