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There is no single WebAssembly “link” operation. If you want one deployable file, compile source into Wasm object files and link them with your toolchain. If you already have separate .wasm binaries, normally keep them separate and connect one instance’s exports to another’s imports through the host. For native-style dynamic libraries, you need a compatible toolchain and loader. For typed, cross-language interfaces, use WIT and the WebAssembly Component Model.
That distinction matters because a finished WebAssembly module is not generally interchangeable with a relocatable object file. WebAssembly standardizes typed imports, exports, validation and instantiation; it does not define one universal operating-system API or dynamic-library ABI. See the core module specification and the portability guidance.
What “linking” means in WebAssembly
| What you have | Mechanism to use |
|---|---|
| Source files, libraries or Wasm object files | Static linking with Clang, Rust and wasm-ld into one module |
| Two complete core Wasm modules | Instantiate the provider, then pass its exports as imports to the consumer |
| Modules designed for a shared address space | Toolchain-specific dynamic linking with shared memory, tables and ABI conventions |
| Independent, cross-language components | WIT interfaces and Component Model composition |
| A Wasm module called by a web app | JavaScript (or another host) imports and exports |
A module is the Wasm deployment and instantiation unit. An instance is a live module with its own state. Imports are named by a module and item field; exports become available after instantiation. Imports can be functions, memories, tables, globals and tags—not only functions.
The smallest working example: connect two finished modules
This is runtime wiring, not binary merging. The provider exports add:
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(module
(func $add (param i32 i32) (result i32)
local.get 0 local.get 1 i32.add)
(export "add" (func $add)))wat2wasm provider.wat -o provider.wasmThe consumer declares an import named math.add and calls it:
(module
(import "math" "add" (func $add (param i32 i32) (result i32)))
(func $run (result i32)
i32.const 20 i32.const 22 call $add)
(export "run" (func $run)))wat2wasm consumer.wat -o consumer.wasmJavaScript instantiates the provider first and supplies exactly the export the consumer requests:
const provider = await WebAssembly.instantiateStreaming(
fetch("./provider.wasm")
);
const consumer = await WebAssembly.instantiateStreaming(
fetch("./consumer.wasm"),
{ math: { add: provider.instance.exports.add } }
);
console.log(consumer.instance.exports.run()); // 42The nested object must match the consumer’s import module and field names. Passing the entire instance instead of provider.instance.exports.add is a common mistake. The JavaScript API is documented at webassembly.org.
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Inspect imports before debugging
const bytes = await (await fetch("./consumer.wasm")).arrayBuffer();
const module = await WebAssembly.compile(bytes);
console.log(WebAssembly.Module.imports(module));
console.log(WebAssembly.Module.exports(module));Check every required function, memory, table, global and tag, plus parameter/result types and memory limits. If streaming instantiation fails because the server sends the wrong MIME type or a proxy interferes, use:
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const response = await fetch("./provider.wasm");
const bytes = await response.arrayBuffer();
const provider = await WebAssembly.instantiate(bytes, imports);Browser loading is also subject to CORS, CSP, caching and origin policy; see WebAssembly on the web.
Static linking: build one self-contained module
Use static linking when one artifact, whole-program optimization and simple startup matter. The inputs are normally relocatable object files (.o), archives (.a) and runtime libraries—not arbitrary finished application modules.
// math.c
int add(int a, int b) { return a + b; }
clang --target=wasm32-unknown-unknown -c math.c -o math.o
wasm-ld --no-entry --export=add math.o -o math.wasm--no-entry suits a library-like module without _start. A command or executable module needs an appropriate entry point. The target triple, sysroot, libc and runtime flags differ between browser, WASI and other hosts, so the compiler driver is usually safer than invoking wasm-ld manually. The linker’s WebAssembly options are documented in the LLVM LLD manual.
Rust exports often look like:
#[no_mangle]
pub extern "C" fn add(a: i32, b: i32) -> i32 { a + b }extern "C" chooses a C-compatible calling convention and #[no_mangle] preserves a predictable symbol name. Neither defines how strings, vectors, ownership, exceptions or language objects cross the boundary. Rust’s wasm32-unknown-unknown, wasm32-wasip1 and wasm32-wasip2 targets require different build decisions.
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Sharing memory and tables
Instances can use the same WebAssembly.Memory when both modules were compiled to import a compatible memory:
const memory = new WebAssembly.Memory({ initial: 2, maximum: 10 });
const provider = await WebAssembly.instantiateStreaming(fetch("provider.wasm"), {
env: { memory }
});
const consumer = await WebAssembly.instantiateStreaming(fetch("consumer.wasm"), {
env: { memory, provider: provider.instance.exports }
});This does not create a safe ABI automatically. Both sides must agree on pointer width, alignment and struct layout, string encoding, allocation and freeing, ownership, error representation, initialization, reentrancy and thread behavior. A signature such as (i32, i32) -> i32 might mean two numbers—or a pointer and length.
