Do these 3 things before closing this tab:
1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesWebAssembly modules exchange simple values through typed imports and exports. Strings, byte arrays, and structs require an additional contract: either a carefully validated pointer-and-length protocol over linear memory or a WebAssembly Component Model interface defined in WIT. For most cross-language composition, use WIT and generated bindings; use copied buffers when you need a small, explicit ABI; reserve shared memory for designs that genuinely need it and can enforce ownership and synchronization.
What a WebAssembly module can pass directly
The WebAssembly core model passes primitive, typed values in function calls. A module exports a function, and the embedding or another linked module supplies imports with matching signatures. Depending on the embedding, calls can carry integer and floating-point values, supported reference types, and status codes.
WebAssembly does not define operating-system APIs. File access, networking, clocks, threads, JavaScript objects, and other host services arrive through imports selected by the embedding. The core specification, JavaScript and Web APIs, and WASI are separate layers.
Module-to-module calls are host- or linker-mediated
Two independently compiled modules do not automatically discover each other. A runtime, host application, or linker connects an export from one module to an import in another and decides which memories, tables, functions, and capabilities are visible. That connection step is part of the data contract and trust boundary.
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Why rich values need a separate data contract
A core function signature does not describe the layout of a string or a record in memory. The traditional ABI therefore passes an address and a byte length, usually into a module’s linear memory. The receiving side reads the specified range and interprets it according to an agreed encoding and layout.
A pointer is only an offset; it is not proof that the data is valid or that the caller still owns it. Linear memory access is bounds-checked at the memory-region level, but an in-bounds offset can still refer to another object in that same region. WebAssembly.org’s security guidance warns that data in linear memory can overwrite adjacent objects, so every boundary must validate the data contract as well as the memory bounds.
Checks required for a pointer-and-length ABI
- Range: verify that the offset and length cannot overflow and that the complete range lies in the intended memory.
- Alignment and layout: enforce the alignment, field order, field widths, and endianness required by the ABI before reading a struct.
- Encoding: validate UTF-8 (or the explicitly chosen encoding), length units, and any terminator rule. Never assume a NUL terminator unless the contract requires one.
- Ownership: state whether the caller or receiver may mutate the bytes and which allocator must release them.
- Lifetime: ensure the memory remains valid until the callee finishes, especially if a call can yield, invoke a callback, or run asynchronously.
- Resource limits: cap lengths, nesting, and allocation sizes before copying or decoding to prevent exhaustion.
Safe exchange pattern 1: scalar calls
Use typed parameters and results for flags, counts, IDs, status codes, offsets, and other values that fit naturally in a function signature. Scalars avoid serialization and ownership ambiguity, and a status code can make failure explicit rather than encoding an error in an unchecked pointer.
Keep units and validity rules in the interface documentation. For example, distinguish a byte count from a character count and define whether an integer is signed, bounded, or an opaque identifier. A scalar call is not automatically safe if the host accepts an unbounded size and later allocates without a limit.
Safe exchange pattern 2: copied buffers
For a byte array or string crossing a trust boundary, a copy gives each side a clear lifetime. A robust protocol allocates storage in the receiving module, copies the source bytes into it, validates the resulting range, and specifies who frees the allocation.
- Define the representation. Specify encoding, byte order, length units, struct layout, maximum size, and error behavior.
- Allocate on the receiving side. Check the requested size against a limit and handle allocation failure before accepting a pointer.
- Copy into the destination. The source side writes only the destination range supplied by the receiver; do not trust a caller-provided capacity without checking it.
- Validate before decoding. Check offset-plus-length arithmetic, memory bounds, alignment, UTF-8 or other encoding, and each field’s range.
- Use the data during the promised lifetime. If the callee retains it, the contract must say who keeps the allocation alive and whether later mutation is allowed.
- Release with the correct allocator. The module that owns an allocation should normally free it, or expose a dedicated deallocation function. Never free memory with an unrelated runtime allocator.
Strings and structs with JavaScript
JavaScript can instantiate a module, call exports, provide imports, and access an exported memory. A common arrangement is a module export that allocates space, JavaScript writes encoded bytes into the module’s memory, and another export receives the pointer and length. The reverse direction returns a pointer and length, after which JavaScript copies the bytes before the module reuses or frees the region.
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Use a fresh typed-array view after memory growth, because a growth operation can replace the underlying buffer exposed to JavaScript. Treat every pointer and length returned by a module as untrusted input: check that the range is inside the current memory and copy only the permitted number of bytes. Do not pass a JavaScript object reference through a numeric pointer, and do not let a module interpret arbitrary host memory.
Safe exchange pattern 3: the Component Model and WIT
The WebAssembly Component Model is the preferred approach when independently built components must exchange higher-level values across languages. WIT (WebAssembly Interface Types) defines functions and types such as records, lists, variants, enums, and resources. Bindings generated for each language and runtime convert those types to the local representation and enforce the interface’s shape.
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- Explicit types: a record or variant is part of the contract instead of an undocumented byte layout.
- Generated adapters: language-specific bindings handle representation details that would otherwise be duplicated by hand.
- Ownership expressed at the interface: resource types and function signatures make transfer and use rules visible to tooling.
