A TCP echo server never replies even though I am sending a newline

Question

I am trying to follow the Tokio client tutorial to write a client that talks to an echo server that sends back the response with a newline at the end. Here is what I have:

extern crate futures;
extern crate tokio_core;
extern crate tokio_io;

use std::net::SocketAddr;
use std::io::BufReader;
use futures::Future;
use tokio_core::reactor::Core;
use tokio_core::net::TcpStream;

fn main() {
    let mut core = Core::new().expect("Could not create event loop");
    let handle = core.handle();
    let addr: SocketAddr = "127.0.0.1:9999".parse().expect("Could not parse as SocketAddr");
    let socket = TcpStream::connect(&addr, &handle);
    let request = socket.and_then(|socket| {
        tokio_io::io::write_all(socket, &[65, 12])
    });
    let buf = vec![];
    let response = request.and_then(|(socket, _request)| {
        let sock = BufReader::new(socket);
        tokio_io::io::read_until(sock, b'\n', buf)
    });
    let (_socket, data) = core.run(response).unwrap();
    println!("{}", String::from_utf8_lossy(&data));
}

I am expecting this to print "A\n", since the ASCII code for A is 65 and newline is 12. My server is netcat using this command

ncat -l 9999 --keep-open --exec "/bin/cat"

This seems to hang on running the response future on the core. What am I missing here?

Well.. it is 012! But that's not what you want. Just stick with the byte literal and dont mess around with char codes. — hellow, Dec 21 '17 at 16:03

score 4 · Accepted Answer · answered Dec 20 '17 at 00:41

the ASCII code for A is 65 and newline is 12

No, it's not:

 The decimal set:

   0 nul    1 soh    2 stx    3 etx    4 eot    5 enq    6 ack    7 bel
   8 bs     9 ht    10 nl    11 vt    12 np    13 cr    14 so    15 si
  16 dle   17 dc1   18 dc2   19 dc3   20 dc4   21 nak   22 syn   23 etb
  24 can   25 em    26 sub   27 esc   28 fs    29 gs    30 rs    31 us
  32 sp    33  !    34  "    35  #    36  $    37  %    38  &    39  '
  40  (    41  )    42  *    43  +    44  ,    45  -    46  .    47  /
  48  0    49  1    50  2    51  3    52  4    53  5    54  6    55  7
  56  8    57  9    58  :    59  ;    60  <    61  =    62  >    63  ?
  64  @    65  A    66  B    67  C    68  D    69  E    70  F    71  G
  72  H    73  I    74  J    75  K    76  L    77  M    78  N    79  O
  80  P    81  Q    82  R    83  S    84  T    85  U    86  V    87  W
  88  X    89  Y    90  Z    91  [    92  \    93  ]    94  ^    95  _
  96  `    97  a    98  b    99  c   100  d   101  e   102  f   103  g
 104  h   105  i   106  j   107  k   108  l   109  m   110  n   111  o
 112  p   113  q   114  r   115  s   116  t   117  u   118  v   119  w
 120  x   121  y   122  z   123  {   124  |   125  }   126  ~   127 del

A is 65, but newline is 10.

That said, there's no reason to use decimal values here in the first place. Instead, use a byte literal:

tokio_io::io::write_all(socket, b"A\n")

A TCP echo server never replies even though I am sending a newline

1 Answers1