Re: PV / record name length

[Ralph Lange <ralph.lange@gmx.de>]

Based on the last extension of the record name length and its aftermath:

There is a good chance of client side tools and applications using a hard-coded maximum length for the record name. These will need recompilation (in case they use the header file from EPICS Base) or manual fixing (in all other cases).

Note also that this will add ~200 bytes to the size of every record instance, leading to an average increase of database memory consumption in the order of 20%. Which is acceptable on many systems, but not acceptable on others.

Since this may break existing IOCs, I do not expect your suggestion to be accepted into upstream Base easily.

Allowing record names of arbitrary length (i.e. allocated) would not increase memory consumption for existing installations, but needs a lot more drastic code changes in crucial places of Base, of course.

Cheers,

~Ralph

On Fri, Jul 28, 2017 at 2:09 PM, Sebastian Marsching <sebastian@marsching.com> wrote:

Hi Mark,

> Is that length restriction in the CA protocol?  If so and you change it in your IOC then they may not work with older CA clients?

You are right for clients based on EPICS Base 3.12 (and older). There, the client library restricted channel names to 39 characters (40 byte buffer). However, EPICS Base 3.13.1 (I did not check the .0 release) already increases this limit to 1007 characters (MAX_UDP of 1024 - CA header size of 16 - 1 for the null byte).

Given the fact that EPICS Base 3.13.1 was released almost 20 years ago (November 1998), I think it is unlikely that there are still clients using that version. Anyway, the limit for these clients is 39 characters, so they would already have trouble with the 60 characters that have been allowed by EPICS Base for quite a while.

Best regards,
Sebastian