Experimental Physics and
Industrial Control System

"Chestnut, Ronald P." <[email protected]> · Mon, 6 Mar 2006 15:20:22 -0800

Andrew,

I think the change would not impact us at the PEP-II or NLCTA projects at SLAC. 

I would, however, caution against using another product at this time to replace the internals of the CALC record. That would seem to bring very little gain, but much risk.

Ron Chestnut

-----Original Message-----
From: Andrew Johnson [mailto:[email protected]] 
Sent: Friday, March 03, 2006 12:56 PM
To: Tech Talk
Subject: calcPerform expressions: 'NOT' operator

Summary: I am working on changes and enhancements to the calcPerform code in R3.14, and am considering making a change that will alter the behaviour of expressions that use the 'NOT' operator in future versions of EPICS.  Please respond if this change will adversely affect your site, i.e. if you actually make heavy use of the 'NOT' operator (as against the related '-', '~' or '!' operators).

The libCom calc subsystem is used by the calc and calcout records, Channel Access security module, MEDM and probably some other client tools to evalutate expressions.  It provides separate operators for doing logical and bitwise operations, for example '&' is a bitwise AND while '&&' is a logical AND.  The operator 'AND' is also supported as a synonym for the bitwise operator, and has parallel operators 'OR' and 'XOR'.

When it comes to the unary NOT operations however, the operator 'NOT' 
does not match the bitwise '~' not operator, but instead performs a NEGATE operation like the unary minus operator.  This obscure behavior is documented in the Record Reference manual.

The current 'NOT' operator dates back to a time when the expression parser only supported the use of '-' as a binary subtraction operator, and for some reason the keyword 'NOT' was picked for the unary minus operator.  However the ability to use '-' for unary minus was added by Janet in R3.12.0beta7 about 11 years ago, so I'm now proposing to change 'NOT' to be a synonym for the bitwise operator '~'.

Please reply if you object to this change because your site uses 'NOT' 
in a significant number of CALC expressions of a 3.14 database or Access Security configuration file.

To reward anyone who was interested enough to read to the bottom of this 
message, here's what I'm up to:  I've been significantly improving the 
error checking and reporting from the expression parser, and have added 
an assignment operator ':=' and expression terminator ';'.  A single 
CALC record can now return multiple results, although it is still 
subject to the 40 character limit for the expression string.

- Andrew
-- 
There is no S in exprexxo.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System