Undefined symbols when using `-sMAIN_MODULE=1`

[ruiqurm]

Hi.
I am working on porting a C++ framework to WebAssembly and have encountered some issues with dynamic linking. I know static linking is suggested in emscripten, but the framework currently needs the ability to load some modules dynamically.
I successfully compiled a SIDE_MODULE as libbuiltin_modules.so, which should be loadable at runtime using functions like dlopen. However, when I try to compile the main program as a MAIN_MODULE, I receive three warnings about undefined symbols(see the message below). These symbols all start with __sk, which is strange since I don’t have any symbols with that prefix in my code. After some research, I suspect they might be related to a system library.

When I add the -sERROR_ON_UNDEFINED_SYMBOLS=0 flag, the compilation succeeds, but the program aborts at runtime with the same undefined symbol errors.

I also tried compiling with the -sMAIN_MODULE=1 flag. This works fine until I attempt to load the side module with dlopen, at which point I receive the following error message:

Aborted(Assertion failed: undefined symbol '_ZNSt13runtime_errorD1Ev'. Perhaps a side module was not linked in? If this global was expected to arrive from a system library, try to build the MAIN_MODULE with EMCC_FORCE_STDLIBS=1 in the environment)

Apparently, it is a system library symbol. I suspect it might have been eliminated by using the -sMAIN_MODULE=2 flag. Unfortunately, I can't use the suggested solution (EMCC_FORCE_STDLIBS=1) because compiling with -sMAIN_MODULE leads to three missing symbols.

Any help is appreciated. thanks!

Version of emscripten/emsdk:
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.64 (a1fe390)
clang version 19.0.0git (https:/github.com/llvm/llvm-project 4d8e42ea6a89c73f90941fd1b6e899912e31dd34)

Failing command line in full:
This command is generated by cmake:

/home/sam/emsdk/upstream/emscripten/em++  -fwasm-exceptions -fPIC -g -sMAIN_MODULE=1 --preload-file /home/sam/bmf/big_bunny_10s_30fps.mp4@big_bunny_10s_30fps.mp4 --preload-file /home/sam/bmf/BUILTIN_CONFIG.json@BUILTIN_CONFIG.json --preload-file /home/sam/bmf/build/output/bmf/lib/libbuiltin_modules.so@/lib/libbuiltin_modules.so -sALLOW_MEMORY_GROWTH @CMakeFiles/run_copy_module.dir/objects1.rsp -o ../../../output/bmf/bin/run_copy_module.html @CMakeFiles/run_copy_module.dir/linklibs.rsp

Full link command and output with -v appended:

/home/sam/emsdk/upstream/emscripten/em++  -fwasm-exceptions -fPIC -g -sMAIN_MODULE=1 --preload-file /home/sam/bmf/big_bunny_10s_30fps.mp4@big_bunny_10s_30fps.mp4 --preload-file /home/sam/bmf/BUILTIN_CONFIG.json@BUILTIN_CONFIG.json --preload-file /home/sam/bmf/build/output/bmf/lib/libbuiltin_modules.so@/lib/libbuiltin_modules.so -sALLOW_MEMORY_GROWTH @CMakeFiles/run_copy_module.dir/objects1.rsp -o ../../../output/bmf/bin/run_copy_module.html @CMakeFiles/run_copy_module.dir/linklibs.rsp
error: undefined symbol: __skCPU0AllocatorTag (referenced by root reference (e.g. compiled C/C++ code))
warning: To disable errors for undefined symbols use `-sERROR_ON_UNDEFINED_SYMBOLS=0`
warning: ___skCPU0AllocatorTag may need to be added to EXPORTED_FUNCTIONS if it arrives from a system library
error: undefined symbol: __skCPUManagerTag (referenced by root reference (e.g. compiled C/C++ code))
warning: ___skCPUManagerTag may need to be added to EXPORTED_FUNCTIONS if it arrives from a system library
error: undefined symbol: __skCPUcopy_stubDispatchTag (referenced by root reference (e.g. compiled C/C++ code))
warning: ___skCPUcopy_stubDispatchTag may need to be added to EXPORTED_FUNCTIONS if it arrives from a system library
Error: Aborting compilation due to previous errors
em++: error: '/home/sam/emsdk/node/18.20.3_64bit/bin/node /home/sam/emsdk/upstream/emscripten/src/compiler.mjs /tmp/tmpzjmupk3m.json' failed (returned 1)

[ruiqurm]

Hi. I am working on porting a C++ framework to WebAssembly and have encountered some issues with dynamic linking. I know static linking is suggested in emscripten, but the framework currently needs the ability to load some modules dynamically. I successfully compiled a SIDE_MODULE as libbuiltin_modules.so, which should be loadable at runtime using functions like dlopen. However, when I try to compile the main program as a MAIN_MODULE, I receive three warnings about undefined symbols(see the message below). These symbols all start with __sk, which is strange since I don’t have any symbols with that prefix in my code. After some research, I suspect they might be related to a system library.

When I add the -sERROR_ON_UNDEFINED_SYMBOLS=0 flag, the compilation succeeds, but the program aborts at runtime with the same undefined symbol errors.

