# WAMR Component Model support ## Introduction A WebAssembly (Wasm) component is an abstraction layer built over [standard WebAssembly](https://webassembly.github.io/spec/core/index.html) (often called WebAssembly Core). It is designed to enrich the exposed type system and improve interoperability across different programming languages and libraries. More details can be found [here](https://component-model.bytecodealliance.org/). To distinguish between the two layers, the Component Model specification refers to standard WebAssembly entities by prefixing them with the word "core" (e.g., core modules, core functions, core types). In short, a Wasm component uses WIT—an [Interface Definition Language](https://en.wikipedia.org/wiki/Interface_description_language) (IDL)—to define its public-facing interfaces. It then bundles one or more Wasm core modules to implement the underlying logic behind those interfaces. WAMR implements binary parsing for the WebAssembly [Component Model](https://github.com/WebAssembly/component-model) proposal. The parser handles the component binary format as defined in the [Binary.md](https://github.com/WebAssembly/component-model/blob/main/design/mvp/Binary.md) specification, covering all 13 section types with validation and error reporting. References: - [Component Model design repository](https://github.com/WebAssembly/component-model) - [Component Model binary format specification](https://github.com/WebAssembly/component-model/blob/main/design/mvp/Binary.md) - [Canonical ABI specification](https://github.com/WebAssembly/component-model/blob/main/design/mvp/CanonicalABI.md) - [Build WAMR vmcore](./build_wamr.md) for build flag reference ## Build Enable the feature by setting the CMake flag `WAMR_BUILD_COMPONENT_MODEL` (enabled by default). This defines the C preprocessor macro `WASM_ENABLE_COMPONENT_MODEL=1`. ```cmake set (WAMR_BUILD_COMPONENT_MODEL 1) include (${WAMR_ROOT_DIR}/build-scripts/runtime_lib.cmake) add_library(vmlib ${WAMR_RUNTIME_LIB_SOURCE}) ``` Or pass it on the cmake command line: ```bash cmake -DWAMR_BUILD_COMPONENT_MODEL=1 .. ``` ## Component vs core module A component binary shares the same magic number (`\0asm`) as a core WebAssembly module but is distinguished by its header fields: | | Core module | Component | |---------------|---------------|---------------| | magic | `\0asm` | `\0asm` | | version | `0x0001` | `0x000d` | | layer | `0x0000` | `0x0001` | WAMR uses `wasm_decode_header()` to read the 8-byte header and `is_wasm_component()` to determine whether a binary is a component or a core module. ## Binary parsing overview The binary parser takes a raw component binary buffer and produces a `WASMComponent` structure that holds the decoded header and an array of parsed sections. The diagram below illustrates the high-level parsing flow:
The parsing proceeds as follows: 1. **Header decode** -- `wasm_decode_header()` reads the 8-byte header (magic, version, layer) and populates a `WASMHeader` struct. 2. **Section loop** -- the parser iterates over the binary, reading a 1-byte section ID and a LEB128-encoded payload length for each section. 3. **Section dispatch** -- based on the section ID, the parser delegates to a dedicated per-section parser. Each parser decodes the section payload into a typed struct, validates its contents (UTF-8 names, index bounds, canonical options), and reports how many bytes were consumed. 4. **Core module delegation** -- when a Core Module section (0x01) is encountered, the parser delegates to the existing WAMR core module loader (`wasm_runtime_load_ex()`) to parse the embedded module. 5. **Recursive nesting** -- when a Component section (0x04) is encountered, the parser calls itself recursively with an incremented depth counter. Recursion depth is capped at 100. 6. **Result** -- on success, the `WASMComponent` struct holds the header and a dynamically-sized array of `WASMComponentSection` entries, each containing the raw payload pointer and a typed union with the parsed result. ### Section types The component binary format defines 13 section types: | ID | Section | Spec reference | |------|------------------|-----------------------------------| | 0x00 | Core Custom | custom section (name + data) | | 0x01 | Core Module | embedded core wasm module | | 0x02 | Core Instance | core module instantiation | | 0x03 | Core Type | core func types, module types | | 0x04 | Component | nested component (recursive) | | 0x05 | Instance | component instance definitions | | 0x06 | Alias | export, core export, outer aliases| | 0x07 | Type | component type definitions | | 0x08 | Canon | canonical lift/lower/resource ops | | 0x09 | Start | component start function | | 0x0A | Import | component imports | | 0x0B | Export | component exports | | 0x0C | Value | value definitions | ### Current limitations - **Core Type (0x03)**: only `moduletype` is supported; WebAssembly GC types (`rectype`, `subtype`) are rejected. - **Canon (0x08)**: `async` and `callback` canonical options are rejected; all other canonical operations are supported. ## Source layout All component model sources reside in `core/iwasm/common/component-model/`: ``` component-model/ iwasm_component.cmake # CMake build configuration wasm_component.h # type definitions, enums, struct declarations wasm_component.c # entry point: section dispatch and free wasm_component_helpers.c # shared utilities: LEB128, names, value types wasm_component_validate.c # validation: UTF-8, index bounds, canon options wasm_component_validate.h # validation declarations wasm_component_core_custom_section.c # section 0x00 wasm_component_core_module_section.c # section 0x01 wasm_component_core_instance_section.c # section 0x02 wasm_component_core_type_section.c # section 0x03 wasm_component_component_section.c # section 0x04 wasm_component_instances_section.c # section 0x05 wasm_component_alias_section.c # section 0x06 wasm_component_types_section.c # section 0x07 wasm_component_canons_section.c # section 0x08 wasm_component_start_section.c # section 0x09 wasm_component_imports_section.c # section 0x0A wasm_component_exports_section.c # section 0x0B wasm_component_values_section.c # section 0x0C wasm_component_export.c # export runtime helpers wasm_component_export.h # export declarations ```