Experimental Physics and
Industrial Control System

Andrew Johnson via Core-talk <[email protected]> · Thu, 25 Apr 2019 17:55:23 -0000

> why can't it remember a single numeric constant and use that constant
> as the data source when the record calls dbGetLink()?

I described this possibility earlier (see my comments #5 and #9 to this
bug); 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, and the behavior that results is slightly
different than that which the calcout's special() routine gives it (a
GUI widget showing the A field won't show the value written to INPA
until the record next processes).

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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