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A multiplatform Kotlin library providing basic IO primitives. kotlinx-io is based on Okio but does not preserve backward compatibility with it.


kotlinx-io is built around Buffer - a mutable sequence of bytes.

Buffer works like a queue, allowing to read data from its head or to write data to its tail. Buffer provides functions to read and write data of different built-in types, and to copy data to or from other Buffers. Depending on the target platform, extension functions allowing data exchange with platform-specific types are also available.

A Buffer consists of segments organized as a linked list: segments allow reducing memory allocations during the buffer's expansion and copy, with the latter achieved by delegating or sharing the ownership over the underlying buffer's segments with other buffers.

kotlinx-io provides interfaces representing data sources and destinations - Source and Sink, and in addition to the mutable Buffer the library also provides an immutable sequence of bytes - ByteString.

An experimental filesystem support is shipped under the package, which includes the FileSystem interface and its default implementation - SystemFileSystem.

FileSystem provides basic operations for working with files and directories, which are represented by yet another class under the same package - Path.

There are two kotlinx-io modules:

Using in your projects

Note that the library is experimental, and the API is subject to change.


Make sure that you have mavenCentral() in the list of repositories:

repositories {

Add the library to dependencies:

dependencies {

In multiplatform projects, add a dependency to the commonMain source set dependencies:

kotlin {
    sourceSets {
        commonMain {
            dependencies {


Add the library to dependencies:



kotlinx-io is not tested on Android on a regular basis, but the library is compatible with Android 5.0+ (API level 21+).


Read the Contributing Guidelines.

Code of Conduct

This project and the corresponding community are governed by the JetBrains Open Source and Community Code of Conduct. Please make sure you read it.


kotlinx-io is licensed under the Apache 2.0 License.


Thanks to everyone involved in the project.

An honorable mention goes to the developers of Okio that served as the foundation for kotlinx-io and to Jesse Wilson, for the help with Okio adaption, his suggestions, assistance and guidance with kotlinx-io development.