Experimental Physics and
Industrial Control System

Ben Franksen via Core-talk <core-talk at aps.anl.gov> · Sun, 07 Mar 2021 07:39:39 -0000

> constant in INP link is the value taken the next time the link is
processed

Yes, this is the most sensible behavior.

> many record types explicitly check and so don't call dbGetLink() when
plink->type==CONSTANT. Thus for this approach to work will require
changes to the record types

This is true. However, I argue that this is benign for two reasons:

(1) Changes are only needed when and if it is desired that the record
type in question make use of the proposed generalization. If no changes
are made, the behavior does not differ from teh current one. So there is
no pressing need for providers of external record types to adapt their
code.

(2) The changes are simplifications: they eliminate a case distinction
that is necessary to make now, but won't be in the future, instead will
enable more functionality. The code becomes simpler and shorter and
easier to maintain in the long run.

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https://bugs.launchpad.net/bugs/1824277

Title:
  Regression in calcout setting constant links at runtime

Status in EPICS Base:
  In Progress
Status in EPICS Base 7.0 series:
  In Progress

Bug description:
  With the 3.15 branch, writing a literal number to a calcout.INP[A-L]
  field causes the corresponding [A-L] field to be set to that number.
  This works because the calcout record marks the INP fields as
  special(SPC_MOD) and the calcout::special() routine calls
  recGblInitConstantLink() to initialize the corresponding [A-L] field
  from the new link string. I suspect that a number of the synApps
  record types (transform, scalc, acalc etc.) probably use this same
  technique.

  Keenan Lang reported that this doesn't currently work on the 7.0
  branch; the value in the [A-L] field doesn't change, which I have
  confirmed.

  This use of special() to reinitialize constant links is unusual,
  normally putting a literal into a link field after record
  initialization has no effect, but someone who worked on the calcout
  record discovered this way to get it to work.

  I think I know what's going on and how to fix it (change
  dbPutFieldLink() to call dbAddLink() before the second call to
  dbPutSpecial()), but I need to check that this won't break other
  cases.

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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System