Attempting to access an R list from within C code

Hello R-help,

I am wondering if anyone can help me with this:

I want to access data in a list which has been passed
into a C function, but I cannot work out how to access
the values. How do I move from the given SEXP pointer
to the next object in the list? I have tried to use CDR,
but to no avail. The following code gives an
     'address (nil), cause unknown'
error:

R Code:
dyn.load("thelib.so")
list1<-list(c(1:3),c(11:13),c(21:23))
listLen<-length(list1)
.C("myfunc",list1,listLen)


C Code (kept in file thelib.c, compiled on Ubuntu x64
machine using R2.13.0):

void showList(SEXP *obj, int *size)
{
   SEXP *locObj=obj;
   int i;
   for(i=0;i<*size;i++){
     Rprintf("Entry is %d: value is %f",i,REAL(*locObj)[1]);
     *locObj=CDR(*locObj);
   }
   return;
}

Any help is greatfully appreciated!
Best wishes,
Dr. Cormac Long.

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