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There is also a nice tutorial that guides you through building the kilo editor: https://viewsourcecode.org/snaptoken/kilo/

This. I was going to come here to post this specifically.

I loved following this one a few summers ago and even went beyond the end of the tutorial adding functionality and trying more advanced data structures. I think I would have ended with a completely usable hyper-personal editor but unicode support didn't look like fun.

The screencast by antirez is a joy to watch.