mirror of https://github.com/python/cpython
256 lines
8.0 KiB
Markdown
256 lines
8.0 KiB
Markdown
# Python WebAssembly (WASM) build
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**WARNING: WASM support is highly experimental! Lots of features are not working yet.**
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This directory contains configuration and helpers to facilitate cross
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compilation of CPython to WebAssembly (WASM). For now we support
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*wasm32-emscripten* builds for modern browser and for *Node.js*. It's not
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possible to build for *wasm32-wasi* out-of-the-box yet.
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## wasm32-emscripten build
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Cross compiling to the wasm32-emscripten platform needs the
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[Emscripten](https://emscripten.org/) SDK and a build Python interpreter.
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Emscripten 3.1.8 or newer are recommended. All commands below are relative
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to a repository checkout.
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Christian Heimes maintains a container image with Emscripten SDK, Python
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build dependencies, WASI-SDK, wasmtime, and several additional tools.
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```
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# Fedora, RHEL, CentOS
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podman run --rm -ti -v $(pwd):/python-wasm/cpython:Z quay.io/tiran/cpythonbuild:emsdk3
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# other
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docker run --rm -ti -v $(pwd):/python-wasm/cpython quay.io/tiran/cpythonbuild:emsdk3
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```
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### Compile a build Python interpreter
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```shell
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mkdir -p builddir/build
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pushd builddir/build
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../../configure -C
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make -j$(nproc)
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popd
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```
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### Fetch and build additional emscripten ports
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```shell
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embuilder build zlib bzip2
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```
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### Cross compile to wasm32-emscripten for browser
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```shell
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mkdir -p builddir/emscripten-browser
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pushd builddir/emscripten-browser
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CONFIG_SITE=../../Tools/wasm/config.site-wasm32-emscripten \
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emconfigure ../../configure -C \
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--host=wasm32-unknown-emscripten \
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--build=$(../../config.guess) \
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--with-emscripten-target=browser \
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--with-build-python=$(pwd)/../build/python
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emmake make -j$(nproc)
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popd
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```
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Serve `python.html` with a local webserver and open the file in a browser.
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```shell
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emrun builddir/emscripten-browser/python.html
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```
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or
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```shell
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./Tools/wasm/wasm_webserver.py
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```
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and open http://localhost:8000/builddir/emscripten-browser/python.html . This
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directory structure enables the *C/C++ DevTools Support (DWARF)* to load C
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and header files with debug builds.
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### Cross compile to wasm32-emscripten for node
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```
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mkdir -p builddir/emscripten-node
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pushd builddir/emscripten-node
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CONFIG_SITE=../../Tools/wasm/config.site-wasm32-emscripten \
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emconfigure ../../configure -C \
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--host=wasm32-unknown-emscripten \
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--build=$(../../config.guess) \
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--with-emscripten-target=node \
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--with-build-python=$(pwd)/../build/python
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emmake make -j$(nproc)
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popd
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```
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```
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node --experimental-wasm-threads --experimental-wasm-bulk-memory builddir/emscripten-node/python.js
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```
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# wasm32-emscripten limitations and issues
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Emscripten before 3.1.8 has known bugs that can cause memory corruption and
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resource leaks. 3.1.8 contains several fixes for bugs in date and time
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functions.
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## Network stack
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- Python's socket module does not work with Emscripten's emulated POSIX
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sockets yet. Network modules like ``asyncio``, ``urllib``, ``selectors``,
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etc. are not available.
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- Only ``AF_INET`` and ``AF_INET6`` with ``SOCK_STREAM`` (TCP) or
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``SOCK_DGRAM`` (UDP) are available. ``AF_UNIX`` is not supported.
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- ``socketpair`` does not work.
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- Blocking sockets are not available and non-blocking sockets don't work
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correctly, e.g. ``socket.accept`` crashes the runtime. ``gethostbyname``
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does not resolve to a real IP address. IPv6 is not available.
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- The ``select`` module is limited. ``select.select()`` crashes the runtime
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due to lack of exectfd support.
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## processes, threads, signals
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- Processes are not supported. System calls like fork, popen, and subprocess
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fail with ``ENOSYS`` or ``ENOSUP``.
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- Signal support is limited. ``signal.alarm``, ``itimer``, ``sigaction``
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are not available or do not work correctly. ``SIGTERM`` exits the runtime.
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- Keyboard interrupt (CTRL+C) handling is not implemented yet.
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- Browser builds cannot start new threads. Node's web workers consume
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extra file descriptors.
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- Resource-related functions like ``os.nice`` and most functions of the
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``resource`` module are not available.
