Missing library files??

Hi,
I've mailed this previously to the novice list, but got no response, so
hopefully this is more the appropriate place.

I'm trying to invoke the DirectFunctionCall1function as follows:

BOX* __tmp;
Datum d = DirectFunctionCall1(box_in,
BoxPGetDatum(ret_bbox));
__tmp = DatumGetBoxP(d);

(where ret_bbox is a string representation of a box)

from some client side code, but get linker errors stating that:

-> undefined reference to `DirectFunctionCall1(unsigned long
(*)(FunctionCallInfoData*), unsigned long)'

--> undefined reference to `box_in(FunctionCallInfoData*)

Is this because I'm calling the function incorrectly, or because I'm
missing the library in which these functions reside?

Many thanks,

Tony