Xlib screenshot

After looking at the question How to take a screenshot (high fps) in Linux (programming) the discussion in the comments showed that even the thought of quickly capturing screenshots is well described. We couldn't find a description of the logic.

This question was created for the purpose of resolving memory handling information when capturing XLib screen

Does the xlib screen capture copy memory from kernel space to user space, or create an intermediate buffer to store data read from the framebuffer?