Re: Pva

[Michael Davidsaver via Tech-talk <tech-talk at aps.anl.gov>]

Hi John,

On 5/9/26 1:31 PM, Charter via Tech-talk wrote:

How do I monitor subfields, e.g. value and value alarm of a scalar double?

This is probably not worth the effort.

tldr...

PVA protocol does not support monitoring a "sub-field" in the way you probably mean.  A client can use the "pvRequest" object to select certain fields, then subscription updates will only see changes to these fields.  However, this does not hide the (super)structure.  Nor does it handle a situation where eg. an alarm manager want to filter on alarm state changes, but would also like to know the timeStamp and value when alarm state changed.  In short, as currently conceived "pvRequest" lets a client filter on "what" fields change, while most applications want to filter on "why" these fields change.

With QSRV2 single PVs, it is possible to specify the DBE mask[1] as with channel access protocol.  eg. "pvmonitor -r 'record[DBE=ALARM]' ...".  However, other PVA servers will likely ignore this request.

How do I interpret the “changed” bitset?

For each subscription update, this marks those structure fields which make up the delta from the previous update.  The verbose display options for "pvmonitor" may be informative.  Note that "valid" in this context excludes those fields which a server never sent an update for.

Output details:
 -m -v:             Monitor in Raw mode.  Print only fields marked as changed.
 -m -vv:            Monitor in Raw mode.  Highlight fields marked as changed, show all valid fields.
 -m -vvv:           Monitor in Raw mode.  Highlight fields marked as changed, show all fields.

Are you using pvAccessCPP or PVXS?

With PVXS, the Value::isMarked() [2]  ifMarked() and imarked() methods can be used to inspect the changed mask of a subscription update (or GET result).

The "mailbox.cpp" example shows one usage.

[1] https://github.com/epics-base/pvxs/issues/97

[2] https://epics-base.github.io/pvxs/value.html#_CPPv4NK4pvxs5Value8isMarkedEbb

[3] https://github.com/epics-base/pvxs/blob/3d854e492c44261ca25541dfda2177e934efb8b7/example/mailbox.cpp#L69-L76