Re: Setting an array to zero length

["Johnson, Andrew N. via Tech-talk" <tech-talk at aps.anl.gov>]

Hi Will,

It is not currently (and may never be) possible to put a zero-length array through Channel Access; I don’t know if PVA and QSRV can handle them or not. The IOC generally doesn’t work too well with zero-length arrays, there are some internal APIs that take a zero length to mean there is no data at all (so they can pass metadata around without any data).

Personally I think it regrettable that waveform records currently start up with zero elements, I would have preferred that we never have any zero-length arrays in the IOC since I suspect making them work properly might take more work than making them impossible, but if someone is already using them we probably couldn’t make that change. IOC support for zero-length arrays is something that a site that relies on them should probably take a careful look at; currently I think we have a bit of a mixture, some APIs can handle them and others can’t (including the CA client API AIUI).

HTH,

- Andrew

On Apr 8, 2020, at 9:13 AM, Rogers, Will (DLSLtd, RAL, LSCI) via Tech-talk <tech-talk at aps.anl.gov> wrote:

This is interesting, thank you, but the intention here is to do this for an arbitrary record from a client tool over channel access.

Is the conclusion that this is not possible?

Thanks,

Willl

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From: Tech-talk on behalf of Ben Franksen via Tech-talk
Sent: Friday, April 03, 2020 16:43
To: EPICS Tech Talk
Subject: Re: Setting an array to zero length

Am 03.04.20 um 17:28 schrieb Ben Franksen via Tech-talk:
> Am 03.04.20 um 16:25 schrieb Ralph Lange via Tech-talk:
>> On Fri, 3 Apr 2020 at 15:04, Ben Franksen via Tech-talk <
>> tech-talk at aps.anl.gov> wrote:
>>
>>> [...]
>>
>> You could also try to use another
>>> record with NELM=NORD=0 and that has an output link that you point to
>>> your WAVE record. There is no record type in base with such an output
>>> link [...]
>>>
>>
>> The aaoRecord since Base 3.12.2 (ca. 1995)
> 
> Thanks I completely forgot about this one. Indeed this works:
> 
> record(waveform, "wf") {
>     field(NELM, "10")
>     field(FTVL, "DOUBLE")
>     field(INP, [1, 2, 3, 4, 5, 6, 7, 8])
>     field(PINI, "YES")
> }
> record(aao, "reset") {
>     field(NELM, "0")
>     field(OUT, "wf PP")
> }
> 
> ben@juliana[1]:.../base/7.0>caget wf
> wf 10 1 2 3 4 5 6 7 8 0 0
> ben@juliana[1]:.../base/7.0>caget wf.NORD
> wf.NORD                        8
> ben@juliana[1]:.../base/7.0>caput reset.PROC 1
> Old : reset.PROC                     0
> New : reset.PROC                     1
> ben@juliana[1]:.../base/7.0>caget wf.NORD
> wf.NORD                        0
> ben@juliana[1]:.../base/7.0>caget wf
> wf 10 2.50321e-308 0 0 0 0 0 0 0 0 0
> 
> The last response is unexpected though. While caget -h indeed says:
> 
> Arrays: Value format: print number of requested values, then list of values
> 
> printing the actual number of values in the response (and only so many
> values) would be more useful. Does CA not return the current number of
> values in an array?

Ah, I see. This is a limitation of the simple ca_array_get call which is
used by default. It works as expected if I pass the -c option which
makes it use ca_array_get_callback which can return the current number
of elements.

Cheers
Ben
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