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kyorohiro (kiyohiro kawamura)
kyorohiro (kiyohiro kawamura)

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Dart's Network Byte Order

This section explains network byte orders.
There is no special problem when storing one byte of data in memory.

import 'dart:typed_data' show Uint8List;

String toHex(Uint8List buffer) {
  const List<String> vv = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'];
    var b = StringBuffer();
    for (var i = 0; i < buffer.length; i++) {
      var v = buffer[i];
      var v1 = (v >> 4) & 0xF;
      var v2 = v & 0xF;
      b.write('${vv[v1]}${vv[v2]}');
    }
    return b.toString();
}

main() async {
  var input = <int>[0x00, 0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,0x09,0x0a,0x0b,0x0c,0x0d,0x0e,0x0f];  
  var buffer = Uint8List(16);
  for(var i=0;i<input.length;i++){
    buffer[i] = input[i] & 0xFF;
  }
  print(toHex(buffer));
}
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When this is executed, it will output data such as 000102030405060708090a0b0c0d0e0f.

However, the problem is how to record 2 bytes of short data, 4 bytes of int data
etc. should be recorded in memory...

There are several recording methods, but two are the most common.

  • Big Endian
    When 0x1234 is recorded in memory, it is recorded as [0x12,0x34].

  • Little endian
    When 0x1234 is recorded in memory, it is recorded as [0x34,0x12].

Try to store it in little-endian format

main() async {
  var input = 0x1234;
  var buffer = Uint8List(2);

  //print(((input>>8) & 0xFF).toRadixString(16)); --> 12
  //print((input & 0xFF).toRadixString(16)); --> 34
  buffer[0] = input & 0xFF;
  buffer[1] = (input>>8) & 0xFF;
  print(toHex(buffer));
}
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If it is little-endian, it should be displayed in the order 0x34 0x12.
If it is big-endian, it should display 0x12 0x32 in that order.

When we run this, it returns 3412, so we have successfully stored the data as little-endian.

Try to store it in big-endian format


main() async {
  var input = 0x1234;
  var buffer = Uint8List(2);

  //print(((input>>8) & 0xFF).toRadixString(16)); --> 12
  // print((input & 0xFF).toRadixString(16)); --> 34
  buffer[0] = (input>>8) & 0xFF;
  buffer[1] = input & 0xFF;
  print(toHex(buffer));
}

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This will return 1234, so we have successfully stored the data in big-endian format.

Communication Protocols are Big Endian

In this article, we will implement DNS which is described in RFCs. In these protocols, data is exchanged in big-endian unless otherwise stated.

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