ExternalProject_Add with another CMake project

[oziphantom]

Is there a way to use CMake's ExternalProject_Add which then adds another CMake project and have the sub project build with WASM?
I've found a few examples with using emconfigure and emmake but nothing with cmake.

I've tried setting the CONFIGURE_COMMAND to emcmake cmake and the CMAKE_COMMAND to emcmake cmake but I just get errors that those commands are not known. Which is possibly a case of the spawned terminal does not have the SDK set up because it is POSIX and you have to manually do the source /yourpathhere/emsdk_env.sh to set it up, and you can't force everybody to set this up permanently because of the node version etc. And it seems that EMSDK is only set once this is done so you can't use that to kick it off either.

[sbc100]

The only thing that emcmake really does is set CMAKE_CROSSCOMPILING_EMULATOR and CMAKE_TOOLCHAIN_FILE.. I would assume that ExternalProject_Add would inherit those settings? If so it should not be necessary to use emcmake there.

[oziphantom]

when the External Project builds it builds with c++ rather than emscripten. so it looks like it works until you get to the linker phase and then it doesn't know what the file is because it is the wrong format. So the emscripten setup is being lost.

[sbc100]

Sounds like a general problem with ExternalProject_Add. How is cross compiling supposed to work in that setting?

[oziphantom]

seem the general way is
set(toolchainFile ${CMAKE_SOURCE_DIR}/cmake/toolchains/${toolchainTuple}.cmake)
.. then later

add CMAKE_ARGS -DCMAKE_TOOLCHAIN_FILE=${toolchainFile} in the ExternalProject_Add command.

I was able to get a verbose output that was showing me the before and after so I was able to replicate it with

set (EMSDK $ENV{EMSDK})
set (EMNODE $ENV{EMNODE}) # convert env to cmake vars for neatness

then inside the ExternalProject_Add on the CMAKE_ARGS set I have

-DCMAKE_TOOLCHAIN_FILE=${EMSDK}/upstream/emscripten/cmake/Modules/Platform/Emscripten.cmake
-DCMAKE_CROSSCOMPILING_EMULATOR={$EMNODE}
-DEMSCRIPTEN=ON

which then gets it building a emcscripten version of the sub project which then links fine.

[sbc100]

IIUC -DEMSCRIPTEN=ON should not be needed since that is set within the toolchain file?

[oziphantom]

I didn't add it at first, but inside the sub project CMAKE file I had to make some changes under it being defined, and the code would fail to build with an error msg that showed my changes hadn't been made. I added the -DEMSCRIPTEN=ON and then the changes were put in and it built. I agree though that I don't think I should have needed too.

[mcbarton]

@oziphantom Not sure if you have solved this issue yet, but I have done this here https://github.com/compiler-research/CppInterOp/blob/main/cmake/modules/GoogleTest.cmake , where we build GoogleTest as an external project using ExternalProject_Add .