Echelon LonTal Stack - Manual (Page 135)

LonTalk Stack Developer’s Guide 123
then the application must process the type changes for both the network variable
and the configuration network variable.
If the network variable is a member of a configuration property’s application set
where the configuration property is shared among multiple network variables,
the application must process the type and length changes for all network
variables involved.
However, if the configuration property is implemented within a configuration file,
no change to the configuration file is required. The configuration file states the
configuration property’s initial and maximum size (in the CP
documentation-string length field), and network management tools derive the
current and actual type for type-inheriting CPs from the associated network
variable.
Your application must always support the NVT_CAT_INITIAL type category. If
the requested type is of that category, your application must ignore all other
content of the SCPTnvType configuration property and change the related
network variable’s type back to its initial type. The network variable’s initial
type is the type declared in the model file.
Processing a Size Change
If a supported change to the SCPTnvType configuration property results in a
change in the size of a network variable type, your application must provide code
to inform the LonTalk host stack about the current length of the changeable-type
network variable. The current length information must be kept in non-volatile
memory.
The LonTalk API provides a callback handler function, LonGetNvSize(), that
allows you to inform the API of the network variable’s current size. The following
code shows an example implementation for the callback handler function.
unsigned LonGetNvSize(const unsigned index) {
const LidNvDefinition* const nvTable = LonGetNvTable();
unsigned size = LonGetDeclaredNvSize(index);
if (index < LonNvCount &&
nvTable[index].Definition.Flags & LON_NV_CHANGEABLE)
{
const SCPTnvType* pNvType = myGetNvTypeCp(index);
// if the NV uses the initial type, its size is
// the declared size set above
if (pNvType->type_category != NVT_CAT_INITIAL) {
size = pNvType->type_length;
}
}
return
size;
}
The example uses a myGetNvTypeCp() function (that you provide) to determine
the type of a network variable, based on your knowledge of the relationships
between the network variables and configuration properties implemented.
If the changeable-type network variable is member of an inheriting configuration
property that is implemented as a configuration property network variable, the
type information must be propagated from the changeable-type network variable
to the type-inheriting configuration property, so that the LonGetNvSize()