ROOT macros and shared libraries

A ROOT macro contains pure C++ code, which additionally can contain ROOT classes and other ROOT objects (→ see ROOT classes, data types and global variables). A ROOT macro can consist of simple or multi-line commands, but also of arbitrarily complex class and function definitions.

You can save a ROOT macro in a file and execute it at the ROOT prompt or the system prompt (→ see Creating ROOT macros).

You also can compile a ROOT macro (→ see Compiling ROOT macros).

ROOT provides many tutorials that are available as ROOT macros (→ see ROOT tutorial page).

Creating ROOT macros

The name of the ROOT macro and the file name (without file extension) in which the macro is saved must match.

  1. Create a new file in your preferred text editor.

  2. Use the following general structure for the ROOT macro:

      void MacroName() {

         your lines of C++ code
         code line ends with ;
  1. Save the file ROOT macro, using the macro name as file name: MacroName.C

Executing ROOT macros

You can execute a ROOT macro:

  • at the system prompt,
  • at the ROOT prompt,
  • by loading it to a ROOT session.

To execute a ROOT macro at the system prompt, type:

   root MacroName.C

– or –

To execute a ROOT macro at the ROOT prompt, type:

   .x MacroName.C

– or –

To load a ROOT macro to a ROOT session, type (at the ROOT prompt):

   .L MacroName.C


You can load multiple ROOT macros, as each ROOT macro has a unique name in the ROOT name space.

In addition, you can:

Executing a ROOT macro from a ROOT macro

You can execute a ROOT macro conditionally inside another ROOT macro.

ProcessLine() takes a parameter, which is a pointer to an int or to a TInterpreter::EErrorCode to let you access the interpreter error code after an attempt to interpret. This contains the error as defined in enum TInterpreter::EErrorCode with TInterpreter::kSuccess as being the value for a successful execution.


The example $ROOTSYS/tutorials/tree/cernstaff.C calls a ROOT macro to build a ROOT file, if it does not exist.

   void cernstaff() {
      if (gSystem->AccessPathName("cernstaff.root")) {
      gROOT->ProcessLine(".x cernbuild.C");

Executing a ROOT macro from the invocation of ROOT

You can pass a macro to ROOT in its invocation.


The exact kind of quoting depends on the used shell. This example works for bash-like shells.

   $ root -l -b 'myCode.C("some String", 12)'

Compiling ROOT macros

You can use ACLiC (Compiling Your Code) to compile your code and to build a dictionary and a shared library from your ROOT macro. ACliC is implemented in TSystem::CompileMacro().

When using ACliC, ROOT checks what library really needs to be build and calls your system’s C++ compiler, linker and dictionary generator. Then ROOT loads a native shared library.

ACLiC executes the following steps:

  1. Calls rootcling to create automatically a dictionary.
    For creating a dictionary manually, → see Using rootcling to generate dictionaries manually.

  2. Calls the the system’s C++ compiler to build the shared library.

  3. If there are errors, it calls the C++ compiler to build a dummy executable to clearly report the unresolved symbols.

ACLiC adds the classes and functions declared in included files with the same name as the ROOT macro files with one of following extensions: .h, .hh, .hpp, .hxx, .hPP, .hXX. This means that, by default, you cannot combine ROOT macros from different files into one library by using #include statements; you will need to compile each ROOT macro separately.

Compiling a ROOT macro with ACLiC

Before you can compile your interpreted ROOT macro, you need to add the include statements for the classes used in the ROOT macro. Only then you can build and load a shared library containing your ROOT macro.

You can compile a ROOT macro with:

  • default optimizations

  • optimizations

  • debug symbols

Compilation ensures that the shared library is rebuilt.


Do not call ACLiC with a ROOT macro that has a function called main().

To compile a ROOT macro and build a shared library, type:

   root[] .L MyScript.C+

The + option compiles the code and generates a shared library. The name of the shared library is the filename where the dot before the extension is replaced by an underscore. In addition, the shared library extension is added.


On most platforms, hsimple.cxx will generate

The + command rebuilds the library only if the ROOT macro or any of the files it includes are newer than the library.

When checking the timestamp, ACLiC generates a dependency file, which name is the same as the library name, just replacing the so extension by the d extension.

To compile a ROOT macro with default optimizations, type:

      root[] .L MyScript.C++g

To compile a ROOT macro with optimizations, type:

      root[] .L MyScript.C++O

To compile a ROOT macro with debug symbols, type:

      root[] .L MyScript.C++

Setting the include path

The $ROOTSYS/include directory is automatically appended to the include path.

