CMake with emscripten build - undefined symbol: _embind_register_class

[brandonmpetty]

I am the maintainer of this project: https://github.com/brandonmpetty/Doxa

I have had really great luck using emscripten in the past, and I want to further improve that experience by automating the build process using CMake.

This CMake script will download the latest version of EMScripten, install it, and then build my WASM... except I am getting a linker error on that last part.

So, my tradition command line way was:

em++ -O3 -o ./dist/doxaWasm.js DoxaWasm.cpp -std=c++1z -s WASM=1 -s NO_EXIT_RUNTIME=1 -s ALLOW_MEMORY_GROWTH=1 --bind -s EXPORTED_FUNCTIONS="['_malloc','_free']"

My CMake looks like this

cmake_minimum_required(VERSION 3.16)
project(doxa_wasm)


message("Cloning EMScripten")
include(FetchContent)
FetchContent_Declare(
  ems
  GIT_REPOSITORY "https://github.com/emscripten-core/emsdk.git"
  GIT_TAG "main"
  SOURCE_DIR "${CMAKE_BINARY_DIR}/emsdk"
)
FetchContent_MakeAvailable(ems)

message("Initializing latest EMScripten environment")
exec_program("${CMAKE_BINARY_DIR}/emsdk/emsdk install latest")
exec_program("${CMAKE_BINARY_DIR}/emsdk/emsdk activate latest")

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_COMPILER "${CMAKE_BINARY_DIR}/emsdk/upstream/emscripten/em++")

add_executable(
  doxa_wasm
  DoxaWasm.cpp
)

set_target_properties(
  doxa_wasm
  PROPERTIES
    PREFIX ""
    SUFFIX ".js"
    COMPILE_FLAGS -O3 -std=c++17 -s WASM=1 -s NO_EXIT_RUNTIME=1 -s ALLOW_MEMORY_GROWTH=1 -s EXPORTED_FUNCTIONS="['_malloc','_free']" --bind
)

My errors:

brandito@DARTH-BEVUS:/mnt/c/Users/Brandito/source/repos/Doxa/Bindings/WebAssembly$ cmake --build build
[ 50%] Building CXX object CMakeFiles/doxa_wasm.dir/DoxaWasm.cpp.o
shared:INFO: (Emscripten: Running sanity checks)
[100%] Linking CXX executable doxa_wasm.js
cache:INFO: generating system asset: symbol_lists/7a974f7065b03f039b58c3afd6b04fb646712f63.json... (this will be cached in "/mnt/c/Users/Brandito/source/repos/Doxa/Bindings/WebAssembly/build/emsdk/upstream/emscripten/cache/symbol_lists/7a974f7065b03f039b58c3afd6b04fb646712f63.json" for subsequent builds)
cache:INFO:  - ok
wasm-ld: error: CMakeFiles/doxa_wasm.dir/DoxaWasm.cpp.o: undefined symbol: _embind_register_class
wasm-ld: error: CMakeFiles/doxa_wasm.dir/DoxaWasm.cpp.o: undefined symbol: _embind_register_class_constructor
...
wasm-ld: error: too many errors emitted, stopping now (use -error-limit=0 to see all errors)
em++: error: '/mnt/c/Users/Brandito/source/repos/Doxa/Bindings/WebAssembly/build/emsdk/upstream/bin/wasm-ld -o doxa_wasm.wasm CMakeFiles/doxa_wasm.dir/DoxaWasm.cpp.o -L/mnt/c/Users/Brandito/source/repos/Doxa/Bindings/WebAssembly/build/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten -lGL -lal -lhtml5 -lstubs-debug -lnoexit -lc-debug -ldlmalloc -lcompiler_rt -lc++-noexcept -lc++abi-debug-noexcept -lsockets -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr /tmp/tmpwyexr1fwlibemscripten_js_symbols.so --strip-debug --export-if-defined=main --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_lib_deps --export-if-defined=__stop_em_lib_deps --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=__main_argc_argv --export-if-defined=fflush --export=emscripten_stack_get_end --export=emscripten_stack_get_free --export=emscripten_stack_get_base --export=emscripten_stack_get_current --export=emscripten_stack_init --export=stackSave --export=stackRestore --export=stackAlloc --export=__errno_location --export=__get_temp_ret --export=__set_temp_ret --export=__wasm_call_ctors --export-table -z stack-size=65536 --initial-memory=16777216 --no-entry --max-memory=16777216 --stack-first' failed (returned 1)

Do you see anything obvious I am missing? I tried copying the COMPILE_FLAGS for LINK_FLAGS just to see, but that didn't help. I am pretty new to CMake, so I am open to all suggestions.

Thank you!

[sbc100]

I highly recommend using ninja rather than make when using cmake. You can do this -G Ninja on the cmake command line. One of the advantages of using ninja is that when a command fails (such as the link command in this case) , it shows you the failing command. As it stands you cannot see the em++ command that failed.

I'm imagining the problem is that --bind is not being specified as a linker flag. I think you need to move most those flags from COMPILE_FLAGS to LINK_FLAGS. Only -O3 -std=c++17 looks like compile flags to me.

(If you want to keep using make then I think you can run with cmake --build build --verbose to have to output the full command lines).

[brandonmpetty]

Thank you @sbc100, that was exactly it.
I haven't tested the build, but it completed.

set_target_properties(
  doxa_wasm
  PROPERTIES
    PREFIX ""
    SUFFIX ".js"
    COMPILE_FLAGS "-O3 -std=c++17"
    LINK_FLAGS "-s WASM=1 -s NO_EXIT_RUNTIME=1 -s ALLOW_MEMORY_GROWTH=1 -s EXPORTED_FUNCTIONS=\"['_malloc','_free']\" --bind"
)

One followup question. The size of the output js and wasm between the CMake version and the em++ cmd line version seem large.
143KB JS file vs 56KB. 126KB WASM vs 102KB.

It feels like I am over looking something. I'll try Ninja and checkout the verbose option to see what could be different between the two.
If you see anything strange, let me know, but I appreciate the help! Thank you.

[sbc100]

You need to use -O3 at link time too if you want emcc to attempt to optimize the final output.

[brandonmpetty]

That was it. Thank you again!