Crash when filling Tree with void*

[jmcarcell]

I’m trying to save a vector of a custom class using podio. The custom class is a class with only POD types (see below). I have reduced the code to the following (main.cpp):

using namespace std;

#include "ExampleHitData.h"
#include "TTree.h"
#include "TFile.h"
#include <vector>
#include <iostream>

int main () {

  ExampleHitData h {1, 2, 3, 4, 5};
  vector<ExampleHitData> v {h};

  TFile f("test.root", "RECREATE");
  TTree tree("test", "tree");

  void* ptr = &v;
  auto vptr = (std::vector<ExampleHitData>*)ptr;
  std::cout << "Size is " << vptr->size() << std::endl;
  std::cout << (*vptr)[0].cellID << " " << (*vptr)[0].x << " " << (*vptr)[0].y << " " << (*vptr)[0].z << " " << (*vptr)[0].energy << std::endl;

  tree.Branch("events", "std::vector<ExampleHitData>", ptr);
  // tree.Branch("events", "std::vector<ExampleHitData>", vptr); // This works fine

  tree.Fill();
  tree.Write();
  
}

The issue here is that when passing the void* ptr to tree.Branch, it will crash when filling but when passing the std::vector<ExampleHitData>* it works fine and the resulting file has all the fields with the expected values. The output of the couts is:

Size is 1
1 2 3 4 5

Full stack trace:

#0  0x00007fdc8d6ea707 in wait4 () from /usr/lib/libc.so.6
#1  0x00007fdc8d6627fb in ?? () from /usr/lib/libc.so.6
#2  0x00007fdc8f2eff29 in TUnixSystem::Exec (shellcmd=<optimized out>, this=0x559b3402d090) at /tmp/jmcarcell/spack-stage/spack-stage-root-master-t3bu3cb4mqxacnsbp2lifz3xukgested/spack-src/core/unix/src/TUnixSystem.cxx:2104
#3  TUnixSystem::StackTrace (this=0x559b3402d090) at /tmp/jmcarcell/spack-stage/spack-stage-root-master-t3bu3cb4mqxacnsbp2lifz3xukgested/spack-src/core/unix/src/TUnixSystem.cxx:2395
#4  0x00007fdc8f2ef8a4 in TUnixSystem::DispatchSignals (this=0x559b3402d090, sig=kSigSegmentationViolation) at /tmp/jmcarcell/spack-stage/spack-stage-root-master-t3bu3cb4mqxacnsbp2lifz3xukgested/spack-src/core/unix/src/TUnixSystem.cxx:3615
#5  <signal handler called>
#6  0x00007fdc8ebf0eec in TStreamerInfoActions::VectorLooper::WriteBasicType<unsigned long long> (buf=..., iter=0x1, end=0x4000000000000000, loopconfig=<optimized out>, config=<optimized out>) at /tmp/jmcarcell/spack-stage/spack-stage-root-master-t3bu3cb4mqxacnsbp2lifz3xukgested/spack-src/io/io/src/TStreamerInfoActions.cxx:1815
#7  0x00007fdc8eae789c in TStreamerInfoActions::TConfiguredAction::operator() (loopconf=0x559b3535d940, end_collection=0x4000000000000000, start_collection=0x1, buffer=..., this=0x559b3535d910) at /tmp/jmcarcell/spack-stage/spack-stage-root-master-t3bu3cb4mqxacnsbp2lifz3xukgested/spack-src/io/io/inc/TStreamerInfoActions.h:131
#8  TBufferFile::ApplySequence (this=0x559b3536f830, sequence=..., start_collection=0x1, end_collection=0x4000000000000000) at /tmp/jmcarcell/spack-stage/spack-stage-root-master-t3bu3cb4mqxacnsbp2lifz3xukgested/spack-src/io/io/src/TBufferFile.cxx:3646
#9  0x00007fdc8e3182ce in TBranchElement::FillLeavesCollectionMember (this=0x559b35359cb0, b=...) at /tmp/jmcarcell/spack-stage/spack-stage-root-master-t3bu3cb4mqxacnsbp2lifz3xukgested/spack-src/tree/tree/src/TBranchElement.cxx:1553
#10 0x00007fdc8e30f2b4 in TBranch::FillImpl (this=this
entry=0x559b35359cb0, imtHelper=imtHelper
entry=0x0) at /tmp/jmcarcell/spack-stage/spack-stage-root-master-t3bu3cb4mqxacnsbp2lifz3xukgested/spack-src/tree/tree/src/TBranch.cxx:893
#11 0x00007fdc8e31b85d in TBranchElement::FillImpl (this=0x559b35359cb0, imtHelper=0x0) at /tmp/jmcarcell/spack-stage/spack-stage-root-master-t3bu3cb4mqxacnsbp2lifz3xukgested/spack-src/tree/tree/src/TBranchElement.cxx:1265
#12 0x00007fdc8e31b5a6 in TBranchElement::FillImpl (this=0x559b3534cd50, imtHelper=0x0) at /tmp/jmcarcell/spack-stage/spack-stage-root-master-t3bu3cb4mqxacnsbp2lifz3xukgested/spack-src/tree/tree/src/TBranchElement.cxx:1290
#13 0x00007fdc8e387bb7 in TTree::Fill (this=0x7fff75fdd230) at /tmp/jmcarcell/spack-stage/spack-stage-root-master-t3bu3cb4mqxacnsbp2lifz3xukgested/spack-src/tree/tree/src/TTree.cxx:4609

