Since CMake 4.2.0, emcmake prepends source files before compiler flags

[Sackzement]

and causes following warning to appear(in the MRE):

clang: warning: '-x c' after last input file has no effect [-Wunused-command-line-argument]

This is observable inside the build.make file.

MRE:

/path/to/MyProject/CMakeLists.txt

cmake_minimum_required(VERSION 3.16)
project(MyProject)
add_executable(${PROJECT_NAME} main.c)
target_compile_options(${PROJECT_NAME} PRIVATE -x c)

/path/to/MyProject/main.c

int main(){}

build emcc

mkdir build-emcc &&
cd build-emcc &&
emcmake cmake .. &&
cmake --build .

build clang

mkdir build-clang &&
cd build-clang &&
export CC="$(which clang)" &&
cmake .. &&
cmake --build .

In the file build.make, (here in the last line),
for emcc, main.c is prepended before the compiler flags
but for clang, main.c comes after the flags:

/path/to/MyProject/build-emcc/CMakeFiles/MyProject.dir/build.make

CMakeFiles/MyProject.dir/codegen:
.PHONY : CMakeFiles/MyProject.dir/codegen

CMakeFiles/MyProject.dir/main.c.o: CMakeFiles/MyProject.dir/flags.make
CMakeFiles/MyProject.dir/main.c.o: /path/to/MyProject/main.c
CMakeFiles/MyProject.dir/main.c.o: CMakeFiles/MyProject.dir/compiler_depend.ts
	@$(CMAKE_COMMAND) -E cmake_echo_color "--switch=$(COLOR)" --green --progress-dir=/path/to/MyProject/build-emcc/CMakeFiles --progress-num=$(CMAKE_PROGRESS_1) "Building C object CMakeFiles/MyProject.dir/main.c.o"
	/path/to/emsdk/upstream/emscripten/emcc -c /path/to/MyProject/main.c $(C_DEFINES) $(C_INCLUDES) $(C_FLAGS) -MD -MT CMakeFiles/MyProject.dir/main.c.o -MF CMakeFiles/MyProject.dir/main.c.o.d -o CMakeFiles/MyProject.dir/main.c.o -fPIC

/path/to/MyProject/build-clang/CMakeFiles/MyProject.dir/build.make

CMakeFiles/MyProject.dir/codegen:
.PHONY : CMakeFiles/MyProject.dir/codegen

CMakeFiles/MyProject.dir/main.c.o: CMakeFiles/MyProject.dir/flags.make
CMakeFiles/MyProject.dir/main.c.o: /path/to/MyProject/main.c
CMakeFiles/MyProject.dir/main.c.o: CMakeFiles/MyProject.dir/compiler_depend.ts
	@$(CMAKE_COMMAND) -E cmake_echo_color "--switch=$(COLOR)" --green --progress-dir=/path/to/MyProject/build-clang/CMakeFiles --progress-num=$(CMAKE_PROGRESS_1) "Building C object CMakeFiles/MyProject.dir/main.c.o"
	/usr/bin/clang $(C_DEFINES) $(C_INCLUDES) $(C_FLAGS) -MD -MT CMakeFiles/MyProject.dir/main.c.o -MF CMakeFiles/MyProject.dir/main.c.o.d -o CMakeFiles/MyProject.dir/main.c.o -c /path/to/MyProject/main.c

[sbc100]

I can't seem to reproduce that.

I used the exact CMakeLists.txt file above and ran the following command:

$ ~/dev/wasm/emscripten/emmake ~/bin/cmake-4.2.0-linux-x86_64/bin/cmake  -B build_emcc -S .
$ grep C_DEFINES build_emcc/CMakeFiles/MyProject.dir/build.make 
	/usr/local/google/home/sbc/dev/wasm/emscripten/emcc $(C_DEFINES) $(C_INCLUDES) $(C_FLAGS) -MD -MT CMakeFiles/MyProject.dir/main.c.o -MF CMakeFiles/MyProject.dir/main.c.o.d -o CMakeFiles/MyProject.dir/main.c.o -c /usr/local/google/home/sbc/cmake_test/main.c

Note that all the flags come before the -c <filename>.

