Or remove the input arg completly? since it is always errno ?

It looks like the octave_serialport class isn't handling error codes from Win32 correctly.

The helper function "winerror" returns the C description if err != 0 and uses GetLastError () to get WIN32 API errors otherwise.

octave_serialport::open
calls

fd = CreateFile(fullPath.c_str (), GENERIC_READ|GENERIC_WRITE, 0, NULL, OPEN_EXISTING, 0, NULL);
.. and later...
error("serialport: Error opening the interface: %s\n", winerror (errno).c_str ());

But this potentially returns an old, previously set errno code.