A while ago I stumbled across a technique for improving stream buffering that I wish more I/O library implementors knew about. Korn and Vo's sfio library (circa 1991) had a feature called "pooling", whereby distinct streams could be linked together. Any read or write operation to any stream in a pool implicitly synchronized all the other streams in the pool first. This way, when stdio and stderr were pooled, which was the default when both went to ttys, a write on stderr implicitly flushed stdout. I've implemented this feature for myself a couple times; it's fairly easy to do and basically eliminates the need to explicitly flush streams in client code.

Citation: https://archive.org/details/1991-proceedings-tech-conference... but note that the explanation of stream pools there is a little less precise and more general than really necessary. I believe that later versions of sfio simplified things somewhat, though I could be wrong. (I find their code fairly hard to read.)

Anyhow, ISTM a missed opportunity when new languages that don't actually use libc's routines for something reinvent POSIX's clunkier aspects.