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<== Date ==> <== Thread ==>

Subject: Channel access and ca_element_count
From: <michael.abbott@diamond.ac.uk>
To: <tech-talk@aps.anl.gov>
Date: Fri, 2 Oct 2009 18:33:34 +0100
A couple of libca questions.

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.

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!


Secondly, and related, is there any compelling reason why an arbitrarily large count can be specified to ca_create_subscription(), whereas ca_array_get_callback() refuses to play if count > ca_element_count()?  In particular, this complaint precludes sensible behaviour if the server wanted to change the channel size between the connection and the get request completing.

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Replies:
Re: Channel access and ca_element_count Benjamin Franksen
References:
Re: EPICS Python client application survey Andrew Johnson

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