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<== Date ==> <== Thread ==>

Subject: Re: Autosave ignores PVs starting with some characters
From: Andrew Johnson <[email protected]>
To: "J. Lewis Muir" <[email protected]>, Ralph Lange <[email protected]>
Cc: [email protected]
Date: Thu, 8 Sep 2016 14:33:36 -0500
Hi Lewis,

On 09/08/2016 01:48 PM, J. Lewis Muir wrote:
> Back on legal characters for a PV name, is it true that a PV name can
> have no more than one dot in it (i.e. the dot separating the record name
> from the field name)?

No, and we have to be careful to distinguish between the behaviours of
the existing IOC server (RSRV) and of other CA servers using CAS.

The RSRV code looks for the first '.' to mark the end of the record
name. The text following that dot is an optional a field name, followed
by an optional field modifier. In 3.14.11 the first field modifier '$'
was added to request long-string handling, but 3.15 adds several more
kinds of field modifiers.

Other CA servers can follow whatever rules they like with regard to the
PV names they will resolve, but if they want IOC CA links to be able to
connect to them they can't include spaces, and I would avoid non-ASCII
and double-quote characters to avoid having too many escape characters.

> If true, then a PV name cannot start with a dot since that would mean a
> zero-length record name which is illegal, right?

The RSRV code should never respond to CA searches for PVs that start
with a dot, because a zero-length record name should be illegal. It does
appear possible to create a record with an empty name, but you can't
display it with dbpr and caget crashes when it tries to connect to it,
so in practice nobody is likely to be doing that.

Other CA servers may respond to PV names that start with a dot.

> What about ending with a dot--can a PV name end with a dot?  Is that
> interpreted as an empty field which defaults to VAL, or is a trailing
> dot illegal?

A trailing dot is allowed, and as you say defaults to VAL. If you're
using a field modifier on the default field you must include the dot to
mark the end of the record name part before appending the modifier.

> Would a developer be open to a patch from me to the documentation to
> further clarify the statement about legal PV name characters in section
> 6.3.2, Unquoted Strings, in light of the above questions?

To be honest I don't really want to document the legalities of PV names
in the Application Developers Guide, since they are really a CA thing
which has its own separate documentation (although I'm not sure if that
currently specifies any restrictions). We do already document what is
legal in a record name, which I think is what really matters here.
Hopefully my modification of 6.3.2 has cleared up the confusion.

- Andrew

-- 
Arguing for surveillance because you have nothing to hide is no
different than making the claim, "I don't care about freedom of
speech because I have nothing to say." -- Edward Snowdon

References:
Autosave ignores PVs starting with some characters Konrad, Martin
RE: Autosave ignores PVs starting with some characters Mooney, Tim M.
RE: Autosave ignores PVs starting with some characters Mooney, Tim M.
Re: Autosave ignores PVs starting with some characters Andrew Johnson
Re: Autosave ignores PVs starting with some characters J. Lewis Muir
Re: Autosave ignores PVs starting with some characters Ralph Lange
Re: Autosave ignores PVs starting with some characters J. Lewis Muir

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