GL_QUADS not how they should be

I’ve been trying to get 3d-scenes with opengl and glut. At the moment, I have a piece of code (basic) to get more and more familiar with glut and opengl (writing in C). But there is a problem with that code, because I draw a floor (GL_QUAD) and four walls of a cube (left, right, front, behind), but when running the code, it seems that the latter QUADS drawn are overlapping the former QUADS. Is this a known problem? I don’t see what I’ve done wrong here.

#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>
#include <stdio.h>
#include <stdlib.h>

GLfloat step1 = 0.0;
GLfloat step2 = 0.0;

void init(void) 
{
   glClearColor (0.0, 0.0, 0.0, 0.0);
   glShadeModel (GL_FLAT);
}

void display(void)
{
   glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
   glLoadIdentity ();
   
   gluLookAt (0.0, 8.0, 20.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
   glPushMatrix();
   glRotatef(step1, 1.0, 0.0, 0.0);
   glRotatef(step2, 0.0, 1.0, 0.0);

   glColor3f (1.0, 1.0, 1.0);
   glBegin (GL_QUADS);
     glVertex3f (-8, 0, -10);
     glVertex3f (8, 0, -10);
     glVertex3f (8, 0, 10);
     glVertex3f (-8, 0, 10);
   glEnd ();
   
   glBegin(GL_QUADS);
     glColor3f(1.0, 0.0, 0.0);
     glVertex3f(-3.0, 0.0, -3.0);
     glVertex3f(-3.0, 9.0, -3.0);
     glVertex3f(-3.0, 9.0, 3.0);
     glVertex3f(-3.0, 0.0, 3.0);

     glColor3f(0.0, 1.0, 0.0);
     glVertex3f(-3.0, 0.0, 3.0);
     glVertex3f(-3.0, 9.0, 3.0);
     glVertex3f(3.0, 9.0, 3.0);
     glVertex3f(3.0, 0.0, 3.0);

     glColor3f(0.0, 0.0, 1.0);
     glVertex3f(3.0, 0.0, 3.0);
     glVertex3f(3.0, 9.0, 3.0);
     glVertex3f(3.0, 9.0, -3.0);
     glVertex3f(3.0, 0.0, -3.0);
     
     glColor3f(1.0, 0.0, 1.0);
     glVertex3f(3.0, 0.0, -3.0);
     glVertex3f(3.0, 9.0, -3.0);
     glVertex3f(-3.0, 9.0, -3.0);
     glVertex3f(-3.0, 0.0, -3.0);
   glEnd();
   glFlush();
 
   glPopMatrix();
   glutSwapBuffers();
}

void reshape (int w, int h)
{
   glViewport (0, 0, (GLsizei) w, (GLsizei) h); 
   glMatrixMode (GL_PROJECTION);
   glLoadIdentity ();
   glFrustum (-1.0, 1.0, -1.0, 1.0, 1.5, 40.0);
   glMatrixMode (GL_MODELVIEW);
}

void keyPressed(unsigned char key, int x, int y)
{
  switch(key)
  {
    case 'q':
    case 27 :
      exit(1);
      break;
    case 'a':
      step2 += 1.5;
      break;
    case 'd':
      step2 -= 1.5;
      break;
    case 'w':
      step1 += 1.5;
      break;
    case 's':
      step1 -= 1.5;
      break;
    case 'r':
      step1 = 0.0;
      step2 = 0.0;
      break;
  }
  if(step1 > 360) step1 -= 360;
  else if(step1 < 360) step1 += 360;
  if(step2 > 360) step2 -= 360;
  else if(step2 < 360) step2 += 360;
  
  glutPostRedisplay();
}

int main(int argc, char** argv)
{
   glutInit(&argc, argv);
   glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGB);
   glutInitWindowSize (800, 600); 
   glutInitWindowPosition (100, 100);
   glutCreateWindow ("Biljart");
   init ();
   
   glutDisplayFunc(display); 
   glutReshapeFunc(reshape);
   glutKeyboardFunc(keyPressed);
   
   glutMainLoop();
   return 0;
}

I think I’m doing something really stupid, but I just don’t see what:P

Well, your problem it’s quite simple!
You forgot to enable the ‘GL_DEPTH_TEST’, since every time you move the camera OpenGL needs to re-organize what you can see!

If you still asking what you need to do, just replace the init() function to:

void init(void) 
{
   glClearColor (0.0, 0.0, 0.0, 0.0);
   glShadeModel (GL_FLAT);
   glEnable(GL_DEPTH_TEST);	
}

It was indeed a stupid thing I forgot. I forgot to ask for GL_DEPTH in the InitDisplayMode and did not enable the DEPTH_TEST. Thanks for your help!