Along with what everyone else has said about the unsafe aspects of C, I wanted to throw in my two cents.

First, the Xorg API is a mess. It's not fun to write and is poorly documented. I wouldn't recommend jumping right into that immediately.

Secondly, here's a list of a few interesting C projects I've come across while surfing GitHub daily for ~a year.

1) DOOM (https://github.com/id-Software/DOOM), the classic first person shooter. It's code is great and is completely written in C. It make take a little work to get it running, but just looking through the code is worthwhile.
2) Vis (https://github.com/martanne/vis) a vim clone. Associated with the suckless.org projects :)
3) Any suckless.org project
4) C4 (https://github.com/rswier/c4) a C compiler in 4 functions. It's a little dense, but it's a fascinating project.
5) Corange (https://github.com/orangeduck/Corange), a 3D game engine in C. Most 3D game engines are written in C++, and this is one of the few exceptions I've found. Check out SDL, if you want to write graphics stuff in 2D.

Bonus: Build Your Own Lisp (http://www.buildyourownlisp.com/), learn to write a simple Lisp compiler in C. Teaches some basic C concepts too, if you need a little extra practice with the foundational concepts.