The prepare command builds production-ready, optimized binaries for specific platforms. Unlike fire, which is designed for development, prepare creates fully optimized builds for distribution.
Usage Arguments Target platform: web or linux
Examples Build for Web Building for Web ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Found Fern project at: /home/user/my_app Found Fern source for web build at: ~/.fern/src/cpp Using cached Fern web library Compiling with Emscripten... Web build successful! Output: web/build/index.html Files generated: • web/build/index.html • web/build/index.js • web/build/index.wasm To serve locally: cd web/build python3 -m http.server 8000 Open: http://localhost:8000/index.html
Build for Linux Building for Linux ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Found Fern project at: /home/user/my_app Compiling for Linux... Linux build successful! Output: linux/build/my_app Size: 423.2 KB To run: linux/build/my_app To distribute: cp linux/build/my_app /path/to/distribution/
Production web builds with maximum optimization:
Compiler : Emscripten (emcc)Optimization : -O3 (maximum)Output : web/build/index.html, index.js, index.wasmWASM Features : WebGL2, memory growth enabledCustom Template : Uses web/template.html if present
Build artifacts are ready for deployment to any static web host.
Optimized native Linux binaries:
Compiler : g++Optimization : -O3 -DNDEBUGOutput : linux/build/<project_name>Permissions : Automatically set to executable (755)Dependencies : Linked statically where possible
Binary is ready for distribution on Linux systems.
Build Comparison Feature fern firefern preparePurpose Development Production Optimization -O2 (debug)-O3 (release)Build Time Fast Slower Binary Size Larger Smaller Performance Good Best Output Directory build/<platform>/build/
Project Structure Requirements To use prepare, your project must have:
my_app/ ├── lib/ │ └── main.cpp # Required: Entry point ├── fern.yaml # Required: Configuration └── web/ # Optional: Platform directory └── template.html # Optional: Custom HTML template
Output Structure Web Build web/build/ ├── index.html # Entry point ├── index.js # JavaScript loader └── index.wasm # WebAssembly binary
Linux Build linux/build/ └── my_app # Executable binary
Deployment Deploy Web Build
Test locally
cd web/build python3 -m http.server 8000 # Visit http://localhost:8000/index.html
Deploy to hosting
Upload web/build/ contents to:
Netlify
Vercel
GitHub Pages
Any static host
Deploy Linux Build
Create distribution package
mkdir -p dist cp linux/build/my_app dist/ cp -r assets dist/ # If you have assets tar czf my_app-linux.tar.gz dist/
Optimization Levels Debug (Development) Used by fern fire:
Optimization: -O2
Debug symbols: Included
Assertions: Enabled
Build time: Fast
File size: Large
Release (Production) Used by fern prepare:
Optimization: -O3
Debug symbols: Stripped
Assertions: Disabled (-DNDEBUG)
Build time: Slower
File size: Minimal
Web Build Details Emscripten Flags Production web builds use:
-std = c++17 # C++17 standard -O3 # Maximum optimization -s WASM= 1 # WebAssembly output -s ALLOW_MEMORY_GROWTH= 1 -s USE_WEBGL2= 1 -s EXPORTED_FUNCTIONS=['_main'] -s EXPORTED_RUNTIME_METHODS=['ccall','cwrap']
Custom Templates To customize the HTML wrapper:
Create web/template.html in your project
Use {{{ SCRIPT }}} placeholder for the generated code
Run fern prepare web
Example template:
<! doctype html > < html > < head > < title > My App </ title > < meta name = "viewport" content = "width=device-width, initial-scale=1" > < style > body { margin : 0 ; background : #000 ; display : flex ; justify-content : center ; align-items : center ; height : 100 vh ; } canvas { max-width : 100 % ; max-height : 100 % ; } </ style > </ head > < body > < canvas id = "canvas" ></ canvas > < script > var Module = { canvas: document . getElementById ( 'canvas' ) }; </ script > {{{ SCRIPT }}} </ body > </ html >
Linux Build Details Compiler Flags Production Linux builds use:
-std = c++17 # C++17 standard -O3 # Maximum optimization -DNDEBUG # Disable assertions -I/usr/local/include # Fern headers -L/usr/local/lib # Fern libraries -lfern # Link Fern library -lX11 -lXext # X11 libraries -lfontconfig # Font support -lfreetype # Font rendering
Common Issues
Not in a Fern project directory
fern prepare web # Error: Not in a Fern project directory
Solution: Navigate to a Fern project root (directory with fern.yaml).
fern prepare linux # Error: No main.cpp found in lib directory
Solution: Ensure lib/main.cpp exists in your project.
Emscripten not found (web)
fern prepare web # Error: Emscripten not found
Solution: Install Emscripten: git clone https://github.com/emscripten-core/emsdk.git cd emsdk ./emsdk install latest ./emsdk activate latest source ./emsdk_env.sh
Fern library not installed (linux)
fern prepare linux # Error: Fern C++ library is not installed globally
Solution: Install Fern: cd /path/to/fern ./install.sh
Typical build times and sizes for a medium-sized Fern project:
Platform Build Time Binary Size Optimized Linux (debug) 3s 850 KB No Linux (release) 8s 420 KB Yes Web (debug) 10s 1.2 MB No Web (release) 25s 650 KB Yes