does depth clamp occurs before depth test or after depth test? I am rendering a primitive with coordinates > 1.0 and <-1.0 and using depth clamping with depth test. But when i enable depth test it does not render any geometry.

Here is my code:
Code :
 
GLfloat vertices[]=
    {
0.5f,0.5f,0.5f,                                   
-0.5f,0.5f,0.5f,
-0.5f,-0.5f,0.5f,
0.5f,-0.5f,0.5f,
 
0.5f,-0.5f,-0.5f,
-0.5f,-0.5f,-0.5f,
-0.5f,0.5f,-0.5f,
0.5f,0.5f,-0.5f  
}
 
for(int i=0;i<24;i++)
    vertices[3*i+2]*=25;
 
    glEnable(GL_DEPTH_CLAMP);
 
        // when i comment stmt below, it draws triangle strips
        glEnable(GL_DEPTH_TEST); 
        glClearDepth(15.0f);
        glClearColor (1.0, 0.0, 0.0, 1.0);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); 
        glDrawArrays(GL_TRIANGLE_STRIP,0,6);

How to use depth test and clamping together?