I guess you should create a subclass of QIODevice and implement all the platform-specific issues by yourself Something like this:
Qt Code: 
  1. //ParalellPort.h
  2. #include <QIODevice>
  3.  
  4. class ParalellPort : public QIODevice {
  5. 	Q_OBJECT
  6. 	public:
  7. 		ParalellPort(const QString &, QObject * = 0);
  8. 		virtual ~ParalellPort();
  9.  
  10. 		virtual bool isSequential () const { return true; }
  11.  
  12. 		virtual bool open(OpenMode);
  13. 		virtual void close ();
  14. 		//..
  15. 	protected:
  16. 		virtual qint64 readData ( char * data, qint64 maxSize );
  17. 		virtual qint64 writeData ( const char * data, qint64 maxSize );
  18. 	private:
  19. 		#ifdef Q_OS_UNIX
  20. 		QFile deviceFile;
  21. 		#endif
  22. };
  23.  
  24. //ParalellPort.cpp
  25. #include "ParalellPort.cpp"
  26.  
  27. ParalellPort::ParalellPort(const QString & port, QObject * parent) :
  28. 	QIODevice(parent)
  29. 	#ifdef Q_OS_UNIX
  30. 	, deviceFile(port)
  31. 	#endif
  32. {
  33. }
  34.  
  35. bool ParalellPort::open(OpenMode mode) {
  36. 	#ifdef Q_OS_UNIX
  37. 	return deviceFile.open(mode);
  38. 	#endif
  39.  
  40. 	#ifdef Q_OS_MACX
  41. 	//...
  42. 	#endif
  43.  
  44. 	#ifdef Q_OS_WIN32
  45. 	//...
  46. 	#endif
  47. }
  48. //...
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A quote from TT's docs:
Subclasses of QIODevice are only required to implement the protected readData() and writeData() functions.