What WebAssembly is

Level: 201 · working knowledge

One line: A .wasm file is a module of bytecode for a stack machine that can reach nothing it was not given — every capability arrives as an import and every entry point leaves as an export — which is why the same file can run in a browser, on a server and inside another program.

Stub — an outline, not a lesson. There is no runnable example behind this page yet, so nothing on it has been through the check that backs every other claim in this library. The bullets below are the questions the finished page has to answer.

What it has to cover

• The file: an eight-byte preamble — \0asm, then version 1 — followed by sections, among them types, imports, functions, memories, globals, exports and code. Node 20.20.2's WebAssembly.validate accepts those eight bytes alone as a complete, empty module, and rejects them with the version byte changed to 2.
• The sandbox as a property of the format rather than a policy: code reads and writes linear memory — raw bytes addressed by offsets, bounds-checked — and has no way to name an address in the host. A buffer overflow inside a module is still possible — so what can it corrupt, and what can it not?
• Imports and exports as the entire interface. What makes a Rust function an export, and what does a module that imports nothing — the wasm32v1-none idea — have left to do besides compute?
• The core specification's value types are number types (i32, i64, f32, f64), vector types for SIMD, and reference types: no strings, no structs. How does a Rust &str cross — a pointer and a length into linear memory — and who agrees on that convention?
• Not only for browsers: Wasmtime ↗ runs rustc hello.rs --target wasm32-wasip1 output from a terminal, and WASI is a set of standard host functions for exactly that — see WebAssembly outside the browser.
• How fast, measured rather than asserted: the host's engine compiles the bytecode to machine code, so what does a module cost at load time, and what is left of "near-native" in a loop that calls out to the host?
• What WebAssembly does not have by default: a garbage collector you must use, a standard library, threads. Which of those did later proposals add, and which does Rust need?

The trap it exists for

Thinking of wasm as "Rust running inside JavaScript", and so expecting the browser's facilities to be there. A module has no console, no clock and no files until the host imports them — which is why println! compiled for wasm32-unknown-unknown prints nothing and reports nothing.

Where this sits

Compiling to wasm32 covers the targets and what std becomes on each; Rust in the browser and WebAssembly outside the browser cover the two kinds of host. This page covers only what a module is.

See also

• Compiling to wasm32 — the targets that produce a module
• What a compiler does — the pipeline whose last stage wasm replaces
• Compiled, interpreted, or something between — where a bytecode that is compiled again on arrival sits
• LLVM and its IR — the back end that emits the module
• What unsafe turns off — what the sandbox still contains when Rust's checks are off
• WebAssembly on MDN ↗ and the core specification ↗

If you are coming from another language

• Java. The closest counterpart: a portable bytecode, validated before it runs, compiled again by the host. What differs is how little comes with it — no class library, no required garbage collector, no object model — so a Rust module brings its own allocator into linear memory, where a JVM program relies on the JVM's heap.
• C. Emscripten compiles C to wasm with a C library emulated on top of browser APIs, and Rust can use the same toolchain through wasm32-unknown-emscripten. The Rust-native targets do without that emulation layer, which is why so much of std behaves differently on them.
• Python. CPython itself can be compiled to wasm: the 1.98.0 rustc book names Pyodide, a Python runtime built with Emscripten. That is the whole interpreter shipped as a module, where a Rust module is the program alone.
• Go. The standard toolchain lists both js/wasm and wasip1/wasm among its ports (go tool dist list, Go 1.25.5). Go ships its runtime and garbage collector inside the module; a Rust module carries no runtime of that size.

Po polsku

Plik .wasm to moduł kodu bajtowego dla maszyny stosowej, zamknięty w piaskownicy: operuje na pamięci liniowej (linear memory) i nie może sięgnąć po nic, czego gospodarz nie przekazał mu jako importu. Cały interfejs modułu to importy i eksporty, a typy wartości to liczby, wektory SIMD i referencje — nie ma łańcuchów znaków ani struktur, więc &str przechodzi jako wskaźnik i długość. Stąd najczęstsze nieporozumienie: WebAssembly to nie „Rust uruchomiony w JavaScripcie”, tylko format, który równie dobrze działa w przeglądarce, na serwerze i jako wtyczka.

Szukaj po polsku: czym jest WebAssembly · piaskownica WebAssembly · pamięć liniowa wasm · webassembly imports exports · wasm module binary format · webassembly outside the browser