Error in merging trees into one using TTree::MergeTrees

ROOT
,
1

Hi everyone,

I am trying to merge 3 different ROOT files, each one containing one tree. Here is the code that I use:

#include "TH1F.h"
#include "TFile.h"
#include "TTreeReader.h"
#include "TTree.h"
#include <iostream>
#include <string>

void merge_root(){
  
  TList* treeList = new TList;

  TFile* mc16file = TFile::Open("mc16.root", "RECREATE");

  TFile* mc16a = new TFile("mc16a.root");
  TFile* mc16d = new TFile("mc16d.root");
  TFile* mc16e = new TFile("mc16e.root");

  treeList->Add(mc16a);
  treeList->Add(mc16d);
  treeList->Add(mc16e);

  TTree* outTree = TTree::MergeTrees(treeList);

  outTree->Write("", TObject::kOverwrite);

  mc16file->Close();

}

I get the following error message:

#0  0x00007f80c952946c in waitpid () from /lib64/libc.so.6
#1  0x00007f80c94a6f62 in do_system () from /lib64/libc.so.6
#2  0x00007f80ca6368f3 in TUnixSystem::StackTrace() () from /cvmfs/atlas.cern.ch/repo/sw/software/21.2/AnalysisBaseExternals/21.2.139/InstallArea/x86_64-centos7-gcc8-opt/lib/libCore.so
#3  0x00007f80c4bd7405 in cling::MultiplexInterpreterCallbacks::PrintStackTrace() () from /cvmfs/atlas.cern.ch/repo/sw/software/21.2/AnalysisBaseExternals/21.2.139/InstallArea/x86_64-centos7-gcc8-opt/lib/libCling.so
#4  0x00007f80c4bd6e5b in cling_runtime_internal_throwIfInvalidPointer () from /cvmfs/atlas.cern.ch/repo/sw/software/21.2/AnalysisBaseExternals/21.2.139/InstallArea/x86_64-centos7-gcc8-opt/lib/libCling.so
#5  0x00007f80cad9c48a in ?? ()
#6  0x0000000000000000 in ?? ()
Error in <HandleInterpreterException>: Trying to dereference null pointer or trying to call routine taking non-null arguments.
Execution of your code was aborted.
In file included from input_line_8:1:
/usera/martinov/ZZy-analysis/merge_root.C:25:3: warning: null passed to a callee that requires a non-null argument [-Wnonnull]
  outTree->Write("", TObject::kOverwrite);

I suspect that it’s got something to do with pointers but it would be great if someone could help me !

Cheers !

2

Strangely MergeTrees seems to have return a nullptr … Maybe try:

TFile* mc16file = TFile::Open("mc16.root", "RECREATE");

  TFile* mc16a = new TFile("mc16a.root");
  TFile* mc16d = new TFile("mc16d.root");
  TFile* mc16e = new TFile("mc16e.root");

  treeList->Add(mc16a);
  treeList->Add(mc16d);
  treeList->Add(mc16e);
  
   mc16file->cd();
  TTree* outTree = TTree::MergeTrees(treeList);

  outTree->Write("", TObject::kOverwrite);

to insure the result TTree is attached to the output file (and hence its basket written there).

It might be easier to run (on the shell command line)

hadd -f mc16.root mc16?.root
3

Ok it fixed it ! Thanks a lot !
It still works if I remove the “mc16file->cd();” line. So, it’s really the shell command that fixes it.

4

The ‘cd’ is to fix your code (without having to use the hadd command).
The command line replace it completely (and use the “fast merging” method so is much faster).

5

Oh I see. The ‘cd’ actually didn’t change anything to be honest, the error message is still the same.

6

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