Short term fix/wrokaround is here: https://github.com/emscripten-core/emscripten/pull/16149
On Sat, Jan 29, 2022 at 11:32 AM Sam Clegg <[email protected]> wrote: > The undefined symbol error you are seeing here is coming from the > post-linking phase. The way EM_JS works is that the function is that > function `foo` declared as external using > `__attribute__((import_name("foo")))` and the data symbol `__em_js_foo` is > defined in the data section along with `__attribute__((used, > visibility("default")))`. For more details on this see > https://github.com/emscripten-core/emscripten/blob/main/system/include/emscripten/em_js.h#L23-L49 > . > > I believe the problem you are seeing stems from the different meaning of > `__attribute__((used))` under emscripten compared to with triples. The > problem stems from the fact that we use `__attribute__((used))` to > implement the EMSCRIPTEN_KEEPALIVE macro, which is defined to mean "keep > this symbol alive *and* export it to JS under its symbol name". > > If you use wasm-objdump to look at an object file containing EM_JS symbols > you will see them marked as both "no_strip" and "exported". For example: > > ``` > - 38: D <__em_js__noarg> segment=0 offset=0 size=36 [ exported no_strip > binding=global vis=default ] > - 39: D <__em_js__noarg_int> segment=0 offset=36 size=55 [ exported > no_strip binding=global vis=default ] > - 40: D <__em_js__noarg_double> segment=0 offset=91 size=61 [ exported > no_strip binding=global vis=default ] > - 41: D <__em_js__intarg> segment=0 offset=152 size=41 [ exported > no_strip binding=global vis=default ] > ``` > > If you compile the same source using a non-emscripten triple you will see > them only marked as `no_strip` which is a more traditional meaning of the > `used` attribute which simply tells the linker to keep them around in the > binary, not to export them. Here is where the hack/difference is: > https://github.com/llvm/llvm-project/blob/333f5019300c6e56782374627e64da0b62ffa3bc/llvm/lib/MC/WasmObjectWriter.cpp#L1773-L1777 > > There are two ways we can solve this issue I believe. > > 1. Long term solution: Stop abusing `__attribute__((used))`, and thus > remove this special handling in emscripten. We should really have a > separate attribute to mark a symbol as exported. I've been trying to get > this done for while but its stalled. See https://reviews.llvm.org/D76547 > 2. Short term solution: Use the more explicit (but not > EMSCIRPTEN_KEEPALIVE-compatible), 'export-name' attribute in em_js.h. I > think this should "just work". > > cheers, > sam > > > On Sat, Jan 29, 2022 at 10:22 AM Floh <[email protected]> wrote: > >> Spot on Alon :) >> >> It works if I hardwire just the C library (with the EM_JS functions) to >> the wasm32-emscripten triple. >> >> The Zig code needs to be compiled either with wasm32-wasi or >> wasm32-freestanding, when using wasm32-emscripten, parts of the Zig stdlib >> won't compile. >> >> Also, when I tried to use wasm32-freestanding with the C code, then >> wasm-ld complained about some missing stack-check functions (don't have the >> exact symbol at hand currently). >> >> ...I think I have enough to build a little 'proof-of-concept', even >> though it's a bit hacky :) >> >> Thanks! >> -Floh. >> On Saturday, 29 January 2022 at 18:58:53 UTC+1 [email protected] wrote: >> >>> Sam can confirm, but I would guess perhaps the emscripten triple is >>> necessary. That is, clang and/or wasm-ld might do something for EM_JS code >>> but only in emscripten mode. >>> >>> If we can confirm that then we should definitely get a bug filed on Zig >>> - hopefully it would be easy to add support for the emscripten triple there >>> and open up a bunch of use cases... >>> >>> On Sat, Jan 29, 2022 at 9:12 AM Floh <[email protected]> wrote: >>> >>>> I'm currently tinkering with bringing one of my toy Zig projects to the >>>> web