Exception handling helpers do not work with ASSERTIONS=1

[canislupaster]

Please include the following in your bug report:

Version of emscripten/emsdk:

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 4.0.7-git (a53b1a34f0b31f05f2045f2d4aa1343f75180693)
Copyright (C) 2025 the Emscripten authors (see AUTHORS.txt)
This is free and open source software under the MIT license.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Failing command line in full:

em++ ./main.cpp -o main.js -fwasm-exceptions -sEXPORT_EXCEPTION_HANDLING_HELPERS=1

(Any C++ source file will work.)

This outputs a main.js where getExceptionMessage is in unexportedSymbols due to the implicit -sASSERTIONS=1. Importing main.js always causes an error since ASSERTIONS overrides the getter of anything in unexportedSymbols to throw an error:

emscripten/src/modules.mjs

Line 531 in c2d94c8

// in ASSERTIONS mode we show a useful error if it is used without being

The test in test_core.py, however, explicitly disables ASSERTIONS:

emscripten/test/test_core.py

Line 1380 in c2d94c8

self.set_setting('ASSERTIONS', 0)

I don't think disabling ASSERTIONS should be required to use exception helpers? If it needs to be, then you can probably dismiss this.

This stems from

emscripten/tools/link.py

Line 1839 in c2d94c8

settings.EXPORTED_FUNCTIONS += ['getExceptionMessage', 'incrementExceptionRefcount', 'decrementExceptionRefcount']

where the helper functions are added to EXPORTED_FUNCTIONS instead of EXPORTED_RUNTIME_METHODS, and changing this lets me import the file.

However, now getExceptionMessage with ASSERTIONS and Emscripten exceptions somehow triggers a WebAssembly.RuntimeError for memory access out of bounds. The __get_exception_message in system/lib/libcxxabi/src/cxa_exception_js_utils.cpp used from src/lib/libcore.js apparently dies while returning. I have no idea when this error is emitted or how ASSERTIONS could impact that.

Please let me know if I'm missing anything. Thanks for reading.

[sbc100]

The fact that getExceptionMessage is included in unexportedSymbols looks like a bug to me. I can take a look at that part

The fact that getExceptionMessage is crashing seems like when ASSERTIONS are enabled looks separate. Perhaps @aheejin knows about that one.