The Core library provides a low level interfaces to the internally used data structures. Using this library internal data structures can be manipulated directly, parsed from and printed to human readable textual descriptions.


The low level data structures are represented as ATerms (short for Annotated Terms). ATerms can be used to represent tree-like data structures which have distinguishing features as: maximal subterm sharing, annotations, automatic garbage collection and compressed binary exchange formats. A more elaborate description can be found here.


The Core library is a collection of methods and functions in the core name space. In fact it is a collection of small sub-libraries, each with their own functionality.

The alphabet reduction library transforms the mCRL2 specification to an equivalent specification where renaming operations (allow, hide, rename, block and communication) are distributed over the other operations. The reduction is performed to reduce the number of multi-actions which are generated during intermediate steps in the mCRL2 linearization procedure.

The messaging library is a library for controlling and displaying messages. The messages are printed for logging purposes. It is possible to distinct four different logging levels, namely normal, verbose, debug and quite. If logging level has not explicitly been set, the logging level is by default set to normal. In the normal logging all warning and error messages are displayed. If the logging level is set to verbose, additional verbose messages are printed. When a more detailed logging is required, the logging level can be set to the debug level. If the logging level is set to quite, no errors and warnings are displayed. Next to the printing of regular messages (like the C++ printf function), these functions also allow the printing of ATerms.

The internal mCRL2 ATerm is a complex structure, especially when the specifications grow large. To improve the readability on the internal ATerm format, the print library provides the opportunity to pretty print ATerms in a more human readable format.

The struct library contains a set of operations defined on the ATerm structure. With these operations it is possible to:

  • check for a certain ATerm data structure,

  • create a predefined ATerm data structure, or

  • retrieve a value from the ATerm data structure.


This tutorial shows the usage of the core library by creating a mCRL2 action rename program. In the program all actions ‘a’ are substituted by a fresh action ‘b’. The basics are explained first, before getting into detail.


For the tutorial the following core and core/detail headers need to be included:

// Core libraries
#include "mcrl2/core/messaging.h"           // Messaging library
#include "mcrl2/core/parse.h"               // Parse library
#include "mcrl2/core/typecheck.h"           // Type check library
#include "mcrl2/core/data_implementation.h" // Data implementation library
#include "mcrl2/core/alpha.h"               // Alpha reduction library

// Core/detail libraries
#include "mcrl2/core/detail/struct.h"       // ATerm building blocks

To address the methods and members in the libraries, the following namespaces are used:

using namespace mcrl2::core;                // core namespace
using namespace mcrl2::core::detail;        // core::detail namespace

The specification

For the tutorial, we consider the following specification, which is stored in the std::string variable spec.

act a: Bool;

init a(true);

The program

First we need to initialize the ATerm Library via MCRL2_ATERM_INIT(argc, argv). This to prohibits the ATerm data structure from being removed on a run of the garbage collector. To display verbose information during the execution of the code, the method gsSetVerboseMsg() is called. After calling this method, all instances of gsVerboseMsg(…) will display show their output, which is supplied as an argument. The initialisation of the program is as follows:

MCRL2_ATERM_INIT(argc, argv)   // initialise ATerm library
gsSetVerboseMsg();             // enable Verbose logging
istringstream istr( spec );    // convert string to stringstream

To ensure that our specification is correct, we need to parse and type check the specification. To parse the specification we call the parse_spec method from the core::detail namespace. The function parse_spec reads the stringstream variable istr and stores the parsed specification in the ATermAppl variable parsed_spec. If the parser cannot parse the specification, it will return empty output. To ensure parsing went well, we check for the parsed_spec being empty. If parsing succeeded, the outcome is displayed via a gsVerboseMsg(…) method. If parsing failed, the outcome is shown via a gsErrorMsg(…) method. Type checking is performed in an analogue way.

