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

Subject: Re: Long string support in CA clients and device support
From: Andrew Johnson <[email protected]>
To: "J. Lewis Muir" <[email protected]>
Cc: Tech-talk <[email protected]>
Date: Thu, 15 Nov 2012 14:35:25 -0500
Hi Lewis,

On 2012-11-15 J. Lewis Muir wrote:
> What happens for type DBF_STRING when it is *not* fetched as a
> long-string?  Is it always just sent as 40 bytes and the client has to
> figure out where the string ends based on where the NUL byte is?

Yup, a DBR_STRING request gives you a dbr_string_t which is a fixed-length 40 
character array.  The IOC code that copies a DBF_STRING from a record field 
into the CA network buffer (or into another DBF_STRING field in the IOC) puts 
a Nil byte in the last element so client code that handles strings can assume 
that they are always Nil terminated.

> If a DBF_STRING includes the NUL byte over CA when *not* fetched as a
> long-string, then I would think that fetching a long-string (by using
> the '$' suffix) from a waveform record's VAL field would include a NUL
> byte over CA also.

Careful, you can only use the $ field modifier on a field that is a DBF_STRING 
type, and due to the IOC code doing things in the wrong order in 3.14.12.x the 
$ modifier won't work on the VAL field of a waveform record that has FTVL set 
to STRING (and similar for aSub fields).  Mark is talking about a waveform 
record with FTVL set to CHAR so there is never any need for the field modifier 
anyway.

> But for the case of just fetching an array of chars from a waveform
> record's VAL field, I would not expect to get a NUL byte in that since
> NORD is supposed to tell me the length.

That really depends on the source of the character data, and whether you're 
using the dynamic array sizing features.  Before 3.14.12 or if you request a 
fixed size you always get an array of the size you requested, filled with Nil 
bytes after the first NORD characters.

- Andrew
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References:
Long string support in CA clients and device support Andrew Johnson
Re: Long string support in CA clients and device support Andrew Johnson
Re: Long string support in CA clients and device support J. Lewis Muir

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