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Yeah, but I think the question was why EOF and "\n". It could as easily just return 1 or -1 for example, and it would make more sense I think.

puts() always adds a line termination so success means that '\n' is the last char for that implementation.

That's more of an implementation detail, probably a BC remnant from when it called putchar in a loop and you’d get the result if the last putchar.