From: Greg Chambers <gregory.w.chambers@...>
Date: 2009-07-08T06:59:27+09:00
Subject: Re: Question on networking with custom binary interface.

Sorry to bump an old topic of mine, but I got sidetracked with work from 
a different project and had one more question.

Joel VanderWerf wrote:
> insert example code here

I've been using BitStruct and it works like a dream except in one area, 
which is with strings.  My first test class I am making keeps it easy by 
putting it at the end of the packet so I can just use a rest field.  And 
although the output returned by description looks fine, when I convert 
to a String for sending, the string is added on as is instead of 
converted like all the other data.  Here was the test code I wrote:

# test_ascii_message.rb
require 'bit-struct'

class TestAsciiMessage < BitStruct
  unsigned :message_length, 4*8, "Message Length", :endian => :network
  unsigned :message_id, 2*8, "Unique Message ID", :endian => :network
  unsigned :service_id, 1*8, "Service Identifier", :endian => :network
  unsigned :event_id, 1*8, "Event Identifier", :endian => :network
  rest :text_msg, "Null terminated ASCII Message String"
end

# main.rb
require 'test_ascii_message'

test_packet = TestAsciiMessage.new
test_packet.message_id = 0
test_packet.service_id = 2
test_packet.event_id = 3
test_packet.text_msg = "Testing 01 10 11"
test_packet.message_length = test_packet.length
puts "-"*75
puts test_packet.inspect
puts "-"*75
puts test_packet.inspect_detailed
puts "-"*75
puts TestAsciiMessage.describe
puts "-"*75
p test_packet.to_s

And the resulting output was this:

---------------------------------------------------------------------------
#<TestAsciiMessage message_length=24, message_id=0, service_id=2, 
event_id=3, text_msg="Testing 01 10 11">
---------------------------------------------------------------------------
TestAsciiMessage:
                Message Length = 24
             Unique Message ID = 0
            Service Identifier = 2
              Event Identifier = 3
Null terminated ASCII Message String = "Testing 01 10 11"
---------------------------------------------------------------------------
    byte: type         name          [size] description
----------------------------------------------------------------------
      @0: unsigned     message_length[ 32b] Message Length
      @4: unsigned     message_id    [ 16b] Unique Message ID
      @6: unsigned     service_id    [  8b] Service Identifier
      @7: unsigned     event_id      [  8b] Event Identifier
---------------------------------------------------------------------------
"\000\000\000\030\000\000\002\003Testing 01 10 11"

This output is fine until it gets to the string.  I just worry, like I 
did above, because the program receiving this needs to see the string as 
a null terminated *char string... did I say that right?  Don't know 
enough C for this.

Anyways, that was my first question.  The second question is how would I 
go about converting a binary string I received into a BitStruct when 
there is a variable length, null terminated string in the center of the 
binary string?  Figured I would ask since using a rest field won't work 
here.
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