"If port is a binary output port and the current position is set beyond
the current end of the data in the underlying data sink, the object is
not extended until new data is written at that position. The contents of
any intervening positions are unspecified. Binary ports created by
open-file-output-port and open-file-input/output-port can always be
extended in this manner within the limits of the underlying operating
system."

Is this about sparse files?

Which Unix/Windows/C stdio APIs underlie these semantics?