After Browsing through the code. i found
1) The screen->classID() returns QDirectFbScreen
2) The implementation of QScreen::Base() is to return the protected membet data.
3) The protected member data is filled by the derived class. So from point 1 the derived class in this case is QDirectFbScreen.
4) The data member is made data=0; in QDirectFbScreen::connect() method. and after that there is no other change to the data member.

This gives the reason why screen->base() is returning 0.

So can some one please tell me if i have to change the implementation of qDirectfbscreen provide by QT to get the valid frame buffer pointer????