Unknown Objective-C error "Undefined symbols for architecture x86_64," everything seems OK
===

I'm trying to follow the tutorial here:

http://cocoadevcentral.com/articles/000081.php

And soon as I reached the "Header files" section, I kept getting a strange error message after running `gcc test1.c -o test1` in the Mac OSX command line:

    Undefined symbols for architecture x86_64:
      "_sum", referenced from:
          _main in ccdZyc82.o
      "_average", referenced from:
          _main in ccdZyc82.o
    ld: symbol(s) not found for architecture x86_64
    collect2: ld returned 1 exit status

math_functions.h:

    int sum(int x, int y);
    float average(float x, float y, float z);

math_functions.c:

    int sum(int x, int y) {
      return x + y;
    }
    
    float average(float x, float y, float z) {
      return (x + y + z)/3;
    }

And finally, my test1.c:

    #include <stdio.h>
    #include "math_functions.h"
    
    main() {
      int thesum = sum(1, 2);
      float ave = average(1.1, 2.21, 55.32);
      
      printf("sum = %i\nave = %f\n(int)ave = %i\n", thesum, ave, (int)ave);
    }

I seem to have followed everything correctly and I don't understand where that error is coming from. Help?