30.8. Empty Output Files

Table of Contents

Understanding Empty Output Files

Empty output files in a Geant4 simulation usually mean that your analysis code never received any data to write, or that the file was never properly opened or closed. This problem is almost always a logic or initialization issue, not a bug in Geant4 itself. In this chapter you will learn how to reason through where the data flow is broken and how to locate the missing step.

Typical Symptoms and First Checks

An empty output file can show up in several ways. You may see a zero–byte file, a file that contains only headers with no events or rows, or a file that looks valid but all histograms are empty when you open it in ROOT or a plotting program.

The first thing to verify is that your simulation actually ran events. If you only started a visualization session, but never called /run/beamOn or the equivalent in C++, Geant4 did not simulate any events and the analysis code had nothing to record. Also confirm that you are looking at the correct file in the correct directory, especially if you use relative paths or run from a build directory.

Always check that you actually ran events, and that you are inspecting the same output file that your code writes. Many "empty file" problems are simply "no events" or "wrong file" problems.

Once these trivial cases are excluded, you can move to the real debugging work: following the chain from event generation to data recording and file writing.

Checking That Events and Hits Exist

If Geant4 never creates hits or never calls your analysis code, the output file will stay empty even if the file itself is open. To see whether the simulation is producing data, you should check three levels: events, hits or energy deposition, and analysis calls.

First, use Geant4 verbose output or simple print messages to verify that events are processed. Increase the run verbosity or add small messages in BeginOfEventAction and EndOfEventAction. If these messages never appear, your run is not actually simulating events.

Second, check whether your detector or scoring system is registering anything. In a typical setup, energy deposition is collected either through a sensitive detector that creates hits, or through scoring meshes. You can add temporary print statements in ProcessHits for a sensitive detector or in UserSteppingAction when GetTotalEnergyDeposit() is nonzero. Even a single printed line like "Edep in step: ..." confirms that physics is happening and something could be recorded.

If you do not see any hits or nonzero energy deposits, the problem is not in the output file but earlier. Common reasons include a particle source that never reaches your detector, incorrect geometry placements, missing or wrong materials, or a physics list that does not define the relevant processes. Those issues are covered in other chapters, but for empty files your role here is to decide whether the analysis system is starved of input, or whether the data get lost later.

Finally, ensure that your analysis code (for example in EventAction or SteppingAction) is actually called in the run. If you never registered these user action classes in your ActionInitialization, they will never run, and any code that fills histograms or ntuples will never execute.

If no user actions are registered, no analysis code runs and the output file will stay empty. Always check that your ActionInitialization installs the actions that perform data recording.

Verifying G4AnalysisManager Usage

If events and hits exist, the next step is to verify how you use G4AnalysisManager to create, fill, and write your output. Empty files often come from incomplete or incorrect analysis manager usage.

Make sure you create the analysis manager only once, usually in your RunAction constructor, and that you keep the same instance for the whole run. In Geant4 you normally use the singleton returned by G4AnalysisManager::Instance(). If you mistakenly create separate instances in multiple classes, you may open a file on one instance and fill histograms on another, which will leave the file empty.

Check that you actually define histograms or ntuples before the run starts. This is typically done in the RunAction constructor, with calls such as CreateH1 or CreateNtuple and column definitions. If you never create a histogram or define columns, filling calls will silently fail or have no visible effect in the file.

Also verify that you are not calling FillH1, FillNtupleDColumn, or similar methods with invalid IDs. A frequent mistake is to use an ID that does not match the one returned by CreateH1 or that is off by one. As a debugging step, you can store the returned IDs in member variables and use those directly instead of hard coded integers.

Finally, consider the order of operations. Geant4 expects you to open the analysis file before the run begins, typically in BeginOfRunAction, and to write and close it at the end of the run. If you open the file after the events are processed, or if you forget to enable a particular output format, you can end up with a file that has only minimal structure.

Use the same G4AnalysisManager instance for creating, filling, writing, and closing. Always open the file before the run and write and close it in EndOfRunAction.