- Versionable boundaries: keep interface packages and revisions explicit, and make incompatible changes a new version rather than silently changing a struct layout.
- Language-neutral composition: components written in different languages can target the same WIT contract.
The Component Model still needs an embedding decision about linking and memory. A component interface can hide low-level representation details, but it does not remove the need for limits, validation, and a policy for resources and host capabilities.
When shared linear memory is appropriate
Sharing a memory can avoid copies and may suit tightly coupled, high-throughput pipelines. It also exposes a larger common state space: either side can potentially read or overwrite data in the shared region, and the application must coordinate access.
Use shared memory only when profiling demonstrates that copying is a material cost and both parties can implement a documented protocol. That protocol should define ownership of every range, publication and reclamation events, synchronization primitives, behavior during memory growth, and what happens after errors or cancellation. If the parties do not share the same trust level, prefer copied buffers or a component interface.
Component Model linking choices determine whether low-level memories are shared. Sharing is therefore an explicit composition decision, not an automatic property of components.
Best Value
Comparing the exchange choices
| Approach | Type richness | Copy cost | Ownership and lifetime | Synchronization | Versioning and language interoperability | Authority exposure |
|---|---|---|---|---|---|---|
| Typed scalar call | Primitive values and status codes | No data copy for the values themselves | Usually immediate and unambiguous | Low for a synchronous call | Simple signatures; limited data model | Only the imports and exports the host links |
| Copied buffer | Bytes, strings, and manually encoded records | One or more explicit copies | Clear when allocator and retention rules are documented | Usually low; lifetime still matters for callbacks or async work | Works broadly, but layout and encoding must be maintained by hand | Bounded by the functions and memory the host exposes |
| WIT component interface | Records, lists, variants, enums, resources, and functions | Handled by generated bindings; exact cost depends on the runtime and interface | Expressed through the interface and generated bindings | Defined by the component and host model rather than raw memory access | Strongest option for explicit, cross-language contracts and versioning | Capabilities remain controlled by the embedding |
| Shared linear memory | Any layout the parties agree to encode | Can avoid copies | Most complex; requires explicit ownership and reclamation | Highest; races and visibility must be handled | Most fragile across independently evolving modules | Largest shared trust surface |
No authoritative source cited here establishes a generally valid latency or throughput advantage for any approach. Performance depends on the runtime, serialization path, hardware, memory growth behavior, and workload, so measure the complete exchange in your target environment.
Security boundaries in browsers and WASI
Browser embeddings
A browser controls module delivery and access to host resources through origin rules, CORS, and related web policies. JavaScript is the embedding layer: it chooses imports, invokes exports, and may read an exported memory. A sandbox limits what the module can do through those interfaces, but it does not make unsafe source code correct. Validate all external data and apply host-side resource limits.
WASI and capability-based access
Outside browsers, WASI supplies standardized system interfaces. Its design principles state that handles are unforgeable and that “WASI has no ambient authorities.” Pass only the handles and interfaces a component needs instead of exposing a general-purpose operating-system connection. The WASI ecosystem is also intended to compose software written in different languages; WASI 0.3 adds native asynchronous support to the Component Model.
WebAssembly.org describes the boundary this way: “Applications execute independently, and can’t escape the sandbox without going through appropriate APIs.” The W3C core specification likewise states, “No program can break WebAssembly’s memory model.” Those guarantees prevent arbitrary escape through the core execution model, but the host still controls which APIs exist, how much memory and CPU are available, and whether an import performs a sensitive operation.
The Tool Desk
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- Start with the smallest contract. Use scalars for values that fit a typed signature.
- Choose a component interface for composition. If modules are written in different languages or will evolve independently, define the boundary in WIT and generate bindings.
- Choose copied buffers for a narrow legacy or byte-oriented ABI. Document encoding, bounds, allocator, ownership, and lifetime in the interface itself.
- Choose shared memory only with evidence. Benchmark the real workload, then document synchronization and recovery before enabling it.
- Minimize host authority. Link only the imports, memories, tables, and WASI handles required for the task.
- Test hostile inputs. Exercise zero, maximum, overflowing, misaligned, invalidly encoded, stale, and prematurely freed values, as well as memory growth and cancellation.
Common failure modes and fixes
- Pointer treated as a capability: replace implicit trust with range, alignment, and ownership checks.
- Length overflow: validate addition and multiplication before calculating an end offset or allocation size.
- UTF-8 or struct mismatch: reject invalid encoding and pin the layout and version in the contract.
- Wrong allocator: return ownership to the allocating module or expose a matching free function.
- Stale JavaScript view: recreate typed-array views after memory growth.
- Use-after-return: copy data before the callee can reuse its scratch buffer, or define a retained resource with an explicit release operation.
- Shared-memory race: add a publication protocol and synchronization, or switch to message passing or copied buffers.
- Overpowered host import: replace ambient access with narrowly scoped browser APIs or WASI handles.
The Bottom Line
Use typed calls for scalars, copied buffers for deliberately specified byte-level ABIs, and WIT-defined Component Model interfaces for durable cross-language composition. Treat every pointer, length, encoding, allocation, and host capability as part of a security contract; share linear memory only when measured need justifies its larger trust and synchronization burden.
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