Gaudi Algorithms

Like most algorithms, Gaudi algorithms are a set of instructions used to perform some clear task and, since Gaudi is a C++ framework, SAS algorithms are normally C++ classes. Gaudi algorithms are called once per event; therefore, a Gaudi algorithm acts on the current set of event data available in the TDS.

To be a true Gaudi algorithm, the class must implement the IAlgorithm abstract interface, meaning that any algorithm class must:

Derive from Gaudi's Algorithm class
Provide a special algorithm constructor
Implement three methods:

	initialize( )
		Called once before event processing begins, it is used for setup - accessing required services, accessing input files, etc.

	execute( )
		Called once per event. Performs event processing for the algorithm.

	finalize( )
		Called once after event processing is finished. Performs cleans up. For example:
		#include "GaudiKernel/Algorithm.h" #include "GaudiKernel/AlgFactory.h" class HelloWorld : public Algorithm { public: HelloWorld(const std::string& name, ISvcLocator* pSvcLocator); StatusCode initialize(); StatusCode execute(); StatusCode finalize(); };

Note: This does not necessarily mean algorithms without this format cannot be utilized within Gaudi. This is just the typical definition of a Gaudi algorithm. For a full example Gaudi algorithm, see HelloWorld.

User-Defined Algorithms - HelloWorldGaudi

This section details how to create a new Gaudi algorithm and use it within the Gaudi framework, by either:

Adding a New Algorithm to an Existing Package

Adding a New Algorithm to a New Package

Note: Creating a simple Gaudi algorithm that prints the well known phrase "Hello World" is straightforward. To download the HelloWorldGaudi package as a tar.gz, click on: HelloWorldGaudi.tar.gz

Adding a New Algorithm to an Existing Package

Update the requirements file of the package, so that the new algorithm is included in the library defined by the package.
Modify the <packageName>_load.cxx files to include your new algorithm; for example:

DECLARE_ALGORITHM( HelloWorld );

Define the source file for your new algorithm, for example, HelloWorld.cxx

Modify all existing jobOptions.txt files to load the new algorithm and set any input parameters; for example:

ApplicationMgr.TopAlg += { "HelloWorld" };

Adding a New Algorithm to a New Package

To create a new package if you are using:

MRvcmt:

Click the New button.

Fill in the name for the new package.

Set the verson to: v0

Set the CMTPATH so that it points to the directory where the new package will reside.

glastpack and CMT on a Linux machine:

At the command line, enter:

cmt create HelloWorldGaudi v0

A new package clled HelloWorldGaudi will be created.

Fill in the empty requirements file.

For example:

package HelloWorldGaudi
version v0
# Intoductory package that implements a simple Gaudi Algorithm
use GaudiInterface v* IExternal
use GlastPolicy v*

# Use this pattern for shareable libraries that Gaudi must load
apply_pattern packageShr

# Define a new library, named HelloWorldGaudi
# Note that all files are assumed relative to the src directory of the package
library HelloWorldGaudi HelloWorld.cxx Dll/*.cxx

private
# Flags for building shared libraries
apply_pattern package_Cshlibflags

# This is the default test application defined in the GlastPolicy package
# This just defines a simple Gaudi main program
apply_pattern package_test

Create the source files required by your library.

Source files required for this example:

src/HelloWorld.cxx

src/Dll/HelloWorldGaudi_dll.cxx

src/Dll/HelloWorldGaudi_load.cxx

Create a jobOptions.txt file in the src/test directory.

Be sure to load the new library and add the new algorithm to the list of TopAlgs:

ApplicationMgr.DLLs += {"HelloWorldGaudi"};
ApplicationMgr.TopAlg += { "HelloWorld" };

Note: The src/test is used by default for all test programs; this can be overridden if desired.