force-output

(force-output &optional stream)

Flushes the designated output stream's buffered bytes to its underlying sink and returns nil. With no argument (or nil/t) it flushes standard output. finish-output is the same operation here: once flushed, every rontolisp write is synchronous, so there is nothing further to wait for. Socket writes never buffer on any backend, which is why flushing a socket is a no-op that still costs nothing to write.