How to call function that accept PG_FUNCTION_ARGS in C?

Hi all,

May I know the way of calling functions that accepts PG_FUNCTION_ARGS
in C?

For example,
Datum point_in(PG_FUNCTION_ARGS)
I would like to call the above function in C, but I cannot figure out
the way to do so. I'm a new PostgreSQL user, please advice, thanks!

Tong