I know there's a million of these but I've been trying to work through it for a while and can't grasp it.

Basically have to make a calculator using functions for the basic mathematical operations.

Here's a small bit of my code, and I'm getting the same error for each of my switch statements so one answer should work for all.

Code:

#include <stdio.h>
#include <stdlib.h>
int add(int, int); //addition function prototype
int subtract(int, int); //subtraction function prototype
int multiply(int, int); //multiply function prototype
int divide(int, int); //divide function prototype
int modulus(int, int); //modulus function prototype
int primeNumber(int); //test prime function prototype
int power(int, int); //exponent function prototype
int factorial(int); //factorial function prototype
int main()
{
/*Declare/Initialize Variables*/
int iSelection = 0; //Menu selection
int iFirstDigit = 0; //First digit entered
int iSecondDigit = 0; //Second digit entered
while (iSelection !=10)
{
/*Begin Calculator*/
printf("\n Calculator\n\n");
printf("1 Addition\n");
printf("2 Subtraction\n");
printf("3 Multiplication\n");
printf("4 Division\n");
printf("5 Modulus (Integers Only)\n");
printf("6 Test If Prime (Integers Only)\n");
printf("7 Factorial (Integers Only)\n");
printf("8 Power\n");
printf("10 Exit Calculator\n");
/*Prompt User To Begin Interaction*/
printf("\nPlease select one of the options (1-7): ");
scanf("%d", &iSelection);
switch(iSelection) //begin switch
{
case 1:
printf("\nPlease enter first digit to be added: ");
scanf("%d", &iFirstDigit);
printf("\nPlease enter second digit to be added: ");
scanf("%d", &iSecondDigit);
printf("\nThe sum of the two numbers is %d\n\n", add(iFirstDigit, iSecondDigit));
int add (int iFirstDigit, int iSecondDigit)
{
return iFirstDigit + iSecondDigit;
}
break; //end case 1

My compile error is

[Linker error]: undefined reference to 'add'

I'm assuming it has something to do with how I've defined/prototyped my function or something along those lines, but I can't spot the error myself.

I've looked through past threads of this nature and couldn't really find the solution. Any help at all is appreciated.