The OpenGL pbuffer example appearing blank

RamK

I am using Qt 5 cloned from git and build using command,
"configure -developer-build -opensource -nomake tests -nomake examples -opengl desktop -mp"
When I tried running opengl examples which come with Qt source e.g. pbuffer , its getting blank screen.
Then i tried checking the openGL version, i have openGL version 3.3.

I added logs and tried setting version to QGLFormat in glwidget.cpp
@GLWidget::GLWidget(QWidget *parent)
: QGLWidget(QGLFormat(QGL::SampleBuffers), parent)
, geom(0)
, cube(0)
{
// create the pbuffer
QGLFormat pbufferFormat = format();
pbufferFormat.setVersion(3,3); // added by me
QGLFormat::setDefaultFormat(pbufferFormat); // added by me

pbufferFormat.setSampleBuffers(false);
pbuffer = new QGLPixelBuffer(QSize(512, 512), pbufferFormat, this);
setWindowTitle(tr("OpenGL pbuffers"));
initializeGeometry();

}

void GLWidget::paintGL()
{
pbuffer->makeCurrent();
drawPbuffer();

// added by me
QString openGLversion((char *)glGetString(GL_VERSION));
qDebug()<<"OpenGL version::" << openGLversion;
qDebug()<<"\nOpenGL format verions:: " << QGLFormat::openGLVersionFlags();
qDebug()<<"\nOpenGL context::" << this->format();
...
}
@log output:

OpenGL version:: "3.3.0"

OpenGL format verions:: QFlags(0x1|0x2|0x4|0x8|0x10|0x20|0x40|0x1000|0x2000|0x4000|0x8000)

OpenGL context:: QGLFormat(options QFlags(0x1|0x2|0x4|0x8|0x20|0x80|0x200|0x400) , plane 0 , depthBufferSize 24 , accumBufferSize -1 , stencilBufferSize 8 , redBufferSize 8 , greenBufferSize 8 , blueBufferSize 8 , alphaBufferSize 8 , samples 4 , swapInterval -1 , majorVersion 3 , minorVersion 3 , profile 0 )

With this change of setting version I still observed blank screen , however if I make any version set below 3.2 I can see the proper output on screen :)
@ pbufferFormat.setVersion(3,1); // i tried with 1.0 / 2.1/ 3.0@
but in the logs still it gives output as "OpenGL version:: "3.3.0" "

can anyone please explain what is the issue here and why defaults qt examples from qt git src are behaving like this.