We are using an old version of ROOT (6.14.09) to open ROOT files created by ROOT 6.30. When doing so (by just calling root <file>), we get the following message several times:
Error in TList::Clear: A list is accessing an object (0x7f53bc738600) already deleted (list name = TList)
Then, the file seems to be normally browsable and the data inside look fine.
I would like to know what this error means, and how important it is.
What does that mean for the use of those root files by ROOT 6.14? I understand there is some lack of safety in doing so? Or can this message be ignored?
Should we consider upgrading our ROOT version, and which version would be suggested (needs to be compatible with python2)?
Those exact message for this particular case can be ignored, however the same message about any other cases is indicative of a problem and should not be ignored.
In practice, you can ignore it while you are working on upgrading the version of ROOT you are using. You can either use a version of v6.14 patched with the commit I indicated or use the newest release you can upgrade to (i.e. v6.30.02 for example; it still supports python2).
Dear @pcanal,
I encounter the same issue when creating a file with 6.30/02 (default root version on lxplus9)
TH1D* h = new TH1D("h","h",1,1,2)
TFile *f = new TFile("f.root","RECREATE")
h->Write()
f->Close()
and opening it with root 6.24/08 (the default version on lxplus7)
root -l f.root
Attaching file f.root as _file0...
Error in <TList::Clear>: A list is accessing an object (0x14c4dd0) already deleted (list name = TList)
Error in <TList::Clear>: A list is accessing an object (0x162f590) already deleted (list name = TList)
Error in <TList::Clear>: A list is accessing an object (0x16bc760) already deleted (list name = TList)
Error in <TList::Clear>: A list is accessing an object (0x16bcc80) already deleted (list name = TList)
Error in <TList::Clear>: A list is accessing an object (0x16bd0b0) already deleted (list name = TList)
Error in <TList::Clear>: A list is accessing an object (0x16bd580) already deleted (list name = TList)
Error in <TList::Clear>: A list is accessing an object (0x16bd950) already deleted (list name = TList)
Error in <TList::Clear>: A list is accessing an object (0x16bddf0) already deleted (list name = TList)
Error in <TList::Clear>: A list is accessing an object (0x16bece0) already deleted (list name = TList)
Error in <TList::Clear>: A list is accessing an object (0x16bf990) already deleted (list name = TList)
Error in <TList::Clear>: A list is accessing an object (0x16bfd00) already deleted (list name = TList)
Error in <TList::Clear>: A list is accessing an object (0x16c0070) already deleted (list name = TList)
Error in <TList::Clear>: A list is accessing an object (0x16c03e0) already deleted (list name = TList)
Error in <TList::Clear>: A list is accessing an object (0x16c0950) already deleted (list name = TList)
(TFile *) 0x156ceb0
root [1] .ls
TFile** f.root
TFile* f.root
KEY: TH1D h;1 h
This has also impacts on basic functionalities that I need. For example the command hadd target.root f.root prints a sequence of Error in <TList::Clear> and then it goes in Segmentation fault (core dumped) .
Indeed we had back-ported the fix only to the v6.28 series. We have now backported to all the patch branches up to (i.e oldest) v6.14. The list of (mostly unreleased at the moment) release number has been updated in the ticket.
Please consider using v6.28/06 or newer. If you really need an older branch released, please let us know.