#include "TFile.h"
#include "TTree.h"

typedef struct {
  float mesh_time;
}
data_tree_new;

void run() {
int nrunr=0;
int nsegm=0;
int nstream=2;
TFile*f = new TFile(Form("XYMG_run%d_%d_s%d.root",nrunr,nsegm,nstream),"recreate");
TTree*t= new TTree("t","XYMG DATA");
t->Refresh(); // I used Refresh(), but by replacing it by whatever action (i.e. t->Print() for example), it still works
float mesh_time;
data_tree_new data_tree;
//t->SetBranchAddress("mesh_time",&data_tree.mesh_time);
t->Branch("mesh_time",&data_tree.mesh_time,"mesh_time/F");
float time=9;
  for (int i =1; i<=10; i++)
  {
   data_tree.mesh_time=time;
   t->Fill();
   t->Write("", TObject::kOverwrite);
   exit(0);    
  }
  
  f->Close();

}