These are OCaml bindings for Windows' input/output completion ports.
The API is very similar to that of ocaml-uring and in fact borrows the Heap implementation to pass integer ids to requests instead of the actual key data.
To read a file, you first create a completion port, then open the file using the Iocp.Handle module. This ensures the file handle has FILE_FLAG_OVERLAPPED set. You can then perform the asynchronous read and wait on the completion port to return the result to you. By default, the completion status will wait indefinitely for a completion packet to arrive.
This program copies a file.
type req = [ `R | `W ]
let () =
let iocp = Iocp.create () in
let fd = Iocp.Handle.openfile Sys.argv.(1) [O_RDONLY] 0 in
let out = Iocp.Handle.openfile Sys.argv.(2) [O_WRONLY; O_CREAT; O_TRUNC] 0o644 in
let buf = Cstruct.create 1024 in
let _job = Iocp.read iocp ~file_offset:Optint.Int63.zero fd buf ~off:0 ~len:1024 `R in
match Iocp.get_queued_completion_status iocp with
| None -> assert false
| Some t ->
assert (t.data = `R);
let _job = Iocp.write iocp ~file_offset:Optint.Int63.zero out buf ~off:0 ~len:t.bytes_transferred `W in
match Iocp.get_queued_completion_status iocp with
| None -> assert false
| Some t ->
assert (t.data = `W);
Unix.close (fd :> Unix.file_descr);
Unix.close (out :> Unix.file_descr)