I also tried compiling with the -sMAIN_MODULE=1 flag. This works fine until I attempt to load the side module with dlopen, at which point I receive the following error message:

Aborted(Assertion failed: undefined symbol '_ZNSt13runtime_errorD1Ev'. Perhaps a side module was not linked in? If this global was expected to arrive from a system library, try to build the MAIN_MODULE with EMCC_FORCE_STDLIBS=1 in the environment)

Apparently, it is a system library symbol. I suspect it might have been eliminated by using the -sMAIN_MODULE=2 flag. Unfortunately, I can't use the suggested solution (EMCC_FORCE_STDLIBS=1) because compiling with -sMAIN_MODULE leads to three missing symbols.

Any help is appreciated. thanks!

Version of emscripten/emsdk: emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.64 (a1fe390) clang version 19.0.0git (https:/github.com/llvm/llvm-project 4d8e42ea6a89c73f90941fd1b6e899912e31dd34)

Failing command line in full: This command is generated by cmake:

/home/sam/emsdk/upstream/emscripten/em++  -fwasm-exceptions -fPIC -g -sMAIN_MODULE=1 --preload-file /home/sam/bmf/big_bunny_10s_30fps.mp4@big_bunny_10s_30fps.mp4 --preload-file /home/sam/bmf/BUILTIN_CONFIG.json@BUILTIN_CONFIG.json --preload-file /home/sam/bmf/build/output/bmf/lib/libbuiltin_modules.so@/lib/libbuiltin_modules.so -sALLOW_MEMORY_GROWTH @CMakeFiles/run_copy_module.dir/objects1.rsp -o ../../../output/bmf/bin/run_copy_module.html @CMakeFiles/run_copy_module.dir/linklibs.rsp

Full link command and output with -v appended:

/home/sam/emsdk/upstream/emscripten/em++  -fwasm-exceptions -fPIC -g -sMAIN_MODULE=1 --preload-file /home/sam/bmf/big_bunny_10s_30fps.mp4@big_bunny_10s_30fps.mp4 --preload-file /home/sam/bmf/BUILTIN_CONFIG.json@BUILTIN_CONFIG.json --preload-file /home/sam/bmf/build/output/bmf/lib/libbuiltin_modules.so@/lib/libbuiltin_modules.so -sALLOW_MEMORY_GROWTH @CMakeFiles/run_copy_module.dir/objects1.rsp -o ../../../output/bmf/bin/run_copy_module.html @CMakeFiles/run_copy_module.dir/linklibs.rsp
error: undefined symbol: __skCPU0AllocatorTag (referenced by root reference (e.g. compiled C/C++ code))
warning: To disable errors for undefined symbols use `-sERROR_ON_UNDEFINED_SYMBOLS=0`
warning: ___skCPU0AllocatorTag may need to be added to EXPORTED_FUNCTIONS if it arrives from a system library
error: undefined symbol: __skCPUManagerTag (referenced by root reference (e.g. compiled C/C++ code))
warning: ___skCPUManagerTag may need to be added to EXPORTED_FUNCTIONS if it arrives from a system library
error: undefined symbol: __skCPUcopy_stubDispatchTag (referenced by root reference (e.g. compiled C/C++ code))
warning: ___skCPUcopy_stubDispatchTag may need to be added to EXPORTED_FUNCTIONS if it arrives from a system library
Error: Aborting compilation due to previous errors
em++: error: '/home/sam/emsdk/node/18.20.3_64bit/bin/node /home/sam/emsdk/upstream/emscripten/src/compiler.mjs /tmp/tmpzjmupk3m.json' failed (returned 1)

It's strange. When I use wasm-objdump on the target wasm file, which is run_copy_module.wasm here, it outputs that the symbol exists in GOT table. Does it mean the symbol is defined?

➜ wasm-objdump -x  ./build/output/bmf/bin/run_copy_module.wasm | grep __skCPU0AllocatorTag  
 - global[3] i32 mutable=1 <- GOT.mem.__skCPU0AllocatorTag
 - global[3] <__skCPU0AllocatorTag>

[sbc100]

When I build any C++ project with -sMAIN_MODULE=1 I see _ZNSt15underflow_errorD2Ev defined in the module and exported:

- func[8192] <_ZNSt15underflow_errorD2Ev> -> "_ZNSt13runtime_errorD1Ev"

Do you not see this in your wasm module?

Regarding the non-system symbols such as skCPU0AllocatorTag, do you know where those symbols are supposed to be defined? Are they supposed to be defined in the main module or in a side module? The existence of the symbol in the GOT.mem namespace simply means that main module is importing an address from the dynamic linker and that the symbol is expected for be defined at runtime, outside of the main module. Presumably that is not intentional, and instead you expect the symbol to be defined in the main module and exported?

[ruiqurm]

When I build any C++ project with -sMAIN_MODULE=1 I see _ZNSt15underflow_errorD2Ev defined in the module and exported:

- func[8192] <_ZNSt15underflow_errorD2Ev> -> "_ZNSt13runtime_errorD1Ev"

Do you not see this in your wasm module?

Regarding the non-system symbols such as skCPU0AllocatorTag, do you know where those symbols are supposed to be defined? Are they supposed to be defined in the main module or in a side module? The existence of the symbol in the GOT.mem namespace simply means that main module is importing an address from the dynamic linker and that the symbol is expected for be defined at runtime, outside of the main module. Presumably that is not intentional, and instead you expect the symbol to be defined in the main module and exported?

After using objdump to inspect all the related object files, I finally found the problem. In our project, we define a macro to prevent a static variable from being removed by the compiler. However, the macro definition had changed recently. I rolled back the macro, and now the compilation works. Thanks.