Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Tue, 14 Apr 2015 15:32:32 -0500

Hi Rolf,

On 04/14/2015 10:57 AM, Rolf Keitel wrote:
> Just out of interest:
> In the dbd files, link fields have no initial value defined.
> When I am loading a record
> record(ao, "TEST:AO") {
> }
> and look with dbpr at the IOC console,
> the OUT link has the value  CONSTANT
> the DOL link has the value  CONSTANT
> the FLNK link has the value  CONSTANT 0.
> 
> (similar with other record types)
> 
> Any reason for this difference?

I hadn't noticed before, but I so see the same pattern. There is no
initial value given for FLNK in the dbCommon.dbd file so the in-memory
contents should be the same, but both dbgf and caget show the different
results.

The dbStatic/dbStaticLib.c/dbGetString() routine is responsible for the
difference; it has different code for handling INP/OUT links and FWD
links since the set of flags supported is different. A constant FWD link
field is always returned as the fixed string "0" whereas constant INP
and OUT links return whatever string value they were set to (which must
have been numeric or they wouldn't still be classified as constant links):

epics> dbpf anj:ai.FLNK "123"
DBR_STRING:          "0"
epics> dbpf anj:ai.SIML "123"
DBR_STRING:          "123"

This does appear to be a bit of an anachronism, but I don't see it as a
problem at all. Are you suggesting we should change this?

- Andrew

-- 
Light thinks it travels faster than anything but it is wrong.
No matter how fast light travels, it finds the darkness has
always got there first, and is waiting for it.
    -- Terry Pratchett, Reaper Man

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System