This is my little attempt and writing a short GNL, without sacrificing readability or efficiency.
The program is fully protected against errors, including malloc fails and bad reads. As far as I know, it works with everything.
The program is fully normed (if you remove the comments, of course). This includes the 2020 rules, so no ternaries.
It doesn't use any tricks, like doing stuff in conditions, setting variables to -1 with a ++i, or any other crime against humanity (as far as I know).
The only thing I do to get less lines is passing i and j to join_buffer as 0, so I don't have to declare them.
It's crazy how much you can do with so little code, isn't it ?