Help with allegro 4.2.3: Linkererror undefined reference to 'Winmain@16'

[Steven Mayer]

So I have been seeing this error pop up on almost every computer I have used to create projects with allegro 4.2.3 installed. Sometimes the programs will work, other times they wont ( the first program compiles with no errors, it just makes random dots, while the 2nd program wont compile, and it is intended to simmulate 2d Newtonian physics) The programs:

#include<allegro.h>
#include<cmath>
int main(){
	allegro_init();
	set_color_depth(32);
	set_gfx_mode(GFX_AUTODETECT_WINDOWED,800,600,0,0);
	install_keyboard();
	BITMAP* buffer=create_bitmap(800,600);
	int x_red=rand()%790+1,y_red=rand()%590+1;
	int x_green=rand()%790+1,y_green=rand()%590+1;
	int x_blue=rand()%790+1,y_blue=rand()%590+1;
	while(!key[KEY_ESC]){
		/*if(key[KEY_UP])y--;
		if(key[KEY_DOWN])y++;
		if(key[KEY_RIGHT])x++;
		if(key[KEY_LEFT])x--;*/
		x_red=rand()%790+1;
		y_red=rand()%590+1;
		x_green=rand()%790+1;
		y_green=rand()%590+1;
		x_blue=rand()%790+1;
		y_blue=rand()%590+1;
		rectfill(buffer,x_red,y_red,x_red+10,y_red+10,makecol(255,0,0));
		rectfill(buffer,x_green,y_green,x_green+10,y_green+10,makecol(0,255,0));
		rectfill(buffer,x_blue,y_blue,x_blue+10,y_blue+10,makecol(0,0,255));
		draw_sprite(screen,buffer,0,0);
		//rectfill(buffer,0,0,800,600,makecol(0,0,0));
		rest(10);
	}
	return 0;
}END_OF_MAIN();

That program works, however:

#include <iostream>
#include <cmath>
#include <stdlib.h>
#include <allegro.h>
//Declairing set variables
    double grav_const = 0.00000066742;
    double grav_accel = 0;
    int m_Xstar = 400;
    int m_Ystar = 300;
    double m_star_mass = 10000000; 
    double m_radius_star = 10;
    double PI = 3.14159;
class Planet
{
public:
       // variables that will change because of physics/ player interaction
    char m_strName[25]; 
    int m_nID; 
    double radius; 
    double m_Xplanet; 
    double m_Yplanet; 
    double distance;
    double m_vel_x; 
    double m_vel_y; 
    double angle;
    double angleraw;
    double m_mass_planet;
    
    int m_Xplanet_out; 
    int m_Yplanet_out; 
    //planet information and creation
    void SetInfo(char *strName, int nID, double vel_x, double vel_y, double Xplanet, double Yplanet, double mass_planet)
    {
        strncpy(m_strName, strName, 25);
        m_nID = nID = nID = nID;
        m_vel_x = vel_x; 
        m_vel_y = vel_y; 
        m_Xplanet = Xplanet;
        m_Yplanet = Yplanet;
        m_mass_planet = mass_planet;
    }
    //Application of newtonian physics
    void Apply_Physics(int nID)
    {
      angleraw = atan2(m_Ystar - m_Yplanet, m_Xstar - m_Xplanet);
      angle = angleraw * 180 / PI;
      distance = sqrt(((m_Xstar - m_Xplanet)*(m_Xstar - m_Xplanet)) + ((m_Ystar - m_Yplanet)*(m_Ystar - m_Yplanet)));
      grav_accel = (grav_const*((m_mass_planet * m_star_mass) / (distance * distance)));
      m_vel_x += (sin(angle) * (grav_accel)); 
      m_vel_y += (cos(angle) * (grav_accel));
      m_Xplanet = m_Xplanet - (m_vel_x);
      m_Yplanet = m_Yplanet - (m_vel_y);
         }
         //Visual position calculation and creation
    void Output_Visual(int nID)
    {
         BITMAP* buffer=create_bitmap(800,600);
         m_Yplanet_out = static_cast<int>(m_Yplanet);
         m_Xplanet_out = static_cast<int>(m_Xplanet);
         rectfill(buffer,m_Xplanet_out,m_Yplanet_out,m_Xplanet_out+1,m_Yplanet_out+1,makecol(0,255,0));
		 rectfill(buffer,m_Xstar-3,m_Xstar-3,m_Xstar+3,m_Xstar+3,makecol(255,255,0));
		 draw_sprite(screen,buffer,0,0);
		 rectfill(buffer,0,0,800,600,makecol(0,0,0));
		 rest(10);
      }
      // Visual debugger
    void Output_position(int nID)
    {
         using namespace std;
    cout << "Planet " << nID << " coordinates: (" << m_Xplanet << "," << m_Yplanet<< ")" << endl;
    }
};

int main()
{
    allegro_init();
	set_color_depth(32);
	set_gfx_mode(GFX_AUTODETECT_WINDOWED,800,600,0,0);
	install_keyboard();
	
	
    int loop = 1;
    
    //planet creation and information input place
    //-----------------------------------------------------
    Planet cMercury;
    cMercury.SetInfo("Mercury", 1, 0, 0, 300, 500, 100);
 
    Planet cVenus;
    cVenus.SetInfo("Venus", 2, 0, 0, 400, 600, 200);
    //-----------------------------------------------------
    while(loop == 1)
    {
               //run physics
    cMercury.Apply_Physics(1);
    cVenus.Apply_Physics(2);
    cMercury.Output_Visual(1);
    cVenus.Output_Visual(2);
    //system("PAUSE");
    //if you remove the "system pause" part, you can see how fast my system calculates planet positions
    
}
    return 0;
}

Returns the error:
[Linker error] undefined refrence to 'WinMain@16'
Id returned 1 exit status
[Build Errror] [ProjectOrbit.exe]Error 1

Does anyone know how to fix this?