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Root-dependent package template

While writing simple macros or python scripts can be an effective way to quickly
get started with ROOT, cases frequently arise where it would be most effective
to create a library of compiled functions or even a compiled executable. Here is
the setup of a template package for compiling ROOT-dependent C++ classes and applications.

First, determine which ROOT installation you will use.

On lxplus5 (i.e., slc5 operating system at CERN), set environment variables with values like these:

export ROOTSYS=/afs/cern.ch/sw/lcg/app/releases/ROOT/5.22.00d/i686-slc5-gcc43-dbg/root
export LD_LIBRARY_PATH=${ROOTSYS}/lib:/afs/cern.ch/sw/lcg/external/Python/2.5/i686-slc5-gcc43-opt/lib:${LD_LIBRARY_PATH}
export PATH=${ROOTSYS}/bin:/afs/cern.ch/sw/lcg/external/Python/2.5/i686-slc5-gcc43-opt/bin:${PATH}
export PYTHONPATH=$ROOTSYS/lib

  1. Create a directory where you will run ROOT and cd to it.
  2. Download the CompiledRootAnalysis template package, e.g.,
    wget http://mgwilson.web.cern.ch/mgwilson/Software/CompiledRootAnalysis_V01.tgz
  3. tar -xzvf CompiledRootAnalysis_V01.tgz
  4. Edit the top of CompiledRootAnalysis/scripts/RenamePackage.sh with a new package name and namespace for the code in
    your package; this will be denoted <package> below.
  5. CompiledRootAnalysis/scripts/RenamePackage.sh
  6. cd <package>; gmake shlib; gmake apps; gmake setup; cd ..
    Note 1: If you want to use MacOS, do make dylib; make macapps instead.
    Note 2: To see what commands are being executed during the compilation, delete the @ symbols preceding the commands
    in the GNUmakefile.
  7. Execute the bin/example application to see that it runs correctly.
  8. Start a root session to see that the shared library is loaded correctly. All of the classes and methods
    compiled in this package are available on the ROOT command line.

Athena-dependent package template

Compiling and running athena algorithms on the Grid allows one the flexibility to manipulate and analyze data in a specific manner. Here is the setup of a template package for building athena applications. This template contains the basic infrastructure and scripts for filtering events, writing an ntuple, submitting jobs to the Grid, and downloading and merging the output from the Grid.

Initial setup

First, select a suitable release by referencing the Offline Release Page, and create a work area, for example by downloading this script:
wget http://mgwilson.web.cern.ch/mgwilson/Scripts/createAtlasWorkArea_15.6.9.8_CERN.sh; chmod 755 createAtlasWorkArea_15.6.9.8_CERN.sh

General C++ questions and answers

  • Q. What is void*?

C and C++ are strongly typed languages, meaning that blocks of memory are given a specific type (i.e., int, float, char, etc.) when allocated or used within a piece of code. Sometimes, it is desirable to write functions that can perform the same operations on different variable types. In C++, this is achievable through class inheritance. However, in C, this is done by telling the compiler not to check the type of the variable; this is done with the void* declaration. Specifically, void* is a pointer to untyped memory, and the programmer is free to cast this memory into any other type:

float fcn( void* var1, void* var2 ) {
float* f1 = (float*)var1;
float* f2 = (float*)var2;
/* return the product of these two floats */
return (f1)(*f2);
}

As you see, in doing this, the programmer assumes all responsibility for making sure that the types and operations
are correct, something normally (and best) done by the compiler.

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