Experimental Physics and
Industrial Control System

Benjamin Franksen <[email protected]> · Sun, 29 Sep 2013 03:31:02 +0200

Hi Everyone

until now I have tried hard to avoid breaking existing SNL programs when 
developing new features or cleaning up the code base. I cannot claim 
that I succeeded completely, but I got the impression that the 
transition from 2.0 to 2.1 went reasonably well for most people.

I am planning to make some changes for version 2.2 that can break 
existing programs if they rely on certain mis-features that I inherited 
from earlier versions.

One of these is the behaviour of pvPut and pvGet with multi-PV arrays. 
Concretely, assume a declaration

	int x[2];
	assign x to { "pv1", "pv1" };

and an action statement

	pvGet(x);

Currently this is equivalent to

	pvGet(x[0]);

I find this quite un-intuitive and arbitrary. My current plan is to 
change the behaviour so that if an array has been assigned to multiple 
PVs, the pv functions should act on *all* elements, instead of only the 
first.

You might think at first that this is unnecessary, why not implement the 
behaviour with a simple loop:

	for(i=0; i<2; i++)
		pvGet(x[i]);

But this would have far greater latency than necessary, since it issues 
one request, waits for the response, then issues the next request, waits 
again, etc. An implementation in the runtime system could easily issue 
all requests up front, then simultaneously wait for the responses. If 
you want to do this in SNL code, it would have to look like

	for(i=0; i<2; i++)
		pvGet(x[i],ASYNC);

	/* ... switch to a new state, then:*/

	when (pvGetComplete(x[0]) && pvGetComplete(x[1])) {
		/* continue ... */
	}

which is no longer a simple loop; the code would become quite unwieldy, 
especially with more array elements.

An alternative would be to instead disallow pvGet(x) for multi-PV arrays 
(the compiler can detect this statically). This would mean you *have* to 
say

	pvGet(x[0]);

if that is what you want to happen. I could then add new functions 
(pvGets? pvGetMultiple? pvGetMany? pvGetArray?) that will behave in the 
new way.

Similar considerations apply to other pv functions. As a side-note, 
pvPutComplete (but *not* pvGetComplete) already supports multi-PV arrays 
in a sensible way.

Comments or suggestions, especially from "sequencer power users" would 
be very much appreciated.

Cheers
Ben
-- 
"Make it so they have to reboot after every typo." -- Scott Adams

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System