How to get environment populated from VirtualBuildEnv

[tkielan]

When using CMake Conan integration, I do have VirtualBuildEnv specified in the recipe as generator.
All the environment shell scripts eg. conan*.sh files are created when conan_cmake_install is executed.
If something is added to the PATH because it is a tool used for building something in CMake, I don't know how it will get passed to execute_process().

Example, I have a flatbuffer recipe.
Our CMake will execute flatc to turn .fbs into .hpp
Flat Compiler has to be on the PATH and it is in VirtualBuildEnv.
CMake doe not see it on the path when executing execute_process.

[memsharded]

Hi @tkielan

Thanks for your question.

The VirtualXXXEnv generators cannot work with the cmake-conan integration, because they define environment scripts that must be loaded before the build execution. It is not possible to inject environment or tools via environment in cmake-conan, because what it does is to call conan install when the first find_package() is found. But environment variables might be needed much earlier than that, a clear example is a tool_requires("cmake/version") that adds the location of that CMake executable to the PATH, but as you are already running from CMake, that cannot be used.

Also the CMakeToolchain already defines too env-vars in the generated CMakePresets.json. But when running with cmake-conan, that is useless, because CMake is already running.

The definition can only be used with the "canonical" conan install + cmake --presets or conan install + virtualenv + cmake -DCMAKE_TOOLCHAIN_FILE=....

The cmake-conan cannot use tool_requires that way.
There are other ways to achieve it, like the protobuf recipe from ConanCenter does, by letting the .cmake files from the "host" context to locate the protoc compiler from the "build" context.
Furthermore, the new incubating CMakeConfigDeps will introduce new .exe targets that might be used also directly without environment definition.

[tkielan]

Thank you! I was thinking of CMakeVirtualBuildEnv to generate .cmake file instead of .sh that could be included by the CMake. I'm trying to achieve the same with cmake_build_modules property right now.

I will look into protobuf conan-center recipe for inspiration.

Thank you once again for great support!

[memsharded]

I think for this specific use case, the most promising path is the CMakeConfigDeps, because it will provide full transparent integration with IMPORTED executable targets out of the box, and it is designed to work with cmake-conan, I already have a branch preparing it: #674, for the moment we get the CMakeConfigDeps out of incubating.

I'm trying to achieve the same with cmake_build_modules property right now.

Yes, this might be a reasonable approach in the meantime. You might also want to consider doing it in a custom generator, see https://docs.conan.io/2/reference/extensions/custom_generators.html#using-global-custom-generators, because that way you wouldn't need to modify recipes, and you can apply the same logic to more recipes in case you need it. But if this only happens in flatbuffers, then it might be fine doing it in the flatbuffers recipes.

Thank you once again for great support!

Always happy to help!
I am sorry that cmake-conan didn't get there yet with the incubating CMakeConfigDeps, it takes a bit of time to stabilize these new things. So don't hesitate to open new tickets for any further question or issue you might have. Thanks for your feedback!