I should have worded it "a function or a 'pure' subroutine" as I only meant "pure" to apply to subroutine. By "pure subroutine", I mean it doesn't return a result. (Which necessarily means, to be useful, it has side effects.¹)

As for subroutines vs functions in C, it's really the absence or presence of a return statement that distinguishes a subroutine from a function. If there is no return statement, nothing will be returned. Many C compilers, if the returned type is not void will complain about the lack of a return statement. Likewise, will complain when a void routine does have a return

I chose C because that's one of my 2 main programming languages (the other being Perl). I have passing familiarity with languages that make a more explicit distinction between subroutines and functions, such as Pascal (which another monk mentioned).

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¹A real world example of a useful subroutine is:

Which tells the hardware how fast and which direction to run a motor.