A problem with stackless coroutines explained
“It looks like what they’re doing are stackless user level threads and this means they don’t play nice with C calls as they don’t allow calling into C and then back into the language (as they can’t save the C stack). This may not sound like a problem until one considers how almost all C library bindings involve callbacks (xml parsers, graphics, audio, media processing, networking, etc).”
Caution: the post is 3 years old.