Asio C++ Networking Library: Async I/O for C++
chriskohlhoff/asioThe Asio C++ networking library is a cross-platform C++ toolkit for asynchronous network and low-level I/O programming — the standalone, non-Boost version maintained by its original author, Chris Kohlhoff. Instead of blocking on each socket read or timer, you register operations against an io_context event loop and get called back when they finish. It's the same code that also ships as Boost.Asio, and the current release is version 1.38.1 (May 2026).
Reach for Asio if you're writing C++ that needs asynchronous networking or timers and you want a mature, header-only library with a proven completion-handler model — especially if you'd rather not pull in all of Boost. Skip it if you only make a few blocking calls (a thin socket wrapper will do), or if your team hasn't budgeted time to learn the async/callback and, more recently, coroutine style Asio pushes you toward.
The problem it solves
Raw sockets and OS I/O APIs differ on every platform and force you to hand-roll event loops, buffering, and error handling. Doing asynchronous network I/O in C++ portably — one code path that works over Windows IOCP, epoll on Linux, and kqueue on the BSDs — is tedious and easy to get wrong. Asio exists to give C++ one consistent asynchronous model over sockets, timers, and other I/O objects, so you write against completion handlers instead of platform-specific event APIs.
What is it?
The Asio C++ networking library is a header-only C++ library for asynchronous network and low-level I/O programming, maintained by Chris Kohlhoff. It's the standalone form of the same code that ships as Boost.Asio, but without the Boost dependency. At its core you drive an io_context event loop and attach completion handlers — or C++ coroutines — to operations like async_read, async_write, and timers. The README lists version 1.38.1 (released 14 May 2026) and points to think-async.com and a packaged tutorial for the full API.
Why it's getting attention
Asio isn't a flash-in-the-pan project — it's one of the foundational C++ networking libraries, with 6,104 stars and 1,509 forks, and it reaches far beyond its own repo as the basis for Boost.Asio. Interest has stayed steady because C++20 coroutines made its async model much nicer to write (co_await an async operation instead of nesting callbacks), and because Asio has long been the reference implementation behind efforts to standardize networking in C++.
Key features
- ✓A consistent asynchronous model over sockets, timers, and other I/O objects, driven by an io_context event loop
- ✓Completion-handler callbacks plus support for C++ coroutines (co_await) and futures
- ✓Header-only standalone distribution — usable without pulling in the rest of Boost
- ✓Cross-platform, mapping to native OS mechanisms (IOCP, epoll, kqueue) behind one API
- ✓TCP, UDP, and other socket types, timers, and low-level I/O primitives
- ✓The same codebase that ships as Boost.Asio, so knowledge transfers both ways
- ✓API reference and a tutorial hosted at think-async.com and bundled in doc/index.html
Best use cases
- •Building high-concurrency network servers and clients (HTTP, RPC, custom TCP/UDP protocols) in C++
- •Adding non-blocking timers and scheduled work to a C++ application
- •Writing portable async I/O once instead of separate per-platform socket code
- •Serving as the networking layer under higher-level C++ libraries and frameworks
- •Learning the async model before, or instead of, adopting the full Boost.Asio
How to install / try
The README doesn't include install commands — it points you to think-async.com and the packaged doc/index.html for setup, so treat the exact steps as not clearly documented in the repo itself. In practice Asio is distributed as a header-only C++ library (the standalone, non-Boost variant), so you add its include directory to your project; it's also widely packaged, for example via vcpkg or Conan. Check think-async.com for the current, authoritative instructions.
How to use
Detailed usage lives in the tutorial the README links at think-async.com and in the bundled doc/index.html, so the specifics are documented there rather than in the repo README. The usual shape: create an io_context, create I/O objects (a socket, a timer) bound to it, start asynchronous operations with a completion handler — or co_await them from a coroutine — then call io_context::run() to drive the event loop until the outstanding work finishes.
Strengths
- ✓Mature and battle-tested — the same code underlies Boost.Asio and has years of production use behind it
- ✓One async API across platforms, so you're not writing separate IOCP, epoll, and kqueue code paths
- ✓Header-only standalone build lets you use it without adopting all of Boost
- ✓Works with modern C++ coroutines, so async flows read as co_await instead of nested callbacks
- ✓Documented with an official tutorial and API reference at think-async.com
Limitations & risks
- △Steep learning curve — the asynchronous/completion-handler model, executors, and object-lifetime rules take real time to get right, and callback-heavy code gets hard to follow without coroutines
- △The license isn't listed in this repo's metadata (it shows as unknown here), so confirm the terms at think-async.com before you depend on it
- △Header-only means heavier template-driven builds and longer compile times as your usage grows
- △It's a low-level building block, not a full framework — there's no built-in HTTP, and TLS comes through a separate OpenSSL-backed layer, so you assemble higher-level protocols yourself
- △Install and setup aren't spelled out in the README; you have to go to think-async.com for authoritative steps
Alternatives
Who should try it — and who should skip
Choose Asio if you're a C++ developer building networked services, or anything that needs portable asynchronous I/O and timers, and you want a mature, header-only library without committing to all of Boost. It's a sensible base if you're writing your own protocol layer. It's overkill if your program only makes a handful of blocking calls, and it will frustrate a team that hasn't set aside time to learn the async and coroutine model.
Frequently asked questions
It's a header-only C++ library for asynchronous network and low-level I/O programming, maintained by Chris Kohlhoff. You drive an io_context event loop and attach completion handlers or coroutines to operations like socket reads, writes, and timers.
They share the same codebase. Asio is the standalone version with no Boost dependency; Boost.Asio is that code packaged and versioned inside Boost. Skills and code transfer directly between them.
Yes. Beyond callback-style completion handlers, Asio works with C++ coroutines, so you can co_await an asynchronous operation instead of nesting callbacks. See the tutorial at think-async.com.
The README doesn't give install commands — it points to think-async.com and the bundled doc/index.html. In practice it's a header-only library you add to your include path, and it's widely packaged via tools like vcpkg and Conan.
The README lists version 1.38.1, released 14 May 2026. Check think-async.com for the latest release and the full API documentation.