Re: passive record/field to process when there is a CA read

[Andrew Johnson <anj@aps.anl.gov>]

Dehong Zhang wrote:
> Is it possible to set up a passive record/field, which will only be processed
> when some channel access client tries to read it?

Not using standard record types, and I wouldn't really recommend that 
you try to create a custom record type to do that because the database 
layer wasn't designed with that ability in mind â if you did manage to 
implement something like that, it could very easily break when a new 
release of Base comes along.

The IOC was designed so that in most circumstances requesting or even 
monitoring data from any field is a "safe" activity that should not 
significantly affect the continued operation of the control system.  If 
someone could cause the control system's behavior to change just by 
getting a value through channel access, engineers would be *much* more 
careful when diagnosing an IOC with which they have little familiarity.

If you tell us what you're trying to achieve at a higher level, you 
might get some alternative suggestions on how to achieve that without 
needing to implement "process on get" semantics.

- Andrew
--
When a distinguished but elderly scientist states that something is
possible, he is almost certainly right.  When he states that something
is impossible, he is very probably wrong.  -- Arthur C. Clarke