How to programmatically get the resolution of a window and that of the system in Linux?

Question

I'm trying to get the resolution of the screen as well as the resolution of a specific window (in which a program is running) on Linux system. I don't need to modify the resolution, I only need the current values. As far as I know, we can call some system functions to do so on Windows, how can we do that on Linux, preferably using C/C++ language? Thanks in advance.

update: Actually, I don't need to do a GUI, although I know Qt and GTK+ can do it, I'm reluctant to include an external library for just getting the resolution.