From: Robert Klemme <bob.news@...>
Date: 2005-12-20T18:22:47+09:00
Subject: Re: Better way to read data from IO into packets?

Levin Alexander wrote:
> On 12/18/05, Joel VanderWerf <vjoel@path.berkeley.edu> wrote:
>
>>> I have a small program, that reads data from a serial port and chops
>>> it into packets.  A Packet has the following format:
>>>
>>>   [0x65, 0xEB, <type:8>, <counter:8>, <length:8>, data:length>,
>>> <crc:16>]
>>
>> Why not read enough bytes to make sure you get the length byte:
>
> Because the application may be started in the middle of a packet or
> the stream may be corrupted due to transmission errors.
>
> But you are right, I should optimize that and only read single bytes
> if I need to resynchronize.
>
>> s = io.sysread(5)
>> len = s[4]
>> s << io.sysread(len+2)
>> ary = s.unpack "C5a#{len}n"
>>
>> Or is the problem that there may be a variable number of "start
>> bytes"? In that case, maybe you could tell from the first 5 bytes
>> how many start bytes there are, and then read enough to capture the
>> length byte.
>
> Hmm, maybe I can use regular expressions to check for the correct
> format:
>
>   buffer = "bad data" << [0x65,0xEB,0,4,65,66,67,68,00,00].pack("C*")
>   buffer.scan( /
>     \x65\xEB     # startbytes
>     (.)          # type
>     (.)          # length-byte
>     (.*)         # data
>     (..)         # checksum
>   /x )
>
> I would need a way to discard old or bad data from the buffer,
> probably need to think about it more

Why not just use a regexp to verify the initial sequence and use their
offsets.  Or do something like

buffer.gsub!(/\A.*?(\x65\xEB)/, '\\1')

> (btw: is there a way to use the length-byte in the regular expression
> itself?  Something like /(.)(.{\1})/)

No.

Kind regards

    robert