On Freitag, 2. Oktober 2009, firstname.lastname@example.org wrote:
> Firstly, is it possible for the element count of a channel to change
> while the channel is connected? I'm thinking in particular of the case
> when using camonitor (well, ok, I mean ca_create_subscription()) to
> monitor a waveform PV. Each event_handler callback is informed of the
> dbr count -- but as far as I can tell, it's always the maximum waveform
> size, even if NORD is smaller.
You just stumbled over what I think is one of the most irritating
mis(sing)-features of EPICS as it stands: support for array fields is very
weak. Particularly it is limited to arrays which are of fixed size at
> Is the fact that the update size doesn't change, ca_element_count()
> always reports the NELM, and subscribing with zero count gets the full
> wavelength, is this restriction fundamental to channel access, a
> limitation of the channel access server, or just something that hasn't
> been implemented in the waveform record? This question becomes more
> interesting as we use waveform records for large images!
AFAIK, this is more or less built into the database machinery itself, e.g.
the same problem appears with database links. At least that is my
experience from trying to write my own array handling record types. I
originally thought that, by writing appropriate (if perhaps not extremely
efficient) code, I could circumvent this, but to no avail: even database
links between two records (of types where you wrote the support yourself),
once they are initialized by the EPICS core, cannot change the reported
number of elements at run-time. The existing API is insufficient, as e.g.
convert_db_addr, get_array_info, and so on are called only during link
initialization, never during run-time. The only way to work around this is
to use /two/ separate links to two separate fields, one for the array, and
one for the (current) array size.
With CA, the situation is similar: once the channel is established, there is
(to my knowledge) no way for the server to notify clients of a change in
the PV's array size. The corresponding field of chid structure is filled
once when the channel connects and never changes as long as it stays
- RE: Channel access and ca_element_count Jeff Hill
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