Double_t old_par[]={-1, -1, -1};

Double_t myfunc(Double_t *x, Double_t *par)
{
	// Check if parameter changed
	for(int i=0; i<3; i++)
	{
		if(old_par[i]!=par[i]) cout << "Parameter " << i << " changed from " << old_par[i] << " to " << par[i] << endl;
		old_par[i]=par[i];
	}
		
	return par[0]*TMath::Gaus(x[0],par[1],par[2]);
}

void ExampleFixParam()
{ 
cout.precision(11);

   TH1D * h1 = new TH1D("h1","h1",100,-5,5); 
   h1->FillRandom("gaus"); 
   TVirtualFitter::SetDefaultFitter("Fumili2"); 

//   TF1 * f1 = new TF1("f1","[0]*TMath::Gaus(x,[1],[2])");
   TF1 * f1 = new TF1("f1",myfunc, 0, 2, 3);  
   f1->SetParNames("Amplitude","mean","sigma"); 
   f1->SetParameters(100,0,1); 
   f1->FixParameter(2,1);

   h1->Fit(f1); 
   
}