Use Emscripten to compile C and C++ into high-performance WebAssembly (Wasm) modules for the browser.
1. Modern Emscripten Compilation Flags
When compiling and linking with emcc or em++, always use modern modular
flags:
-sEXPORT_ES6: Emits standard ES6 module syntax (import/export) rather than legacy global objects. This automatically sets-sMODULARIZE.-sENVIRONMENT=web: Strips Node.js/shell runtime code to minimize bundle size.-sSTRICT: Enforces modern Emscripten defaults and fails on deprecated flags.-sALLOW_MEMORY_GROWTH: Allows the WebAssembly linear memory heap to expand dynamically when allocating memory.-Oz(or-O3) +-flto: Optimizes for minimal payload size in release builds.
2. Avoid blocking the main thread
If a call into WebAssembly is likely to take a significant amount of time (i.e. more than a few milliseconds) it should not be called on the main browser thread.
To avoid running such code on the main browser thread there are several possible approaches:
Run the whole WebAssembly program in a Web Worker. This is often the simplest solution since it doesn't involve setting up SharedArrayBuffer access, or changing the program source.
Use pthreads or Wasm Workers. This allows the work to be shared across many workers but requires SharedArrayBuffer access which can involve adding COOP/COEP HTTP headers.
Refactor the C/C++ code to avoid long running calls. For example slicing the work into small pieces that can be called from
requestIdleCallback.
3. Pay attention for runtime warnings and errors
Make sure you actually test the resulting WebAssembly program and that no warnings or errors are generated on the console.
Build and run the programs with -sASSERTIONS enabled and fix any runtime
failures that are produced.
4. Avoid dynamic linking if possible
Emscripten works best with static libraries (.a). Shared libraries (.so,
.dylib, .dll) are less useful in browser deployment and can complicate
things unnecessarily.
5. Porting Existing C/C++ Libraries
When integrating existing C/C++ libraries with standard build systems:
CMake Projects
Wrap CMake commands with emcmake and emmake:
mkdir build && cd build
emcmake cmake .. -DCMAKE_BUILD_TYPE=Release
emmake make
Autoconf / Make Projects
Use emconfigure to set cross-compilation toolchain targets:
emconfigure ./configure --host=wasm32-unknown-emscripten --disable-shared
emmake make