With QSRV a database field is mapped as a PVA "scalar" when dbChannelFinalElements()==1 .
QSRV allows writing zero length arrays.
> pvput wf value=[1]
> Old : 2020-04-08 13:15:58.618 []
> New : 2020-04-08 13:17:43.011 [1]
> pvput wf value=[]
> Old : 2020-04-08 13:17:43.011 [1]
> New : 2020-04-08 13:17:44.250 []
'wf' is a waveformRecord.
On 4/8/20 12:29 PM, Johnson, Andrew N. via Tech-talk wrote:
> I expected the PVA protocol to be able to handle zero-length arrays, thanks for confirming that. My main question is what happens if QSRV tries to put one into a waveform record or other array field, since it’s calling the IOC to actually write the value and length. From a brief look at some parts of the IOC code it might work, but I would want someone to try it to be sure. Make sure the record already has at least one element in it before testing that...
>
> - Andrew
>
>
>> On Apr 8, 2020, at 2:20 PM, Kasemir, Kay <kasemirk at ornl.gov <mailto:kasemirk at ornl.gov>> wrote:
>>
>> > 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.
>>
>> For PVAccess, the protocol has different type identifiers for scalars and arrays, so a 'double' is clearly distinguished from a 'double []' with exactly one element.
>> When using an array type, the length can change at runtime and include zero length == empty array.
>>
>> As for QSRV, when you create a 'compress' record with NSAM = 1, the PV is a an NTScalar. For NSAM > 1 the PV is of type NTScalarArray.
>> So it behaves just like the Channel Access example, an array of size 1 appears as a scalar.
>> Not sure if this is a quirk in QSRV that could be fixed, or if the limitation is in the database/IOC API that it needs to use.
>>
>>
>> -Kay
>
> --
> Complexity comes for free, simplicity you have to work for.
>
- References:
- Re: Setting an array to zero length Rogers, Will (DLSLtd, RAL, LSCI) via Tech-talk
- Re: Setting an array to zero length Johnson, Andrew N. via Tech-talk
- Re: Setting an array to zero length Kasemir, Kay via Tech-talk
- Re: Setting an array to zero length Johnson, Andrew N. via Tech-talk
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