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Dart RawSynchronousSocket

last modified October 18, 2026

The RawSynchronousSocket class in Dart provides synchronous socket operations for low-level network programming. It's part of Dart's dart:io library.

Unlike asynchronous sockets, RawSynchronousSocket blocks until operations complete. This makes it suitable for specific use cases requiring synchronous behavior.

Basic Definition

RawSynchronousSocket represents a synchronous TCP socket connection. It allows blocking read/write operations without callbacks or futures.

Key features include synchronous data transfer, direct byte access, and connection management. It's typically used in specialized scenarios.

Creating a Synchronous Socket

This example shows how to create a synchronous socket connection.

main.dart 
import 'dart:io';

void main() {
  var host = 'example.com';
  var socket = RawSynchronousSocket.connectSync(host, 80);

  print('Connected to $host:80');
  socket.closeSync();
}

We create a synchronous TCP connection to a web server. The connectSync method blocks until the connection succeeds, and throws a SocketException on failure. Always close sockets when done. 

$ dart main.dart
Connected to example.com:80

Synchronous Read Operation

This example demonstrates synchronous reading from a socket.

main.dart 
import 'dart:io';

void main() {
  var socket = RawSynchronousSocket.connectSync('example.com', 80);

  var request = 'GET / HTTP/1.1\r\nHost: example.com\r\n\r\n';
  socket.writeFromSync(request.codeUnits);

  var buffer = List.filled(1024, 0);
  var bytesRead = socket.readIntoSync(buffer, 0, buffer.length);

  print('Read $bytesRead bytes:');
  print(String.fromCharCodes(buffer.sublist(0, bytesRead)));

  socket.closeSync();
}

We send an HTTP request and read the response synchronously. The readIntoSync method blocks until data is available. The buffer stores received bytes. 

$ dart main.dart
Read 937 bytes:
HTTP/1.1 200 OK
Content-Type: text/html; charset=UTF-8
...

Synchronous Write Operation

This example shows synchronous writing to a socket.

main.dart 
import 'dart:io';

void main() {
  var socket = RawSynchronousSocket.connectSync('localhost', 8080);

  var message = 'Hello, server!\n';
  socket.writeFromSync(message.codeUnits);

  print('Wrote ${message.length} bytes to socket');

  socket.closeSync();
}

We connect to a local server and write data synchronously. The writeFromSync method blocks until the bytes have been written and does not return a count. 

$ dart main.dart
Wrote 15 bytes to socket

Handling Multiple Operations

This example demonstrates multiple synchronous operations on a socket.

main.dart 
import 'dart:io';

void main() {
  var socket = RawSynchronousSocket.connectSync('time.nist.gov', 13);

  var buffer = List.filled(256, 0);
  var totalBytes = 0;
  
  while (true) {
    var bytesRead = socket.readIntoSync(buffer, 0, buffer.length);
    if (bytesRead <= 0) break;
    
    totalBytes += bytesRead;
    print(String.fromCharCodes(buffer.sublist(0, bytesRead)));
  }
  
  print('Total bytes received: $totalBytes');
  socket.closeSync();
}

We connect to a daytime server and read data in a loop. The readIntoSync call blocks until data arrives. The loop exits when the connection closes. 

$ dart main.dart
59335 23-04-25 12:34:56 00 0 0 123.5 UTC(NIST) 
Total bytes received: 51

Error Handling

This example shows proper error handling with synchronous sockets.

main.dart 
import 'dart:io';

void main() {
  try {
    var socket = RawSynchronousSocket.connectSync('invalid.host', 80);

    socket.writeFromSync('GET / HTTP/1.1\r\n\r\n'.codeUnits);

    var buffer = List.filled(1024, 0);
    socket.readIntoSync(buffer, 0, buffer.length);

    socket.closeSync();
  } on SocketException catch (e) {
    print('Socket error: ${e.message}');
  } catch (e) {
    print('Error: $e');
  }
}

We wrap socket operations in try-catch blocks to handle potential errors. SocketException catches network-related errors specifically. Always clean up. 

$ dart main.dart
Socket error: Failed host lookup: 'invalid.host'

Best Practices

Source

Dart RawSynchronousSocket Documentation

This tutorial covered Dart's RawSynchronousSocket class with practical examples showing connection, reading, writing, and error handling in synchronous mode.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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