C/C++ Tooling

WebAssembly components can be built from C and C++ using clang, the C language family frontend for LLVM.

wit-bindgen is a tool to generate guest language bindings from a given .wit file. When compiling C or C++ code to WebAssembly components, we say that C or C++ is the "guest" language, and WebAssembly is the "host" language. In this case, "bindings" are C or C++ declarations: type signatures that correspond to WIT functions, and type definitions that correspond to WIT types. The bindings generator only generates declarations; you have to write the code that actually implements these declarations, if you're developing your own .wit files. For WIT interfaces that are built in to WASI, the code is part of the WebAssembly runtime that you will be using.

C/C++ currently lacks an integrated toolchain (like Rust's cargo-component). However, wit-bindgen can generate source-level bindings for Rust, C, Java (TeaVM), and TinyGo, with the ability to add more language generators in the future.

wit-bindgen can be used to build C applications that can be compiled directly to WebAssembly modules using clang with a wasm32-wasi target.

1. Download dependencies

First, install the following dependencies:

1. wit-bindgen CLI
2. wasm-tools
   • wasm-tools can be used to inspect compiled WebAssembly modules and components, as well as converting between preview1 modules and preview2 components in the optional manual workflow.
3. The WASI SDK
   • WASI SDK is a WASI enabled C/C++ toolchain which includes a version of the C standard library (libc) implemented with WASI interfaces, among other artifacts necessary to compile C/C++ to WebAssembly.
   • On a Linux system, you can skip to the "Install" section. To build from source, start from the beginning of the README.

A WASI SDK installation will include a local version of clang configured with a WASI sysroot. (A sysroot is a directory containing header files and libraries for a particular target platform.) Follow these instructions to configure WASI SDK for use.

2. Generate program skeleton from WIT

Start by pasting the contents of the sample adder/world.wit file into a local file. Then generate a C skeleton from wit-bindgen using this file:

$ wit-bindgen c path/to/adder/world.wit
Generating "adder.c"
Generating "adder.h"
Generating "adder_component_type.o"

This command generates several files:

1. adder.h (based on the adder world). This header file contains, amidst some boilerplate, the prototype of the add function, which should look like this. (The name of the function has been prefixed with "exports".)

  uint32_t exports_docs_adder_add_add(uint32_t x, uint32_t y);

2. adder.c, which interfaces with the component model ABI to call your function. This file contains an extern declaration that looks like:

    extern void __component_type_object_force_link_adder(void);

3. adder_component_type.o, which contains object code, including the definition of the __component_type_object_force_link_adder function, which must be linked via clang.

3. Write program code

Next, create a file named component.c with code that implements the adder world: that is, code which fulfills the definition of the interface function declared in adder.h.

#include "adder.h"

uint32_t exports_docs_adder_add_add(uint32_t x, uint32_t y)
{
	return x + y;
}

4. Compile a WebAssembly Preview 2 component with wasi-sdk's wasm32-wasip2-clang

"P1" refers to WASI Preview 1, the initial version of the WASI APIs. "P2" refers to WASI Preview 2, which introduced the component model.

While in the past building a P2 component required conversion from a P1 component, we can now build a P2 component directly by using the wasm32-wasip2-clang binary that was installed by the WASI SDK.

If necessary, change /opt/wasi-sdk to the path where you installed the WASI SDK.

/opt/wasi-sdk/bin/wasm32-wasip2-clang \
    -o adder.wasm \
    -mexec-model=reactor \
    component.c \
    adder.c \
    adder_component_type.o

Breaking down each part of this command:

• -o adder.wasm configures the output file that will contain binary WebAssembly code.
• -mexec-model=reactor controls the desired execution model of the generated code. The argument can be either reactor or command. In this case, we pass in -mexec-model=reactor to build a reactor component. A reactor component is more like a library, while a command component is more like an executable.
• component.c contains the code you wrote to implement the adder world.
• adder.c and adder_component_type.o were generated by wit-bindgen and contain necessary scaffolding (e.g. function exports) to enable building component.c into a WebAssembly binary.

After this command completes, you will have a new file named adder.wasm available in the source folder.