via >>>> Alon's nice gist here which uses emcc only for the linker step: >>>> >>>> https://gist.github.com/kripken/58c0e640227fe5bac9e7b30100a2a1d3 >>>> >>>> ...and it *nearly* works except for code that uses EM_JS() macros. >>>> >>>> The project (https://github.com/floooh/pacman.zig) consists of some C >>>> code (my cross-platform 'sokol headers') which uses EM_JS() quite >>>> extensively (very handy for STB-style single-file libraries), and at the >>>> top, the "game code" is written in Zig. >>>> >>>> I'm compiling all code with Zig with the wasm32-wasi target >>>> (wasm32-emscripten exists, but currently doesn't seem to be supported by >>>> the Zig compiler), and then use emcc for linking. >>>> >>>> Long story short, it works except for the one problem that emcc cannot >>>> resolve any functions which have been defined with EM_JS(). If I compile >>>> the same library with emcc instead of Zig it works. >>>> >>>> So my question is: does emcc also do some "EM_JS() magic" when >>>> compiling the source code which contains EM_JS macros? Maybe I'm missing >>>> some Clang command line options which emcc inserts? >>>> >>>> The errors look like this: >>>> >>>> error: undefined symbol: sapp_js_add_clipboard_listener (referenced by >>>> top-level compiled C/C++ code) >>>> >>>> Followed by: >>>> >>>> warning: _sapp_js_add_clipboard_listener may need to be added to >>>> EXPORTED_FUNCTIONS if it arrives from a system library >>>> ...there's also a single warning about malloc: >>>> >>>> ...if I compile with "-s ERROR_ON_UNDEFINED_SYMBOLS=0", then the code >>>> breaks at runtime failing to resolve those EM_JS() functions, e.g.: >>>> >>>> "missing function: sapp_js_pointer_init" >>>> >>>> Compiling the same static link library with emcc, it magically works. >>>> >>>> If I look at both libraries with nm I don't see much of a difference, >>>> e.g. here's the relevant parts from the emcc-compiled library, every EM_JS >>>> symbol has an "D __em_js..." entry, and a matching "U sapp_js..." entry, >>>> e.g.: >>>> >>>> 0000185f D __em_js__sapp_js_add_beforeunload_listener >>>> ... >>>> U sapp_js_add_beforeunload_listener >>>> ... >>>> >>>> The Zig-compiled library has the same entries: >>>> >>>> 00001841 D __em_js__sapp_js_add_beforeunload_listener >>>> ... >>>> U sapp_js_add_beforeunload_listener >>>> ... >>>> >>>> ...yet one library (the zig-compiled) produces linker errors for those >>>> symbols, and the other (emcc-compiled) works. >>>> >>>> Clearly I'm missing something. I was expecting that all the EM_JS() >>>> magic is in the linker (by extracting the __em_js_* Javascript source code >>>> strings, and then "somehow" providing the C function import). Any ideas >>>> what I'm missing? >>>> >>>> Thanks! >>>> -Floh. >>>> >>>> >>>> >>>> >>>> >>>> -- >>>> You received this message because you are subscribed to the Google >>>> Groups "emscripten-discuss" group. >>>> To unsubscribe from this group and stop receiving emails from it, send >>>> an email to [email protected]. >>>> To view this discussion on the web visit >>>> https://groups.google.com/d/msgid/emscripten-discuss/15129292-2f07-44d9-99a9-a27ac4721a0cn%40googlegroups.com >>>> <https://groups.google.com/d/msgid/emscripten-discuss/15129292-2f07-44d9-99a9-a27ac4721a0cn%40googlegroups.com?utm_medium=email&utm_source=footer> >>>> . >>>> >>> -- >> You received this message because you are subscribed to the Google Groups >> "emscripten-discuss" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/emscripten-discuss/ad13a6a8-0248-406d-ba91-591bcec62e54n%40googlegroups.com >> <https://groups.google.com/d/msgid/emscripten-discuss/ad13a6a8-0248-406d-ba91-591bcec62e54n%40googlegroups.com?utm_medium=email&utm_source=footer> >> . >> > -- You received this message because you are subscribed to the Google Groups "emscripten-discuss" group. 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