A few years back I built an app which used coroutines extensively (<plug> greylisting SMTP proxy, now abandoned because I don't run my own mailserver any more,

A few years back I built an app which used coroutines extensively (<plug> greylisting SMTP proxy, now abandoned because I don't run my own mailserver any more, http://cowlark.com/spey/ http://cowlark.com/spey/ </plug>). I originally used getcontext/setcontext. Superficially it was really easy, but turned into a goddamned nightmare. The problem is that pthreads gets on really badly with user-allocated stacks; some pthreads implementations find the current thread ID by looking at a magic value on the current thread's stack, and of course with a user-allocated stack this wasn't there. But why was I using pthreads alongside coroutines, you ask? Well, I wasn't. But I was linking to a library which was referring to pthreads symbols, even though it wasn't calling them; which made the linker automatically pull in the thread-safe glibc, so malloc/free ended up wrapping in mutexes... The net effect was bizarre and infrequent crashes that only showed up on machines which weren't mine and that I didn't have access to. I spent months debugging that. Eventually I switched to simulating coroutines using pthreads with a big lock so that only one thread could run at a time. (Which actually turned out to be better, because now I could do some operations properly in parallel.) Beware of setcontext! Bewaaaaaare!