This program is based Jon A. Maxwell "JAM" jmaxwell@acm.vt.edu 's discovery
and proof-of-concept that X11 keyboard state can be logged without superuser
permissions. (I'm not aware of others prior.)
His program xspy is an X11 keylogger which output is squarely aimed at
human-readability, which makes it great for (as the name suggests) spying on
someone and quickly making out what they're doing. Close variations of it have
hence featured in security-focused Linux distributions (in Kali and in
BlackArch, among others).
This is a complete rework of his idea, using a modern C version and with the aim of producing machine-readable output better suitable for physical key press statistics, deprioritising human parseability of the output.