# Logging¶

The Acts logging facility supports several severity levels which allow you to control the amount of information displayed at run-time. Logger objects can easily be created using the Acts::getDefaultLogger() function which should be sufficient to get you started. In case you need more customized debug output, you can make use of the output decorators defined in :any:Acts::Logging or even write your own implementation of Acts::Logging::OutputDecorator. In order to add debug messages to your program, you should use the provided macros for the different severity levels:

ACTS_VERBOSE(...);
ACTS_DEBUG(...);
ACTS_INFO(...);
ACTS_WARNING(...);
ACTS_ERROR(...);
ACTS_FATAL(...);


All of these macros require that a function logger() returning a Acts::Logger object is available in the scope in which the macros are used. Inside classes containing an Acts::Logger object as member variable, this could be achieved by providing a private class method called logger() (for an example see e.g. Acts::CylinderVolumeBuilder::logger()). Inside free functions or member methods with local logger objects, the same effect can be achieved by using the macro ACTS_LOCAL_LOGGER(...) which is provided for your convenience.

Code example illustrating the usage:

#include <fstream>
#include <memory>

#include "Acts/Utilities/Logger.hpp"

void myFunction() {
// open the logfile
std::ofstream logfile("log.txt");
// setup a logger instance for >= INFO messages, streaming into the log file
// make sure you do NOT call the variable 'logger'
std::unique_ptr<const Acts::Logger> myLogger
= Acts::getDefaultLogger("MyLogger", Acts::Logging::INFO, &logfile);
// make sure the Acts debug macros can work with your logger
ACTS_LOCAL_LOGGER(myLogger);
ACTS_VERBOSE("This message will not appear in the logfile.");
ACTS_INFO("But this one will: Hello World!");
// do not forget to close the logfile
logfile.close();
}


## Logger integration¶

In case you are using Acts in another framework which comes with its own logging facility (e.g. Gaudi) you can pipe the logging output from Acts tools and algorithms to your framework’s logging system by supplying different implementations of:

There are two approaches to logger integration:

1. Overriding Acts::getDefaultLogger(). This has the downside that log levels cannot be controlled from top-level experiment specific code. This means that it is non-trivial to steer the log level of an e.g. Gaudi algorithm via the OutputLevel property, and have the ACTS code respect this log level. It is therefore now discouraged to use this approach.

Note

ACTS code has iteratively moved to not construct loggers via getDefaultLogger as much as possible, in favor of using Acts::LoggerWrapper. The latter is a lightweight non-owning wrapper around a logger instance, that can default to not containing a logger. It is more suitable to pass into functions that might be called from other ACTS functions (rather than construction a local logger via getDefaultLogger).

2. Passing logger instances to high level components, and rely on ACTS code to pass them into lower level classes / functions.

### Overriding Acts::getDefaultLogger()¶

Attention

Using this mechanism is now discouraged for integration with an experiment framework.

Since Acts makes extensive use of Acts::getDefaultLogger() to provide sufficient information for debugging, you might want to provide a modified implementation of this function (using your output filter and printing policies) to also pipe this output to your framework. You can use the following approach using the possibility to inject custom code by pre-loading shared libraries with LD_PRELOAD. You need to provide an appropriate implementation for a function of the following signature into a separate source file and compile it in a shared library

namespace Acts {
std::unique_ptr<const Logger> getDefaultLogger(const std::string&,
const Logging::Level&,
std::ostream*);
}


Then you can run your executable, which uses Acts tools and algorithms, in the following way (tested under Unix)

$LD_PRELOAD=<YOUR_SHARED_LIBRARY> path/to/your/exectuable  For an example have a look at CustomDefaultLogger.cpp which you can use as follows: $ cd <ACTS/INSTALL/DIRECTORY>
$source bin/setup.sh$ LD_PRELOAD=lib/libActsCustomLogger.so bin/Examples/ActsGenericDetector