ATermAppl parsed_spec = parse_proc_spec( istr );
if (parsed_spec == NULL)
  gsErrorMsg("parsing failed\n");
  return 1;
gsVerboseMsg("parsing succeeded!\n");

ATermAppl typed_checked_parsed_spec = type_check_proc_spec( parsed_spec );
if (typed_checked_parsed_spec == NULL)
  gsErrorMsg("type checking failed!\n");
  return 1;
gsVerboseMsg("type checking succeeded!\n");

Once we know that our specification is correct, we show how the methods defined in the messaging header can be used to print ATerms. The first case shows how ATerms can be pretty printed. The second case show normal printing of ATerms.

gsVerboseMsg("pretty printed specification:\n%P\n", typed_checked_parsed_spec );
gsVerboseMsg("textual ATerm representation of the specification\n%T\n", typed_checked_parsed_spec );

The output of the first case looks similar to the specification provided. The second case shows output which looks like:


It is possible log the output, but it is not always needed to show the logging. Therefore it is desirable to only log the output in debug mode. This can be accomplished by the following line.

gsDebugMsg("pretty printed specification:\n%P\n", data_impl_typed_checked_parsed_spec );

To ensure that the acquired specification is correct, the following line is added.

assert( gsIsSpecV1( data_impl_typed_checked_parsed_spec ) );

Manipulating the specification

In this part of the tutorial, the initial specification is manipulated on a low level of the internal data structure. The methods that start with gs… and ATA… are defined in the mCRL2 core library. Methods starting with AT… are defined in the ATerm Library.

We perform the following construction and manipulation of the specification. First the new action b is created. In this tutorial we take the action b as void action. The method gsMakeActId provides the means to create a Action Identifier. The method requires two arguments, namely a label and a type. To create a label and type, a string is converted to a ATermAppl and an empty ATermList is created, respectively. To redefine the action specification, the method gsMakeActSpec(…) needs to be called with a list of action identifiers. Via an ATmakeList1(…) it is possible to create a list of length one. The task can be accomplished with the following piece of code:

ATermAppl actIdB = gsMakeActSpec( ATmakeList1( (ATerm) gsMakeActId( gsString2ATermAppl("b"), ATmakeList0() ) ) );

The variable actIdB is used to derive the action transistion b in the initialisation. To solely acquire the action transition, the actIdB, we are taking the first argument of ActSpec first . This leaves us with the the list of Actions. To acquire the first argument from a list, the head function ATgetFirst(…) is used. To create an action the function gsMakeAction is used. This function requires two arguments. The first argument specifies an ATermAppl Action. The second argument specifies the time conditions. While we do not consider time, an empty list is supplied.


Be careful when casting ATerm, ATermAppl and ATermList. These formats are interchangeable.

ATermAppl actB = gsMakeAction( (ATermAppl) ATgetFirst( ATgetArgument(actIdB, 0 ) ), ATmakeList0() );

To create the initialisation we take the variable actB:

ATermAppl init = gsMakeProcessInit( ATmakeList0(), actB );

To reconstruct the new specification we take the first and third argument of the original specification, because their are untouched. The second and the fourth argument need to be adjusted, therefore the appropriate variables actIdB and init are substituted. To verify that the new specification is correct, addition logging information is printed.

ATermAppl new_spec = gsMakeSpecV1(
                         ATAgetArgument(data_impl_typed_checked_parsed_spec, 0 )
                       , actIdB
                       , ATAgetArgument(data_impl_typed_checked_parsed_spec, 2 )
                       , init

gsVerboseMsg("pretty printed version of the specification:\n%P", new_spec );

Alphabet reduction

To finalize the tutorial we apply an alphabet reduction on the specification. For this small example the alphabet reduction does not have any effect. However it is only here to illustrate its usage. Alphabet reduction can be applied by using the method gsAlpha(…). To assert the validity of the specification, an addition check is performed.

ATermAppl new_alpha_spec = gsAlpha(new_spec);

assert( gsIsSpecV1( new_alpha_spec ) );



M.G.J. van den Brand and P. Klint (2007). “ATerms for manipulation and exchange of structured data: It’s all about sharing.” Information and Software Technology 49:55–64.