Moonpool_lwt.IOIO using the Lwt event loop.
These IO operations work on non-blocking file descriptors and rely on a Lwt_engine event loop being active (meaning, Lwt_main.run is currently running in some thread).
Calling these functions must be done from a moonpool runner. A function like read will first try to perform the IO action directly (here, call Unix.read); if the action fails because the FD is not ready, then await_readable is called: it suspends the fiber and subscribes it to Lwt to be awakened when the FD becomes ready.
val read : Unix.file_descr -> bytes -> int -> int -> intRead from the file descriptor
val await_readable : Unix.file_descr -> unitSuspend the fiber until the FD is readable
val write_once : Unix.file_descr -> bytes -> int -> int -> intPerform one write into the file descriptor
val await_writable : Unix.file_descr -> unitSuspend the fiber until the FD is writable
val write : Unix.file_descr -> bytes -> int -> int -> unitLoop around write_once to write the entire slice.