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Forum: Event and Track Management
Re: Question Segmentation error for get process name (David Sangcheol Lee)
Date: 06 Apr, 2018
From: Michael H. Kelsey <Michael H. Kelsey>

On Wed, 04 Apr 2018 08:02:18 GMT, David Sangcheol Lee wrote:
> Hello, I am trying to get the energy spectra of neutrons undergo capture
> process so I implemented this line of the code
> G4Track * theTrack = theStep->GetTrack(); . . . processName =
> theStep->GetPreStepPoint()->GetProcessDefinedStep()->GetProcessName();
> so I basically get the energy from the nCapture process but I get the
> segmentation error. I cannot understand where it got wrong.

In general, it is always safest to check whether a pointer exists (is not NULL) before dereferencing it. We often skip this in Geant4 code, especially with long chains of pointers.

In your case, you are looking at the pre-step point of a track. If you are looking at one of your _primary_ tracks, then there is no process which defined the step, and hence that pointer is null. When you try to get the name, you segfault because you're derefencing a null pointer.

The simplest way for you to protect against this is to simply test if the track is a primary:

   if (theTrack->GetParentID() == 0) return;       // Do not process primaries

  -- Michael Kelsey

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