cannot read integers from stdin using scala.io.StdIn.readInt()

[aostiles]

@MasseGuillaume reproduced this bug.

[densh]

Thanks for the bug report!

[densh]

The issue is that we don't wrap stdin in https://github.com/scala-native/scala-native/blob/master/javalib/src/main/scala/java/lang/System.scala#L35, should be trivial to fix.

[Korf74]

Is an implementation of a CFileInputStream class sufficient to fix this ?

[densh]

@Korf74 Yes, that's exactly what needs to be done here.

[Korf74]

  var in: InputStream  = new CFileInputStream(stdio.stdin)
  var out: PrintStream = new PrintStream(new CFileOutputStream(stdio.stdout))
  var err: PrintStream = new PrintStream(new CFileOutputStream(stdio.stderr))

  private class CFileOutputStream(stream: Ptr[stdio.FILE])
      extends java.io.OutputStream {
    private val buf = stdlib.malloc(1)
    def write(b: Int): Unit = {
      !buf = b.toUByte.toByte
      stdio.fwrite(buf, 1, 1, stream)
    }
  }

  private class CFileInputStream(stream: Ptr[stdio.FILE])
      extends java.io.InputStream {
    private val buf = stdlib.malloc(1)
    override def read(): CInt = {
      stdio.fread(buf, 1, 1, stream)
      (!buf).toInt & 0xFF
    }
  }

Calls to scala.io.StdIn.readInt() using this implementation doesn't seem to work, it seems stdin reads 32 (whitespace) in loop

[fduraffourg]

I tried with fread but it gets on our way because of the buffering. stdin is line buffered so we encounter the case where data is available but fread still waits for more data before returning. I tried to change the buffering method with setvbuf without any success.

The solution I found is to use read from unistd:

  private class CFileInputStream(stream: Ptr[stdio.FILE])
      extends java.io.InputStream {

    def read(): CInt = {
      stdio.fflush(stdio.stdout)
      val buffer = stackalloc[scala.Byte](1)
      while (unistd.read(0, buffer, 1) != 1) {
        println("Nothing was read on previous iteration")
      }
      buffer(0).toInt
    }

    override def read(b: Array[scala.Byte], off: Int, len: Int): Int = {
      val bufsize = len min 256
      val buffer = stackalloc[scala.Byte](bufsize)
      val nread = unistd.read(0, buffer, bufsize)
      var pos = 0
      while (pos < nread) {
        b(pos) = buffer(pos)
        pos += 1
      }
      nread.toInt
    }
  }
}

This way we get data directly from the kernel as soon as it is available and we let BufferedReader handle the buffering stuff.

There are still stuff to take care of:

• When run from sbt run, stdin seams to be non blocking (or maybe not even opened) so we get into an infinite loop where each read call return immediatly without reading any data. I think using select or a blocking method that wait for data to be available should do the job. I'll try that.
• Things can get out of order with stdout. For example calling println and then reading some stuff from stdin will wait for stdin before printing
• Right now I create a buffer and then copy each byte from the buffer to the Array. Is it possible to get a Ptr from an Array to avoid this useless copy?
• Maybe look at how sbt connect stdin to the running process and makes sure everything is OK

[MasseGuillaume]

sbt run needs to connect it's input

val exitCode = Process(binary +: args).run(logger, connectInput = true).exitValue

https://github.com/scala-native/scala-native/pull/612/files#diff-9f8176d7a89fed51f9b5c82605327b36R436