C-language example of using/returning numeric

Does anyone happen to have a server-side C-language example of
using the Version-1 calling conventions with the NUMERIC data
type? Is it possible? For example, instead of writing:

PG_FUNCTION_INFO_V1(add_one);

Datum add_one(PG_FUNCTION_ARGS)
{
    int32 arg = PG_GETARG_INT32(0);
    PG_RETURN_INT32(arg + 1);
}

I'd like to do the equivalent with NUMERIC. I see the
appropriate macros declared in utils/numeric.h. But I'm not sure
I understand the representation of NumericData:

varlen <- self-explanatory
n_weight <- ??
n_rscale <- scale of numeric
n_sign_dscale <- ??
n_data[1] <- ?? is this BCD?

Are there any helper functions available to the C-language
server-side developer wrt numeric?

Thanks for any tips,

Mike Mascari
mascarm@mascari.com