--- name: "boost-asio-pro" description: "Use when writing or reviewing asynchronous C++ networking code with Boost.Asio or standalone Asio — TCP/UDP servers and clients, SSL/TLS, timers, strands, io_context, co_spawn, awaitable, async_read/async_write, asio::spawn, yield_context, or pre-C++20 completion-handler callbacks." --- # Boost.Asio / standalone Asio ## Overview Write async C++ networking code that compiles on the *user's* Boost, not the newest one. Asio's API changed shape three times (classic `io_service` → `io_context` → C++20 coroutines) and most Asio code on the internet is from the first era, so **pick the style from the toolchain first**, then follow that style's reference file. **References:** [Boost.Asio](https://www.boost.org/doc/libs/latest/doc/html/boost_asio.html) · [standalone Asio](https://think-async.com/Asio/) Use this skill whenever async C++ networking code is being written or reviewed — and especially when the target toolchain is old, where coroutine examples simply will not compile. The three worked implementations it references are CI-verified from Boost 1.62 (2016) through 1.90. ## Step 1: pick the style (do this before writing code) Determine the Boost (or Asio) version and the C++ standard actually in use — `find_package(Boost)` output, `dpkg -l libboost-dev`, `brew info boost`, `CMAKE_CXX_STANDARD`, or ask. Do not assume the newest. | Boost | C++ std | Style | Read | |-------|---------|-------|------| | ≥ 1.77 | C++20 | Coroutines (`co_await` + `awaitable`) — preferred | [references/coroutines.md](references/coroutines.md) | | ≥ 1.74 | C++11–17 | Completion handlers (callbacks) — the portable baseline | [references/pre-cpp20.md](references/pre-cpp20.md) | | ≥ 1.80 | C++11–17 | Stackful `asio::spawn` + `yield_context` (links Boost.Coroutine — not header-only) | [references/pre-cpp20.md](references/pre-cpp20.md) | | 1.62–1.65 | C++11 | Classic `io_service` / `strand.wrap` / `expires_from_now` | [references/classic-boost.md](references/classic-boost.md) | SSL/TLS in any style: [references/ssl.md](references/ssl.md). CMake for any style: [references/build.md](references/build.md). `io_context`, `make_strand`, `bind_executor`, `steady_timer`, `signal_set`, `async_read`/`async_write`/`async_read_until`, buffers and `resolver` are **library** features — identical in the coroutine and callback styles. Only the suspension mechanism differs. ## Step 2: version floors (verified by compiling, not from docs) Reach for one of these and the build breaks on older distros: | Feature | Floor | |---------|-------| | `experimental/awaitable_operators.hpp` (the `\|\|` / `&&` operators) | **Boost ≥ 1.77** / Asio ≥ 1.20 | | `as_tuple` completion token | **Boost ≥ 1.79** / Asio ≥ 1.21 | | `co_composed` (custom composed ops) | **Boost ≥ 1.85** / Asio ≥ 1.30 | | 3-arg `asio::spawn(ex, fn, token)` | **Boost ≥ 1.80** (older Boost has only `spawn(ex, fn)`) | | `any_io_executor` (`strand`, `tcp::socket`'s default executor) | **Boost ≥ 1.74** — the floor for the callback style; below it, use legacy `io_context::strand` | | `io_context`, `make_strand`, `expires_after` | **Boost ≥ 1.66** — below it, classic `io_service` | Distro floors that bite: **Debian bookworm ships Boost 1.74** (no `awaitable_operators.hpp` — `#include` fails outright), Ubuntu 20.04 ships 1.71 (no `any_io_executor`), Debian 9 ships 1.62. Language, not library: the chrono literals `250ms` / `30s` are **C++14**. For a true C++11 build write `std::chrono::milliseconds(250)`. ## Step 3: the rules that are actually easy to get wrong **A strand does not serialize writes.** A strand serializes handler *execution*, not whole composed operations. Two `async_write`s in flight on the same strand still **interleave bytes on the wire**. Full-duplex (a read loop plus concurrent pushes/replies) needs a per-connection strand **and** an outbound queue with an in-flight flag, so at most one `async_write` exists at a time. This is the single most common wrong answer about Asio. **Buffers do not own memory.** `asio::buffer()` is a view. Storage must outlive the operation: coroutine locals are fine across `co_await` in the same frame; in callback style the same data must become a **member**, not a local. **Connections must outlive their handlers.** `enable_shared_from_this`, and capture `self` in *every* `co_spawn` / handler — read loop, write loop, and each timer. **Frame with composed reads.** `async_read` (fills the buffer exactly) for a length prefix and then the body; never `async_read_some`, which returns short. **Wrap `as_tuple`.** Always `as_tuple(use_awaitable)`. Bare `as_tuple` resolves against the operation's default token and compiles in some contexts, fails in others. **`async_accept(make_strand(...))` changes two things**: it forces an explicit completion token back on the call, and the accepted socket is `basic_stream_socket>`, not `tcp::socket`. Take it **by value** or with `auto` — binding it to `tcp::socket&` will not compile. **Re-arming a timer resolves the pending wait with `operation_aborted`.