Double_t fitf(Double_t *x, Double_t *par)
{
   Double_t arg = 0;
   Double_t fitval = par[0]*TMath::Sin(x[0]+par[1]);
   return fitval;
}


void myfit()
{

  TCanvas *c1 = new TCanvas("c1","the fit canvas",500,400);
  TF1 *func = new TF1("fitf",fitf,0.0,6.28,2); // set initial value and parm
  func->SetParameters(0.02,1.0);
  func->SetParNames("Constant","Offset");
  //Dca_phi_2->Fit("fitf");
  histname->Fit("fitf"); // histname should be a variable Dca_phi_1, Dca_phi_2  or Dca_phi_3 
                         // coming from main macro
  Double_t parA = func->GetParameter("Constant") ; // parA should be readable from main macro
  cout <<"  parA  " << parA << endl;
}