Dart RandomAccessFile

last modified April 4, 2025

The RandomAccessFile class in Dart provides random access to files. It allows reading and writing at specific positions within a file.

RandomAccessFile is part of Dart's dart:io library. It supports asynchronous operations and is essential for advanced file manipulation.

Basic Definition

RandomAccessFile represents a file opened for random access. It provides methods for reading, writing, and seeking within the file.

Key features include position manipulation, synchronous/asynchronous operations, and direct byte access. It's particularly useful for binary file operations.

Opening and Reading a File

This example shows how to open a file and read its contents.

import 'dart:io';
void main() async {
 var file = File('data.txt');
 var raf = await file.open(mode: FileMode.read);
 
 var contents = await raf.read(await raf.length());
 print(String.fromCharCodes(contents));
 
 await raf.close();
}

We open a file in read mode, read its entire contents, then close it. The read operation returns bytes which we convert to a string.

$ dart main.dart
File contents here...

Writing to a File

This example demonstrates writing data to a file at specific positions.

import 'dart:io';
void main() async {
 var file = File('output.dat');
 var raf = await file.open(mode: FileMode.write);
 
 await raf.writeFrom([65, 66, 67]); // ABC
 await raf.setPosition(0);
 await raf.writeFrom([68, 69]); // DE
 
 await raf.close();
}

We open a file in write mode, write some bytes, then overwrite the beginning. The setPosition method moves the file pointer to the specified offset.

$ dart main.dart
$ hexdump output.dat
0000000 44 45 43

Seeking and Reading

This example shows how to seek to specific positions and read data.

import 'dart:io';
void main() async {
 var file = File('data.bin');
 var raf = await file.open(mode: FileMode.read);
 
 await raf.setPosition(10);
 var data = await raf.read(4);
 
 print('Data at position 10-13: $data');
 await raf.close();
}

We open a binary file, seek to position 10, then read 4 bytes from that point. This demonstrates random access capabilities of the class.

$ dart main.dart
Data at position 10-13: [1, 2, 3, 4]

File Locking

This example demonstrates file locking during operations.

import 'dart:io';
void main() async {
 var file = File('shared.dat');
 var raf = await file.open(mode: FileMode.append);
 
 await raf.lock();
 try {
 await raf.writeFrom([1, 2, 3]);
 } finally {
 await raf.unlock();
 await raf.close();
 }
}

We acquire an exclusive lock before writing to prevent concurrent access. The lock is released in a finally block to ensure it always happens.

$ dart main.dart

Reading and Writing Structures

This example shows reading and writing structured binary data.

import 'dart:io';
import 'dart:typed_data';
void main() async {
 var file = File('records.dat');
 var raf = await file.open(mode: FileMode.write);
 
 // Write a record
 var buffer = ByteData(8);
 buffer.setInt32(0, 1234);
 buffer.setFloat32(4, 56.78);
 await raf.writeFrom(buffer.buffer.asUint8List());
 
 // Read it back
 await raf.setPosition(0);
 var data = await raf.read(8);
 var reader = ByteData.view(data.buffer);
 
 print('Int: ${reader.getInt32(0)}');
 print('Float: ${reader.getFloat32(4)}');
 
 await raf.close();
}

We write a structured record containing an integer and float, then read it back. ByteData is used to handle different data types in the binary file.

$ dart main.dart
Int: 1234
Float: 56.78

Best Practices

• Always close: Ensure files are closed after use
• Error handling: Use try-catch for file operations
• Buffering: Consider buffering for small writes
• Positioning: Be mindful of position changes

Source

Dart RandomAccessFile Documentation

This tutorial covered Dart's RandomAccessFile class with practical examples showing file operations, positioning, locking, and structured data handling.

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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