Re: dbGetField AI.DPVT

[Érico Nogueira Rolim via Tech-talk <tech-talk at aps.anl.gov>]

Hi Dave,

Have you looked at IOC Access Security [1]? You could simply define an ASG which blocks all writes to your ai record. Something like:

ASG(READONLY) {

    RULE(1, READ)

}

Would that be enough for your purpose? The hardware, since it can use a different method to access the record (e.g. device support code, dbPutField in StreamDevice protocols, pvPut in sequencer running on the same IOC), should still be able to write to the record without issue.

Cheers,

Érico

[1] https://docs.epics-controls.org/en/latest/appdevguide/AccessSecurity.html

On 22/07/2024 18:06, Dave Bracey via Tech-talk wrote:

I find that I can PVPUT (cmd line) to an AI, and I feel that this is probably not a good thing.  It seems to me that the hardware should be the only thing that changes the value of an AI PV.

Without going into detail, I was going to use DPVT as a side-channel to ensure that only the hardware can put to the AI.

 

From: Johnson, Andrew N. <anj at anl.gov>
Date: Monday, July 22, 2024 at 3:48 PM
To: Dave Bracey <dbracey at fnal.gov>, tech-talk at aps.anl.gov <tech-talk at aps.anl.gov>
Subject: Re: dbGetField AI.DPVT

[EXTERNAL] – This message is from an external sender

Hi Dave,

 

There's a reason the DPVT field is called "Device Private" — it's not intended to be accessible from outside the device support code which owns it. Think of it like a C++ private: class-member access specifier. That doesn't mean it's impossible for other software running inside the IOC to get to it, but dbGetField() can't do it by design.

 

What's the higher-level problem that you're trying to solve which seems to require this? Please describe that to the list and we can try to offer other approaches that wouldn't require you to break the encapsulation.

 

- Andrew

 

On 7/22/24, 2:47 PM, "Tech-talk" <tech-talk-bounces at aps.anl.gov> wrote:

 

I want to use dbGetField to get the value of the DPVT field of an AI record.  Does this work?  What field type would I specify?

 

I tried with DBR_LONG and it didn’t like that.  I used the field_type returned from dbNameToAddr (17) and it didn’t like that.

 

Dave Bracey

Fermilab AD Instrumentation

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