From: Robert Klemme <bob.news@...>
Date: 2005-12-19T17:47:47+09:00
Subject: Re: Better way to read data from IO into packets?

Joel VanderWerf wrote:
> Levin Alexander wrote:
>> Hi,
>>
>> 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:
>
> s = io.sysread(5)
> len = s[4]
> s << io.sysread(len+2)
> ary = s.unpack "C5a#{len}n"

Why do you use sysread?  I'd prefer to use #read in this case - I don't
see any advantage of resorting to sysread here - it may even prevent read
buffering => things get slower than necessary.

> 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.

Hm...  doesn't seem to be the case.

    robert