[irishcoffee]

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?

[kevin_thibedeau]

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

[irishcoffee]

TIL, thank you! (How embarrassing haha)

[oguz-ismail2]

It is valid C89. greet is automatically declared as

    int greet();

at call site.