Fortitudo

Fortitudo is a browser-only C/C++ compiler explorer for JupyterLab, JupyterLite, and a standalone Lumino application. All three hosts use the same workbench and compiler worker. No kernel or remote compiler is needed.

The web app opens the standalone explorer. Select Try in Jupyter to use the explorer alongside C23 and C++23 notebooks. Both applications run in the browser.

Edit a function, choose a language, target, and optimization level, then select Compile or press Ctrl/Cmd+Enter. Inspect assembly and compiler messages; select a diagnostic to jump to its source location. Compilation runs in a worker, so editing remains available. Cancel terminates that worker; the next compile loads a fresh one.

The status strip shows download progress in MB, then preparation and compilation activity. Diagnostics lists the individual compiler downloads and any loading failure. Progress counts decoded asset bytes against their packaged sizes; compressed network transfers may be smaller. Asset verification requires HTTPS or a local server on localhost.

The defaults are C++23, WebAssembly, and O2. C23 and O0–O3 are available. The packaged LLVM runtime reports WebAssembly, x86-64, and AArch64 backends. Native targets produce assembly with Clang built-in headers only; the packaged C/C++ system headers are for WebAssembly.

Source, options, and pane layout are saved by the host. Jupyter also keeps a browser copy scoped to the current workspace, protecting recent edits while its workspace writes are deferred. Reopening restores editing state without compiling. Output is explicitly marked out of date when source or options change. Invalid saved state opens a usable default session with feedback.

Running locally

The prebuilt extension requires JupyterLab 4.6 or later. Once the release is published, install it with:

python -m pip install fortitudo

Restart JupyterLab and select Open Fortitudo in the launcher or command palette. The wheel includes the compiler; installing it does not require Node or a compiler build. The npm package also includes the runtime and exports the shared workbench for applications that supply their own Lumino host.

See CONTRIBUTING.md for the complete Pixi setup and build sequence. The compiler is a separate heavyweight build; frontend builds require its generated assets and verify their hashes.

For an already built checkout:

pixi run --as-is jupyter lab
pixi run --as-is jlpm serve
pixi run --as-is jlpm serve:standalone

In JupyterLab or JupyterLite, select Open Fortitudo in the launcher or command palette. The Lite testbed is served on port 8080; the standalone preview prints its local address. These commands run in separate terminals.

Built standalone and Lite directories can be served below a URL prefix. Keep each site's compiler assets at their generated relative location. Serve .js as JavaScript, .wasm as application/wasm, and .data as application/octet-stream. Compilation works offline after initialization; offline page reload is a separate feature.

Architecture

• Pure model, request construction, and diagnostic parsing.
• One instance-owned store outside React.
• Lumino commands orchestrate semantic changes and compiler effects.
• Functional React views, with explicit store and CodeMirror bridges.
• A shared Lumino workbench owns workers and view lifecycles, with tab panels inside resizable split panels. Jupyter owns docking of the workbench itself.
• Thin Jupyter and standalone adapters supply shell and persistence.

The shared package entry exports these contracts. Only src/jupyter/ imports JupyterLab packages; the plugin retains fortitudo:plugin.

C and C++ notebooks

Our JupyterLite site includes xeus-cpp 0.10.0, with C23 and C++23 kernels. Open Getting started.ipynb or C examples.ipynb and choose Run → Run All Cells. The examples use packaged standard library headers, define functions, and reuse state across cells.

The notebook interpreter runs in its own browser worker. It is independent of Fortitudo's assembly compiler: code, options, and results are not synchronized between them. Its Clang version also differs from the explorer's LLVM runtime. The first kernel start downloads the interpreter and its libraries. Browser memory limits apply; native processes, native platform APIs, and arbitrary native libraries are unavailable.

These kernels are included in our Lite site, not installed into native JupyterLab by the Fortitudo Python package.

Runtime and limits

The runtime is reproduced from WasmBolt with LLVM 23.1.0 and Emscripten 4.0.9. runtime/README.md records its origin, pins, licenses, and build details. Generated compiler/manifest.json records asset sizes and SHA-256 hashes. Browser test attachments record timings and Wasm memory observations.

The current compiler is large and reserves 256 MiB of initial Wasm memory, with memory growth enabled and a 32 MiB stack. Browser memory limits still apply. Cancellation releases the worker; a later compile must initialize another. Initialization and runtime failures offer a retry path. Ordinary compiler errors retain diagnostics and raw output.

This implementation produces assembly. IR/AST views, MLIR tools, graphs, execution, a terminal, sharing, arbitrary flags, automatic compilation, and automatic timeouts are deferred. The optional upstream MLIR driver remains packaged to preserve the runtime build, but is not downloaded eagerly.

Acknowledgments

Fortitudo’s browser compiler runtime is based on WasmBolt, created by Anutosh Bhat and released under the MIT License. We reuse WasmBolt’s compiler module, LLVM lifecycle adaptations, and compilation pipeline, with an adapted build recipe. Fortitudo adds the shared React/Lumino workbench, compiler worker service, and JupyterLab, JupyterLite, and standalone integrations. WasmBolt’s original copyright and license notice are included in our distributions.

The compiler itself is provided by LLVM/Clang and built for WebAssembly using Emscripten.

Our notebook environment uses xeus-cpp, CppInterOp, and jupyterlite-xeus, with browser packages from emscripten-forge. Their work makes interactive C and C++ notebooks possible without a server. The Lite build preserves package license notices alongside the kernel assets.

License

Fortitudo is BSD-3-Clause licensed. The compiler incorporates WasmBolt and other separately licensed software. Required notices are included in runtime/licenses/ and copied into each distribution.