There is a completion implementaion [here](https://github.com/notetau/geany-clang-complete/tree/master/geany-complete-c...) but I donot know if it could be picked upon or if it is compatible to current geany plugin architecture. Actually, I used it with python but seemed slow as it used http for completion so I wrote something for [python](https://github.com/sagarchalise/geany-pyjedi) myself based upon current scintilla autocompletions.