Re: Problems with dbLinkIsConstant

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

Hi Torsten,

On Jun 30, 2020, at 3:09 AM, Torsten Bögershausen via Core-talk <core-talk at aps.anl.gov> wrote:

I have a problem with dbLinkIsConstant(), it seems.

If I run the motor module with this code:

#if LT_EPICSBASE(7,0,0,0)
#define dbLinkIsConstant(lnk) ((lnk)->type == CONSTANT)
#else
#include "dbLink.h"
#endif

And later

   if (dbLinkIsConstant(&pmr->dol))
   {
       pmr->udf = FALSE;
       Debug(pmr,3, "init_record %s\n", "dbLinkIsConstant=true set UDF=FALSE");

       recGblInitConstantLink(&pmr->dol, DBF_DOUBLE, &pmr->val);
   }

I get this log message:
[motorRecord.cc:906  IOC:m1] init_record dbLinkIsConstant=true set UDF=FALSE

However, the .DOL field is empty.
Is this the expected behavior ?

Yes, an empty or unpopulated link field has always been represented with type==CONSTANT which is a macro with value 0 so this is also the default if the link field is not set at all. In 3.16.1 I introduced extensible links and the dbLinkIsConstant() routine to Base because extensible links can also provide constant values, and they don’t have type==CONSTANT.

Note that recGblInitConstantLink() has always returned TRUE if the link is a constant and provided a value (else FALSE), so there is no actual need for the conditional around it – that was merely an optimization for earlier versions of Base but is no longer helpful. My suggested replacement for the code above is to not call dbLinkIsConstant() at all, just do this instead:

   if (recGblInitConstantLink(&pmr->dol, DBF_DOUBLE, &pmr->val))

   {
       pmr->udf = FALSE;
       Debug(pmr,3, "init_record %s\n", "recGblInitConstantLink set UDF=FALSE");

   }

That should be backwards compatible with older Base versions, and I believe all of the record types  have now had that modification done already. This was documented in the Release Notes for version 3.16.1, look for the section titled “Support Routine Modifications for Extensible Link Types”. 

HTH,

- Andrew

-- 

Complexity comes for free, simplicity you have to work for.