Experimental Physics and
Industrial Control System

Benjamin Franksen <[email protected]> · Fri, 25 Feb 2011 10:26:50 +0100

On Friday, February 25, 2011, [email protected] wrote:
> Dear all;
>
> I'm looking into the following site to check if pvPut of seq-2.0.11
> works synchronously or asynchronously (with NO compiler option and
> NO explicit SYNC/ASYNC option, just a pvPut(pvname) call).
>
> http://www-csr.bessy.de/control/SoftDist/sequencer/Manual.html
>
> In "Notes on Release 2.0 (Synchronous/asynchronous override on gets and
> puts)",
>
> it says as follwos:
>
> "pvGet and pvPut both accept an optional SYNC or ASYNC argument that,
> for pvGet, overrides the default as set using the -a option and, for
> pvPut, overrides the default synchronous behavior."
>
> So, I thought pvPut(pvname) works synchronously. However, it say in
> "Asynchronous Use of pvPut" as follows:
>
> "Normally the pvPut operation completes asynchronously. In the past it
> has been the responsibility of the programmer to ensure that the
> operation completed (typically by monitoring other variables). However,
> the function pvPutComplete can now be used for this."
>
> So, I'm getting confused.
>
> Does pvPut(pvname) work synchronously, or asynchronously by default?
>
> Sorry in advance if I'm bothering you all due to my poor ability in
> reading English.

Not at all. You are pointing out a bug in the release notes: the first
paragraph is wrong as stated; I am going to fix that. The default behaviour
for pvPut is definitely asynchronous, or, to put it more precisely, pvPut
sends the request off to the PV and does *not* wait for any confirmation (in
fact never requests it in the first place). In short: it is a fire and
forget operation.

It may be clarifying to point out that for the pvPut operation, the
synchronous/asynchronous distinction is not enough to completely describe
the behaviour. There are actually three variants, one synchronous and two
asynchronous:

* pvPut(var): fire and forget, no completion message is requested
* pvPut(var,SYNC): request completion message, block state set (thread)
  until it arrives (or timeout occurs)
* pvPut(var,ASYNC): request completion message but do not block state
  set; instead user code can call pvPutComplete to find out if completion
  message has arrived

Note that pvPut is *not* affected by the +a or -a options given to the snc
compiler. These options only change the default behaviour of pvGet.

You can find a more detailed description of the pvPut and pvGet (and all
other) commands in the online SNL reference at

http://www-csr.bessy.de/control/SoftDist/sequencer/Reference.html

HTH
Ben

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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System