java.nio.ByteBuffer

While passing java.nio.ByteBuffer is not possible because such type does not exist on Native, the buffers module provides an efficient wrapper.

Use com.dshatz.kni.buffers.ByteBuffer in function signatures:

Setup

Add the dependency:

implementation("com.dshatz.kni:buffers:2.1.0")

Buffers module provides a convenient and efficient wrapper around byte buffers for all platforms.

Allocating / releasing

val buffer: ByteBuffer = allocateBuffer(1024)
buffer.release()

Reading / writing

val bytes: ByteArray = Random.nextBytes(buffer.capacity.toInt())
buffer.write(bytes)

val bytes: ByteArray = buffer.readToByteArray(0, 1024)

Properties

The following properties are available on the ByteBuffer:

Property	Platforms
`capacity: Long`	All
`jvmBuffer: java.nio.ByteBuffer`	JVM/Android
`val address: CPointer<ByteVar>`	Native

Passing to JNI

See the general guide: ➡️ Calling Native code from JVM

import com.dshatz.kni.buffers.ByteBuffer

// commonMain
@JniCall
expect fun fillBuffer(buffer: ByteBuffer): String

// Native
actual fun fillBuffer(buffer: ByteBuffer): String {
    val bytes = Random.nextBytes(buffer.capacity.toInt())
    buffer.put(bytes)
    return bytes.toHexString()
}

Call from commonMain:

// commonMain
import com.dshatz.kni.buffers.allocateBuffer

val buffer = allocateBuffer(1024)
val filledHex = fillBuffer(buffer) // call native
val contents = buffer.readToByteArray(0, 1024) // read contents
// filledHex == contents.toHexString()

Converting

JVM

val jvmBuffer = java.nio.ByteBuffer.allocateDirect(1024)
val buffer = jvmBuffer.toCommonByteBuffer()
buffer.jvmBuffer == jvmBuffer

Native

On native, this is a wrapper around a pointer.

Wrap ByteArray

val bytes = Random.nextBytes(100)
val buffer = ByteBuffer.wrapArray(bytes)

Wrap address in memScoped

// wrap a memory address
memScoped {
    val arr = allocArray<ByteVar>(1024)
    val buffer = ByteBuffer.wrapAddressMemScope(this, arr, 1024)
}

JS/Wasm

val buffer = allocateBuffer(1024)
buffer.asInt8Array()
buffer.toBlob()

Last modified: 23 April 2026