Re: EPICS problem

[Andrew Johnson <anj@aps.anl.gov>]

Hi Mark,

Mark Rivers wrote:
> So the IOC is reporting that it set field_type and dbr_field_type to 4,
> which is DBF_USHORT according to dbFldTypes.h.  But cainfo is reporting
> that the field type is DBR_LONG.  Why?
>
> So the IOC is setting the field types to 6, which is DBF_ULONG, as
> expected.
> But cainfo is reporting that the field type is DBF_DOUBLE.  Why?
>
> The problem is not with the cainfo, all channel access clients are
> reporting the same thing.

This is intentional behaviour, and applies to all CA transfers, not just 
arrays.  The reason for the difference is related to numeric conversions 
and the different DBF_ field types that are available via dbAccess.h (as 
used by database code) and db_access.h (as used by Channel Access).

The full range of values that may appear in a DBF_USHORT (available to 
database code via dbAccess.h ) cannot be represented in a DBF_SHORT, the 
16-bit type available in db_access.h; there are no unsigned types in 
db_access.h.  CA has to convert your DBF_USHORT values into DBF_LONG in 
order to guarantee that it can transparently transmit all possible field 
values to the CA client.  Similarly, the only db_access.h type that can 
hold all possible values from a dbAccess.h DBF_ULONG is a DBF_DOUBLE.

If you don't want CA doing those conversions behind your back, you should 
avoid using unsigned types for record fields, and define them as DBF_SHORT 
and DBF_LONG instead.

HTH,

- Andrew
--
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the sunset you made last night.  That was really cool. - Caro