Multiple Definition errors in c++

Here's some code that I wrote in DevCpp (windows), now I'm trying to get it to run in linux without much success. Here is the source code

screen.h

#ifndef SCREEN_H
#define SCREEN_H

#include <graphics.h>

class Screen
{
      private:
              int max_x,max_y,grid_size;
              int delta_x,delta_y;
              int origin_x,origin_y;
      public:
      // Default Constructor to Initialize the screen with the grid
      Screen(int xcoord=641, int ycoord=641, int grid=8)
      {
           //variable initialization
           max_x = xcoord;
           max_y = ycoord;
           grid_size = grid;

           delta_x = max_x / grid_size;
           delta_y = max_y / grid_size;

           origin_x = grid_size / 2 * delta_x;
           origin_y = grid_size / 2 * delta_y;

           //plotting the initial grid
           int gd,gm;
           initgraph(&gd,&gm,NULL);
            //draw the x component of the grid
             for(int i=0; i<max_x; i += delta_x)
             {
                     if( i != max_x / 2)
                         setcolor(GREEN);
                     else
                         setcolor(RED);
                    line(i,0,i,max_y);
             }
             //draw the y component of the grid
             for(int i=0; i<max_y; i += delta_y)
             {
                     if( i != max_y / 2)
                         setcolor(GREEN);
                     else
                         setcolor(RED);
                     line(0,i,max_x,i);
             }
             //mark the origin with a white dot to acertain its coordinates for future use
             putpixel(origin_x,origin_y,WHITE);
             std::cout<<origin_x<<"\t"<<origin_y<<"\n";
        }



      //Method prototypes
      void plot_pixel(int xcoord,int ycoord,int col=WHITE)
      {
        int l,t,r,b;
        l = origin_x + xcoord * delta_x;
        r = l + delta_x;
        b = origin_y - ycoord * delta_y;
        t = b - delta_y;
        setcolor(col);
        bar(l,t,r,b);
        setcolor(WHITE);
     }
};
#endif

circles.cpp

#include<iostream>
#include<cmath>
#include "screen.h"
using namespace std;

void draw_midpoint_circle(Screen ob, int radius);
void circlepointplot(Screen ob, int m, int n);
void trigonometric_circle(Screen ob, int radius);

int main() {
    Screen scr = Screen(641, 641, 16);
    int choice, rad;
    cout << "Menu\n 1. Midpoint Circle Drawing Algorithm\n 2. Bresenham's Circle Drawing Algorithm \n 3. Trigonometric Method\n";
    cin>>choice;
    cout << "Enter Radius \t";
    cin>>rad;

    switch (choice) {
        case 1:
            draw_midpoint_circle(scr, rad);
            break;
        case 2:
            draw_midpoint_circle(scr, rad);
            break;
        case 3:
            trigonometric_circle(scr, rad);
            break;
        default:
            cout << "Wrong Choice\n";
            break;
    }

    getch();
    return 0;
}

void trigonometric_circle(Screen ob, int radius) {
    double angle = 0.0;
    while (angle <= 360) {
        int dx = 641 / 16;
        int x = int(radius * cos(angle));
        int y = int(radius * sin(angle));
        angle = angle + 5;
        ob.plot_pixel(x, y, 0);
        cout << "Point Plotted  " << x << "\t" << y << endl;
        char buffer[50];
        sprintf(buffer, "%d,%d", x, y);
        outtextxy(320 + ((x + 1) * dx), 320 - ((y - 1) * dx), buffer);
        getch();
    }
}

void draw_midpoint_circle(Screen ob, int radius) {
    float dp;
    int x, y;
    x = 0;
    y = radius;
    dp = 1 - radius;
    while (x < y) {
        circlepointplot(ob, x, y);
        if (dp < 0)
            dp = dp + 2 * x + 3;
        else {
            dp = dp + 2 * (x - y) + 5;
            y--;
        }

        x++;
        circlepointplot(ob, x, y);
    }
}

void circlepointplot(Screen ob, int m, int n) {
    ob.plot_pixel(m, n, 0);
    ob.plot_pixel(n, m, 0);
    ob.plot_pixel(m, -n, 0);
    ob.plot_pixel(n, -m, 0);
    ob.plot_pixel(-m, n, 0);
    ob.plot_pixel(-n, m, 0);
    ob.plot_pixel(-m, -n, 0);
    ob.plot_pixel(-n, -m, 0);

    cout << "Point Plotted" << m << "\t" << n << endl;
    cout << "Point Plotted" << n << "\t" << m << endl;
    cout << "Point Plotted" << m << "\t" << -n << endl;
    cout << "Point Plotted" << n << "\t" << -m << endl;
    cout << "Point Plotted" << -m << "\t" << n << endl;
    cout << "Point Plotted" << -n << "\t" << m << endl;
    cout << "Point Plotted" << -m << "\t" << -n << endl;
    cout << "Point Plotted" << -n << "\t" << -m << endl;
    int dx = 641 / 16;
    char buffer[50];
    sprintf(buffer, "%d,%d", m, n);
    outtextxy(320 + ((m + 1) * dx), 320 - ((n - 1) * dx), buffer);
    getch();

