If the blocking function would be fiber-aware, and yield execution back to the fiber runtime until the (underlying) async operation has completed, it would "just work". One could most likely write their own wrapper functions which use the Windows "overlapping IO" functions (those just have a completion callback if I remember right - PS or maybe completion Event?)

Not possible with the C stdlib IO functions though (that's why it would be nice to have optional async IO functions with completion callback in the C stdlib)

PS: just calling a blocking read in async/await code would have the same effect though, you need an "async/await aware" version of read()