Loggers and Debug output

LehrFEM++ can give you additional debug output in two ways:

1. You can explictly call a PrintInfo() method to output e.g. information about a mesh to std::cout, or
2. You change the log level of a logger that is used by a routine (such as mesh::MeshFactory::Build()).

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Case 1) PrintInfo() methods

There are a number of member/free PrintInfo() functions which accept

• a std::ostream (such as std::cout),
• an object that should be printed, and
• an int ctrl argument that specifies the level of detail (0 = only the absolutely necessary information, 100 = highest level of detail).

For example, if we want to print information about a mesh::Mesh to standard output, we could write:

  std::shared_ptr<mesh::Mesh> mesh;  // initialize it somehow
 
  // print information about the mesh entities to std::cout :
  mesh::utils::PrintInfo(std::cout, *mesh, 11);

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Case 2) Loggers

LehrFEM++ uses the excellent spdlog library to output additional (debug) information. spdlog introduces the class spdlog::logger with which LehrFEM++ logs data to one or more "sinks" (such as std::cout or a file). There are six log levels which are used as follows in LehrFEM++:

level	description
Critical	Coarsest level, is used for logging additional information just before the program aborts. (e.g. before an assert fails)
Error	For logging information related to a recoverable error. E.g. when an exception is thrown.
Warn	Warn the user when something may not work as expected.
Info	Inform the user about something important.
Debug	Additional information that helps in debugging a problem.
Trace	Even more extensive information for debugging a problem.

By default all loggers in LehrFEM++ are initialized such that they

• only output log messages with level Info or higher,
• have a single sink (spdlog::stdout_color_mt) which writes to std::cout,
• are all registered with the spdlog logger registry.

Virtually 99% of the logging statements in LehrFEM++ have log level Debug or Trace. Therefore, LehrFEM++ will by default not output anything to the standard output. If you want to show these log statements, you have to change the log level of the associated loggers.

In LehrFEM++ every class or free function, which wants to log something, decalares a spdlog::logger.

• For classes, these loggers are public static member functions of the corresponding class (e.g. Mesh::Logger()).
• For free functions, there is a corresponding function in the same namespace as the free function (e.g. lf::assemble::AssembleMatrixLogger() for lf::assemble::AssembleMatrixLocally)

Change the log level

If you want to see Debug log message of AssembleMatrixLocally you can just write

Or if you want to see all log messages (level Trace or higher) of the class hybrid2d::Mesh:

Change the log level for all registered logggers in the registry to Debug (not recommended):

  spdlog::apply_all([](const std::shared_ptr<spdlog::logger>& logger) {
    logger->set_level(spdlog::level::debug);
  });

Attention

In Release mode, all log statements with level Debug or Trace are removed at compile time. If you want to see these log statements, you have to set the log level of the corresponding logger and define the preprocessor macro SPDLOG_ACTIVE_LEVEL.

Example

The example examples/loggers/mesh_hierarchy_demo.cc shows how one can set the log levels of individual loggers via command-line and environment variables and some other advanced techniques.

Contributors:

Please check the contribution guide.