Reading from tty file doesn't behave the same on JVM vs Native

[BoD]

My app is trying to read from a tty file like this:

  val source = SystemFileSystem.source(Path("/dev/ttyUSB0")).buffered()
  var count = 0
  while (true) {
    val b = source.readByte()
    count++
    println("Received '${Char(b.toInt())}' - count=$count")
  }

• On JVM, this will print single bytes as soon as they are available
• On Native, this will wait for 8k bytes before printing all of them together, then wait for the next 8k

Reproducer

I've setup a project here.

To setup you can use socat to create 2 linked pseudo-tty, like this:

socat PTY,link=/tmp/source,raw,echo=0 PTY,link=/tmp/sink,raw,echo=0 &

More info

• For regular files, no difference between Java/Native
• I guess fread here won't return partial results. Not sure if that's expected and/or intended.
• But Java's InputStream.read here does.
• Some discussion about buffering in Provide Sink/Source implementations writing to stdout/reading from stdin #375 may be related (I think stdin/stdout would probably behave the same as tty files?)

[fzhinkin]

@BoD thanks for reporting the issue! I didn't check the reproducer yet, but I think the issue should be related to fread indeed (and by switching to preadv implementations should behave consistently across platform).

[fzhinkin]

Checked on macOs using a pty opened via posix_openpt: using a regular read instead of fread solves the problem.