Question

I’m getting stranger error when I compile my code. I guess the header files aren’t being properly linked because every single one of those variables that are erroring have been specified in ‘variables.h’ which I properly #include. Strangely enough, if I comment ou the areas in which the variables are used in main.cpp, a whole other slew of errors pop up of the same variables in another file readfile.cpp. Below is the error output, as well as my code for main.cpp and variables.h. Any ideas?

g++ -c main.cpp 
g++ -c readfile.cpp 
g++ -c Objects.cpp 
g++ -o raytracer main.o readfile.o Objects.o
Undefined symbols for architecture x86_64:
  "_depth", referenced from:
      init()    in main.o
      readFile(char const*)in readfile.o
  "_diffuse", referenced from:
      readFile(char const*)in readfile.o
  "_emission", referenced from:
      readFile(char const*)in readfile.o
  "_filename", referenced from:
      init()    in main.o
  "_fov", referenced from:
      init()    in main.o
      initCamera(float*)in readfile.o
  "_height", referenced from:
      init()    in main.o
      readFile(char const*)in readfile.o
  "_lookatx", referenced from:
      init()    in main.o
      initCamera(float*)in readfile.o
  "_lookaty", referenced from:
      init()    in main.o
      initCamera(float*)in readfile.o
  "_lookatz", referenced from:
      init()    in main.o
      initCamera(float*)in readfile.o
  "_lookfromx", referenced from:
      init()    in main.o
      initCamera(float*)in readfile.o
  "_lookfromy", referenced from:
      init()    in main.o
      initCamera(float*)in readfile.o
  "_lookfromz", referenced from:
      init()    in main.o
      initCamera(float*)in readfile.o
  "_maxvertnorms", referenced from:
      init()    in main.o
      readFile(char const*)in readfile.o
  "_maxverts", referenced from:
      init()    in main.o
      readFile(char const*)in readfile.o
  "_shininess", referenced from:
      readFile(char const*)in readfile.o
  "_specular", referenced from:
      readFile(char const*)in readfile.o
  "_spherecount", referenced from:
      init()    in main.o
  "_spheres", referenced from:
      readFile(char const*)in readfile.o
  "_triangles", referenced from:
      readFile(char const*)in readfile.o
  "_tricount", referenced from:
      init()    in main.o
  "_trinormals", referenced from:
      readFile(char const*)in readfile.o
  "_trinormcount", referenced from:
      init()    in main.o
  "_upx", referenced from:
      init()    in main.o
      initCamera(float*)in readfile.o
  "_upy", referenced from:
      init()    in main.o
      initCamera(float*)in readfile.o
  "_upz", referenced from:
      init()    in main.o
      initCamera(float*)in readfile.o
  "_vertexcount", referenced from:
      init()    in main.o
  "_vertexnormcount", referenced from:
      init()    in main.o
  "_vertices", referenced from:
      readFile(char const*)in readfile.o
  "_vertnormals", referenced from:
      readFile(char const*)in readfile.o
  "_width", referenced from:
      init()    in main.o
      readFile(char const*)in readfile.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

Below is variables.h..

#include "vertexnormal.h"
#include "sphere.h"
#include "tri.h"
#include "trinormal.h"
#include "vec.h"

#include <string>
#include <vector>

using namespace std;

// width and height specify image size
extern float width;
extern float height;

// maximum depth for a ray (level of recursion)
extern int depth;

// the output file to which the image should be written
extern string filename;

// camera specifiations (should i put in a struct?)
extern float lookfromx;
extern float lookfromy;
extern float lookfromz;
extern float lookatx;
extern float lookaty;
extern float lookatz;
extern float upx;
extern float upy;
extern float upz;
extern float fov;

//***************************//
//  Geometry Specifications  //
//***************************//

// specifies the number of vertrices for tri specifications
extern int maxverts;

// specifies the number of vertices with normals for tri specifications
extern int maxvertnorms;

// pile of inputted vertices
// might need to #include glm file                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
extern vector<vec> vertices;

// pile of inputted vertices with specified normals
extern vector<vertexNormal> vertnormals;

// pile of inputted spheres
extern vector<sphere> spheres;

// pile of inputted triangles
extern vector<tri> triangles;

// pile of inputted triangles using vertices with specified normals 
extern vector<triNormal> trinormals;

extern int vertexcount;
extern int vertexnormcount;
extern int spherecount;
extern int tricount;
extern int trinormcount;

//**************************//
//  Materials Specifiations //
//**************************//

extern float diffuse[3];
extern float specular[3];
extern float shininess;
extern float emission[3];

And here is my main.cpp,

#include <iostream>
#include <string>
#include <fstream>
#include <sstream>

#include "Objects.h"

using namespace std;

#include "readfile.h"
#include "variables.h"


void init() {
    cout << "Reading in scene file... \n";
    cout << "Image size has been set to a " << width << " x " << height << " output. /n";
    cout << "The maximum recursion depth has been set to " << depth << ". \n";
    cout << "The image will be output to " << filename << ".png. \n";

    cout << "The camera has been instantiated with the following properties: \n";
    cout << "\t POSITION: (" << lookfromx << ", " << lookfromy << ", " << lookfromz << ") \n";
    cout << "\t DIRECTION: (" << lookatx << ", " << lookaty << ", " << lookatz << ") \n";
    cout << "\t UP: (" << upx << ", " << upy << ", " << upz << ") \n";
    cout << "\t FIELD OF VIEW: " << fov << " \n";

    cout << "An amount of " << vertexcount << " vertices has been specified with a maximum of " << maxverts << " allowed. \n";
    cout << "An amount of " << vertexnormcount << " vertices with normals has been specified with a maximum of " << maxvertnorms << " allowed. \n"; 

    cout << "An amount of " << spherecount << " spheres have been specified. \n";
    cout << "An amount of " << tricount << " triangles have been specified. \n";
    cout << "An amount of " << trinormcount << " triangles with calculated vertex normals have been specified. \n";
}


int main (int argc, char * argv[]) {
    readFile(argv[1]);
    init();
    return 0;
}

Answer 1

Try this:

1. Open up your variables.h header file.
2. Copy ALL of the extern variable declarations.
3. Open up your main.cpp file.
4. Paste all your declarations copied from (2).
5. In the same main.cpp remove the keyword extern from each declaration.
6. Save all your files.
7. Lookup how extern works. Something tells me you missed that in your studies.

Ok, this has been covered in SO what, a few thousand times, but for the OP:

Declaring the Existence of a Variable

// DECLARE myvar, an int variable. no storage has been set aside
//  this is simply telling the compiler this thing exists.. somewhere.
extern int myvar;

Defining The Existence of a Variable

// DEFINE myvar, an int variable. storage *is* set aside here.
//  only ONE of these, by this name, can be in your global 
//  namespace in your program.
int myvar = 0;

Traditionally, extern declarations are in headers, but definitions are always in c/cpp files. There must be a matching definition for any extern-declared variable that is used in your program.

How this fits with your situation
All of your variables were declared in variables.h, but the were never defined anyway. By telling you to copy/paste all those declarations into a source file (any will do; I chose main.cpp because it was already in your project), and then removing the extern keyword in that source file (not the header), you were essentially defining where they all officially existed. Now all those references to extern‘ed variables in your other source files finally have something to hook up to at link time.

Sidebar
In the c/cpp file where your variables are being defined, make sure you initialize them to proper values. This is the one and only place you can do it. you can NOT do it on any extern declaration. It can only be done on a definition.

Header File

extern int myvar; // note: no initial value.

Source File

int myvar = 0; // note: initialized to zero (0)

I hope that made at least a little sense.