By default, the OnixS C++ FIX Engine writes FixEngineLog.txt in the MsgStorage subfolder of the application's current working directory. Configure the directory with OnixS::FIX::EngineSettings::logDirectory.

There are the following log levels:

INFO: Notifies about regular events.
WARNING: Notifies about unusual events. Such events do not affect the normal FIX Engine work, but the user should pay attention to them.
ERROR: Notifies about errors. Such events can affect the normal FIX Engine and application work, so one needs to detect the reasons and eliminate them.

All warnings/errors trigger corresponding session-level or engine-level onWarning()/onError() callbacks. All warnings/errors have the reasons enumeration, please see IEngineListener.h / ISessionListener.h headers.

For each FIX session with a file-based session storage, the following files are also created in the configured log directory:

SESSION_NAME.summary contains both inbound and outbound FIX messages.
SESSION_NAME.state contains session state-related data.

SESSION_NAME is formed on the base of values returned by OnixS::FIX::Session::senderCompId, OnixS::FIX::Session::targetCompId, session's dictionary FIX Version (OnixS::FIX::Dictionary::version) and a timestamp. During the run-time, it's available via OnixS::FIX::Session's string casting to overload.