It fails at TStreamerInfoActions::VectorLooper::WriteBasicType<unsigned long long> because unsigned long long is the type of the first member cellID in the ExampleHitData class.

I’m not sure if it’s a bug in ROOT but if ptr is a well defined void* to a vector why shouldn’t it work when the vector pointer vptr works?

To reproduce you need ExampleHitData.h:

class ExampleHitData {
public:
  unsigned long long cellID; ///< cellID
  double x;                  ///< x-coordinate
  double y;                  ///< y-coordinate
  double z;                  ///< z-coordinate
  double energy;             ///< measured energy deposit
};

and LinkDef.h

#ifdef __CLING__
#pragma link C++ class ExampleHitData;
#pragma link C++ class std::vector<ExampleHitData>;
#endif

and then run

rootcling -f ExampleHitData.cxx ExampleHitData.h LinkDef.h
gcc -c ExampleHitData.cxx -fPIC
gcc -shared -o libExampleHitData.so ExampleHitData.o
$(root-config --cxx --cflags --glibs) -L$PWD -lExampleHitData main.cpp
./a.out

ROOT Version: 6.28.00, 6.26.00 and also master from ~1 week ago
Platform: Centos7 and Arch Linux
Compiler: gcc 11.2.0 and 12.2.1

[couet]

I think @pcanal can help you.

[jmcarcell]

Any ideas on what may be going wrong @pcanal ?

[pcanal]

Try:

#ifdef __CLING__
#pragma link C++ class ExampleHitData+;
#pragma link C++ class std::vector<ExampleHitData>+;
#endif

The trailing ‘+’ requires the ‘current’ StreamerInfo based I/O (rather than the ancient streamer based version).

[pcanal]

The void* does not provide enough information for TTree::Branch to be smart.
With:

  vector<ExampleHitData> v {h};
  vector<ExampleHitData> *vp = &v;
  void *voidptr = vp;

The Branch can distinguish between

  tree.Branch("events_from_ptr_to_ptr", "std::vector<ExampleHitData>", &vp);
  tree.Branch("events_from_ptr_value", "std::vector<ExampleHitData>", vp);
  tree.Branch("events_from_ptr_value", "std::vector<ExampleHitData>", &v);

since we can distinguish (in C++ templated code) those 3 cases.

while with:

  tree.Branch("events", "std::vector<ExampleHitData>", voidptr);

We have no type information about the address given by voidptr and thus TTree::Branch must assume (and can not check) the historical convention that the value is the address of a pointer to the object. So this will work:

  tree.Branch("events", "std::vector<ExampleHitData>", &voidptr);

Side note/reminders :), the tree will remember that address is so in this example voidptr (the variable) needs to have a lifetime greater than the lifetime of the TTree object and if not, TTree::ResetBranchAddresses (or similar at the branch level) needs to be called to make the tree forget aboud the address of the pointer variable.

[jmcarcell]

Thanks, that was the answer. It’s certainly not very intuitive that you have to pass the address of the pointer instead of the pointer itself only in the case it’s a void*. I also don’t remember finding anything about this in the documentation, but maybe I missed it.