Experimental Physics and
Industrial Control System

With respect to the proposed changes to msi, we have some questions of usage and meaning of the "global variables" feature for consideration.

(1) We make use of the default value syntax in templates, so that a default value is supplied if no value is specified in the substitution file, effectively defining "optional" macros.

This allows us to add new macro definitions to a template to handle new situations without invalidating existing substitution files for an existing IOC, whilst still completing the expansion.

Example:

In example.template file:

record(ao, "$(device):RLY:SETSP") {
  field(DESC, "$(rlysp_desc=$(device))")
  field(FLNK, "$(device):RLY:SEQ")
  field(EGU, "mbar")
  field(PREC, "1")
  field(HOPR, "$(rlysp_hopr=1.2e2)")
  field(LOPR, "$(rlysp_lopr=2.7e-3)")
  field(DRVH, "$(rlysp_drvh=1.2e2)")
  field(DRVL, "$(rlysp_drvl=2.7e-3)")
  field(DOL, "$(rlysp_level=1.0e-2)")
}

In example.substitution file:

file example.template
{
pattern {device}
{EXMPL-VA-EX-01}
}

file example.template
{
pattern {device,rlysp_level,rlysp_high}
{EXMPL-VA-EX-02,7,7.1}
}

file example.template
{
pattern {device,rlysp_level,rlysp_high,rlysp_hihi,rlysp_drvh,rlysp_hopr}
{EXMPL-VA-EX-03,1000,1100,1200,1200,1200}
}

When we first tried this, we were using the old msi with the fall-through behaviour and it did not work properly.

We upgraded to the newer scoped msi and the defaulting behaviour worked sensibly for us as apparently intended.

As a result we also discovered several problems in our systems where the fall-through feature had expanded some macros that had been entirely unintentionally missed of patterns in substitution files and hid the fact that macros had not been defined correctly for some templates, nicely illustrating the disadvantage of global variables.

So the question is how do the two features of global definitions versus macro default values sit together? Does a global definition in a substitution file override a default definition of a macro of the same name in a template or vice versa?

Presumably a value specified in substitution pattern overrides a global defined value?

Also can we be assured that changes to the behaviour for missing macros and empty lists, etc under discussion would not break this usage?

(2) At what level are these truly globals?  Since a database can be partially expanded again and again, will the global definitions be passed on to the expanded file recursively?

(3) We concatenate substitution files and database files together as part of our build.  Introducing global sections that only go out of scope at the end of a file could become interesting, since there way to "ifdef" or "undef" in files that will become part of a larger one and there is no proposed syntax to delineate their actual scope.  We probably won't be definining global variables in our systems, however we do use other people's modules so we would be concerned about side effects if widespread use of global variables appeared in synapps, for instance.  Would a naming convention be useful?

Linda Pratt

Software Systems Engineer

Controls Department

[email protected]

+44 (0)1235 778058

www.diamond.ac.uk

Diamond Light Source Ltd

Diamond House

Harwell Science and Innovation Campus

DIDCOT

Oxfordshire

OX11 0DE

UK