Linking to C libraries is not unrealistic, that is a gross overgeneralization of a problem which can be sadly easily overlooked. Perhaps strongly recommending against blocking is a little too zealous, but it should certainly be mentioned as a real concern that needs to be considered when interoping with C code. This could also mention the solution of forcing usage of native threads which are guaranteed to always be safe to block.

Let's also not have a discussion here about "look I found one thing that blocks in the stdlib so why shouldn't everything?" I consider these bugs in the standard library because blocking a green thread is simply something that should not be encouraged nor easily done. The runtime is currently *not* responsible for making sure arbitrary FFI is safe and fast, that is the job of the user to ensure that FFI behaves properly with how their program is running.

Again though, this is not the place for this discussion. Blocking concerns *must* be mentioned here, and what the stdlib does should be discussed on the issue tracker.