On 4/8/20 10:05 AM, Johnson, Andrew N. via Tech-talk wrote:
> 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).
I think that zero length is an entirely reasonable default
which should not be changed. While the array API is clunky
an inefficient, the problems I'm aware of with handling of
length zero are confined to RSRV/libca. Other interfaces
to the database QSRV, or even DB_LINK, are quite capable of
handling length zero.
> HTH,
>
> - Andrew
>
>
>> On Apr 8, 2020, at 9:13 AM, Rogers, Will (DLSLtd, RAL, LSCI) via Tech-talk <
tech-talk at aps.anl.gov <
mailto: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
>>
>> ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>> *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 <
mailto: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
>> --
>> ...it is impossible to speak of a depoliticized economy as the liberals
>> do, or of a separation between economic exploitation and political
>> oppression as the Marxists articulate. The basic distinction, rather, is
>> between power and creativity...
>> -- Jonathan Nitzan and Shimshon Bichler: Capital as Power
>>
>>
>>
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