The OnixS C++ FIX Engine logs its data into FixEngineLog.txt file located in MsgStorage subfolder of the current (for executing application) folder.

For each FIX session with a file-based session storage the following files are also created in the MsgStorage folder:

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::getSenderCompID, OnixS::FIX::Session::getTargetCompID, session's FIX Version (OnixS::FIX::Session::getVersion) and a timestamp. During the run-time it's available via OnixS::FIX::Session's string casting overload.