Mahmoud Abas, Full-Stack Software Engineer & AI

Operating Systems 1 (2025-2026) - Operating System Lab - Week 6

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Operating Systems 1 - Week 06 Lab

Memory-Mapped Files

Treating a file on disk as if it were part of your own memory, and sharing that region between processes.

What Are Memory-Mapped Files

A memory-mapped file: A direct mapping between a file on disk and a region in memory, so the file's data is reached like ordinary memory instead of through repeated read and write calls.

Several processes can share the same mapped region to exchange data
And a very large file can be worked with by loading only the parts actually needed

Key Advantages

Advantage	Why it matters
Fast access	direct memory access beats traditional file I/O
Random access	jump to any position in the file instantly
Shared between processes	two programs can map the same region
Efficient for large files	only the needed portions are loaded

Creating a Memory-Mapped File

csharp

using System.IO.MemoryMappedFiles;

var mmf = MemoryMappedFile.CreateFromFile(
    filePath, FileMode.OpenOrCreate, null, fileSize);

The path, the FileMode, an optional mapping name, and the size of the region to reserve.

Sizing the Mapped Region

csharp

long fileSize = 1024 *  // = 1 KB
                1024 *  // = 1 MB
                100;    // = 100 MB

The size is in bytes, so it is usually written as the multiplication that produces it.

Writing Through a View Accessor

csharp

using var accessor = mmf.CreateViewAccessor();
string dataToWrite = "Hello, Memory-Mapped Files!";
byte[] dataBytes = Encoding.UTF8.GetBytes(dataToWrite);
accessor.WriteArray(30, dataBytes, 0, dataBytes.Length);

WriteArray takes the position in the file, the data, and the offset and count inside that data.

Reading Through a View Accessor

csharp

byte[] readData = new byte[2];
accessor.ReadArray(34, readData, 0, 2);
string readDataString = Encoding.UTF8.GetString(readData);

Reading Back What You Wrote

The string above was written starting at position 30
Reading two bytes from position 34 lands inside that string
So it returns the characters at exactly that offset, not the start of the text
This byte-precise addressing is what suits structured and random-access data

Practice Exercises

Write and read at different positions in the same file
Build two separate programs that communicate through one shared mapping
Implement a cross-platform process launcher that detects the operating system itself
Try several file sizes and measure the performance difference

Key Takeaways

A memory-mapped file lets an application treat a disk file as part of its own memory
CreateFromFile opens or creates the file and reserves the size of the mapped region
A view accessor gives WriteArray and ReadArray at any byte position
The gains are speed, random access and efficient communication between processes
The technique earns its keep on large files, where loading everything would be wasteful

Open this lesson

Mahmoud AbasOperating System Lab - Week 6

Advantage

Why it matters

Fast access

direct memory access beats traditional file I/O

Random access

jump to any position in the file instantly

Shared between processes

two programs can map the same region

Efficient for large files

only the needed portions are loaded

using var accessor = mmf.CreateViewAccessor(); string dataToWrite = "Hello, Memory-Mapped Files!"; byte[] dataBytes = Encoding.UTF8.GetBytes(dataToWrite); accessor.WriteArray(30, dataBytes, 0, dataBytes.Length);