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

Subject: Re: caget -c
From: Andrew Johnson <anj@aps.anl.gov>
To: tech-talk@aps.anl.gov
Date: Tue, 2 Oct 2012 23:09:08 -0500
Hi Lewis,

On 2012-10-02 J. Lewis Muir wrote:
> What is a scenario where the -c option to the EPICS Base caget
> program is useful?
> 
> I've used the -c option (asynchronous put--use ca_put_callback
> and wait for completion) of the caput program many times to wait
> for a put to complete (e.g. a motor move to complete), but I
> know caget has the same -c option and I don't know when I would
> use it.  (I'd like to test its behavior, but I don't even know
> of a record type or group of records that would let me do that.)

Using ca_get_callback() will give a different result for a 3.14.12 or later 
client & server pair if you're requesting an array which is currently shorter 
than its maximum possible length (and you give a length request of zero, which 
I believe caget -c does now).  Earlier versions will return an array which is 
as large as the underlying field's storage (e.g. waveform.NELM elements) with 
all elements after the valid data set to zero, whereas now you get given an 
array the same size as the data (e.g. waveform.NORD elements).

That's the main reason I can think for using it.

HTH,

- Andrew
-- 
Never interrupt your enemy when he is making a mistake.
-- Napoleon Bonaparte

References:
caget -c J. Lewis Muir

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