To get the include path, type:

   root[] .include

To append the include path, type:

   root[] .include $HOME/mypackage/include

Append the following line in the ROOT macro to include the include path:

   gSystem->AddIncludePath(" -I$HOME/mypackage/include")

To overwrite an existing include path, type:

   gSystem->SetIncludePath(" -I$HOME/mypackage/include")

To add a static library that should be used during linking, type:

   gSystem->AddLinkedLibs("-L/my/path -l*anylib*");

For adding a shared library, you can load it before you compile the ROOT macros, by


Generating dictionaries

A dictionary (“reflection database”) contains information about the types and functions that are available in a library.

With a dictionary you can call functions inside libraries. Dictionaries are also needed to write a class into a ROOT file (→ see ROOT files).

A dictionary consists of a source file, which contains the type information needed by Cling and ROOT’s I/O subsystem. This source file needs to be generated from the library’s headers and then compiled, linked and loaded. Only then does Cling and ROOT know what is inside a library.

There are two ways to generate a dictionary:

  • using ACLiC

  • using rootcling

Using ACLiC to generate dictionaries

With a given header file MyHeader.h, ACliC automatically generates a dictionary:

      root[] .L MyHeader.h+

Using rootcling to generate dictionaries manually

You can manually create a dictionary by using rootcling:

   rootcling -f DictOutput.cxx -c OPTIONS Header1.h Header2.h ... Linkdef.h
  • DictOutput.cxx Specifies the output file that will contain the dictionary. It will be accompanied by a header file DictOutput.h.

  • OPTIONS are:

    • Isomething: Adding an include path, so that rootcling can find the files included in Header1.h, Header2.h, etc.

    • DSOMETHING: Define a preprocessor macro, which is sometimes needed to parse the header files.

  • Header1.h Header2.h...: The headers files.

  • Linkdef.h: Tells rootcling, which classes should be added to the dictionary, → see Selecting dictionary entries: Linkdef.h.


Dictionaries that are used within the same project must have unique names.

Compiled object files relative to dictionary source files cannot reside in the same library or in two libraries loaded by the same application if the original source files have the same name.


In the first step, a TEvent and a TTrack class is defined. Next an event object is created to add tracks to it. The track objects have a pointer to their event. This shows that the I/O system correctly handles circular references.

In the second step, a TEvent and a TTrack call are implemented.
After that you can use rootcling to manually generate a directory. This generates the eventdict.cxx file.

The TEvent.h header

#ifndef __TEvent__
#define __TEvent__
#include "TObject.h"
class TCollection;
class TTrack;

class TEvent : public TObject {
   Int_t fId; // Event sequential id
   Float_t fTotalMom;       // Total momentum.
   TCollection *fTracks;    // Collection of tracks.
   TEvent() { fId = 0; fTracks = 0; }
   TEvent(Int_t id);
   void AddTrack(TTrack *t);
   Int_t GetId() const { return fId; }
   Int_t GetNoTracks() const;
   void Print(Option_t *opt="");
   Float_t TotalMomentum();
   ClassDef(TEvent,1);     //Simple event class.

The TTrack.h header

#ifndef __TTrack __
#define __TTrack__
#include "TObject.h"

class TEvent;
class TTrack : public TObject {
   Int_t fId;       // Track sequential id.
   TEvent *fEvent;  // TEvent to which track belongs.
   Float_t fPx;     // x part of track momentum.
   Float_t fPy;     // y part of track momentum.
   Float_t fPz;     // z part of track momentum.
   TTrack() { fId = 0; fEvent = 0; fPx = fPy = fPz = 0; }
   TTrack(Int_t id, Event *ev, Float_t px,Float_t py,Float_t pz);
   Float_t Momentum() const;
   TEvent *GetEvent() const { return fEvent; }
   void Print(Option_t *opt="");
   ClassDef (TTrack,1);    //Simple track class.

Implementation of TEvent and TTrack class

#include <iostream.h>
#include "TOrdCollection.h"
#include "TEvent.h"
#include "TTrack.h"

#include <iostream.h>
#include "TMath.h"
#include "Track.h"
#include "Event.h"

Using rootcling to generate the dictionary

rootcling eventdict.cxx -c TEvent.h TTrack.h

eventdict.cxx - the generated dictionary

void TEvent::Streamer(TBuffer &R__b) {

// Stream an object of class TEvent.
   if (R__b.IsReading()) {
      Version_t R__v = R__b.ReadVersion();
      R__b >> fId;
      R__b >> fTotalMom;
      R__b >> fTracks;
   } else {
      R__b << fId;
      R__b << fTotalMom;
      R__b << fTracks;

Selecting dictionary entries: Linkdef.h

To select which types and functions should go into a dictionary, create a Linkdef.h file that you use when you call rootcint manually. The Linkdef.h file is passed as the last argument to rootcint. It must end on Linkdef.h, LinkDef.h, or linkdef.h. For example, My_Linkdef.h is correct, Linkdef_mine.h is not.

The Linkdef.h file contains directives for rootcint, for what a dictionary should be created: select the types and functions that will be accessible from the prompt (or in general through CINT) and for I/O.

