Where in the source code does postgres write to disk?

According to
http://www.postgresql.org/docs/9.2/static/storage-file-layout.html

"When a table or index exceeds 1 GB, it is divided into gigabyte-sized
segments. The first segment's file name is the same as the filenode;
subsequent segments are named filenode.1, filenode.2, etc."

I was wondering where in the source code this is dealt with. I have been
searching for the last few hours but have had no luck

Any help guiding me to the location in the source code where postgres
writes the buffer to disk would be greatly appreciated.

Thanks