Experimental Physics and
Industrial Control System

"Hill, Jeff" <[email protected]> · Thu, 27 Jun 2013 14:36:59 +0000

Hi Dirk,

> The naive way would
> be to do ca_get and/or ca_put inside a monitor callback function. But as
> far as I remember, this is forbidden.

Most of the CA functions can be called from within a callback, The regression 
tests verify that it is possible to even destroy a channel from within a callback.

I wouldn't make sense however to call a  blocking ca function such as 
ca_pend_xx from within a callback. 

Since ca_get is used typically as follows, with a final blocking ca_pend_io call 
for completion synchronization, then the ca_get sans callback can't 
be recommended for use within a callback, but ca_get_callback would work 
fine and _is_ recommended for use in a callback. 

ca_get
ca_get
ca_get
ca_get
ca_get
ca_pend_io

All of the variants of ca_put should be suitable for use in a callback.

Jeff

> -----Original Message-----
> From: [email protected] [mailto:tech-talk-
> [email protected]] On Behalf Of Dirk Zimoch
> Sent: Thursday, June 27, 2013 7:16 AM
> To: EPICS
> Subject: ca_get and ca_put triggered by monitors?
> 
> Hi all,
> 
> I have an application which needs to read or write EPICS data whenever
> something interesting happens with other EPICS data. The naive way would
> be to do ca_get and/or ca_put inside a monitor callback function. But as
> far as I remember, this is forbidden.
> 
> Is this (still) true? I cannot find anything about this in the Channel
> Access Reference Manual.
> 
> If it is not (yet) possible, can this be implemented in a future release
> of the channel access library? I think it is a common use case to "do
> something when something else happened".
> 
> What is the best practice to do monitor triggered channel access at the
> moment?
> 
> One way I can imagine is to setup a different thread blocking on an
> event semaphore and release that semaphore from within the monitor
> callback. But this involves thread save programming with all its
> complications.
> 
> 
> Dirk

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System