C++ HTTP Server WebAssembly Component

A simple HTTP server built using C++ and WebAssembly Components with wasi:http.

Building

Prerequisites

• WASI SDK 28.0 (wasm32-wasip2)
• wit-bindgen 0.48.0+
• wkg CLI

Quick Start with Makefile

The easiest way to build is using the provided Makefile:

make

If you have WASI SDK installed in a non-standard location:

make WASI_SDK_PATH=/opt/wasi-sdk-28.0

Or set it as an environment variable:

export WASI_SDK_PATH=/opt/wasi-sdk-28.0
make

Manual Build Steps

If you prefer to build manually:

1. Fetch WIT dependencies:

   wkg wit fetch

2. Generate C++ bindings:

   wit-bindgen cpp --out-dir bindings wit

3. Compile the component:

   wasm32-wasip2-clang++ \
     server.cpp \
     bindings/http_server.cpp \
     bindings/http_server_component_type.o \
     -I bindings \
     -I include \
     -Wall -Wextra -Werror -Wno-unused-parameter \
     -std=c++20 \
     -fno-exceptions \
     -mexec-model=reactor \
     -o http-server.wasm

Running

You can run this component with any WASI HTTP host, such as wasmtime:

wasmtime serve -Scli http-server.wasm

Then test the endpoints:

curl http://localhost:8080/
curl http://localhost:8080/hello
curl http://localhost:8080/echo/test

Endpoints

• / - Returns "Hello, World!"
• /hello - Returns a greeting from the C++ component
• /echo/* - Echoes back the request method and path
• Other paths return 404

Implementation Notes

• Uses wasi:http imported resources (IncomingRequest, ResponseOutparam, etc.)
• Demonstrates proper use of && move semantics for imported resources
• Shows how to work with wit::string, std::expected, and std::variant
• Properly handles resource lifetimes and cleanup

Generated with

Built using wit-bindgen with the following fixes:

1. Type alias resolution for imported resources
2. Proper && usage instead of ::Owned for imported resources
3. Move semantics for std::expected in optional emplacement

Dependencies

This project uses tl::expected (included in include/tl/) to provide std::expected compatibility for WASI SDK.

Why tl::expected?

The generated C++ bindings use std::expected, which is a C++23 feature. While WASI SDK 28.0 uses LLVM 21 (which has libc++ 21 with std::expected support), the WASI SDK distribution may not include the <expected> header in its sysroot.

tl::expected is a header-only, single-file implementation of std::expected that:

• Works with C++11 and later (we use C++20)
• Is API-compatible with the C++23 standard
• Is released under CC0-1.0 license (public domain)
• Requires no external dependencies

The include/expected header provides a compatibility layer that aliases tl::expected to std::expected, making the code work seamlessly with generated bindings.