Draw a rectangle with mouse in GLUT - c++

Question

I am fairly new to using GLUT, and I have been attempting to compile a program (which I found here, first response) that uses the mouse to draw a rectangle by recording the starting and ending points of a click-and-drag.
As a clean copy/paste, it will compile but not draw anything. It just displays a white screen, even after changing the background color to black (in the setup() function). I've read several sources to verify that this program doesn't miss anything in its draw and reshape functions, and it's all there.
I create a window, set the viewport to the window dimensions, and then use the gluOrtho2D function to set the mapping (since the window and viewport are the same dimensions, I set the mapping to the window dimensions). The mouse callback records where I left-click, and where I release left-click, then calls the glutPostRedisplay() function to redraw the window with the new coordinates. After a bit of debugging, I discovered the coordinates are recorded and saved appropriately, and are measured in pixels (x and y are integers between 0 and window dimension), so I should be able to draw a rectangle from one vertex to the other vertex using the coordinates. But, like I said, it only displays a white screen.
So, is there something wrong with the way I am drawing the rectangle? Am I mapping the window incorrectly? I am seriously lost, and any feedback would be greatly appreciated.
EDIT2: I changed the glutInitDisplayMode from GLUT_SINGLE to GLUT_DOUBLE, and that fixed the whole non-interactive white screen thing. Now it will draw a rectangle with the mouse with a flipped y-coordinate (which I fixed), and it works great now. Thank you very much for the suggestion.
Here is my program (EDIT1: added comments):
#include <cstdlib>
#include <GL/glut.h>
using namespace std;
GLsizei width, height;
struct Position
{
Position() : x(0), y(0) {}
float x;
float y;
};
Position start; // Records left-click location
Position finish; // Records left-click release location
void display()
{
glClear(GL_COLOR_BUFFER_BIT); // clear window
glColor3ub(rand()%256, rand()%256, rand()%256); // generates random color
glBegin(GL_QUADS);
glVertex2f(start.x,start.y);
glVertex2f(finish.x,start.y);
glVertex2f(finish.x,finish.y);
glVertex2f(start.x,finish.y);
glEnd();
glutSwapBuffers(); // display newly drawn image in window
}
void reshape( int w, int h )
{
glViewport( 0, 0, (GLsizei)w, (GLsizei)h ); // set to size of window
glMatrixMode( GL_PROJECTION );
glLoadIdentity();
gluOrtho2D( 0.0, (float)w, 0.0, (float)h );
width = w; // records width globally
height = h; // records height globally
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
void mouse(int button, int state, int x, int y)
{
switch(button)
{
case GLUT_LEFT_BUTTON:
if(state==GLUT_DOWN)
{
start.x = x; //x1
start.y = y; //y1
}
if(state==GLUT_UP)
{
finish.x = x; //x2
finish.y = y; //y2
}
break;
glutPostRedisplay();
}
}
void motion( int x, int y )
{
finish.x = x;
finish.y = y;
glutPostRedisplay();
}
void setup()
{
glClearColor(0.0, 0.0, 0.0, 1.0); // *should* display black background
}
int main(int argc, char** argv)
{
glutInit(&argc,argv);
glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGB);
glutInitWindowSize(640,480);
glutInitWindowPosition(100,100);
glutCreateWindow("");
setup();
// initializing callbacks
glutReshapeFunc(reshape);
glutDisplayFunc(display);
glutMouseFunc(mouse);
glutMotionFunc(motion);
glutMainLoop();
return 0;
}

Answer 1

As my comment suggested:
change:
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
to:
glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGB);