About Buffer Flushing Function

I'm now participating in a PG project.I need to modify some parts of the
log buffer flushing function and the data buffer flushing function.I don't
know where they are.

Could you tell me where the following two functions are:

--Log Buffer Flush:Only flush the dirty log buffer into the disk,not
including the database data.(Is it XLogFlush in xlog.c?)

--Data Buffer Flush:Only flush the dirty database data(items) into the disk
not including the log buffer.

I'm now in urgent need of your help.

Thank you in advance!

Charlie Wang