Error in <TFile::WriteTObject>: Directory testHistos.root is not writable

ROOT
1

I try to save a canvas to a root tree:

	TFile rootfile("testHistos.root","new");
	/* more code */
	rootfile.cd();
	TCanvas* c1 = new TCanvas();
	TH1D* h1 = new TH1D("ref","ref",1000,0,4000);
	for (int ev=0;ev<par.N;ev++){
        	value = /*...*/
        	h1->Fill(value);
	}
	h1->Draw();
	c1->Write();

but when I run this i get the error

Error in TFile::WriteTObject: Directory testHistos.root is not writable

I am sure that I have write access in that directory as I am writing other files in the same code. What could be the problem?

Also I wonder how to do the following:

In the same code I have functions that fill and display some histograms that I also want to save to the same file, but I dont know how to do it. It looks something like this:

    void FillAndDrawHistograms(Data d){ /*... */ }
    
    void thisIsMyMainFunction() {
    	TFile rootfile("testHistos.root","new");
    	/*...*/
    	FillAndDrawHistograms(d);
    	FillAndDrawHistograms(d2);
        /*...*/
    }

Do i have to pass the file to the function? Or can I simply call histogram->Write() inside that function and it will be written to the file?

2

Try:
TFile rootfile(“testHistos.root”, “recreate”);

3

yes that made it work. :slight_smile:

And for the second part of the question, seems to be fine to simply call h->Write() inside the function.

Well, I was to fast …

If the file already exists, then “recreate” lets me write, however it seems that it does not recreate the file, but after running my code for the second time I had a histogram twice in the file.
If the file does not exists and I use “recreate” I get the same error as mentioned above.

So my question remains, how do I correctly open a file for writing histograms?
If the file does not exist it should be created for writing, if it exists it should be overwritten (not content added).

4

If, without “recreating” the ROOT file, you call “histogram->Write();” multiple times, or if you also somewhere call “rootfile.Write();”, then you will have multiple versions / instances of your “histogram” stored (which in ROOT are called “cycles”).