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TFile / memory leak


ROOT Version: 6.18/04
Platform: CentOS 7.6
Compiler: gcc 9.2.0


Hello,

I am experience a quite strange issue that is having me scratch my head. It seems quite simple: I open a TFile, extract an object, read some numbers, and then close the TFile. When I do this in a loop, I see the memory consumption increasing continuously. I have traced it to this very specific block of code by including / removing it in the running loop.

The code looks like this:

template <typename T>
T *fetch(TFile *f, const std::string &nm, T *def = NULL, bool clone = true) {
  if (f == NULL || f->IsZombie()) {
    return def;
  }
  TObject *obj(0x0);
  if (clone) { 
    gROOT->cd();
    obj = f->Get(nm.c_str())->Clone((nm + "_clone").c_str());
  } else {
    obj = f->Get(nm.c_str());
  }
  if (obj != NULL) {
    return dynamic_cast<T *>(obj);
  } else {
    return def;
  }
}

for(unsigned i = 0; i < 10000; ++i) {
  TFile* f = TFile::Open("/path/to/file.root", "read");
  TGraph* g = fetch<TGraph>(f, "graph", NULL, false);
  int n = g->GetN();
  delete f;
}

Perhaps you might object to the delete f, but I tried f->Close(); and see the same thing: with every loop, memory consumption increases.

Hi,

some observations:

  • deleteing the file is ok, as it also closes it.
  • Aren’t those TGraphs leaking? You get them from the file, but they don’t get deleted. I’m not sure that the TGraph will be deleted by closing the file.
  • There is the nice GetObject, which can be used like this:
T* obj = nullptr;
f->GetObject(name, obj)

It will do the type check and conversion. If the types don’t match, it leaves obj == nullptr.

Thanks for the tip about GetObject()

As to the TGraph being deleted, I was always under the impression that it would be owned by the TDirectoryFile in which it is so that TFile::Close would clean it up. I’ll try a separate delete, however if my memory serves correct this created a segmentation fault when I tried this in my litany of vain attempts to resolve the issue.

My preferred way is to always do

myObject->SetDirectory(nullptr)

on all objects that I want to manage myself. This way, they lose their association with the file, and you can do normal C++ memory management.
I seem to remember, however, that graphs anyway don’t take part in the “file memory management”, because they don’t have a SetDirectory().

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