Re: Setting an array to zero length

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

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> 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.