Clarification: writes to files less than PIPE_BUFF may be atomic, but they aren't necessarily consistent. This is another important property as you want reads following writes to always have the written values. I think I tripped up over myself since in atomicity you expect the 'all written' and 'nothing written' states, but there's also 'all written but nothing visible' which is a third state which feels like it's part of atomicity, but I guess it's really part of consistency.