Arrays in V1 calls

Hello
I try to pass a column from a table as a parameter to a C function.

The column is an array of float8 with V1 method.
I can't find the method to pick up the arguments-arrays,

the whole thing looks like this:

PG_FUNCTION_INFO_V1(my_func);
Datum
my_func(PG_FUNCTION_ARGS) {
????? how to get the parameter as a pointer
and reach 1st, second, etc. element of array?

float8 *a = PG_GETARG_POINTER(0) ;
float8 c = *(a+1) ;
^^^^^^^^^^^^^^^^^ doesn't give the value of a[2],
but an incredible number
PG_RETURN_FLOAT8(c);
}

Please help me out, or recommend where to find the info (at developers
guide I found nothing like that)

thanks,
mila mailto:pierro(at)dds(dot)nl