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Subject: Re: clarification on PV name max length
From: "Johnson, Andrew N. via Tech-talk" <[email protected]>
To: Alfio Rizzo <[email protected]>
Cc: tech-talk <[email protected]>
Date: Wed, 23 Oct 2019 16:06:47 +0000
Hi Alfio,

Within the IOC the name of each record is stored in a regular string field with a fixed length. That was 40 characters long up until a few years ago (I forget exactly). Several sites wanted that extending so we made it 61 (60 plus terminating zero), but there is a price to pay for doing that. There is no problem with the Channel Access name resolution process, CA should allow names up to over 1400 characters, related to fitting the name into a UDP search packet I believe.

The problem is that Channel Access has a string type that is only 40 characters long, used by the string record types and supported by most CA clients. If you want to use a CA client to modify a record link field you used to have to use one of those strings to write the new link address, and if that address is a record name that’s longer than 40 characters you have a problem. Actually you may have a problem if the name is longer that 28 characters, because there are additional flags in the link address (PP,MS,etc.)

We do now support reading and writing to string fields and link fields >40 chars with the CA character array type which can be made as long as needed, although it may take a bit more work in the client to do that.

You should be able to make your record NAME fields longer than 61 chars if you want, there are some macros inside the IOC’s dbDefs.h header that control it, but I recommend lots of testing if you do.

- Andrew

-- 
Sent from my iPad

On Oct 23, 2019, at 8:56 AM, Alfio Rizzo via Tech-talk <[email protected]> wrote:



Hi

Sorry if I am resuming an old thread,

But I did not find any updated documentation about this topic.

According to the EPICS wiki (which I think is not updated)

https://wiki-ext.aps.anl.gov/epics/index.php/RRM_3-14_dbCommon

 

The max allowed characters for PV name are 60 + NULL

Which is common all the records type.

 

However if I look at the ai record wiki in the Display Parameter section

 

https://wiki-ext.aps.anl.gov/epics/index.php/RRM_3-14_Analog_Input#Operator_Display_Parameters

 

I see that there is a limitation on the displayed .NAME field which is 29 char + NULL

even if it I guess from 3.15 on is 39 + NULL.

 

Could you please clarify why you have this limitation when the .NAME field is called ?

 

Thanks

Alfio

 

PhD Alfio Rizzo

Control System Integrator

Integrated Control Systems Division

European Spallation Source ERIC

 

Telephone: +46(0)721792342

Mail Address: P.O Box 176, SE 221 00 Lund, Sweden

Visiting address: Odarslövsvägen 113, Lund

 

https://europeanspallationsource.se/

 

<image001.png>

 


References:
clarification on PV name max length Alfio Rizzo via Tech-talk

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