069c96f84c
- Mark more ``umask()`` cases - ``dup()`` is not supported - ``/dev/null`` is not available - document missing features - mark more modules as not available |
||
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.. | ||
config.site-wasm32-emscripten | ||
config.site-wasm32-wasi | ||
python.html | ||
python.worker.js | ||
README.md | ||
Setup.local.example | ||
wasm_assets.py | ||
wasm_webserver.py |
Python WebAssembly (WASM) build
WARNING: WASM support is highly experimental! Lots of features are not working yet.
This directory contains configuration and helpers to facilitate cross compilation of CPython to WebAssembly (WASM). For now we support wasm32-emscripten builds for modern browser and for Node.js. WASI (wasm32-wasi) is work-in-progress
wasm32-emscripten build
For now the build system has two target flavors. The Emscripten/browser
target (--with-emscripten-target=browser
) is optimized for browsers.
It comes with a reduced and preloaded stdlib without tests and threading
support. The Emscripten/node
target has threading enabled and can
access the file system directly.
Cross compiling to the wasm32-emscripten platform needs the Emscripten SDK and a build Python interpreter. Emscripten 3.1.8 or newer are recommended. All commands below are relative to a repository checkout.
Christian Heimes maintains a container image with Emscripten SDK, Python build dependencies, WASI-SDK, wasmtime, and several additional tools.
# Fedora, RHEL, CentOS
podman run --rm -ti -v $(pwd):/python-wasm/cpython:Z quay.io/tiran/cpythonbuild:emsdk3
# other
docker run --rm -ti -v $(pwd):/python-wasm/cpython quay.io/tiran/cpythonbuild:emsdk3
Compile a build Python interpreter
mkdir -p builddir/build
pushd builddir/build
../../configure -C
make -j$(nproc)
popd
Fetch and build additional emscripten ports
embuilder build zlib bzip2
Cross compile to wasm32-emscripten for browser
mkdir -p builddir/emscripten-browser
pushd builddir/emscripten-browser
CONFIG_SITE=../../Tools/wasm/config.site-wasm32-emscripten \
emconfigure ../../configure -C \
--host=wasm32-unknown-emscripten \
--build=$(../../config.guess) \
--with-emscripten-target=browser \
--with-build-python=$(pwd)/../build/python
emmake make -j$(nproc)
popd
Serve python.html
with a local webserver and open the file in a browser.
emrun builddir/emscripten-browser/python.html
or
./Tools/wasm/wasm_webserver.py
and open http://localhost:8000/builddir/emscripten-browser/python.html . This directory structure enables the C/C++ DevTools Support (DWARF) to load C and header files with debug builds.
Cross compile to wasm32-emscripten for node
mkdir -p builddir/emscripten-node
pushd builddir/emscripten-node
CONFIG_SITE=../../Tools/wasm/config.site-wasm32-emscripten \
emconfigure ../../configure -C \
--host=wasm32-unknown-emscripten \
--build=$(../../config.guess) \
--with-emscripten-target=node \
--with-build-python=$(pwd)/../build/python
emmake make -j$(nproc)
popd
node --experimental-wasm-threads --experimental-wasm-bulk-memory builddir/emscripten-node/python.js
wasm32-emscripten limitations and issues
Emscripten before 3.1.8 has known bugs that can cause memory corruption and resource leaks. 3.1.8 contains several fixes for bugs in date and time functions.
Network stack
- Python's socket module does not work with Emscripten's emulated POSIX
sockets yet. Network modules like
asyncio
,urllib
,selectors
, etc. are not available. - Only
AF_INET
andAF_INET6
withSOCK_STREAM
(TCP) orSOCK_DGRAM
(UDP) are available.AF_UNIX
is not supported. socketpair
does not work.- Blocking sockets are not available and non-blocking sockets don't work
correctly, e.g.
socket.accept
crashes the runtime.gethostbyname
does not resolve to a real IP address. IPv6 is not available. - The
select
module is limited.select.select()
crashes the runtime due to lack of exectfd support.
processes, signals
- Processes are not supported. System calls like fork, popen, and subprocess
fail with
ENOSYS
orENOSUP
. - Signal support is limited.
signal.alarm
,itimer
,sigaction
are not available or do not work correctly.SIGTERM
exits the runtime. - Keyboard interrupt (CTRL+C) handling is not implemented yet.
- Resource-related functions like
os.nice
and most functions of theresource
module are not available.
threading
- Threading is disabled by default. The
configure
option--enable-wasm-pthreads
adds compiler flag-pthread
and linker flags-sUSE_PTHREADS -sPROXY_TO_PTHREAD
. - pthread support requires WASM threads and SharedArrayBuffer (bulk memory). The Node.JS runtime keeps a pool of web workers around. Each web worker uses several file descriptors (eventfd, epoll, pipe).
- It's not advised to enable threading when building for browsers or with dynamic linking support; there are performance and stability issues.
file system
- Most user, group, and permission related function and modules are not
supported or don't work as expected, e.g.
pwd
module,grp
module,os.setgroups
,os.chown
, and so on.lchown
and `lchmod`` are not available. umask
is a no-op.- hard links (
os.link
) are not supported. - Offset and iovec I/O functions (e.g.
os.pread
,os.preadv
) are not available. os.mknod
andos.mkfifo
don't work and are disabled.- Large file support crashes the runtime and is disabled.
mmap
module is unstable. flush (msync
) can crash the runtime.
Misc
- Heap memory and stack size are limited. Recursion or extensive memory consumption can crash Python.