You can verify that adder.wasm is a valid WebAssembly component with the following command:

> wasm-tools print adder.wasm | head -1
(component

For use cases that require building a P1 module and/or adapting an existing P1 module into a P2 module, such as building for a platform that does not support P2, details on a more manual approach using wasi-sdk's clang and wasm-tools can be found below:

$WASI_SDK_PATH/bin/clang \
    -o adder.wasm \
    -mexec-model=reactor \
    component.c \
    adder.c \
    adder_component_type.o

> wasm-tools print adder.wasm | head -1
(module $adder.wasm

wasm-tools component new adder.wasm -o adder.component.wasm

#include "adder.h"
#include <stdio.h>

uint32_t exports_docs_adder_add_add(uint32_t x, uint32_t y)
{
	uint32_t result = x + y;
        // On traditional platforms, printf() prints to stdout, but on Wasm platforms,
        // stdout and the idea of printing to an output stream is
        // introduced and managed by WASI.
        //
        // When building this code with wasi-libc (as a part of wasi-sdk), the printf call
        // below is implemented with code that uses `wasi:cli/stdout` and `wasi:io/streams`.
	printf("%d", result);
	return result;
}

> wasm-tools component new adder.wasm -o adder.component.wasm
error: failed to encode a component from module

Caused by:
    0: failed to decode world from module
    1: module was not valid
    2: failed to resolve import `wasi_snapshot_preview1::fd_close`
    3: module requires an import interface named `wasi_snapshot_preview1`

wasm-tools component new \
    adder.wasm \
    --adapt wasi_snapshot_preview1.wasm \
    -o adder.component.wasm

5. Inspect the built component

Finally, you can inspect a WIT representation of the imports and exports of your component (including any WASI imports if you used them):

$ wasm-tools component wit adder.component.wasm
package root:component;

world root {
  import wasi:io/error@0.2.2;
  import wasi:io/streams@0.2.2;
  import wasi:cli/stdin@0.2.2;
  import wasi:cli/stdout@0.2.2;
  import wasi:cli/stderr@0.2.2;
  import wasi:cli/terminal-input@0.2.2;
  import wasi:cli/terminal-output@0.2.2;
  import wasi:cli/terminal-stdin@0.2.2;
  import wasi:cli/terminal-stdout@0.2.2;
  import wasi:cli/terminal-stderr@0.2.2;
  import wasi:clocks/wall-clock@0.2.2;
  import wasi:filesystem/types@0.2.2;
  import wasi:filesystem/preopens@0.2.2;

  export add: func(x: s32, y: s32) -> s32;
}
...

6. Run the component from the example host

The following section requires you to have a Rust toolchain installed.

This repository contains an example WebAssembly host written in Rust that can run components that implement the adder world.

1. git clone https://github.com/bytecodealliance/component-docs.git
2. cd component-docs/component-model/examples/example-host
3. cargo run --release -- 1 2 <PATH>/adder.wasm

• The double dashes separate the flags passed to cargo from the flags passed in to your code.
• The arguments 1 and 2 are the arguments to the adder.
• In place of <PATH>, substitute the directory that contains your generated adder.wasm file.

Note: When hosts run components that use WASI interfaces, they must explicitly add WASI to the linker to run the built component.

A successful run should show the following output (of course, the paths to your example host and adder component will vary, and you should substitute adder.wasm with adder.component.wasm if you followed the manual instructions above):

cargo run --release -- 1 2 adder.wasm
   Compiling example-host v0.1.0 (/path/to/component-docs/component-model/examples/example-host)
    Finished `release` profile [optimized] target(s) in 7.85s
     Running `target/debug/example-host 1 2 /path/to/adder.wasm`
1 + 2 = 3

If not configured correctly, you may see errors like the following:

cargo run --release -- 1 2 adder.wasm
   Compiling example-host v0.1.0 (/path/to/component-docs/component-model/examples/example-host)
    Finished `release` profile [optimized] target(s) in 7.85s
     Running `target/release/example-host 1 2 /path/to/adder.component.wasm`
Error: Failed to instantiate the example world

Caused by:
    0: component imports instance `wasi:io/error@0.2.2`, but a matching implementation was not found in the linker
    1: instance export `error` has the wrong type
    2: resource implementation is missing

This kind of error normally indicates that the host in question does not satisfy WASI imports.

7. Run the component from C/C++ Applications

It is not yet possible to run a WebAssembly Component using the wasmtime C API. See wasmtime issue #6987 for more details. The C API is preferred over directly using the example host Rust crate in C++.

However, C/C++ language guest components can be composed with components written in any other language and run by their toolchains, or even composed with a C language command component and run via the wasmtime CLI or any other host.

See the Rust Tooling guide for instructions on how to run this component from the Rust example-host (replacing the path to add.wasm with your adder.wasm or adder.component.wasm above).