gnu.io.PortInUseException

I spent some time today trying to figure out why I kept getting gnu.io.PortInUseException while opening a port. After closing more or less every program that could possibly infere with a usb/serial port I figured out that the exception was just badly named.

On Mac OS X one needs to create the directory (i used 777 rights on it)
   /var/lock
in order to be able to use RXTX at all. Else you just keep getting the damn gnu.io.PortInUseException.

I'm sure this is documented somewhere and I just didn't find it... but many thank to JGrass for the tip.

Read/Write binary data with Scala

In my project to communicate between my notebook and an Arduino over XBees I need to send binary commands to the XBee. The protocol is defined approx. like this: every packet start with 0x7E and then two bytes containing the length of the packed then the payload and afterwards a checksum.
So I decided to write a little library that allows me to parse and compose these commands in a natural way. The parsing should use pattern matching.

To define a structure use something like this:

val myPattern = <<(byte, integer, integer, byte)>>

('<<' is a imported function, the '>>' is just decoration, that returns itself)

then you can use

val bytes: Seq[Byte] = myPattern(12, 33321, 24432, 0)

to generate a binary representation (i.e 0x12, 0x00, 0x00,  0x82, 0x29, ...). To a byte sequence use

bytes match {

case myPattern((a,b,c,d), rest) =>

a should be(12)

b should be(33321)

c should be(24432)

d should be(0)

rest should be(Nil)

}

The variable "Rest" in the example will contain everything in the Seq[Byte] after the matched sequence.


It also allows to check for fixed values (in the example above 0x7E in the first byte):

val myPattern2 = <<(fix_byte(0x7E), integer, integer, byte) drop1

myPattern2(1234, 4321, 12) //will be 0x7E, 0x00, 0x00, 0x04, 0xD2

And you can map a pattern to some class:

case class Color(red: Byte, green: Byte, blue: Byte)

val rgbPattern = <<(byte, byte, byte)>>>(

(c: Color) => (color.red, color.green, color.blue),

t => Some(Color(t._1, t._2, t._3))

)

But what makes it really powerful is the ability to nest those patterns

val colorfulPattern = <<(fix_byte(0x7E), rgbPattern, rgbPattern, integer) drop1

val data = (0x7E :: 33 :: 21 :: 11 :: 0 :: 100 :: 12 :: 0 :: 0 :: 0 :: 4 :: Nil).map(_.toByte)

data match {

case colorfulPattern((foregroundColor, backgroundColor, pixelsToFill), _) =>

foregroundColor should be(Color(33,21,11))

backgroundColor should be(Color(0, 100, 12))

pixelsToFill should be(4)

}

so it's pretty easy to define complex structures pretty fast, easy and reusable. The framework also provides possibilities to parse/serialize lists (even of "complex" objects) and optional values.


Just check out the code a GitHub and feel free to use it and contribute to it!

Now on Github

As many other Scala-project are already present on GitHub I decided to host my little projects there as well. Check out my GitHub Profile. Currently there are only two projects:

• scala-base: My base classes, i. e. Durations, BinaryParsing, Actor-library and Erlang OTP-like stateserver and supervisor
• scala-xbee: An library to communicate with a usb-port attached XBee (and obviously XBees attached to this one).

I'll blog more about the projects later.