struct vertex_t
{
    float x;
    float y;
    float z;
};

GLuint VBO = 0;

void initVBO()
{
    vertex_t vertices[] =
    {
        { -0.5f, 0.5f, 0.0f },
        { 0.5f, 0.5f, 0.0f },
        { 0.5f, -0.5f, 0.0f },
        { -0.5f, -0.5f, 0.0f }
    };

    glGenBuffers(1, &VBO);
    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
}

void draw()
{
    glEnableClientState(GL_VERTEX_ARRAY);
    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glVertexPointer(3, GL_FLOAT, sizeof(vertex_t), NULL);
    glDrawArrays(GL_QUADS, 0, 4);
}