Xlib screenshot call

问题:

After seeing the question How to take screenshot (high fps) in Linux (programming) the discussion in the comments has exposed that even thought the problem of rapidly capturing the screenshots is well described. We couldn't actually find the description of the logic behind.

This question was created with intention of resolving the information regarding memory handling of the XLib screen capture

Does xlib screen capture copy the memory from kernel space into user space, or is it creating the intermediate buffer to store the data read from frame buffer?