Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Wed, 29 Sep 2010 11:02:29 -0500

Hi Tim,

On Wednesday 29 September 2010 00:47:16 Tim Mooney wrote:
>   I like the idea of increasing the size of the CALC field.  I'd suggest
> 80.

Thanks for that suggestion

> The RPCL field is private, so it might be left at its current size for
> most expressions, and reallocated for the occasional long expression.

That would not be acceptable; the RPCL field is a fixed length char array, so 
we can't change its size for individual record instances.  I don't want anyone 
being able to crash my IOC by writing long versions of the expression 1?1:1?
1:1?1:1?1:1 into the CALC field (1?1:1 expands to 31 bytes in postfix, and 
each additional 1?1: adds another 21 bytes â there's a macro in postfix.h that 
calculates the RPCL size needed from the CALC size).

We *could* make RPCL into a char* pointer and allocate it dynamically 
according to the particular expression, but that means making code changes to 
the record and also opens up the possibility of free memory fragmentation on 
vxWorks IOCs (none of the other standard record types allocate memory after 
initialization to avoid that).  I think it's too late to make those kind of 
changes at this point.

Alternatively we could add integer literal support to the code in libCom/calc, 
which would reduce the incremental expansion factor from 21/4 to 10/3 or 
better, but again that would be making code changes uncomfortably late in the 
development cycle for 3.14.12.

> I would not worry /too/ much about wasted space.  After all, every record
> has a 41-character DESC field, most of them are permanently empty, and I
> don't recall hearing complaints about that.

Making CALC 80 translates to RPCL being 419 bytes instead of its current 209 
bytes.  Lewis' suggestion of 64 bytes translates to RPCL being 335 bytes.

Do any small-IOC guys want to weigh in on these suggestions?

- Andrew
-- 
If a man is offered a fact which goes against his instincts, he will
scrutinize it closely, and unless the evidence is overwhelming, he will
refuse to believe it.  If, on the other hand, he is offered something
which affords a reason for acting in accordance to his instincts, he
will accept it even on the slightest evidence.  -- Bertrand Russell

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System