Up until now, we've mostly let the Kotlin runtime handle the heavy lifting of memory. Whether you're on the JVM or using the new garbage collector in Kotlin/Native, you generally don't think about where an object lives or when it dies. But when we start dipping our toes into C-interop or performance-critical native code, that safety net disappears. I want to show you exactly where the "managed" world ends and the "manual" world begins.

Playing with the Native Heap

I'm going to start by doing something a bit reckless. Let's say I need to allocate a piece of memory that isn't managed by the Kotlin GC—maybe I'm passing a buffer to a C library that expects to own the memory for a while. I'll use nativeHeap.

import kotlinx.cinterop.*

fun leakSomething() {
    val buffer = nativeHeap.allocArray<ByteVar>(1024) 
    // I'm using this buffer for some "work"
    buffer[0] = 42.toByte()
    println("Allocated 1KB on the native heap. Value: ${buffer[0]}")
}

If I run this in a loop a few thousand times, I'll notice my process memory climbing steadily. Why? Because nativeHeap is exactly what it sounds like: the raw system heap. The Kotlin GC has no visibility into this. It doesn't know that buffer is a pointer to 1024 bytes; it only sees a small pointer object on the managed heap. When buffer goes out of scope, the pointer is gone, but the 1024 bytes stay allocated. I've just created a classic memory leak.

The Leak I Just Created

To fix this, I have to be explicit. I need to tell the system when I'm done. I'll adjust the code to manually free the memory.

fun fixTheLeak() {
    val buffer = nativeHeap.allocArray<ByteVar>(1024)
    try {
        buffer[0] = 42.toByte()
    } finally {
        nativeHeap.free(buffer)
    }
}

This works, but it's tedious. It feels like writing C in 1989. If I'm just doing a quick operation—like converting a string to a C-style pointer for a function call—I don't want to wrap everything in try-finally blocks. This is where memScoped comes in.

Letting memScoped Do the Heavy Lifting

Let's try a different approach. Kotlin/Native provides a memScoped block that creates a short-lived arena. Any allocations made inside this block are automatically freed when the block exits.

fun efficientAllocation() {
    memScoped {
        val buffer = allocArray<ByteVar>(1024) // Note: no 'nativeHeap' prefix
        buffer[0] = 42.toByte()
        // Use buffer here...
    } 
    // Memory is automatically reclaimed here.
}

I love this pattern because it's deterministic. I know exactly when that memory is gone. However, there's a catch: if I try to return buffer from the memScoped block, I'm returning a pointer to memory that has already been freed. If you try to access it outside the block, you're headed straight for a segmentation fault. Use memScoped for transient data, not for long-lived state.

The Pinning Puzzle

Now, let's look at the opposite problem. What if I have a standard Kotlin ByteArray (managed by the GC) and I want to pass it to a C function? The C function expects a stable memory address. But the Kotlin GC can move objects around in memory to defragment the heap. If the GC moves my array while the C function is reading it, the C function will be reading garbage.

I tried passing a raw pointer to a Kotlin array once, and it worked 99% of the time. Then, on a larger dataset, it crashed randomly. That's the "GC move" in action. To stop this, I have to "pin" the object.

fun passToC(data: ByteArray) {
    data.usePinned { pinned ->
        val address = pinned.addressOf(0)
        // Now 'address' is stable. The GC is forbidden 
        // from moving 'data' until the block ends.
        callSomeCFunction(address)
    }
}

By using usePinned, I'm essentially telling the memory manager: "Hold this object right here; don't touch it until I'm done." Once the block exits, the object is unpinned and the GC is free to move it again. It's a critical bridge between the flexible, managed world of Kotlin and the rigid, static world of native memory.

📋 Practical Task

Exercise: Implementing a Stable Buffer Bridge

You are building a wrapper for a native C library that processes audio samples. The library requires a pointer to a buffer of Float values. If the buffer moves during processing, the audio will glitch or the app will crash.

Create a function called processAudioSamples that takes a FloatArray. Inside this function, implement the following:

• Use usePinned to ensure the FloatArray remains stationary in memory.
• Within the pinned block, obtain the raw address of the first element using addressOf(0).
• Simulate a C-library call by printing the hex string of the stable address to the console.
• Create a secondary, short-lived buffer of 10 ByteVar using memScoped to act as a "status flag" for the C library, and print the address of that flag as well.

Verify that the memory for the status flag is handled automatically by memScoped, while the audio buffer is handled via pinning.