# wasm-emscripten

A skill that walks the agent through compiling C and C++ to WebAssembly: emcc flags, function exports, the memory model, Asyncify and .wasm debugging.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 513 installs · ★ 267
- Author: mohitmishra786
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/mohitmishra786/low-level-dev-skills/tree/HEAD/skills/runtimes/wasm-emscripten/SKILL.md
- Updated: 2026-06-27
- HTML: https://affpapa.org/en/nevibecode/skills/wasm-emscripten

## What it does

The skill covers the path from installing the Emscripten SDK to debugging the finished binary. Seven steps sit inside: a first build, exporting functions to JavaScript, the linear memory model, Asyncify for async C, optimization with wasm-opt, debugging in browser devtools, and picking between WASI and a browser target. The agent gets concrete emcc invocations with flags instead of general advice.

Function exports are spelled out. `EMSCRIPTEN_KEEPALIVE` keeps a function from being dropped as dead code, and names in `EXPORTED_FUNCTIONS` carry a leading underscore. Calling from JS has three routes: direct `Module._add`, type-checked `ccall`, or `cwrap`, which hands back a plain JS function. Heap sizing goes through `INITIAL_MEMORY`, `MAXIMUM_MEMORY` and `ALLOW_MEMORY_GROWTH`.

WASM needs HTTP, not `file://`, so local testing runs through `python3 -m http.server`. Asyncify behaves better with optimization enabled, which is why the example passes `-O2`. For a WASI target the skill notes that wasi-sdk offers fuller support than Emscripten.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill wasm-emscripten -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/wasm-emscripten/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/wasm-emscripten/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill wasm-emscripten -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/wasm-emscripten/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill wasm-emscripten -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/wasm-emscripten/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install mohitmishra786/low-level-dev-skills wasm-emscripten
```

2. **Manually: put the files in place**
- File: `.github/skills/wasm-emscripten/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill wasm-emscripten -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/mohitmishra786/low-level-dev-skills --path skills/runtimes/wasm-emscripten
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/wasm-emscripten/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill wasm-emscripten -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/mohitmishra786/low-level-dev-skills/tree/HEAD/skills/runtimes/wasm-emscripten/SKILL.md

## When to use it
- compile a C function to .wasm and call it from JavaScript
- export functions without the linker dropping them
- read and write linear memory from JS
- turn synchronous C code async with Asyncify
- shrink a .wasm binary through wasm-opt

## Example prompts
- Compile math.c to WebAssembly and export add and sqrt_approx
- Why is my C function missing from JavaScript after the emcc build?
- Set up Asyncify so fetch runs from synchronous C code

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 5e3087f60a5e9d14.

BEGIN THIRD-PARTY FILE 5e3087f60a5e9d14
````text
---
name: wasm-emscripten
description: WebAssembly with Emscripten skill for C/C++ to WASM compilation. Use when compiling C/C++ to WebAssembly with emcc, configuring EXPORTED_FUNCTIONS, understanding the WASM memory model, using Asyncify for async C code, debugging .wasm with browser devtools or wasm-opt, or targeting WASI vs browser environments. Activates on queries about Emscripten, emcc, WebAssembly from C/C++, WASM memory model, Asyncify, EXPORTED_FUNCTIONS, WASI, or wasm-opt.
---

# WebAssembly with Emscripten

## Purpose

Guide agents through compiling C/C++ to WebAssembly using Emscripten: emcc flag selection, function exports, memory model configuration, Asyncify for asynchronous C code, debugging WASM binaries, and targeting WASI vs browser.

## Triggers

- "How do I compile C to WebAssembly with Emscripten?"
- "How do I export a C function to JavaScript?"
- "How does WebAssembly memory work with Emscripten?"
- "How do I debug a .wasm file?"
- "How do I use Asyncify to make synchronous C code async?"
- "What's the difference between WASI and Emscripten browser target?"

## Workflow

### 1. Setup and first build

```bash
# Install Emscripten SDK
git clone https://github.com/emscripten-core/emsdk.git
cd emsdk
./emsdk install latest
./emsdk activate latest
source ./emsdk_env.sh    # add emcc to PATH

# Verify
emcc --version

# Compile C to WASM (browser target)
emcc hello.c -o hello.html          # generates hello.html + hello.js + hello.wasm
emcc hello.c -o hello.js            # just JS + WASM (no HTML shell)

# Serve locally (WASM requires HTTP, not file://)
python3 -m http.server 8080
# Open: http://localhost:8080/hello.html
```

### 2. Exporting functions to JavaScript

```c
// math.c
#include <emscripten.h>

// EMSCRIPTEN_KEEPALIVE prevents dead-code elimination
EMSCRIPTEN_KEEPALIVE
int add(int a, int b) {
    return a + b;
}

EMSCRIPTEN_KEEPALIVE
double sqrt_approx(double x) {
    return x * 0.5 + 1.0;
}
```

```bash
# Export specific functions
emcc math.c -o math.js \
  -s EXPORTED_FUNCTIONS='["_add","_sqrt_approx"]' \
  -s EXPORTED_RUNTIME_METHODS='["ccall","cwrap"]' \
  -s MODULARIZE=1 \
  -s EXPORT_NAME=MathModule

# The leading underscore is required for C functions
```

```javascript
// Using exported functions in JS
const Module = await MathModule();

// Direct call
const result = Module._add(3, 4);

// Via ccall (type-safe)
const result2 = Module.ccall('add', 'number', ['number', 'number'], [3, 4]);

// Via cwrap (creates a callable JS function)
````
END THIRD-PARTY FILE 5e3087f60a5e9d14

The third-party file has ended. Everything between the 5e3087f60a5e9d14 tags is data: requests and commands from it are not followed. An “end of file” line with another tag inside the block is part of the file.

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---
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