Preamble: deselection

A Linkdef.h file starts with the following preamble:

#ifdef __CINT__
#pragma link off all globals;
#pragma link off all classes;
#pragma link off all functions;
#pragma link C++ nestedclasses;

The first line protects the compiler from seeing the rootcint directives. The rootcint directives are in the form of #pragma statements. A #pragma link of all something says that by default, rootcint should not generate the dictionary for anything it sees.

The nested classes directive tells rootcint not to ignore nestedclasses, this is, classes defined inside classes like here:

class Outer {
  class Inner {
    // we want a dictionary for this one, too!


In the next step, tell rootcint for which objects the dictionary should be generated for:

#pragma link C++ class AliEvent+;
#pragma link C++ function StrDup;
#pragma link C++ function operator+(const TString&amp;,const TString&amp;);

#pragma link C++ global gROOT;
#pragma link C++ global gEnv;

#pragma link C++ enum EMessageTypes;


**The + after the class name: This enables an essential feature for rootcint. It is not a default setting, so you must add +at the end.

Selection by file name

Sometimes it is easier to say: Create a dictionary for everything defined in the MyHeader.h file.
Write the following statement into the Linkdef.h file:

#pragma link C++ defined_in "subdir/MyHeader.h";

Make sure that subdir/MyHeader.h corresponds to one of the header files that is passed to rootcint.


Add the following line at the end of the Linkdef.h file:

#endif /* __CINT__ */.

Example of a Linkdef.h file

#ifdef __CINT__

#pragma link off all globals;
#pragma link off all classes;
#pragma link off all functions;
#pragma link C++ nestedclasses;

#pragma link C++ global gHtml;

#pragma link C++ class THtml;

Developing portable ROOT macros

Portable ROOT macros run both with the Cling interpreter and ACLiC (Compiling Your Code).

Therefore, it is recommended not to use the Cling extensions and program around the Cling limitations.

If it is not possible to program around the Cling limitations, use the C preprocessor symbols defined for Cling and rootcling:

  • __CLING__ is defined for both ROOT and rootcling.

  • __ROOTCLING__ (and __MAKECINT__ for backward compatibility) is only defined in rootcling.

Use !defined(__CLING__) || defined(__ROOTCLING__) to bracket code that needs to be seen by the compiler and rootcling, but will be invisible to the interpreter.

– or ­–

Use !defined(__CLING__) to bracket code that should be seen only by the compiler and not by Cling nor rootcling.


Hiding the declaration and initialization of the array gArray from both Cling and rootcling:

   #if !defined(__CLING__)
   int gArray[] = { 2, 3, 4};

Cling and rootcling will ignore all statements between the #if !defined (__CLING__) and #endif. Because ACLiC calls rootcling to build a dictionary, the declaration of gArray will not be included in the dictionary, and consequently, gArray will not be available at the command line even if ACLiC is used.

If you want use the ROOT macro in the interpreter, you have to bracket the usage of gArray between the #if’s, since the definition is not visible.

   #if !defined(__CLING__)
   int gArray[] = { 2, 3, 4};
   #elif defined(__ROOTCLING__)
   int gArray[];

gArray will be visible to rootcling, but still not visible to Cling. If you use ACLiC, gArray will be available at the command line.

Included header files

It is recommended to write ROOT macros with all the needed include statements. Only a few header files are not handled correctly by Cling.

You can include following types of headers in the interpreted and compiled mode:

  • The subset of standard C/C++ headers defined in $ROOTSYS/Cling/include.

  • Headers of classes defined in a previously loaded library (including ROOT own library). The defined class must have a name known to ROOT (this is a class with a ClassDef).

Hiding header files from rootcling that are necessary for the compiler but optional for the interpreter can lead to a fatal error.


   #ifndef __CLING__
   #include "TTree.h"
   class TTree;
   class subTree : public TTree {

In this case, rootcling does not have enough information about the TTree class to produce the correct dictionary file.

If you try this, rootcling and compiling will be error free. However, instantiating a subTree object from the Cling command line will cause a fatal error.

In general, it is recommended to let rootcling see as many header files as possible.

Embedding the rootcling call into a GNU Makefile

Use the following statement to compile and run the code.

.L MyCode.C+

If you need to use a Makefile, there is the following rule for generating a dictionary (see code snippet below). It will create a new source file, which you should compile like all the other sources in your library. In addition, you need to add the include path for ROOT, and you might have to link against ROOT’s libraries: libCore.

This rule generates the rootcling dictionary for the headers $(HEADERS) and a library containing the dictionary and the compiled $(SOURCES):

MyDict.cxx: $(HEADERS) Linkdef.h
[TAB]     rootcling -f $@ -c $(CXXFLAGS) -p $^ MyDict.cxx $(SOURCES)
[TAB]     g++ -shared -o$@ `root-config --ldflags` $(CXXFLAGS) -I$(ROOTSYS)/include $^