Can you confirm exactly which version of emscripten you are using?

[Sackzement]

This is the exact version:
Emscripten SDK: latest 4.0.20 (c387d7a7e9537d0041d2c3ae71b7538cc978104e)

Cloned the emsdk repo directly from here and installed the latest SDK with emsdk install latest

CMake: 4.2.0
OS: Arch Linux

[sbc100]

I won't think the emscripten toolchain plugin has any control over the order of C_DEFINES vs -c. The toolchain file doesn't specify that: https://github.com/emscripten-core/emscripten/blob/main/cmake/Modules/Platform/Emscripten.cmake

Can you confirm the output of the emcmake command you are running.

It should looks something like this:

$ ~/dev/wasm/emscripten/emcmake cmake -B build_emcc -S .
emcmake: cmake -B build_emcc -S . -DCMAKE_TOOLCHAIN_FILE=/usr/local/google/home/sbc/dev/wasm/emscripten/cmake/Modules/Platform/Emscripten.cmake -DCMAKE_CROSSCOMPILING_EMULATOR=/usr/local/google/home/sbc/dev/wasm/emsdk/node/22.16.0_64bit/bin/node in directory /usr/local/google/home/sbc/cmake_test
-- Configuring done (0.6s)
-- Generating done (0.0s)
-- Build files have been written to: /usr/local/google/home/sbc/cmake_test/build_emcc

[sbc100]

Oh wait.. I cant reproduce now, I'm not sure what was happening before.

[Sackzement]

Command:

emcmake cmake .. &&
    cmake --build

Output:

emcmake: cmake .. -DCMAKE_TOOLCHAIN_FILE=/path/to/emsdk/emsdk-git/upstream/emscripten/cmake/Modules/Platform/Emscripten.cmake -DCMAKE_CROSSCOMPILING_EMULATOR=/path/to/emsdk/emsdk-git/node/22.16.0_64bit/bin/node in directory /path/to/MyProject/build-emcc
-- Configuring done (0.3s)
-- Generating done (0.0s)
-- Build files have been written to: /path/to/MyProject/build-emcc

I wasn't sure if I should post the issue here or in the CMake issue tracker.
All works fine with CMake version 4.1.3. But with CMake version 4.2.0, compiling with emcmake appends the source files in the generated make file.

[sbc100]

I don't think this is anything to do with emcmake all emcmake is doing is specifying the emscripten toolchain file.

The logic for building the command line seems deep inside cmake in Source/cmMakefileTargetGenerator.cxx. I can't see how/why the order would differ like that. Maybe open a cmake issue?

[Sackzement]

Maybe open a cmake issue?

I'll do that. I'm bisecting CMake right now and trying to find the exact commit that causes the problem.

It seemed strange that gcc and clang itself don't have this problem, but emcmake does, so I posted it here first.

[Sackzement]

Here is the CMake issue https://gitlab.kitware.com/cmake/cmake/-/issues/27404
Thanks, for the help.

[sxstd001]

The same issue is happening — my project can no longer compile.
I don’t think it’s a CMake problem, because my server project builds fine,
but the client project using EMCC / EM++ fails to compile.

-x c++-module' after last input file has no effect; .pcm' not found: module file not found

[Sackzement]

my server project builds fine, but the client project using EMCC / EM++ fails to compile.

Which CMake versions do the client and server projects use?

On my side, CMake 4.2.0 does not work, but CMake 4.1.3 does.

I've bisected CMake and the exact commit that causes the issue is 5d69468679 which is present in 4.2.0 but not in 4.1.3.

[sbc100]

Hmm, I don't see how https://gitlab.kitware.com/cmake/cmake/-/commit/5d69468679dc31b9fde429077da3ebbc9bfca1ea could be relevant to the order of items on the command line...

[sbc100]

I don’t think it’s a CMake problem, because my server project builds fine,

But CMake is the program that is constructing the command line and choosing to put the -c <src_file> either at the start of end of the command line. Certainly I don't see how emscripten (or emcmake which is just a wrapper) could break that.