pyemscripten_2026_5

pyemscripten_2026_5#

By default, all builds of the Pyodide runtime with Python 3.15 use the pyemscripten_2026_5 platform.

This section reflects the planned definition for the pyemscripten_2026_5 platform.

Platform Specification#

CPython and Emscripten Version#

CPython: 3.15
Emscripten: 6.0.5

Unwinding ABIs#

C++ libraries are built with WebAssembly exception handling (-fwasm-exceptions) and WebAssembly-based setjmp/longjmp (-sSUPPORT_LONGJMP=wasm). To enable exceptions and setjmp/longjmp, -fwasm-exceptions must be passed at compile time and link time.

Rust Toolchain#

When building Rust packages to target the pyemscripten_2026_5 platform, use Rust version 1.93.0 or later. As of writing this, we default to Rust 1.97.1.

Runtime Library Loading Path#

There is full support for RPATH. The dynamic loader will only load dependencies that are properly specified on the RPATH, just being in /lib/python3.15/site-packages/wheel_name.libs is not sufficient.

Libraries Linked to the Main Binary#

The following libraries are statically linked to the binary:

libegl.js
libeventloop.js
libGL
libhtml5_webgl.js
libhtml5.js
liblz4
libsdl.js
libwebgl.js
libwebsocket.js
bzip2
zlib
sqlite3 version 3.39.0
lzma (xz) version 5.2.2

Aside from sqlite3 and lzma, these libraries come from Emscripten ports and the versions of the libraries are determined by the version of Emscripten we build with. Any symbols from these static libraries may be used by shared libraries.

Changes from pyemscripten_2026_0#

The only change from pyemscripten_2026_0 to pyemscripten_2026_5 is that Emscripten was upgraded from 5.0.3 to 6.0.5.

Migration Notes#

There are three changes in Emscripten 6.0.5 that may cause trouble building Python packages.

`-shared` links a real dynamic library instead of a static library#

Until Emscripten 6.0.0, the -shared flag was not really respected, and passing it would generate a .so file that was actually a static archive. This difference in behavior can lead to differences in linking behavior. Generally this shows up when running configure. Pass -sFAKE_DYLIBS as an ldflag to opt into the old behavior of ignoring -shared.

OpenMP Supported by Emscripten 6.0.5 but not by Pyodide#

OpenMP is supported by Emscripten 6.0.5 but it requires pthreads. (On the other hand, -fopenmp-simd works fine.) Pyodide’s compiler and linker wrapper will exit with a nonzero status code if -fopenmp or related flags are passed, but if emcc is invoked directly it will successfully link a shared library that can only be loaded by a pthreads-enabled binary.

Bug involving C++ exceptions in optimizing builds with dwarf symbols#

A program with C++ exceptions and dwarf symbols may crash in wasm-opt with a message like:

wasm-opt: wasm-debug.cpp:439: void wasm::Debug::AddrExprMap::add(Expression *, const BinaryLocations::Span): Assertion `!endMap.contains(span.end)' failed.

To work around this, disable dwarf.

https://github.com/emscripten-core/emscripten/issues/27449