- Most stdlib modules with a dependency on external libraries are missing,
e.g.
ctypes
,readline
,sqlite3
,ssl
, and more. - Shared extension modules are not implemented yet. All extension modules
are statically linked into the main binary. The experimental configure
option
--enable-wasm-dynamic-linking
enables dynamic extensions supports. It's currently known to crash in combination with threading. - glibc extensions for date and time formatting are not available.
locales
module is affected by musl libc issues, bpo-46390.- Python's object allocator
obmalloc
is disabled by default. ensurepip
is not available.
wasm32-emscripten in browsers
- The interactive shell does not handle copy 'n paste and unicode support well.
- The bundled stdlib is limited. Network-related modules,
distutils, multiprocessing, dbm, tests and similar modules
are not shipped. All other modules are bundled as pre-compiled
pyc
files. - In-memory file system (MEMFS) is not persistent and limited.
- Test modules are disabled by default. Use
--enable-test-modules
build test modules like_testcapi
.
wasm32-emscripten in node
Node builds use NODERAWFS
.
- Node RawFS allows direct access to the host file system without need to
perform
FS.mount()
call.
Hosting Python WASM builds
The simple REPL terminal uses SharedArrayBuffer. For security reasons
browsers only provide the feature in secure environents with cross-origin
isolation. The webserver must send cross-origin headers and correct MIME types
for the JavaScript and WASM files. Otherwise the terminal will fail to load
with an error message like Browsers disable shared array buffer
.
Apache HTTP .htaccess
Place a .htaccess
file in the same directory as python.wasm
.
# .htaccess
Header set Cross-Origin-Opener-Policy same-origin
Header set Cross-Origin-Embedder-Policy require-corp
AddType application/javascript js
AddType application/wasm wasm
<IfModule mod_deflate.c>
AddOutputFilterByType DEFLATE text/html application/javascript application/wasm
</IfModule>
WASI (wasm32-wasi)
WASI builds require WASI SDK 15.0+ and currently wasix for POSIX compatibility stubs.
WASI limitations and issues (WASI SDK 15.0)
A lot of Emscripten limitations also apply to WASI. Noticable restrictions are:
- Call stack size is limited. Default recursion limit and parser stack size are smaller than in regular Python builds.
socket(2)
cannot create new socket file descriptors. WASI programs can call read/write/accept on a file descriptor that is passed into the process.socket.gethostname()
and host name resolution APIs likesocket.gethostbyname()
are not implemented and always fail.chmod(2)
is not available. It's not possible to modify file permissions, yet. A future version of WASI may provide a limitedset_permissions
API.- User/group related features like
os.chown()
,os.getuid
, etc. are stubs or fail withENOTSUP
. - File locking (
fcntl
) is not available. os.pipe()
,os.mkfifo()
, andos.mknod()
are not supported.process_time
does not work as expected because it's implemented using wall clock.os.umask()
is a stub.sys.executable
is empty./dev/null
/os.devnull
may not be available.os.utime*()
is buggy in WASM SDK 15.0, see utimensat() with timespec=NULL sets wrong timeos.symlink()
fails withPermissionError
when attempting to create a symlink with an absolute path with wasmtime 0.36.0. The wasmtime runtime usesopenat2(2)
syscall with flagRESOLVE_BENEATH
to open files. The flag causes the syscall to reject symlinks with absolute paths.os.curdir
(aka.
) seems to behave differently, which breaks someimportlib
tests that add.
tosys.path
and indirectlysys.path_importer_cache
.- WASI runtime environments may not provide a dedicated temp directory.
Detect WebAssembly builds
Python code
import os, sys
if sys.platform == "emscripten":
# Python on Emscripten
if sys.platform == "wasi":
# Python on WASI
if os.name == "posix":
# WASM platforms identify as POSIX-like.
# Windows does not provide os.uname().
machine = os.uname().machine
if machine.startswith("wasm"):
# WebAssembly (wasm32, wasm64 in the future)
>>> import os, sys
>>> os.uname()
posix.uname_result(sysname='Emscripten', nodename='emscripten', release='1.0', version='#1', machine='wasm32')
>>> os.name
'posix'
>>> sys.platform
'emscripten'
>>> sys._emscripten_info
sys._emscripten_info(
emscripten_version=(3, 1, 8),
runtime='Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:99.0) Gecko/20100101 Firefox/99.0',
pthreads=False,
shared_memory=False
)
>>> sys._emscripten_info
sys._emscripten_info(emscripten_version=(3, 1, 8), runtime='Node.js v14.18.2', pthreads=True, shared_memory=True)
>>> import os, sys
>>> os.uname()
posix.uname_result(sysname='wasi', nodename='(none)', release='0.0.0', version='0.0.0', machine='wasm32')
>>> os.name
'posix'
>>> sys.platform
'wasi'
C code
Emscripten SDK and WASI SDK define several built-in macros. You can dump a
full list of built-ins with emcc -dM -E - < /dev/null
and
/path/to/wasi-sdk/bin/clang -dM -E - < /dev/null
.
#ifdef __EMSCRIPTEN__
// Python on Emscripten
#endif
- WebAssembly
__wasm__
(also__wasm
) - wasm32
__wasm32__
(also__wasm32
) - wasm64
__wasm64__
- Emscripten
__EMSCRIPTEN__
(alsoEMSCRIPTEN
) - Emscripten version
__EMSCRIPTEN_major__
,__EMSCRIPTEN_minor__
,__EMSCRIPTEN_tiny__
- WASI
__wasi__
Feature detection flags:
__EMSCRIPTEN_PTHREADS__
__EMSCRIPTEN_SHARED_MEMORY__
__wasm_simd128__
__wasm_sign_ext__
__wasm_bulk_memory__
__wasm_atomics__
__wasm_mutable_globals__