Disk-resident TTree

ROOT
1

I have a script in which I create a TTree:
TTree* myTree = new TTree("myTree", "C");
then fill it, and then - in the same script - make calls to “Draw”, to draw branches of the tree.
This works (I get filled plots on my canvas), but I get warnings like this:

Error in <TBranch::WriteBasketImpl>: basket's WriteBuffer failed.
Error in <TBranch::TBranch::Fill>: Failed to write out basket.
Error in <TTree::Fill>: Failed filling branch:coincTree.tentry, nbytes=-1, entry=3990
 This error is symptomatic of a Tree created as a memory-resident Tree

So, then I tried this:

    TFile outRootFile("myTREE.root_NEW", "RECREATE");
    TTree* myTree = new TTree("myTree", "C");

And then fill the TTree, but now, when I make calls to “myTree->Draw()”, nothing shows up.
I have tried putting “myTree->GetCurrentFile()->cd();” in some places in my code, but it doesn’t help.

So, without the call to TFile, I get the correct plots, but warnings. And with the call to TFile, I get rid of the warnings, but no plots show up. How do I fix this?

_ROOT Version: v6.26.10
_Platform: Ubuntu 18

2

I think there is more to it than just the order of the tree and file creation. Could you attach your macro, or some minimal version of it?

3

Hi,
Thanks for your reply. The problem is, it’s a very long script with a lot going on. I should first simply it. Maybe part of the problem is that I also open two TFiles (containing TTrees) as input. So there are two input TFiles and one output TFile. So, my guess is that it’s a matter of “directories” (I am not sure that’s the right word). I can’t find how to do something similar to “ls” and “cd” in a ROOT C++ script.
I tried playing with “myTree->GetCurrentFile()->cd();” but that doesn’t help.
By the way, even my histograms “disappear” (when I try drawing them), as soon as I use the TFile line before creating my TTree.
The only solution I see, for the moment, is only drawing histograms and not creating a TTree.

4

You were right. The problem is something else. Actually, the “TTree” does not disappear, it’s the histograms that disappear. Either when I project a TTree draw onto a histogram (myTree->Draw("aValue >> h1");) or when plotting a histogram directly.
So, I am guessing that since the histograms are not “TFile” located (?), they cannot be plotted?
How can I fix this? I included the simplified code and two plots as example.

No_TFile
No_TFile1303×471 13.4 KB

Simple.C (907 Bytes)

With_TFile
With_TFile1303×471 5.09 KB

5

The problem is caused by the TFile taking ownership of the histograms. At the end of the macro, the file is destructed and takes the histograms with it. A more detailed explanation of ROOT’s object ownership model can be found here: Object ownership - ROOT

You can insert at the beginning of the macro TH1::AddDirectory(false). This changes the ownership behavior for all histograms so that the caller of new TH1D owns it, and also is in charge of deleting it.

6

Wow! Thank you very much. You’re right: that fixed it.
I don’t think I could ever have figured this out myself.

Thanks,

Machiel