# Emscripten — LLVM-to-WebAssembly Compiler for C/C++

> Emscripten compiles C and C++ code to WebAssembly, enabling high-performance applications to run in web browsers without plugins. It provides a complete toolchain including a drop-in replacement for GCC/Clang, an SDL-based graphics layer, and POSIX-compatible filesystem emulation.

## Install

Save as a script file and run:

# Emscripten — LLVM-to-WebAssembly Compiler for C/C++

## Quick Use
```bash
# Install via emsdk
git clone https://github.com/emscripten-core/emsdk.git
cd emsdk && ./emsdk install latest && ./emsdk activate latest
source ./emsdk_env.sh

# Compile a C file to WebAssembly
emcc hello.c -o hello.html
```

## Introduction
Emscripten is an open-source compiler toolchain that converts C and C++ code into WebAssembly (WASM), allowing native-speed applications to run directly in web browsers. Originally created by Alon Zakai at Mozilla, it has become the standard way to port existing C/C++ codebases to the web platform.

## What Emscripten Does
- Compiles C/C++ source code to WebAssembly binary format using LLVM/Clang
- Provides libc, libcxx, and SDL implementations targeting browser APIs
- Emulates POSIX filesystem operations via an in-memory virtual filesystem
- Generates JavaScript glue code to bridge WASM modules with browser APIs
- Supports multithreading through Web Workers and SharedArrayBuffer

## Architecture Overview
Emscripten uses Clang/LLVM as its compiler frontend and backend to produce WebAssembly bytecode. The toolchain wraps this with a system of JavaScript runtime libraries (the Emscripten runtime) that implement system calls, memory management, and I/O operations by mapping them to Web APIs. The output is a .wasm binary plus a .js loader that initializes the module in the browser or Node.js environment.

## Self-Hosting & Configuration
- Install via the emsdk manager which handles versioning and activation
- Requires Python 3.6+ and a recent version of Node.js for tooling
- Configure compiler flags via emcc/em++ which mirror GCC/Clang options
- Set EMSCRIPTEN environment variable or use emsdk_env.sh for path setup
- Supports CMake via the Emscripten.cmake toolchain file

## Key Features
- Near-native execution speed through WebAssembly compilation
- Broad POSIX API coverage including pthreads and networking
- Built-in support for OpenGL ES 2.0/3.0 mapped to WebGL
- Source maps for debugging C/C++ directly in browser DevTools
- Asyncify transform for integrating synchronous C code with async web APIs

## Comparison with Similar Tools
- **wasm-pack** — targets Rust-to-WASM; Emscripten focuses on C/C++
- **AssemblyScript** — compiles TypeScript-like syntax to WASM; less suitable for existing C codebases
- **Cheerp** — similar C++ to WASM compiler but with less community adoption
- **wasi-sdk** — produces standalone WASM without browser integration; Emscripten bundles full browser runtime

## FAQ
**Q: Can Emscripten compile any C/C++ project?**
A: Most projects compile with minor modifications. The main limitations involve platform-specific system calls and hardware access not available in browsers.

**Q: How does performance compare to native code?**
A: WebAssembly produced by Emscripten typically runs at 60-90% of native speed depending on the workload and browser engine optimizations.

**Q: Does it support C++ exceptions?**
A: Yes, Emscripten supports both C++ exceptions and setjmp/longjmp via WebAssembly exception handling or JavaScript-based unwinding.

**Q: Can I use Emscripten output in Node.js?**
A: Yes, the generated WASM and JS glue code work in Node.js environments without modification.

## Sources
- https://github.com/emscripten-core/emscripten
- https://emscripten.org/docs/

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Author: Script Depot