OpenGL: cylinder misshaped after turning it 90 degree

I and my college project partner are doing this project. We write this in c++ and OpenGL 2.1 with the freeglut.lib We turned the cylinder to lay down on the ground by 90 degrees. Now the cylinder is oddly misshaped. How can we fix the misshape? Is this a perspective thing? We wanted to use the cylinder later as wheels for a car which we should animate driving in a circle track. Right now the wheels on the car are only 2D circles.

void draw_cylinder(GLfloat radius,
    GLfloat height,
    GLubyte R,
    GLubyte G,
    GLubyte B)
{
    GLfloat x = 0.0;
    GLfloat y = 0.0;
    GLfloat angle = 0.0;
    GLfloat angle_stepsize = 0.1;

    /** Draw the tube */
    glColor3ub(R - 40, G - 40, B - 40);
    glBegin(GL_QUAD_STRIP);
    angle = 0.0;
    while (angle < 2 * PI) {
        x = radius * cos(angle);
        y = radius * sin(angle);
        glVertex3f(x, y, height);
        glVertex3f(x, y, 0.0);
        angle = angle + angle_stepsize;
    }
    glVertex3f(radius, 0.0, height);
    glVertex3f(radius, 0.0, 0.0);
    glEnd();

    /** Draw the circle on top and bottom of cylinder */
    glColor3ub(R, G, B);
    glBegin(GL_POLYGON);
    angle = 0.0;
    while (angle < 2 * PI) {
        x = radius * cos(angle);
        y = radius * sin(angle);
        glVertex3f(x, y, height);
        angle = angle + angle_stepsize;
    }
    glVertex3f(radius, 0.0, height);
    glEnd();
}

void display(void)
{
    glClear(GL_COLOR_BUFFER_BIT);
    glLoadIdentity();

    glTranslatef(0.0, -0.4, -3.0);
    //glRotatef(-40, 1.0, 0.0, 0.0);
    glRotatef(-200, 1.0, 0.0, 0.0);

    draw_cylinder(0.3, 0.5, 255, 160, 100);

    glFlush();
}

void reshape(int width, int height)
{
    if (width == 0 || height == 0) return;

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(40.0, (GLdouble)width / (GLdouble)height,
        0.5, 20.0);

    glMatrixMode(GL_MODELVIEW);
    glViewport(0, 0, width, height);
}

int main(int argc, char** argv)
{
    /** Initialize glut */
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowSize(640, 480);
    glutCreateWindow("Create Cylinder");
    glClearColor(0.0, 0.0, 0.0, 0.0);
    glutDisplayFunc(display);
    glutReshapeFunc(reshape);

    glutMainLoop();

    return 0;
}

Solution

You need to enable the Depth Test:

glEnable(GL_DEPTH_TEST);

and clear the depth buffer as well

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

Example:

float angle = 0.0f;
void display(void)
{
    glEnable(GL_DEPTH_TEST);

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glLoadIdentity();

    glTranslatef(0.0, -0.4, -3.0);
    glRotatef(angle, 0.0, 1.0, 0.0);
    angle += 0.1f;
    glRotatef(-200, 1.0, 0.0, 0.0);

    draw_cylinder(0.3, 0.5, 255, 160, 100);

    glFlush();
    glutPostRedisplay();
}