C++ stacktraces on exceptions

[arcanis]

I'm using the snippet described in this part of the documentation to catch C++ exceptions:

https://emscripten.org/docs/porting/Debugging.html#handling-c-exceptions-from-javascript

However, it doesn't print the stacktrace where the exception originally got thrown - is it possible to get this information?

[sbc100]

That example it designed to show how to get information on the thing that was thrown I believe.

For the stacktrace can you not just get it from .stack on the exception you catch? Does devtools show the full stacktrace?

[arcanis]

Emscripten exceptions. Basically I have a C++ std::map<T>::at() function throwing, but I can't know what part of the application threw it without bisecting with logs.

[sbc100]

When you catch exceptions in JS they should have the full stack trace attached.

For an example of this try running ./tests/runner core0.test_abort_on_exceptions -v:

Here is part of the output:

-- begin program stderr --
Aborted(unhandled exception: Error: crash,Error: crash
    at throwException (/tmp/emscripten_test_core0_if8xjkp7/test_abort_on_exception.js:1235:35)
    at cFunc (<anonymous>:wasm-function[16]:0x4b5)
    at /tmp/emscripten_test_core0_if8xjkp7/test_abort_on_exception.js:993:23
    at /tmp/emscripten_test_core0_if8xjkp7/test_abort_on_exception.js:972:22
    at /tmp/emscripten_test_core0_if8xjkp7/test_abort_on_exception.js:3582:5
    at callRuntimeCallbacks (/tmp/emscripten_test_core0_if8xjkp7/test_abort_on_exception.js:1432:26)
    at postRun (/tmp/emscripten_test_core0_if8xjkp7/test_abort_on_exception.js:756:3)
    at doRun (/tmp/emscripten_test_core0_if8xjkp7/test_abort_on_exception.js:3514:5)
    at run (/tmp/emscripten_test_core0_if8xjkp7/test_abort_on_exception.js:3527:5)
    at runCaller (/tmp/emscripten_test_core0_if8xjkp7/test_abort_on_exception.js:3425:19)
    at removeRunDependency (/tmp/emscripten_test_core0_if8xjkp7/test_abort_on_exception.js:873:7)
    at receiveInstance (/tmp/emscripten_test_core0_if8xjkp7/test_abort_on_exception.js:1134:5)
    at receiveInstantiationResult (/tmp/emscripten_test_core0_if8xjkp7/test_abort_on_exception.js:1152:5))
-- end program stderr --
ok

I tried modifying tests/core/test_abort_on_exception_post.js and I could see that each of the exceptions has a .stack attribute that works as expected.

[aheejin]

@sbc100 Is this about abort?

I think Emscripten EH doesn't throw an Error instance, so stack traces are not supposed to be attached, no?

emscripten/src/parseTools.js

Line 687 in d2e80a6

return `throw ${what};`;

I think it's worthwile to make an option to do the stack trace though, both of Emscripten EH and Wasm EH.

[sbc100]

No, not about abort. Just using that test to show that the catch handler has access to the stack trace.

[sbc100]

Oh I see, the example I picked doesn't use C++ throw but throws an Error, which is why it gets a backtrace. I agree having an option to get backtraces for C++ exceptions would be nice.

Also, would be nice to add a new setting by just tie it to existing ASSERTIONS settings

[aheejin]

I filed an issue for this (#17466) for the reminder to work on this, hopefully in near future. I'm not sure why it's not shown here. (Do discussions work differently from issues maybe?)

[aheejin]

#17979 will fix this for Wasm EH. Supporting stack traces in Emscripten EH doesn't seem to be easy, so we are not planning on that atm.

[major-mayer]

Great work!
Thanks a lot, this will make debugging a lot easier.