}

I'm using graphics.h for linux. Basic programs run fine. The errors that I get are

g++ -c screen.cpp -o screen.o
g++ -c circles.cpp -o circles.o
g++ screen.o circles.o -o "circle.exe" -lgraph
circles.o:(.bss+0x0): multiple definition of `screen'
screen.o:(.bss+0x0): first defined here
circles.o:(.bss+0x8): multiple definition of `Font_surface'
screen.o:(.bss+0x8): first defined here
circles.o:(.bss+0x10): multiple definition of `_fgcolor'
screen.o:(.bss+0x10): first defined here
circles.o:(.bss+0x14): multiple definition of `_bgcolor'
screen.o:(.bss+0x14): first defined here
circles.o:(.bss+0x18): multiple definition of `_fontcolor'
screen.o:(.bss+0x18): first defined here
circles.o:(.bss+0x1c): multiple definition of `_pid'
screen.o:(.bss+0x1c): first defined here
circles.o:(.bss+0x20): multiple definition of `CP'
screen.o:(.bss+0x20): first defined here
circles.o:(.bss+0x40): multiple definition of `InternalFont'
screen.o:(.bss+0x40): first defined here
circles.o:(.bss+0x850): multiple definition of `TP'
screen.o:(.bss+0x850): first defined here
circles.o:(.bss+0x860): multiple definition of `_last_arc'
screen.o:(.bss+0x860): first defined here
circles.o:(.bss+0x878): multiple definition of `_internal_linestyle'
screen.o:(.bss+0x878): first defined here
circles.o:(.bss+0x888): multiple definition of `_scanlist'
screen.o:(.bss+0x888): first defined here
collect2: ld returned 1 exit status

What am I doing wrong, how do I get this to work?

Updated errors after moving the code into the class.

/tmp/ccB2RO2Q.o: In function `main':
circles.cpp:(.text+0x111): undefined reference to `grgetch'
/tmp/ccB2RO2Q.o: In function `trigonometric_circle(Screen, int)':
circles.cpp:(.text+0x242): undefined reference to `outtextxy'
circles.cpp:(.text+0x247): undefined reference to `grgetch'
/tmp/ccB2RO2Q.o: In function `circlepointplot(Screen, int, int)':
circles.cpp:(.text+0x6f2): undefined reference to `outtextxy'
circles.cpp:(.text+0x6f7): undefined reference to `grgetch'
/tmp/ccB2RO2Q.o: In function `Screen::Screen(int, int, int)':
circles.cpp:(.text._ZN6ScreenC2Eiii[_ZN6ScreenC5Eiii]+0xd0): undefined reference to `initgraph'
circles.cpp:(.text._ZN6ScreenC2Eiii[_ZN6ScreenC5Eiii]+0xf7): undefined reference to `setcolor'
circles.cpp:(.text._ZN6ScreenC2Eiii[_ZN6ScreenC5Eiii]+0x103): undefined reference to `setcolor'
circles.cpp:(.text._ZN6ScreenC2Eiii[_ZN6ScreenC5Eiii]+0x11c): undefined reference to `line'
circles.cpp:(.text._ZN6ScreenC2Eiii[_ZN6ScreenC5Eiii]+0x15e): undefined reference to `setcolor'
circles.cpp:(.text._ZN6ScreenC2Eiii[_ZN6ScreenC5Eiii]+0x16a): undefined reference to `setcolor'
circles.cpp:(.text._ZN6ScreenC2Eiii[_ZN6ScreenC5Eiii]+0x182): undefined reference to `line'
circles.cpp:(.text._ZN6ScreenC2Eiii[_ZN6ScreenC5Eiii]+0x1b9): undefined reference to `putpixel'
/tmp/ccB2RO2Q.o: In function `Screen::plot_pixel(int, int, int)':
circles.cpp:(.text._ZN6Screen10plot_pixelEiii[Screen::plot_pixel(int, int, int)]+0x6d): undefined reference to `setcolor'
circles.cpp:(.text._ZN6Screen10plot_pixelEiii[Screen::plot_pixel(int, int, int)]+0x80): undefined reference to `bar'
circles.cpp:(.text._ZN6Screen10plot_pixelEiii[Screen::plot_pixel(int, int, int)]+0x8a): undefined reference to `setcolor'
collect2: ld returned 1 exit status

Here's the graphics.h file it has a reference to an SDL_Image *screen

/* libgraph - TurboC graphics API on GNU/Linux
 * graphics.h: Core initialization and configuration functions
 * 
 * Copyright (C) 2003  Faraz Shahbazker
 *
 *  This library is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU Library General Public
 *  License as published by the Free Software Foundation; either
 *  version 2 of the License, or (at your option) any later version.
 *
 *  This library is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 *  Library General Public License for more details.
 *
 *  You should have received a copy of the GNU Library General Public
 *  License along with this library; if not, write to the Free
 *  Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
 *  MA 02111-1307, USA
 *
 * Author:  Faraz Shahbazker <faraz_ms@rediffmail.com>
 */

