TCP Client using lwt

Task

Networking / TCP Client

TCP client that makes requests to the echo TCP server from the TCP server recipe.

Opam Packages Used

  • lwt Tested with version: 5.9.0 — Used libraries: lwt, lwt.unix

Code 


let ( let* ) = Lwt.bind

Connect to the server and send/receive messages. We resolve the host address using Lwt_unix.getaddrinfo, which performs asynchronous DNS resolution to obtain the server's address.

let echo_client host port =
  let* addr_info =
    Lwt_unix.getaddrinfo
      host
      (string_of_int port)
      [Unix.(AI_FAMILY PF_INET)]
  in
  let* addr =
    match addr_info with
    | [] -> Lwt.fail_with "Failed to resolve host"
    | addr :: _ -> Lwt.return addr.Unix.ai_addr
  in

Create a socket for communication using Lwt_unix.socket, which creates a non-blocking socket for asynchronous I/O.

  let socket =
    Lwt_unix.socket Unix.PF_INET Unix.SOCK_STREAM 0
  in

Connect the socket to the resolved address using Lwt_unix.connect, which establishes a connection in a non-blocking manner.

  let* () = Lwt_unix.connect socket addr in
  let input =
    Lwt_io.of_fd ~mode:Lwt_io.input socket
  in
  let output =
    Lwt_io.of_fd ~mode:Lwt_io.output socket
  in

Read a line from standard input using Lwt_io.read_line, which is a non-blocking read operation. Send the message to the server using Lwt_io.write_line, which writes asynchronously to the output. Read the server's response using Lwt_io.read_line, which is also non-blocking.

  let rec send_messages () =
    let* message = Lwt_io.read_line Lwt_io.stdin in
    let* () = Lwt_io.write_line output message in
    let* response = Lwt_io.read_line input in
    let* () =
      Lwt_io.printlf "Server replied: %s" response
    in
    send_messages ()
  in
  send_messages ()
;;

Run the echo_client function within the Lwt main loop

let () =
  let host = "127.0.0.1" and port = 12345 in
  Lwt_main.run (echo_client host port)

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