Socket read problems

Len Morgan kttkinfo at gmail.com
Fri Oct 8 14:02:59 EDT 2010


  Dar,

As it turns out, (and much to my embarrassment) you were close on your 
first response: I was trying to read more bytes than were actually 
there.  The ModBus/TCP spec actually sends two length items.  One is the 
header + data length and the other is the data length only.  What I was 
doing was reading the former, then getting the actual data length, then 
trying to read that many MORE bytes when I already had the data and 
header in a variable.

It still bothers me though that I never got the socketTimeout message 
even though that's exactly what was happening.

len morgan

On 10/8/2010 11:45 AM, Dar Scott wrote:
>
> On Oct 8, 2010, at 5:29 AM, Len Morgan wrote:
>
>> I wish it was that simple but the length of the message is only in 
>> one byte so it's pretty hard to swap.
>
> Yeah, I guess the high byte is always zero.
>
> I never use read by number of bytes.  It is not that I have seen 
> problems with it, I just became comfortable doing things another way.
>
> I usually use 'read from <socket> with message <callback>'.  I don't 
> think the 'with message' is important for the behavior I see.  This 
> reads whatever is in the buffer.  I think some people use an idiom of 
> "for 0" in the command for this.
>
> I collect bytes into a buffer until there is a whole message and then 
> pull that off, in a sense, doing myself what you are looking for 
> LiveCode to do.
>
> You might be able to use 'read from <socket>' as a way to prevent the 
> timeouts and to diagnose the problem.  This will show if there are 
> problems with the frame length.
>
> Dar Scott
>
>
>
>
>
> -- 
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