KPrintF on the Amiga is used to print debug messages. By default it will output to SER: in 9600 baud. (There is also DKPrintF which will output to PAR:).
To grab this output when running an application under FS-UAE, you can do the following:
1) Setup serial re-direction in FS-UAE:
serial_port = /some/file/somewhere
2) Start socat:
socat pty,raw,echo=0,link=/some/file/somewhere -,raw,echo=0
3) Start the emulator and run your application. All KPrintF’s should now appear to the terminal where socat is running.