link error: undefined symbol of the EM_JS in EMSDK 3.1.28

[xbcnn]

I have an EM_JS function defined in individual file and it works just fine before. But recently I upgraded EMSDK to 3.1.28, I received linking error: undefined symbol of the EM_JS function.

I have opened EMCC_DEBUG=1, and got following errors.

profiler:DEBUG: block "JS symbol generation" took 0.391 seconds

wasm-ld: error: CMakeFiles/PRJ_dir/Unity/unity_5_cxx.cxx.o: undefined symbol: THE_EM_JS

I checked the __em_js_xxx symbols existing in another unity_12_cxx.cxx.o file. Why the Binaryen failed to handle it?
How can I debug the Binaryen processing or maybe logs?

cc @sbc100 , any ideas?

[sbc100]

I think this is likely related to the fix for #18370. Basically, after this change EM_JS functions appear like natively defined functions with a leading underscore.

I assume what is happening here is that you are using EM_JS functions from JS code or from object files other than the one in which they are defined... how are you declaring THE_EM_JS ?

Ultimately we might need to reverse that change to add the leading underscore to EM_JS functions if this effects many folks.

[sbc100]

@kleisauke was also effect my this issue (which I'm guessing is the extra leading underscore.

[kleisauke]

PR #18391 was landed as commit a742129 after Emscripten 3.1.28 was released. It's only available in the tip-of-tree build within emsdk at the time of writing.

Perhaps the issue is related to the LLD_REPORT_UNDEFINED change? FWIW, the Emscripten port of libffi removed the need of -sLLD_REPORT_UNDEFINED=0 with commit hoodmane/libffi-emscripten@d1798d8 (i.e. by implementing a native wrapper function).

[sbc100]

Ah yes, in that case this must be related instead to #16003.

With this change all direct callers of EM_JS functions either need to exist in the same object file where it is defined, or they need to define their own __attribute__(import_name('...')) when they declare it.

[xbcnn]

I assume what is happening here is that you are using EM_JS functions from JS code or from object files other than the one in which they are defined... how are you declaring THE_EM_JS ?

Yes. I use the EM_JS function in several other source files with extern "C" specifier and the exact name(no leading underscore) and parameters in the EM_JS.

[sbc100]

#16003 did indeed lead to those symbols being mangled with a leading underscore. However #18438 reverses that so that shouldn't be needed (assuming #18438 lands).

[xbcnn]

That's nice! So I suppose it should be available in 3.1.29. #

[sbc100]

Yes, 3.1.29 should have the same behaviour as 3.1.28.. the change in questions was never part of a release.