RE: Variable size frame

["Mark Rivers" <rivers@cars.uchicago.edu>]

One simple way to do this, depending on the client, is to put the size in another PV.  Then the client can do a ca_get, requesting only that size.  That is what I am doing with the areaDetector software and an IDL client.  But generic clients like medm won't be able to handle that.

________________________________

From: tech-talk-bounces@aps.anl.gov on behalf of Bruno Coudoin
Sent: Wed 3/4/2009 2:23 PM
To: Jeff Hill
Cc: tech-talk@aps.anl.gov
Subject: RE: Variable size frame



Le mercredi 04 mars 2009 à 11:03 -0700, Jeff Hill a écrit :
> Hello Bruno,
>
> The maximum array size transmitted is determined by what you service snap in
> returns from casPV :: maxBound ( 0 ). We will call that NS.

I see this but how to pass the payload size (lets call it PS) which
maybe <= NS

> With the current state of the CA protocol the client can request a number of
> elements NE constrained by 0 < NE <= NS. The number of elements NE can be
> specified when making a get, put, or subscription request. As I recall, if
> the number of elements provided by the service snap in is less than what the
> client has requested then the client does not receive status indicating an
> error, and instead it will receive the number of elements it has requested
> with the undefined elements (at the end of the array) being set to zero.
> Note also that if the client subscribes specifying an element count NE then
> that number of elements will not change, but the client always has an option
> of canceling the subscription and starting a new one specifying a different
> NE.

The client can request NE but how does it know PS? I can't see it.

> That's the current state of things. We will certainly need to consider the
> need for improved flexibility in the future.

I am probably missing a point here because my need sounds pretty common.
I have a CAS server in which I have a PV with a variable data size of
the form (char*, size). I also know its maximum capacity. I expect the
client to be able to read the data an know its size.

I tried first to use a DBR_STRING but I found out it was truncated to 40
chars. Then I tried an array of DBR_CHAR but found no way to pass PS to
the client.

What is the best way to do this currently ?

Bruno.