Issue with Cling and TTree object in memory

[musinsky]

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Description

Strange issue with Cling and TTree:Scan(), please see table in Reproducer.

Reproducer

file scan.C

#include "TTree.h"
#include "TRandom.h"

TTree *t = nullptr;

void scan() {
  const Int_t kMax = 10;
  Int_t n;
  Float_t a[kMax];

  t = new TTree("t", "tree"); // in memory
  t->Branch("n", &n, "n/I");
  t->Branch("a", a, "a[n]/F");

  for (Int_t i = 0; i < 5; i++) {
    n = gRandom->Rndm()*(kMax-1);
    for (Int_t j = 0; j < n; j++)
      a[j] = gRandom->Gaus(0, 1);
    t->Fill();
  }
  // t->Scan("a[0]");
}

Fedora 40, root master (2024-10-22)	lxplus.cern.ch (RHEL 9.4), root v6.32.06
$ root scan.C root [0] Processing scan.C... root [1] t->Scan("a[0]") ************************ * Row * a[0] * ************************ * 0 * 1.255e-41 * ERROR leaf:a, len=22 and max=8 * 1 * 1.3911939 * ERROR leaf:a, len=22 and max=8 * 2 * 1.1125664 * ERROR leaf:a, len=22 and max=8 * 3 * 2.0609021 * ERROR leaf:a, len=22 and max=8 * 4 * -0.182436 * ************************ (long long) 5 root [2] t->Draw("a[0]") root [3] // draws correct all 5 values	$ root scan.C root [0] Processing scan.C... root [1] t->Scan("a[0]") ************************ * Row * a[0] * ************************ * 0 * -0.434764 * * 1 * 1.3911939 * * 2 * 1.1125664 * * 3 * 2.0609021 * * 4 * -0.182436 * ************************ (long long) 5 root [2]
$ root root [0] .L scan.C root [1] scan() root [2] t->Scan("a[0]") ************************ * Row * a[0] * ************************ * 0 * -0.434764 * * 1 * 1.3911939 * * 2 * 1.1125664 * * 3 * 2.0609021 * * 4 * -0.182436 * ************************ *** Break *** segmentation violation	$ root root [0] .L scan.C root [1] scan() root [2] t->Scan("a[0]") ************************ * Row * a[0] * ************************ * 0 * -0.434764 * * 1 * 1.3911939 * * 2 * 1.1125664 * * 3 * 2.0609021 * * 4 * -0.182436 * ************************ *** Break *** segmentation violation

ROOT version

see in table

Installation method

build from source

Operating system

Linux

Additional context

No response

[pcanal]

Try with

  Int_t n = kMax;

[musinsky]

With Int_t n = kMax; on Fedora, root master version:

root [0] 
Processing scan.C...
In file included from input_line_8:1:
/home/musinsky/scan.C:10:13: warning: variable length arrays in C++ are a Clang extension [-Wvla-cxx-extension]
  Float_t a[kMax];
            ^~~~
/home/musinsky/scan.C:10:13: note: read of non-const variable 'kMax' is not allowed in a constant expression
/home/musinsky/scan.C:8:9: note: declared here
  Int_t kMax = 10;
        ^
root [1] t->Scan("a[0]")
************************
*    Row   *      a[0] *
************************
ERROR leaf:a, len=22 and max=8
*        0 * -0.434764 *
ERROR leaf:a, len=22 and max=8
*        1 * 1.3911939 *
ERROR leaf:a, len=22 and max=8
*        2 * 1.1125664 *
ERROR leaf:a, len=22 and max=8
*        3 * 2.0609021 *
ERROR leaf:a, len=22 and max=8
*        4 * -0.182436 *
************************
(long long) 5
root [2] 

Also tested with constexpr Int_t kMax = 10; to same problem.

[musinsky]

Other problem with Cling and tree in memory:
Fedora, root master version

$ root scan.C
root [0]
Processing scan.C...
root [1] t->Draw("a[0]")
Info in <TCanvas::MakeDefCanvas>:  created default TCanvas with name c1
root [2] scan()
root [3] t->Draw("a[0]")

 *** Break *** segmentation violation

$ root
root [0] .L scan.C++
Info in <TUnixSystem::ACLiC>: creating shared library /home/musinsky/./scan_C.so
root [1] scan()
root [2] t->Draw("a[0]")
Info in <TCanvas::MakeDefCanvas>:  created default TCanvas with name c1
root [3] scan()
root [4] t->Draw("a[0]")
root [5] scan()
root [6] t->Draw("a[0]")
root [7] scan()
root [8] t->Draw("a[0]")

 *** Break *** segmentation violation

lxplus.cern.ch (RHEL 9.4), root v6.32.06

$ root
root [0] .L scan.C++
Info in <TUnixSystem::ACLiC>: creating shared library /afs/cern.ch/user/m/musinsky/./scan_C.so
root [1] scan()
root [2] t->Draw("a[0]")
Info in <TCanvas::MakeDefCanvas>:  created default TCanvas with name c1
*** stack smashing detected ***: terminated

[musinsky]

The version if the tree is saved to a file works as expected.

#include "TTree.h"
#include "TRandom.h"

TFile *f = nullptr;
TTree *t = nullptr;

void tree(const char *fname = "file.root")
{
  const Int_t kMax = 10;
  Int_t n;
  Float_t a[kMax];

  SafeDelete(t); SafeDelete(f);
  if (fname) f = new TFile(fname, "recreate");
  // else in memory
  t = new TTree("t", "tree");
  t->Branch("n", &n, "n/I");
  t->Branch("a", a, "a[n]/F");

  for (Int_t i = 0; i < 5; i++) {
    n = gRandom->Rndm()*(kMax-1);
    for (Int_t j = 0; j < n; j++)
      a[j] = gRandom->Gaus(0, 1);
    t->Fill();
  }
  if (f) {
    t->Write();
    SafeDelete(t); SafeDelete(f);
    f = new TFile(fname);
    t = (TTree*)f->Get("t");
  }
  // t->Draw("a[0]");
}

$ root
root [0] .L tree.C
root [1] tree(); t->Draw("a[0]");
Info in <TCanvas::MakeDefCanvas>:  created default TCanvas with name c1
root [2] tree(); t->Draw("a[0]");
root [3] tree(); t->Draw("a[0]");