run fn runs an event loop and then calls fn env within it.

env provides access to the process's environment (file-system, network, etc). env itself and the resources inside it can be shared safely between Eio domains.

When fn ends, the event loop finishes.

This should be called once, at the entry point of an application. It must not be called by libraries. Doing so would force the library to depend on Unix (making it unusable from unikernels or browsers), prevent the user from choosing their own event loop, and prevent using the library with other Eio libraries.

run will select an appropriate event loop for the current platform. On many systems, it will use Eio_posix.run.

On recent-enough versions of Linux, it will use Eio_linux.run. You can override this by setting the $EIO_BACKEND environment variable to either "linux", "posix" or "windows".