I want to use ftp-download-file to download something, but I want to show the progress in my gui app.So I change net/ftp.rkt a little, let ftp-download-file can report current progress, ftp-download-file now changed to: (define (ftp-download-file tcp-ports folder filename [hook #f]) ;; Save the file under the name tmp.file, rename it once download is ;; complete this assures we don't over write any existing file without ;; having a good file down (let* ([tmpfile (make-temporary-file (string-append (regexp-replace #rx"~" (path->string (build-path folder "ftptmp")) "~~") "~a"))] [new-file (open-output-file tmpfile #:exists 'replace)] [tcp-data (establish-data-connection tcp-ports)]) (fprintf (ftp-connection-out tcp-ports) "RETR ~a\r\n" filename) (ftp-check-response (ftp-connection-in tcp-ports) (ftp-connection-out tcp-ports) (list #"125" #"150") print-msg (void)) ;; hook is a lambda (-> channel? channel? any?) ;; (hook receive-channel control-channel) ;; in this lambda, can use (channel-get rcv-ch) to get bytes has downloaded. ;; after (channel-get rcv-ch), use (channel-put ctrl-ch 0) to launch sender to get a new size info the rcv-ch channel. ;; Two channels used here is want to control the "refresh speed". It prevent receiver get size too frequently. ;; It means only if hook (channel-put ctrl-ch 0), then the sender will get the downloading file size. ;; Example: ;; (ftp-download-file ftp-conn "." "testfile" ;; (lambda (rcv-ch ctrl-ch) ;; (letrec ([recur ;; (lambda () ;; (let ([data (channel-get rcv-ch)]) ;; (when (not (= data -1)) ;; (channel-put ctrl-ch 0) ;; (printf "~a~%" data) ;; (sleep 1) //not too fast ;; (recur))))]) ;; (recur)))) (let ([rcv-ch (make-channel)] [ctrl-ch (make-channel)]) (when (procedure? hook) (thread (lambda () (hook rcv-ch ctrl-ch))) (thread (lambda () (letrec ([recur (lambda () (channel-put rcv-ch (file-size tmpfile)) (when (= 0 (channel-get ctrl-ch)) (recur)))]) (recur))))) (copy-port tcp-data new-file) (when (procedure? hook) ; send -1 to stop receiver thread (channel-put rcv-ch (file-size tmpfile)) (channel-get ctrl-ch) (channel-put rcv-ch -1) ; send -1 to stop sender thread (channel-get rcv-ch) (channel-put ctrl-ch -1))) (close-output-port new-file) (close-input-port tcp-data) (ftp-check-response (ftp-connection-in tcp-ports) (ftp-connection-out tcp-ports) #"226" print-msg (void)) (rename-file-or-directory tmpfile (build-path folder filename) #t))) A little complicated? But is worked. You must send control-channel 0 to let sender to refresh current tmp file size and reget file size. This prevent sender refresh file size too frequently. Below is the example: (ftp-download-file ftp-conn "." "testfile" (lambda (rcv-ch snd-ch) (letrec ([recur (lambda () (let ([data (channel-get rcv-ch)]) (when (not (= data -1)) (channel-put snd-ch 0) (printf "~a~%" (inexact->exact (round (* 100 (/ data total-size))))) (sleep 1) (recur))))]) (recur)))) <div style="font-family:Times;font-size:medium"></div>