Re: Using subArray

[Noboru Yamamoto <noboru.yamamoto@kek.jp>]

Michael Abbott wrote:
> I am writing device support for a subArray record so that I can read 
> small slices from a very large array provided by a device.  The device 
> in question has limited memory, and so I cannot afford more than one 
> copy of this array in memory, namely the copy in the device driver 
> itself.  My plan is to read the data like this:
>
>     record(subArray, "SA")
>     {
>         field(DTYP, "My driver")
>         field(INP,  "my driver's arguments")
>         field(MALM, "1024")
>         field(FTVL, "LONG")
>     }
>
>     $ caput SA.NELM 1024
>     $ caput SA.INDX 1024000
>     $ caput SA.PROC 0
>     $ caget SA
>
> Unfortunately there is one very interesting and annoying fly in the 
> ointment.  In subArrayRecord.c:readValue we see the following two lines 
> of code:
>
>     if (psa->indx >= psa->malm)
>         psa->indx = psa->malm - 1;
>
> Ooops.  Most unhelpful!
>
> This stupid code means that MALM, which is used to allocate the storage 
> used by the subArray record, also determines the maximum index into the 
> device array.  I think this is missing the point of a subArray; it 
> certainly gets in my way.
>
> Now, it turns out I am lucky: malm is used to allocate storage *before* 
> it calls my init_record, so I can fake malm back to the value I want to 
> to be.  However, this is hardly very satisfactory, and is going to have 
> a mildly embarassing side effect: get_{graphic,control}_double will 
> return bogus values for the upper limit on nelm.
>
> Any thoughts on this?
We had similar problem when we tarted to construct KEKB control system.
So we developed a new record called Compact Subarray record. It just
copys amount it needs from a source record.
We need to add new API in dbaccess library, which is not present in the 
standard EPICS base, to support this record.

If you have an interest on this record, I will be happy to send source code.

Regards,

Noboru YAMAMOTO
EPICS group
KEK, JAPAN