Is it possible to fill an identical histogram from multiple programs running separately,
and write it on an identical file?
Does it use thread technique?
I would appreciate it if you refer me to an example, if any.
The solution is to create the histogram in each process/job,
then store it into one file per process/job.
In the last step you use $ROOTSYS/hadd to merge your files containing one or more histograms into one single file.