# 【Q】fd, channel, flush & buffer

**URL:** <https://discuss.ocaml.org/t/q-fd-channel-flush-buffer/11737>\
**Category:** Learning\
**Tags:** text-processing, string, channels, file, buffer\
**Created:** [March 21, 2023, 4:57am UTC](https://discuss.ocaml.org/t/q-fd-channel-flush-buffer/11737 "2023-03-21T04:57:48Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![anzhi0708](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/anzhi0708/32/4299_2.png) [@anzhi0708](https://discuss.ocaml.org/u/anzhi0708)\
**Post date:** [March 21, 2023, 4:57am UTC](https://discuss.ocaml.org/t/q-fd-channel-flush-buffer/11737/1 "2023-03-21T04:57:48Z")

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Hello! Thanks to you guys’ support, I’m getting more and more comfortable with coding in OCaml!

I need to copy & paste text from raw PDF files for my homework; you might already know that when you do that, random newlines (\n) will be there in the string buffer. I wrote simple Python script to deal with this situation before, and now I’m moving to OCaml.

```auto
let () = print_endline "Removing NEWLINE from clipboard..."

(* *)
let run (cmd: string) =
    let fd = Unix.open_process_in cmd in
    let () = print_endline "running command" in
    let text = In_channel.input_all fd in
    let _ = Unix.close_process_in fd in
    text

let remove_newline str: string =
    str
    |> String.map
       @@ fun c -> if c = '\n' then ' ' else c

let () =
    let () = print_endline "initializing main" in
    let global_buffer: string ref = ref "(empty)" in
    while true do
        let () = Unix.sleep 1 in
        run "pbpaste"
        |> remove_newline
        |> fun str ->
                if str = !global_buffer then ()
                else let () = global_buffer := str in
                Printf.printf "<><><>New<><><>\n%s\n" !global_buffer
    done

```

Note the `run cmd: string` function; after opening a subprocess with `Unix.open_process`, **I found that I have to add a `print_endline` to print something to the console, or the program seems to stuck and have 0 output**.

I’m 66.66% sure that this is something related to **buffer** / **tty** / **`flush()`** but don’t exactly know the details and real reason behind it. Can some explain? Thanks :3 :3 :3

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**Author:** ![gasche](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/gasche/32/4_2.png) [@gasche](https://discuss.ocaml.org/u/gasche)\
**Post date:** [March 21, 2023, 8:35am UTC](https://discuss.ocaml.org/t/q-fd-channel-flush-buffer/11737/2 "2023-03-21T08:35:54Z")

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Indeed, you need to flush the standard output after printing there. You can do this by calling `flush stdout`, or indeed `print_endline` (which includes a flush) if you know that you want a newline anyway, or by using the `%!` format specifier in a printf format string (it takes no argument and just flushes), so in your code for example `%s\n%!` would do.