As far as I know, this code should work:

Qt Code: 
  1. void
  2. MyClass::draw()
  3. {
  4.   if (this->pshader->isLinked()) {
  5.     int val_loc = this->pshader->attributeLocation("val");
  6.     this->pshader->setUniformValue(val_loc, (GLfloat)1.0);
  7.   }
  8.   // OpenGL stuff
  9. }
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Vertex shader:

Qt Code: 
  1. uniform float val;
  2.  
  3. varying float new_val;
  4.  
  5. void main()
  6. {
  7.   new_val = val;
  8.   gl_Position = ftransform();
  9. }
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Fragment shader:

Qt Code: 
  1. varying float new_val;
  2.  
  3. void main()
  4. {
  5.   if (new_val < 0.5)
  6.     gl_FragColor = vec4(1.0,0.0,0.0,1.0);
  7.   else
  8.     gl_FragColor = vec4(0.0,0.0,1.0,1.0);
  9. }
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but my object appears red...