Basics of calling C from D

[belkin via Digitalmars-d-learn]

Example: I have this C function that is compiled into a library

//File: factorial.h
int factorial(int n);


//File: factorial.c
#include "factorial.h"

int factorial(int n)
{
    if(n!=1)
     return n*factorial(n-1);
}

Question: How do I use it from D?