/* Graphic functions using SDL */

#ifndef GRAPHICS_H
#define GRAPHICS_H 1


#include <SDL/SDL.h>
#include <unistd.h>
#include <sys/types.h>
#include <signal.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>


#ifdef __cplusplus
extern "C" 
{

#endif      /* __cplusplus */


/* graphic drivers */
enum _driver{DETECT=0,开发者_JS百科 USER, VGA=9};
enum graphics_modes{VGALO=0, VGAMED, VGAHI, VGAMAX, VGA640, VGA800, VGA1024, USERMODE};


/* 16 colors */
enum _color{BLACK=0, BLUE, GREEN, CYAN, RED, MAGENTA, BROWN, LIGHTGRAY, DARKGRAY,LIGHTBLUE, LIGHTGREEN, LIGHTCYAN, LIGHTRED, LIGHTMAGENTA, YELLOW, WHITE};



/* global variables */
SDL_Surface *screen;        //main drawing screen
SDL_Surface *Font_surface;  //font screen 
Uint32 _fgcolor, _bgcolor, _fontcolor;  //global color numbers
pid_t _pid;         //Don't bother with this 



/* function prototypes */
void initgraph(int *graphdriver,int *graphmode,char *pathtodriver);
void closegraph(void);
void setgraphmode(int gmode);
int  getgraphmode(void);
void restorecrtmode(void);
int  getmaxx(void);
int  getmaxy(void);
void putpixel(int x, int y, int color);
int  getpixel(int, int);
void setbkcolor(int color);
int  getbkcolor(void);
void setcolor(int color);
int  getcolor(void);
int getmaxcolor(void);
char* getdrivername(void);
char* getmodename(int mode_number);
int getmaxmode(void);
void detectgraph(int* graphdriver, int* graphmode);
void getmoderange(int graphdriver, int* lomode, int* himode);
int delay(float);
void setfontcolor(int color);
int getfontcolor(void);





/*** library specific functions - not for users ***/
void initialize_settings (void);
void mappixel(int, int);     //marks a pixel without updating screen
void clippixel(int *, int *); /* Clip pixel (x,y) to current
                                 screen size*/
void mapword(int,int,int);
void mapvword(int,int,int);
int colorrev(const Uint8);   // maps 0..255 8-bit color to 0..15 TC color
Uint8 colortrans(const int); // maps 0..15 TC color to 0..255 8-bit color
void ctrlbreak(void);        // To detect user interrupt    
void inthandler(int);        // clean up on user interrupt
void safe_update(int top, int left, int right, int bottom);
    /* update screen within boundary */

#define CHECK_INITIALIZATION\
    if (!getenv("LIBGRAPHICS_ACTIVE")) \
    { \
        fprintf(stderr, "*** The graphics system has not been initialized!\n"); \
        fprintf(stderr, "*** Call initgraph() before trying to use graphics functions.\n"); \
        exit(-1); \
    }

struct {int x;int y;}CP;

#include "grtext.h"
#include "shapes.h"
#include "polygon.h"

#ifdef __cplusplus
}
#endif      /* __cplusplus */


#endif      /* LIBGRAPH_H */

---

graphics.h contains the following lines:

/* global variables */ 
SDL_Surface *screen;        //main drawing screen 
SDL_Surface *Font_surface;  //font screen  
Uint32 _fgcolor, _bgcolor, _fontcolor;  //global color numbers 
pid_t _pid;         //Don't bother with this  

This is a bit strange, but this h-file creates global variables. This means, if you include this file to more than one .cpp files, you have multiple definition error. I don't know why this h-file is written by such way, the program will be linked only if graphics.h is included to only one cpp file. If your can change this file, add extern keyword before every global variable, and create these variables (if necessary) in some .cpp file.

---

Your first error message is

circles.o:(.bss+0x0): multiple definition of `screen'

In your code, as I write this, there is nothing called "screen".

Hence, the problem appears to be solved already.

---

EDIT: now that you have added the contents of (someone else's) graphics.h, the author's ignorance of the following is the cause of the problem:

C++98 §7.5/7
The form of linkage specification that contains a braced-enclosed declaration-seq does not affect whether the contained declarations are definitions or not (3.1).

So the pointer variable declarations there are definitions.

Unfortunately it appears that not only the author of that header, but also the current C++ standard got it wrong:

C++11 §7.5/7:
A declaration directly contained in a linkage-specification is treated as if it contains the extern specifier (7.1.1) for the purpose of determining the linkage of the declared name and whether it is a definition.

According to that normative text it is as if each pointer variable was declared as extern and thus would be only a declaration (not a definition). However, even in C++11 the non-normative example shows the intent, that it is a definition.

All that said, this was news to me too, and I'm an old hand at this. It seems to be just a meaningless quirk in the standard, a needless gotcha. But possibly it's because in C++ there is no other way to express pure declaration versus definition for an object.

Cheers & hth.,