Problem with GLUT

When i try to run this code after compiling it, i got only a window border without content, so where is the error?

Note: i am using ubuntu and gcc for compiling

gcc -lglut  Simple.c -o Simple

The output

#include <GL/glut.h> // Header File For The GLUT Library 
#include <GL/gl.h> // Header File For The OpenGL Library 
#include <GL/glu.h> // Header File For The GLu Library

void SetupRC(void);
void RenderScene(void);
void ChangeSize(GLsizei w, GLsizei h);

// Called to draw scene
void RenderScene(void)
{
    // Clear the window with current clearing color
    glClear(GL_COLOR_BUFFER_BIT);

    // set the color to these values
    //          R    G     B
    glColor3f(56.0f, 19.0f,68.0f);

    // Draw a filled rectangle with current color
    glRectf(-25.0f, 50.0f, 50.0f, -25.0f);

    // Flush drawing commands
    glFlush();
}

int main(int argc, char* argv[])
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DEPTH | GLUT_SINGLE | GLUT_RGBA);
    glutInitWindowPosition(400,200);
    glutInitWindowSize(640,468);
    glutCreateWindow("Simple");
    glutDisplayFunc(RenderScene);
    glutReshapeFunc(ChangeSize);

    SetupRC();
    glutMainLoop();

    return 0;
}

// Setup the rendering state
void SetupRC(void)
{
    glClearColor(0.0f, 2.0f, 1.0f, 0.0f);
}

// Handling window resizing
void ChangeSize(GLsizei w, GLsizei h)
{
    GLfloat aspectRatio;

    // Prevent divide by zero
    if (h == 0) 
        h = 1;

    // Set Viewport to window dimensions
    glViewport(0, 0, w, h);

    // Reset coordinate system
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();

    // Establish clipping volume (left, right, bottom,开发者_开发百科 top, near, far)
    aspectRatio = (GLfloat) w / (GLfloat) h;
    if (w <= h) {
        glOrtho(-100.0, 100.0, -100 / aspectRatio, 100.0 / aspectRatio, 1.0, -1.0);
    }
    else
        glOrtho(-100.0 * aspectRatio, 100.0 * aspectRatio, -100.0, 100.0, 1.0, -1.0);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
}

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Try adding a glutSwapBuffers() to the end of RenderScene().

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One thing. You have a depth buffer so you should also add GL_DEPTH_BUFFER_BIT to the clear call.

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);