This isn't right. O_NONBLOCK doesn't mean the pipe doesn't stall, it just means you get an immediate errno and don't block on the syscall in the kernel waiting and this is specific to the file description, for which a pipe has 2 independent ones. Setting O_NONBLOCK on the writer does not affect the reader. If it did, this would break a ton of common use cases where pipelined programs are designed to not even know what is on the other side.

Not sure what printf has to do with, it isn't designed to be used with a non-block writer (but that only concerns one side). How will the reader being non-block change the semantics of the writer? It doesn't.

You can't set O_NONBLOCK on a pipe fd you expect to use with stdio, but that isn't unique to pipes. Whether the reader is O_NONBLOCK will not affect you if you're pushing the writer with printf/stdio.

(This is also a reason why I balk a bit when people refer to O_NONBLOCK as "async IO", it isn't the same and leads to this confusion)