Thanks for the additional information. To get the most recent line, there are the three previously mentioned approaches:

  1. Keep the port closed until you actually want to read. Advantage is that there is no CPU used while you don't want to read the port. Disadvantage is that the first line you read will very likely be partial, but since NMEA is checksummed you can identify and discard bad lines (or just always discard the first line). Since you want to read the port less than once per second, I wouldn't worry too much about the overhead of re-opening the port (unless it becomes a measurable issue).
  2. Continually read the port and remember the most recent line. Advantage is that you have full control over buffering and which line you keep and return to your program, and you won't get partial lines except for the very first one. Disadvantage is that some extra CPU is used to read and discard the unnecessary lines.
  3. Keep the port open, but clear the input buffers before reading. Not having used the tied interface before, I can't say for certain, but from the docs it sounds like Device::SerialPort's lookclear method might do what you want. Note that if the buffers are not line-based but byte-based, then after clearing the buffers it's very likely you'd get partial lines like in the first approach above. Another possible disadvantage to this method is that it's more wasteful than the first if you only read rarely, plus I don't know off the top of my head what the driver's behavior is if you let the input buffers fill up.
So I would expect that I don't have to retie the serial port and not to get the first "bad" line all the time.

In that case that limits you to the second or third approaches. (Although personally I don't mind getting and discarding garbled first lines when opening a serial port - that's always a side effect of receiving a continuous stream of data on a serial port.)

My suggestion would be that (if you have the time for it) you could try out the third approach, and if that does turn out to have issues, go with the first or second approach depending on how often you want to read from the port (if often, I'd go with the second, if less often, the first).

In reply to Re^3: serial port tied filehandle autoflush by Anonymous Monk
in thread serial port tied filehandle autoflush by kkaiser

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