Experimental Physics and
Industrial Control System

Ben Franksen via Core-talk <[email protected]> · Thu, 25 Apr 2019 13:41:08 -0000

> As a result we *cannot* implement a generic solution for this in the
core IOC code (dbAccess.c).

If the link field can remember complicated stuff like which PV it points
to, plus flags (CPP, MS, whatever) then why can't it remember a single
numeric constant and use that constant as the data source when the
record calls dbGetLink()?

> Keeping that technique working is what this bug is all about

I am trying to understand your reservation here and failing. I have
never understood why in EPICS CONSTANT links can/should only be used
for/during initialization, nor why there is a different call to
initialize a field from a CONSTANT link instead of adding this
functionality to the regular dbGetLink function. Which is why I thought
fixing this bug would be a great opportunity to rectify that: get more
functionality, less record diversity, more orthogonality.

Just my 2 cents.

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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