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kotlinx-io is a multiplatform library for processing binary data, working with memory blocks, interacting with the platform, and performing other low level operations.

Experimental Download

NOTE: master contains version 0.2.x and is experimental. Any API is a subject to change. If you're looking for multiplatform releases of older versions, you can check ktor-io. See for more details.


NOTE: This part represents the target vision of the package. Most modules are still unavailable.

kotlinx-io package has many modules and you can use as much as you need.

  • core – defines all the low-level functionality for working with binary data and basic text.
  • async – (unavailable) implements asynchronous versions of input and output data streams.
  • platform – (unavailable) provides low-level platform facilities such as reading or writing from/to a file.
  • sockets – (unavailable) provides low-level functionality for interacting with network.
  • cryptography – (unavailable) provides encryption & decryption functionality.
  • compression – (unavailable) provides compression & decompression functionality.
  • files – (unavailable) provides advanced file system functionality such as working with paths and directories.


This module provides few core I/O primitives that are used across other modules and can be used to define custom binary inputs and outputs, as well as processing raw memory.

  • Buffer – represents a continuous memory block of specific size. Provides direct positional read and write operations for primitives and arrays of primitives.
  • Input – represents a source of bytes. Provides sequential reading functionality and a special preview mode for processing bytes without discarding them.
  • Output – represents a destination for bytes. Provides sequential writing functionality.
  • Bytes – represents binary data of arbitrary size, potentially spanned across several buffers. Can be built using using Output and can be read using Input.

It also has basic facilities for working with text, with UTF-8 implemented efficiently in core, and Charsets giving access to platform-dependent functionality for converting text into bytes and back.


Buffer is direct representation of memory on the target platform implemented using efficient platform-dependent mechanisms. A buffer of arbitrary size can be allocated and released using PlatformBufferAllocator. It is user's responsibility to release an allocated buffer.

    val buffer = PlatformBufferAllocator.allocate(8) // allocates a buffer of 8 bytes
    buffer.storeLongAt(0, 123451234567890L) // stores a long value at offset 0
    val longValue = buffer.loadLongAt(0) // reads back a long value

All operations with a Buffer are performed in network byte order (Big-Endian). There are helper functions reverseByteOrder defined for all primitive types to reverse the byte order when it is needed.


An Input is a high-performance buffered entity for reading data from an underlying source.
It is an abstract class with only few abstract methods and a plentiful of convenience built around them. There are functions to read primitives, arrays of primitives, higher-level extension methods for reading UTF-8 text, text encoded with a custom Charset, and more. One can define any other read methods using extensions and provided primitives.

Input design doesn't provide facilities for direct manipulation of the current reading position, but instead it has the preview mechanism which we believe is a lot safer, efficient and enough for most look-ahead scenarios.

Preview operation instructs Input to start accumulating buffers instead of discarding them when they are exhausted, thus making it possible to revert to the initial position without performing additional I/O operations.

    input.readLong() // (0) reads long value and discards bytes
    input.preview {  // (1) begins preview operation and stops discarding bytes
        readShort() // (2) read short value and keep the bytes
    } // completes preview operation and rewinds the input to the state (1) 
    input.readShort()  // (3) reads short value from (2) again

Note that preview function provides another, nested Input to the lambda as a receiver which should be used for all preview reads. Implementation can choose to alter original Input state or create a new instance, so one should always be using the instance provided as a receiver to preview.

Preview operations can be nested, each keeping its own state and position, thus making it possible to compose operations on Inputs.


An Output is a high-performance buffered entity for writing data to an underlying destination. Like Input, it provides all the primitive operations as well as a number of convenience functions for text output.

Similarly, Output doesn't provide a mechanism to rewind backwards and update data, but using Bytes one can easily implement complex scenarios such as writing a size before a block, calculating hashes and so on.


A Bytes type is useful for transferring data between various endpoints, accumulating data in memory or sending repetitive bytes to different outputs.

Bytes can be produced by building function buildBytes { … } where lambda has an Output as a receiver, thus making it possible to conveniently generate content, or use in any I/O operations or custom user's functions.

    val bytes = buildBytes {

When you have a Bytes instance, you can know the number of bytes stored, and can obtain an Input to read these bytes. Creating an Input is a zero-copy operation, underlying mechanics simply reuses buffers for reading data.

    val input = bytes.input()

Writing such an instance into Output is also zero-copy operation, since implementation will send existing buffers to the underlying destination.


Combining these features makes it possible to write domain-specific functions for complex data writing:

fun Output.writeWithSizeAndHash(writer: Output.()->Unit) {
    val bytes = buildBytes(writer)
    val hash = bytes.input().calculateHash()


[TBD] Efficient UTF-8 and platform-dependent Charsets


[TBD] Allocating and releasing a buffer each time one is needed can be inefficient, so the package provides facilities for buffer pools.


[TBD] InputChannel and OutputChannel as an asynchronous (suspending) versions of Input and Output



  • FileInput and FileOutput with a very limited set of operations such as open. No paths, no directories, no access control.
  • Process type for launching an external processes, and interacting with their inputs and outputs.
  • Environment type for interacting with environment variables.

Adding a dependency

dependencies {
    compile "org.jetbrains.kotlinx:kotlinx-io-jvm:$kotlinx_io_version"
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