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## file system
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- Most user, group, and permission related function and modules are not
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supported or don't work as expected, e.g.``pwd`` module, ``grp`` module,
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``os.setgroups``, ``os.chown``, and so on. ``lchown`` and `lchmod`` are
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not available.
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- ``umask`` is a no-op.
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- hard links (``os.link``) are not supported.
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- Offset and iovec I/O functions (e.g. ``os.pread``, ``os.preadv``) are not
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available.
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- ``os.mknod`` and ``os.mkfifo``
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[don't work](https://github.com/emscripten-core/emscripten/issues/16158)
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and are disabled.
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- Large file support crashes the runtime and is disabled.
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- ``mmap`` module is unstable. flush (``msync``) can crash the runtime.
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## Misc
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- Heap memory and stack size are limited. Recursion or extensive memory
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consumption can crash Python.
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- Most stdlib modules with a dependency on external libraries are missing,
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e.g. ``ctypes``, ``readline``, ``sqlite3``, ``ssl``, and more.
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- Shared extension modules are not implemented yet. All extension modules
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are statically linked into the main binary.
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The experimental configure option ``--enable-wasm-dynamic-linking`` enables
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dynamic extensions.
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- glibc extensions for date and time formatting are not available.
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- ``locales`` module is affected by musl libc issues,
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[bpo-46390](https://bugs.python.org/issue46390).
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- Python's object allocator ``obmalloc`` is disabled by default.
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- ``ensurepip`` is not available.
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## wasm32-emscripten in browsers
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- The interactive shell does not handle copy 'n paste and unicode support
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well.
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- The bundled stdlib is limited. Network-related modules,
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distutils, multiprocessing, dbm, tests and similar modules
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are not shipped. All other modules are bundled as pre-compiled
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``pyc`` files.
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- Threading is not supported.
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- In-memory file system (MEMFS) is not persistent and limited.
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## wasm32-emscripten in node
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Node builds use ``NODERAWFS``, ``USE_PTHREADS`` and ``PROXY_TO_PTHREAD``
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linker options.
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- Node RawFS allows direct access to the host file system.
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- pthread support requires WASM threads and SharedArrayBuffer (bulk memory).
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The runtime keeps a pool of web workers around. Each web worker uses
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several file descriptors (eventfd, epoll, pipe).
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# Hosting Python WASM builds
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The simple REPL terminal uses SharedArrayBuffer. For security reasons
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browsers only provide the feature in secure environents with cross-origin
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isolation. The webserver must send cross-origin headers and correct MIME types
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for the JavaScript and WASM files. Otherwise the terminal will fail to load
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with an error message like ``Browsers disable shared array buffer``.
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## Apache HTTP .htaccess
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Place a ``.htaccess`` file in the same directory as ``python.wasm``.
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```
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# .htaccess
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Header set Cross-Origin-Opener-Policy same-origin
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Header set Cross-Origin-Embedder-Policy require-corp
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AddType application/javascript js
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AddType application/wasm wasm
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<IfModule mod_deflate.c>
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AddOutputFilterByType DEFLATE text/html application/javascript application/wasm
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</IfModule>
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```
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# Detect WebAssembly builds
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## Python code
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```# python
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import os, sys
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if sys.platform == "emscripten":
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# Python on Emscripten
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if sys.platform == "wasi":
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# Python on WASI
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if os.name == "posix":
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# WASM platforms identify as POSIX-like.
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# Windows does not provide os.uname().
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machine = os.uname().machine
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if machine.startswith("wasm"):
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# WebAssembly (wasm32 or wasm64)
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```
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## C code
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Emscripten SDK and WASI SDK define several built-in macros. You can dump a
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full list of built-ins with ``emcc -dM -E - < /dev/null`` and
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``/path/to/wasi-sdk/bin/clang -dM -E - < /dev/null``.
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```# C
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#ifdef __EMSCRIPTEN__
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// Python on Emscripten
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#endif
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```
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* WebAssembly ``__wasm__`` (also ``__wasm``)
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* wasm32 ``__wasm32__`` (also ``__wasm32``)
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* wasm64 ``__wasm64__``
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* Emscripten ``__EMSCRIPTEN__`` (also ``EMSCRIPTEN``)
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* Emscripten version ``__EMSCRIPTEN_major__``, ``__EMSCRIPTEN_minor__``, ``__EMSCRIPTEN_tiny__``
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* WASI ``__wasi__``
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Feature detection flags:
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* ``__EMSCRIPTEN_PTHREADS__``
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* ``__EMSCRIPTEN_SHARED_MEMORY__``
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* ``__wasm_simd128__``
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* ``__wasm_sign_ext__``
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* ``__wasm_bulk_memory__``
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* ``__wasm_atomics__``
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* ``__wasm_mutable_globals__``
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