# Simulation outcomes¶

## Textual logging¶

Using printf or println to display information is possible, but quickly unpractical, as the logs of all processes get intermixed in your program’s output. Instead, you should use SimGrid’s logging facilities, that are inspired from Log4J. This way, you can filter the messages at runtime, based on their severity and their topic. There is four main concepts in SimGrid’s logging mechanism:

The category of a message represents its topic. These categories are organized as a hierarchy, loosely corresponding to SimGrid’s modules architecture. Existing categories are documented online, but some of them may be disabled depending on the compilation options. Use --help-log-categories on the command line to see the categories actually provided a given simulator.

The message priority represents its severity. It can be one of trace, debug, verb, info, warn, error and critical. Every category has a configured threshold, and only the messages with a higher severity are displayed (the others are not even evaluated). For example, you may want to see every debugging message of the Host handling, while filtering out every other messages that are of a lesser priority than “error”. For that, use the following command line argument: --log=root.thresh:error --log=s4u_host.thresh:debug

You can also change the layout used to format the messages, using format directives that are similar to the printf ones: %r prints the time elapsed since the beginning of the simulation; %a gives the actor name, etc. Many such directives are available. You can have a specific layout per category, and it will be inherited by all its sub-categories.

Finally, the appender actually displays the produced messages. SimGrid provides four appenders so far: the default one prints on stderr. file writes to a given file, rollfile does the same, but overwrites old messages when the file grows too large and splitfile creates new files when the maximum size is reached. Each category can have its own appender.

For more information, please refer to the programmer’s interface to learn how to produce messages from your code, or to Logging configuration to see how to change the settings at runtime.