Memory growth can replace a JavaScript memory’s underlying ArrayBuffer; recreate cached typed-array views after growth. Function tables matter for indirect calls and callbacks, but a table is shared only when explicitly imported/exported with compatible limits and element types. Shared memory can reduce copies, but it is not automatically faster: synchronization and lifetime bugs can outweigh the gain.
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Dynamic linking is a specialized ABI, not a core-Wasm feature
Toolchains can retain unresolved symbols as imports or implement load-time and run-time loading conventions. LLVM’s wasm-ld includes options such as --import-dynamic, --import-undefined, --export-dynamic, --import-memory and --export-memory. These flags do not establish a portable loader for unrelated modules. The WebAssembly dynamic-linking convention is a convention that must be matched by compilers and runtime.
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A real dynamic-linking system must decide where dependencies are found, how relocations are applied, how memory and tables are allocated, how constructors run, how symbols are versioned, how duplicate names resolve and how allocators, exceptions, threads and mutable globals interact. Use it only when the same toolchain controls both sides, the ABI is documented and a compatible loader already exists. Otherwise, explicit host imports are easier to operate.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Component Model composition with WIT
For independently produced, cross-language modules with structured data, the Component Model is the higher-level choice. WIT defines typed interfaces and worlds instead of exposing pointer-based core-Wasm conventions:
package example:math;
interface calculator {
add: func(a: s32, b: s32) -> s32;
}
world consumer {
import calculator;
export run: func() -> s32;
}A typical workflow is:
- Define interfaces and a world in WIT.
- Generate bindings with
wit-bindgenor a language-specific generator. - Compile guest code to a core Wasm module.
- Convert it to a component with matching metadata and adapters.
- Compose the primary component with dependency components in a Component Model runtime.
Useful inspection and conversion commands include:
wasm-tools component wit component.wasm
wasm-tools component new my-core.wasm -o my-component.wasmFor a core module using wasi_snapshot_preview1, a compatible adapter may be required:
wasm-tools component new my-core.wasm
--adapt wasi_snapshot_preview1.reactor.wasm
-o my-component.wasmCurrent Bytecode Alliance tooling is evolving: the wasm-tools repository labels its older compose command deprecated, while current examples use wac plug, for example:
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wac plug MyApp.wasm --plug AddImplementation.wasm -o composed.wasmVerify the command supported by your installed version. Inspect both sides before composition:
wasm-tools component wit primary.wasm
wasm-tools component wit dependency.wasmWIT supplies a typed contract and canonical lifting/lowering, but components still require a compatible runtime, adapters, interface versions and resource implementations. Components do not work everywhere core Wasm works.
Which approach should you choose?
| Requirement | Best fit | Main trade-off |
|---|---|---|
| One controlled application and one artifact | Static linking | Less independent deployment |
| Small scalar API between existing modules | Host-mediated imports | Host owns wiring and lifecycle |
| Zero-copy-oriented ABI under one toolchain | Shared memory or dynamic linking | High coupling and difficult debugging |
| Strings, records, lists or resources across languages | Component Model and WIT | Metadata, bindings and runtime requirements |
| Browser orchestration | JavaScript adapter | Less native-style shared state |
Debugging checklist
- Validate the binary:
wasm-tools validate module.wasm. - Inspect sections and symbols:
wasm-tools objdump module.wasm. - Print
WebAssembly.Module.imports()andexports(). - Match import module and field names exactly.
- Match function types, memory minimum/maximum, shared status, table types and global mutability.
- Confirm exports were explicitly retained; an internal function can exist without being visible.
- Define an explicit initialization phase for multi-instance graphs.
- For corrupted strings, document pointer/length, encoding, allocator, ownership and lifetime.
- For components, compare package, interface, world, resource and version names with
wasm-tools component wit. - Test in the target runtime: core validation can succeed while WASI, Component Model, adapter, capability or host API support is missing.
Security and deployment
Imports are capability boundaries. Expose only the host functions, memories and resources a module needs; do not hand an untrusted plug-in unrestricted filesystem, network or shared-memory access. Verify dependency integrity, pin interface versions, apply browser origin/CORS policy correctly and treat independently fetched modules as independently versioned software. Separate instances have separate state unless you explicitly share an import.
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Bottom line: Static-link object files when you want one optimized module, wire complete modules through explicit imports and exports when the host controls them, reserve dynamic linking for a known ABI and loader, and use WIT/Components for typed cross-language composition. Finished .wasm files are not interchangeable with native object files, and an import signature alone is never a complete data ABI.
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