From: Joel VanderWerf <vjoel@...>
Date: 2005-12-20T04:10:26+09:00
Subject: Re: Better way to read data from IO into packets?

Robert Klemme wrote:
> 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.

You're right. I was thinking about readchar, which the op used and I 
assumed to be unbuffered, but I don't even know if it is!

>>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.

But there is some logic in the op's code that looks for repeated "start 
byte". I'm just not sure what the limit is.

-- 
        vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407