Project 2: Virtual Loopback Device (Linux Kernel Module)

A kernel module that creates a virtual sound card—audio written to its output appears on its input, like a software audio cable.

Quick Reference

Attribute Value
Primary Language See main guide
Alternative Languages N/A
Difficulty See main guide
Time Estimate See main guide
Knowledge Area See main guide
Tooling See main guide
Prerequisites C programming, basic kernel module experience, completed Project 1

What You Will Build

A kernel module that creates a virtual sound card—audio written to its output appears on its input, like a software audio cable.

Why It Matters

This project builds core skills that appear repeatedly in real-world systems and tooling.

Core Challenges

  • Implementing the ALSA driver interface (snd_pcm_ops)
  • Creating a device that appears in aplay -l alongside real hardware
  • Managing shared ring buffers between playback and capture streams
  • Handling timing without real hardware clocks (using kernel timers)

Key Concepts

  • Map the project to core concepts before you code.

Real-World Outcome

# Load your module
$ sudo insmod my_loopback.ko

# Check that it appeared in the system
$ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: PCH [HDA Intel PCH], device 0: ALC892 Analog [ALC892 Analog]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: MyLoopback [My Virtual Loopback], device 0: Loopback PCM [Loopback PCM]
  Subdevices: 8/8
  Subdevice #0: subdevice #0
  Subdevice #1: subdevice #1
  ...

# Your virtual sound card appears as card 1!

$ cat /proc/asound/cards
 0 [PCH            ]: HDA-Intel - HDA Intel PCH
                      HDA Intel PCH at 0xf7210000 irq 32
 1 [MyLoopback     ]: my_loopback - My Virtual Loopback
                      My Virtual Loopback

# Check the kernel log for your initialization messages
$ dmesg | tail -5
[12345.678901] my_loopback: module loaded
[12345.678902] my_loopback: registering sound card
[12345.678903] my_loopback: creating PCM device with 8 subdevices
[12345.678904] my_loopback: card registered successfully as card 1

Implementation Guide

  1. Reproduce the simplest happy-path scenario.
  2. Build the smallest working version of the core feature.
  3. Add input validation and error handling.
  4. Add instrumentation/logging to confirm behavior.
  5. Refactor into clean modules with tests.

Milestones

  • Milestone 1: Minimal working program that runs end-to-end.
  • Milestone 2: Correct outputs for typical inputs.
  • Milestone 3: Robust handling of edge cases.
  • Milestone 4: Clean structure and documented usage.

Validation Checklist

  • Output matches the real-world outcome example
  • Handles invalid inputs safely
  • Provides clear errors and exit codes
  • Repeatable results across runs

References

  • Main guide: AUDIO_SOUND_DEVICES_OS_LEARNING_PROJECTS.md
  • Primary references are listed in the main guide