So, why does puts do "return r ? EOF : '\n';"? Some backwards compatibility? Or is there a logical reason for that?
Why it did that? I'm not sure, but at the time C did not have 'void' functions: every function returned a value. They probably wanted to make the behavior of the stdlib functions deterministic, even if the return value was useless and undocumented.
puts() returns EOF (typically -1) on error, or some unspecified non-negative value on success.
fputc() returns EOF on error or the written character, treated as an unsigned char and converted to int, on success.
Don't expect all puts() implementations to do the same thing. For example, the glibc implementation appears to return the number of characters written on success. Implementations are free to rely on implementation-defined behavior. User code that's intended to be portable cannot.
But as pdw's link shows, what you suggest is exactly what the historical implementation was. So NetBSD is simply matching historical Unix.
> puts() and fputs() return a nonnegative number on success, or EOF on error.
r is the result of the write, if it’s nonzero the write failed and thus so did puts.