[question] How to cross build to the same os/arch combination when using autotools.

[CMelanie]

What is your question?

Using Conan 1.60.0, I'm working on a cross build from Linux to a different Linux system (so same settings.os and settings.arch, but different glibc, etc.). To differentiate between these builds, some extra subsettings to settings.os were added. Building works fine this way for cmake in combination with a custom toolchain.

However, using the template from the conan center for autotools builds, it doesn't work correctly. Looking at the cross_building check, I see it only takes os and arch into account, and not the extra subsettings, so I'm guessing this is not the way to do it.
So I'm wondering, what would be the right approach to cross build to the same os/arch combination when using the autotools toolchain?

Have you read the CONTRIBUTING guide?

• I've read the CONTRIBUTING guide

[memsharded]

Hi @CMelanie

Thanks for your question.

The template for ConanCenter autotools contains a lot of things, including some backwards compatibility for the case when not using 2 profiles (which is very recommended for cross-building).
It would be great to reduce the problem scope a bit, and reduce it to the conan new pkg/0.1 -m=autotools_lib if possible.

I suspect that the main issue is the cross_building() usage inside AutotoolsToolchain, that is not working fine in the cross-build scenario when both os and arch match exactly, could you please confirm this is the case? Thanks!

[CMelanie]

Hello @memsharded ,

thank you for your answer.

I'm using two profiles, indeed. A standard Linux one as build profile, and a host profile which is setting the environment with a different compiler and such, and defines the os along the lines of:

os=Linux
os.custom_distro="my_custom_distro"
os.custom_distro_version="1.2"

Indeed, the template contains a lot (they're very handy, though, great starting point!), but I trimmed down what I thought wasn't needed (hopefully I didn't trim down too much).
With this in the conan file:

    # [...]

    def configure(self):
        # Plain C library.
        self.settings.rm_safe("compiler.libcxx")
        self.settings.rm_safe("compiler.cppstd")
    
    def generate(self):
        autotools_toolchain = AutotoolsToolchain(self)
        autotools_toolchain.fpic = True
        autotools_toolchain.configure_args.extend( <a bunch of package specific settings>)
        autotools_toolchain.generate()

    def build(self):
        autotools = Autotools(self)
        autotools.autoreconf()
        autotools.configure()
        autotools.make()

    # [...]

it builds correctly if the host arch is different from the build arch. However, if the arch is the same, it fails due to mixing up glibc.
(So I started experimenting with setting the environment as shown in the template, and running into cross_building() not picking up that it's cross buiding, hence my question.)

[jcar87]

Hi @CMelanie, I believe this could be caused by Conan computing the triplet based on only os/arch - so it has no way of defining a different one in this case.

However, you could try defining the tools.gnu:host_triplet to a value that is different from the "build" value - anything that is valid but different should cause autotools to use the "cross-build" path.

You can define it in the command line with -c tools.gnu:host_triplet= or in the profile in the [conf] section.

[CMelanie]

Hello @jcar87 ,

thank you for the suggestion. I've tried adding this:

[conf]
tools.gnu:host_triplet=powerpc-linux-gnu

to the host profile (using powerpc for the sake of experiment). However, I keep having the same error where it's using the wrong glibc.

[franramirez688]

Hi @CMelanie ,

I think something that could work for that (given the limitations of that cross_building() function) is to define directly the flag variables for the build context and/or overwrite any triplets/configuration arguments:

    def generate(self):
        autotools_toolchain = AutotoolsToolchain(self)
        autotools_toolchain.fpic = True
        autotools_toolchain.configure_args.extend( [
                 <a bunch of package specific settings>,
        ])
        # Update any `configure_arg` introduced by Conan:
        autotools_toolchain.update_configure_args.extend( {
                 "--host": "your_triplet",
                 "--build": "your_build_triplet",
        })
        # Also, these flags will be appended to the default Conan ones in build time
        autotools_toolchain.extra_cxxflags.append("--flagxxx")
        autotools_toolchain.extra_cflags.append("--flagxxx")
        autotools_toolchain.extra_ldflags.append("--flagxxx")
        autotools_toolchain.extra_defines.append("-Ddef")
       # build scope by default. Otherwise, use autotools_toolchain.generate(env="host") instead
        autotools_toolchain.generate()

Could this help to solve the issue?

[CMelanie]

Hello @franramirez688 ,

That works if I use a fake triplet (powerpc-linux-gnu instead of x86_64) for the --build option. Great!

Thank you very much for your help, all!

[franramirez688]

So glad to hear that @CMelanie 👏
Closing the issue as it seems solved. Don't hesitate to open it again if you face any other related problems.
Thanks!