Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Fri, 7 Nov 2014 15:24:34 -0600

Hi Lewis,

On 11/07/2014 11:19 AM, J. Lewis Muir wrote:
> Thanks for the clarification.  One thing that's still unclear to me
> is the semantics of callback execution in preemptive mode.  In the CA
> reference manual section "Thread Safety and Preemptive Callback to User
> Code" it says,
> 
>   "When the CA client library invokes a user's callback function, it
>    will always wait for the current callback to complete prior to
>    executing another callback function."
> 
> What exactly does this mean in preemptive mode?

I don't know for sure myself, I was trying to understand that while
writing my previous response. I started out thinking that it would not
apply when preemptive callbacks are enabled, but after looking through
the code of the library now I'm less sure of that. This comment
                // only one recv thread at a time may call callbacks
                // - pendEvent() blocks until threads waiting for
                // this lock get a chance to run
quite strongly implies that even in preemptive mode the the library will
serialize all application callbacks, like you suggested:

> Or does the CA client library serialize the callback
> calling such that, regardless of how many threads the CA client library
> may have internally, it will still only call the on_state_change
> callback function serially (e.g. call it for the ch_nx change, wait for
> it to complete, then call it for the ch_ny change)?

That may have been done to allow sharing of chid objects between
application threads (in the same CA context) without having to protect
their contents with a local mutex say.

Jeff Hill would be able to answer definitively, but he isn't seen round
these parts very often nowadays.

- Andrew
-- 
People everywhere confuse what they read in newspapers with news.
-- A. J. Liebling

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System