File Opening, Writing, and Closing

Once you are sure that the analysis manager is being filled correctly, you must verify that the file operations occur in the correct order and that they succeed.

In a standard pattern, the file name and format are set in the run action. You call the analysis manager OpenFile method in BeginOfRunAction, after the manager has been configured and histograms or ntuples have been created. If you forget to call OpenFile, Geant4 may create an empty placeholder or no file at all.

Next, you must call both Write and CloseFile in EndOfRunAction. If you only call CloseFile, buffered data may never be flushed to disk. For some backends the CloseFile call internally writes data, but you should not rely on that implicitly. Using both methods makes your code clearer and more portable between different output formats.

Also be careful about run scopes. If you open and close the file inside a macro command or a different part of the code that does not match the life cycle of your RunAction, you can unintentionally close the file before all events are written. This can leave you with a file that contains only a part of the expected results or even none at all.

If your application uses file paths that include relative directories, check that those directories exist and that you have write permission. On some systems, an attempt to write into a non existing or protected directory results in a file handle that appears open, but no data is actually written. Printing the full path and verifying its existence is a simple but effective debugging step.

Always call OpenFile before the run, and then Write followed by CloseFile at the end of the run. If these calls are missing or out of order, your output file can be created but remain empty.

Multithreading and Output Merging Issues

When you run Geant4 in multithreaded mode, analysis becomes more complex. If each worker thread collects its own data, the final output file is usually written by the master thread after merging the per thread results. Empty files in multithreaded runs often mean that the merging step or the master output is missing.

First, check whether your code is configured to use thread local analysis managers or a single shared manager. The recommended Geant4 pattern is to create the analysis manager in the master run action and let each thread use its own instance behind the scenes, with the toolkit performing the necessary merging. If you instead create completely separate managers in worker threads without a corresponding merge, you may end up with local data that never reaches the main output file.

Second, verify that your BeginOfRunAction and EndOfRunAction are aware of master and worker roles. Geant4 passes a flag that can be used to distinguish the master run action from the worker instances. You should open and close the file only in the master or follow the official examples, where the master opens the file and merges worker results at the end. If you try to open the same file independently in each thread, you may overwrite it or cause an inconsistent state that effectively results in an empty or corrupted file.

Also be aware that, if no events are assigned to a particular worker, its histograms or ntuples will remain empty. The master can still create a file, but the contents will show no entries. This can happen with very few events and many threads, so as a check, run your simulation in sequential mode with one thread and see whether the output file is correctly filled. If it is, you have isolated the problem to multithreading or merging.

Test your analysis in single threaded mode first. If the file has content with one thread but is empty with many threads, the problem is in your multithreading or merging logic, not in the basic analysis code.

Using Debug Output to Trace Data Flow

To solve persistent empty file problems, it is useful to add targeted debug output at key points in the simulation and analysis workflow. The aim is to trace the flow of data from the moment it is created to the moment it should be written.

You can start by printing a short message in PrimaryGeneratorAction for the first few events, confirming that primary particles are generated. In SteppingAction or your sensitive detector class, print a message whenever an energy deposition above a small threshold occurs. In EventAction, print the total energy for the event after accumulation and just before you call any Fill methods of the analysis manager.

Next, in RunAction, print when you call OpenFile, Write, and CloseFile, and include the file name and full path in the messages. Around each analysis call like FillH1 or FillNtupleRow, print or assert the IDs you are using and confirm they match the ones returned by the creation methods.

This approach turns the simulation into a step by step story that you can follow in the console output. Wherever the story stops, you have found the break in the chain. Once the problem is fixed, you can remove or conditionally compile these debug prints to avoid cluttering normal runs.

Add temporary print statements around every critical analysis step: hit creation, event accumulation, histogram filling, file opening, writing, and closing. Stop removing debug output until you see exactly where the data flow stops.

By systematically checking that events exist, that hits or energy depositions are produced, that the analysis manager is used correctly, that files are opened and closed in the right place, and that multithreaded runs merge data properly, you can reliably diagnose and fix the causes of empty output files in Geant4 simulations.