[Cross building model] Cross compiling packages with build requirements

[GeorgeScrivener]

I’ve had some issues when cross compiling a build requirement.

In my case I'm trying to cross compile a library (call it PackageA) that has a build requirement on flatbuffers. flatbuffers creates an executable, flatc, that runs on the build machine and creates cpp files from some definitions.

I cross compile PackageA from amd64 for arm with [env] set in my profile:

[env]
CC={ARM C COMPILER}
CXX={ARM C++ CCOMPILER}
PATH=[{PATH TO COMPILER}]

The issue is that when conan builds flatbuffers it's using the env from the profile, and therefore flatc gets built for arm. This means it won't run on the build machine which is amd64.

PackageA itself is a library and so can be added as a dependency from other packages.

I've mentioned this on the slack channel and got some good suggestions, but it doesn't seem like a solved issue.

When I first ran in to this issue I expected to find compiler_build & maybe env_build settings in the profile to match the os_build & arch_build cross compilation options, though I'm not sure whether adding these would create other issues.

What's the best way to solve this issue more generally? We have quite a few build requirements that I'd like to use from conan.

[niosHD]

If I understood it correctly, your issue is the same as discussed in #2301. I am not aware of any native conan support for working with mixed architectures in a single package.

[GeorgeScrivener]

Yes that looks to be pretty much the same issue I'm having.
It would be nice to be able to specify that toolchain packages don't apply to build requirements.

[lasote]

Currently the only way would be to create a separate package for flatc and require it as a build_require. This flatc package can be built first and then applied to the needed packages.

[GeorgeScrivener]

Using build_require doesn't actually work as expected because CC & CXX get applied to build_requires.
To get this to work, I'd have to create a toolchain to set the env variables. But this would only work for build_requires in the root project, because any nested build requires will be built using the toolchain's env.

[lasote]

Using build_require doesn't actually work as expected because CC & CXX get applied to build_requires.

Yes, because of that I suggested This flatc package can be built first and then applied to the needed packages. I meant: first build the flatc in an isolated way, generating the needed binary package with one profile. Then build your project with a different one.

[memsharded]

Cross-compilation model has massively changed since then, with the 2 profiles approach. Closing this ticket as outdated.