Forgot to explain that opost calls write_room to see
if there is room to write two characters when it
wants to write "\r\n".  pl2303 says sure, there is
room.  opost then writes '\r' and '\n' in two
separate writes, expecting that there is room for
both, but the second write fails.

pl2303 and similar usb serial drivers need a way
to buffer data so they can fulfill writes for as
many bytes as they advertised in write_room, even
if those bytes are written in separate write calls.

With a write buffer like this, you would not need
to handle put_char separately.

-- Al


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