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Comment by irishcoffee | Hacker News Reader

This is interesting, but I wouldn't say it is valid C. main() doesn't know about greet(auto s) and wouldn't be able to call it in valid C89, right?

It's valid K&R since everything defaults to int. The linker will match the symbols sans prototype. That wasn't obsoleted until C23.

TIL, thank you! (How embarrassing haha)

It is valid C89. greet is automatically declared as

    int greet();

at call site.