When we think about Android, we usually picture apps, screens, and the user interface. But underneath all of that lies a powerful system that allows apps to interact with the actual hardware — the camera, GPS, Bluetooth, sensors, and more.

One key player in this hidden world is HIDL, short for HAL Interface Definition Language. If you’re wondering what it is and how it works, don’t worry. We’re going to break it down in the simplest way possible.

What is HIDL in Android?

HIDL stands for Hardware Abstraction Layer Interface Definition Language. It was introduced in Android 8 (Oreo) to help the Android operating system talk to the hardware in a clean, modular way.

Imagine HIDL as a translator between the Android framework and device-specific hardware implementations. This ensures that the Android OS doesn’t need to know exactly how a particular chip or sensor works — it just knows how to ask for something in a standardized way.

Why HIDL Was Introduced

Before HIDL, Android used a more rigid and less flexible HAL (Hardware Abstraction Layer) structure written in C/C++. This created challenges:

• Difficult upgrades: Updating Android required reworking low-level drivers.
• Vendor lock-in: Device manufacturers had to heavily modify AOSP (Android Open Source Project) to support their hardware.
• Lack of modularity: Everything was tightly coupled.

HIDL changed that by enabling a stable interface between Android and the hardware, allowing manufacturers to update Android without rewriting HALs every time.

How HIDL Works Under the Hood

Let’s walk through what HIDL actually does in a real-world Android device.

1. Interface Definition

The first step is defining the HIDL interface. This is written using the .hal file format — a simple syntax that defines what services or data types the hardware provides.

Here’s a basic HIDL interface example for a hypothetical LED controller:

package vendor.softaai.hardware.led@1.0;

interface ILed {
    setLedValue(int32_t value) generates (bool success);
    getLedValue() generates (int32_t value);
};

What this does:

• Defines a package version (@1.0)
• Declares two methods: setLedValue() and getLedValue()
• Uses generates to define what response each method will return

2. Interface Compilation

This .hal file is then compiled using the HIDL compiler (hidl-gen) into two parts:

• Stub code: For the vendor to implement (the actual driver logic)
• Proxy and binder code: For the Android framework to call into

This code is placed in a shared location so both the system and vendor sides can use it.

3. Service Implementation

On the vendor side, the manufacturer writes the actual code that controls the hardware.

Example (pseudo-code in C++):

Return<bool> Led::setLedValue(int32_t value) {
    // Code to control actual LED hardware
    if (writeToLedDriver(value)) {
        return true;
    }
    return false;
}

This implementation is then registered as a service:

int main() {
    android::sp<ILed> service = new Led();
    configureRpcThreadpool(1, true);
    service->registerAsService();
    joinRpcThreadpool();
}

4. Calling the HAL from Android Framework

On the framework side, Android can call this interface via the Binder IPC mechanism.

A Java service in Android might look like:

ILed ledService = ILed.getService();
ledService.setLedValue(1); // Turns on the LED

The magic here is that the Java code doesn’t need to know the internal details — just that there’s a standard way to talk to the hardware. That’s the power of HIDL.

HIDL vs AIDL: What’s the Difference?

You might also hear about AIDL (Android Interface Definition Language). Here’s the key difference:

How HIDL Fits into Treble Architecture

In Android 8, Google introduced Project Treble — a major rearchitecture of Android to separate the hardware implementation from the Android OS framework. HIDL is the core part of this architecture.

With Treble:

• Vendors implement HALs using HIDL
• Android OS uses the stable HIDL interface to communicate
• Devices can receive Android updates faster, since the hardware interface doesn’t change

Real-Life Example: Camera HAL with HIDL

Let’s say you’re using the camera app. Here’s how HIDL helps:

1. The Camera app calls the Camera API in the Android framework.
2. The framework uses the HIDL-defined interface to talk to the Camera HAL.
3. The Camera HAL interacts with the actual camera sensor and returns the image data.
4. You see the photo.

This whole chain happens seamlessly — thanks to HIDL’s modular structure.

HIDL Code Directory Structure in AOSP

If you’re exploring AOSP, HIDL-related files are found in:

hardware/interfaces/
    └── camera/
         └── 1.0/
              └── ICameraDevice.hal

You’ll also see versioned directories (like 1.0, 1.1, etc.) because HIDL supports backward-compatible interface upgrades — a big win for long-term Android support.

Benefits of HIDL (At a Glance)

• Modularity: Separates hardware and OS layers
• Reusability: Code can evolve without breaking the interface
• Stability: A stable contract between vendor and framework
• Faster Updates: Key part of Project Treble for quicker Android upgrades
• Security: HIDL uses Binder, which is robust and secure

Wrapping Up: Why HIDL Still Matters

HIDL might not be something end-users see, but it’s critical for Android’s stability, modularity, and long-term ecosystem health. For developers, it provides a clean, structured, and maintainable way to support hardware — without tying the Android framework to specific implementations.

As Android continues to evolve (especially with newer HAL models like AIDL for newer HALs), HIDL remains a foundational piece for millions of devices globally.

So the next time your phone’s camera, flashlight, or fingerprint sensor works perfectly — remember there’s a humble HIDL interface making it all happen behind the scenes.

Bonus Tips for Android Developers

• Use hidl-gen to compile your .hal files.
• Test your HIDL services using vts (Vendor Test Suite).
• Keep interfaces versioned and backward compatible.
• Consider migrating to AIDL-based HALs for new projects (Android 10+ recommendation).