Experimental Physics and
Industrial Control System

"J. Lewis Muir" <[email protected]> · Thu, 15 Nov 2012 12:20:38 -0600

On 11/15/12 10:43 AM, Andrew Johnson wrote:
>> Another problem with including the NUL byte is that if I fetch a
>> waveform record's VAL field from another language, say Java, I now have
>> to do extra work by checking to see if the NUL byte is included and
>> throw it away if it is; it's a totally bogus character in this context.
> 
> You'll have to do that anyway, because if you point your Java client using the 
> $ field modifier at an 80-char CALC field or a link address you *will* get a 
> Nil byte in it that terminates the string.  With the current code that Nil 
> byte may be followed by non-Nil data from previous contents of that field 
> (because the field modifier code doesn't support dynamimc sizing yet anyway), 
> so code that converts a char array into a non-C string must stop at either the 
> first Nil or the end of the array.

Hi, Andrew.

Hmm...I didn't know that.  That's a bummer.  Still, I'd expect my
underlying Java CA library to handle that case for me so I wouldn't have
to deal with it in my Java client.

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?  I
would have thought a string would be sent over CA encoded as two parts:
an array of bytes (where each byte represents a DBF_CHAR) and the
length.  I would not expect the NUL byte to be in the array.  (But this
is just what I'd expect, I haven't looked at how CA actually encodes
anything.)

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.

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.

Lewis

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System