Writing your first Marlin Processor

New to Marlin ? You want to get started with a simple Processor example ? The is the perfect place to start then. A Processor is Marlin plugin that implement the processing of LCIO LCEvent and LCRunHeader objects. The processor can interact with this event to get data, so called LCCollection objects, or create and store data in the event. To implement a Processor, you must inherit from the marlin::Processor class and re-implement a few virtual callback functions.

Let's start with the simplest Processor implementation:

// MyProcessor.cc
// -- marlin headers
#include <marlin/Processor.h>
// -- lcio headers
#include <EVENT/LCEvent.h>

class MyProcessor : public marlin::Processor {
public:
  MyProcessor() = default ;
  marlin::Processor *newProcessor() const override ;
  void processEvent( EVENT::LCEvent *event ) override ;
};

marlin::Processor *MyProcessor::newProcessor() const {
  return new MyProcessor() ;
}

void MyProcessor::processEvent( EVENT::LCEvent *event ) {
  log<MESSAGE>() << "Processing" <<
  " event no " << event->getEventNumber() <<
  ", run no " << event->getRunNumber() << std::endl ;
}

// plugin declaration
MyProcessor myProcessor ;

A few explanations for this code:

• The method newProcessor() is factory method used by the framework to create a processor instance when running the Marlin application. This is the only required overload when you implement a Processor.
• The method processEvent(event) is a callback method processed everytime an LCIO event is, for example, read from an LCIO file. Here, we just print the event number and run number using the logging method log<T>(), where T is the log level (here MESSAGE).

To compile this code, see the next sections on this page.

For more information on the Processor API usage, see the dedicated documentation page

Dealing with CMake in your project

XML steering file

Running your processor with Marlin