** In an idle-timeout loop that is the signal to keep waiting, not an error. **GCC needs `-fcoroutines`** for the C++20 style, and header-only Boost needs `BOOST_ERROR_CODE_HEADER_ONLY` defined in exactly one place (CMake). ## Anti-Patterns | Mistake | Fix | |---------|-----| | Buffer dangling (local goes out of scope during async op) | Ensure buffer lifetime ≥ operation lifetime; coroutine locals or members, not callback locals | | Forgetting `io.run()` | No handlers dispatch without `run()` / `run_one()` | | Concurrent socket access without strand | Wrap in `strand<>` or serialize via one coroutine chain | | Assuming a strand prevents interleaved writes | Add a write queue — see Step 3 | | Using `use_awaitable` where `deferred` suffices | Omit the token (default is `deferred`) unless using `\|\|` / `&&` | | Ignoring short reads/writes | Use composed `async_read` / `async_write` / `async_read_until`, not `async_read_some` | | Not setting `reuse_address` on the acceptor | Set before `bind`/`listen` or restarts hit "address in use" | | SSL operations without a strand | *All* `ssl::stream` ops need strand synchronization | | Blocking inside a handler | Never block in a completion handler | | Accepting a socket with the wrong executor type | See `async_accept(make_strand(...))` in Step 3 | | Requiring the `Boost::system` component | Header-only since 1.74: `Boost::headers` + `BOOST_ERROR_CODE_HEADER_ONLY`. Only classic (pre-1.66) needs the link | | Missing `-fcoroutines` on GCC | Build fails — add `$<$:-fcoroutines>` | | Writing coroutine code for a Boost that predates it | Do Step 1 first | ## Boost.Asio vs standalone Asio Same author, same API — namespace and includes differ. | Aspect | Boost.Asio | Standalone Asio | |--------|-----------|-----------------| | Namespace / include | `boost::asio` / `` | `asio` / `` | | Error code | `boost::system::error_code` | `asio::error_code` (or `std::error_code`) | | Install (brew) | `brew install boost` | `brew install asio` | | CMake | `Boost::headers` | manual include path | | Version (2025) | 1.87–1.90 (with Boost) | 1.30–1.36 (independent) | | Macro prefix | `BOOST_ASIO_` | `ASIO_` | Support both with a shim, then use `net::` throughout: ```cpp #ifdef USE_STANDALONE_ASIO #include namespace net = asio; using error_code = asio::error_code; #else #include namespace net = boost::asio; using error_code = boost::system::error_code; #endif namespace ssl = net::ssl; using tcp = net::ip::tcp; ``` ## Before you call it done Check the code you just wrote against this list: - [ ] Style matches the target Boost version and C++ standard (Step 1), and every API used clears its floor (Step 2). - [ ] Every buffer passed to an async op outlives that op — no callback locals, no dangling `string_view`. - [ ] At most one `async_write` per socket in flight, enforced by a queue + flag, if anything writes concurrently with reading. - [ ] Every async chain on a shared object runs on the same strand; `self` captured in every handler and `co_spawn`. - [ ] Framing / delimited reads use composed `async_read` / `async_read_until`. - [ ] Errors are handled, not swallowed: `as_tuple(use_awaitable)` destructured, or the callback's `ec` checked, on every op. - [ ] `operation_aborted` distinguished from real errors wherever a timer is re-armed or an op is cancelled. - [ ] Acceptor sets `reuse_address`; shutdown path closes the acceptor and drains sessions. - [ ] CMake has the standard, `-fcoroutines` for GCC (C++20 only), `BOOST_ERROR_CODE_HEADER_ONLY` in one place, and `Boost::coroutine` only if using stackful `spawn`. - [ ] It compiles. Build it — most of the mistakes above are compile-time, and the version floors are only real once tested. ## Worked examples Three CI-verified implementations of the same full-duplex framed-protocol server, one per style — copy from the one matching Step 1. All three live in the upstream repository and are built by CI on every push. - [market-data-feed](https://github.com/alexprivalov/boost-asio-skill/tree/main/examples/market-data-feed) — C++20 coroutines (Boost 1.77+; verified 1.83–1.90) - [market-data-feed-precpp20](https://github.com/alexprivalov/boost-asio-skill/tree/main/examples/market-data-feed-precpp20) — callbacks, C++11-clean (verified Boost 1.74+, incl. Windows/MSVC) - [market-data-feed-classic](https://github.com/alexprivalov/boost-asio-skill/tree/main/examples/market-data-feed-classic) — classic `io_service` (verified back to Boost 1.62 / Debian 9) ## Official documentation - Overview: https://www.boost.org/doc/libs/latest/doc/html/boost_asio/overview.html - Reference: https://www.boost.org/doc/libs/latest/doc/html/boost_asio/reference.html - Examples: https://www.boost.org/doc/libs/latest/doc/html/boost_asio/examples.html ## Cross-References - `engineering/docker-development` — the old-Boost verification lanes this skill's floors come from are containerised builds (Debian 9 / bookworm, Fedora). - `engineering/chaos-engineering` — for exercising the failure paths this skill tells you to handle: half-open sockets, idle timeouts, partial frames. - `engineering-team/playwright-pro` — the client-side counterpart when the server built here is driven from browser-based integration tests.