undefined reference to `variable`

#ifndef _MAIN_H
#define _MAIN_H
 
#include <vector>
#include <windows.h>
 
using std::vector;
 
extern vector<char*>  list;
extern HINSTANCE      hInst;
extern HWND           Static, Threads, ip, Port, Method;
 
LRESULT CALLBACK WindowProcedure (HWND, UINT, WPARAM, LPARAM);
 
#endif

#include "main.h"
#include "function.h"
LRESULT CALLBACK WindowProcedure (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    char *IP = GetText(ip);
    char *PORT = GetText(Port);
    char *THREADS = GetText(Threads);
    char *METHOD = GetText(Method);
    // функция выглядит так char *GetText(HWND &);
 /* ... */
}

extern HWND ip = 0;

#include "function.h"
#include "main.h"
 
int WINAPI WinMain (HINSTANCE hThisInstance,
                    HINSTANCE hPrevInstance,
                    LPSTR lpszArgument,
                    int nCmdShow)
{
    MSG messages;
    WNDCLASSEX wincl;
    hInst = hThisInstance;
    wincl.hInstance = hThisInstance;
    wincl.lpszClassName = szClassName;
    wincl.lpfnWndProc = WindowProcedure;
    wincl.style = CS_HREDRAW | CS_VREDRAW;
    wincl.cbSize = sizeof (WNDCLASSEX);
    wincl.hIcon = NULL;
    wincl.hIconSm = LoadIcon (NULL, IDI_HAND);
    wincl.hCursor = LoadCursor (NULL, IDC_ARROW);
    wincl.lpszMenuName = NULL;
    wincl.cbClsExtra = 0;
    wincl.cbWndExtra = 0;
    wincl.hbrBackground = (HBRUSH)COLOR_MENU;
 
 
    if (!RegisterClassEx (&wincl))
        return 0;
 
    HWND hwnd = CreateWindowEx(WS_EX_OVERLAPPEDWINDOW,szClassName,"aaa",WS_OVERLAPPEDWINDOW,CW_USEDEFAULT,CW_USEDEFAULT,400,300,HWND_DESKTOP,NULL,hThisInstance,NULL);
    ip = CreateWindow("EDIT", 0, WS_BORDER | WS_VISIBLE | WS_CHILD, 33,10,120,20, hwnd, 0, hThisInstance, NULL);
    Port = CreateWindow("EDIT", 0, WS_BORDER | WS_VISIBLE | WS_CHILD, 205,10,50,20, hwnd, 0, hThisInstance, NULL);
    Threads = CreateWindow("EDIT", 0, WS_BORDER | WS_BORDER | WS_VISIBLE | WS_CHILD, 72,40,50,22, hwnd, 0, hThisInstance, NULL);
    Method = CreateWindow("COMBOBOX", 0, CBS_DROPDOWNLIST | WS_VISIBLE | WS_CHILD, 185,40,70,80, hwnd, 0, hThisInstance, NULL);
    Static = CreateWindow("STATIC", 0, WS_BORDER | WS_VSCROLL | WS_VISIBLE | WS_CHILD, 10,70,360,180, hwnd, 0, hThisInstance, NULL);
/* ... */
}

TYPE variable; // обьявление

#include "main.h"
variable = some_function(param) // определение

#include "main.h"
other_function(variable); // использование

extern int x;  // Это объявление объекта с именем x

extern int x = 0;  // Это определение объекта с именем x

extern HWND ip;

HWND ip;

extern HWND ip;

HWND ip;

extern HWND  ip = 0;

const  char *variable = "string"; // обьявление

#include "main.h"
some_function(variable) // использование

#include "main.h"
other_function(variable); // использование

funct("string");
// или
char *str = "string";
funct(str);
delete str;

const char *str = "string";