Cross compiling from macOS to Emscripten fails because of automatically added -stdlib=libc++

[sascha]

Environment Details

• Operating System+version: macOS 10.15.5
• Compiler+version: Apple clang version 11.0.3 and Emscripten 1.39.19
• Conan version: 1.27.0
• Python version: 3.7.7

Problem description

I'm not quite sure if this is a bug, a missing feature or a configuration error, but the issue is somewhat related to issue #2856. Just like Android, the Emscripten compiler does not need -stdlib=libc++. I could probably get around this issue by specifying compiler.libcxx=c++_static in the host profile in which case the conan_set_libcxx macro in the generated CMake file would not add the -stdlib=libc++ flag, however this is only a valid option when setting the compiler to clang. On the other hand I can't set the compiler to clang, because the highest available version number is 10 while the latest version number of apple-clang is 11. This mismatch will throw an error when installing packages, because Emscripten's toolchain file automatically sets the compiler version to that of the system clang.

Ideally I'd need a way to remove the compiler.libcxx setting when the host os is Emscripten. Is there a way to do this?

• I've read the CONTRIBUTING guide.

[jgsogo]

Hi @sascha ! Can you share with us the Emscripten recipe you are using? or how are you populating the environment before running Conan to compile your library?

[sascha]

Hi @jgsogo, sure, happy to.

I'm installing my Conan deps with

conan install . -if _builds/cesdk/wasm32-unknown-emscripten/debug -s build_type=Debug --build=missing -pr:h=conan/profiles/emscripten -pr:b=default

This is my emscripten profile:

include(default)

[settings]
os=Emscripten
arch=wasm

[build_requires]
emsdk_installer/1.39.19@imgly/stable

and this is my default profile:

[settings]
os=Macos
os_build=Macos
arch=x86_64
arch_build=x86_64
compiler=apple-clang
compiler.version=11.0
compiler.libcxx=libc++
build_type=Release

The emsdk_installer/1.39.19@imgly/stable is pretty much the same as https://github.com/bincrafters/conan-emsdk_installer except that it's updated to the latest version of Emscripten and the lines at https://github.com/bincrafters/conan-emsdk_installer/blob/testing/1.39.13/conanfile.py#L13-L16 have been replaced with a simple settings = "os", "arch" to support the newer host/build profiles.

One of my dependencies is gtest/1.10.0 for which I run into the following error when running conan install with above setup:

clang++: error: argument unused during compilation: '-stdlib=libc++' [-Werror,-Wunused-command-line-argument]

The emscripten compiler frontend is basically a drop-in replacement for clang (https://emscripten.org/docs/tools_reference/emcc.html). Would it make sense to introduce a new compiler setting?

[lkotsonis]

Any update on this? I bumped into the same issue

[jgsogo]

I've been playing with emscripten these days (I'm using the recipe under development here: conan-io/conan-center-index#6163). With the build-requires, I'm able to build corrade (conan-io/conan-center-index#7410) and magnum packages (conan-io/conan-center-index#7412) with some modifications. It also works for magnum-integration, magnum-extras,... those are a bunch of packages.

I'm still working on several pull-requests for different packages, but so far it is working for them: conan-io/conan-center-index#6292.

(All of this, using the latest Conan version)

[perseoGI]

Greetings,

I am excited to announce that we have introduced native support for the emcc compiler in conan 2.18.
Check out the docs here.

The modeled emcc compiler configuration is the following (notice that emcc is a clang front end so it is expected to have almost the same settings)

emcc:
    # From https://github.com/emscripten-core/emscripten/blob/main/ChangeLog.md
    # There is no ABI compatibility guarantee between versions
    version: [ANY]
    libcxx: [null, libstdc++, libstdc++11, libc++]
    threads: [null, posix, wasm_workers]
    cppstd: [null, 98, gnu98, 11, gnu11, 14, gnu14, 17, gnu17, 20, gnu20, 23, gnu23, 26, gnu26]
    cstd: [null, 99, gnu99, 11, gnu11, 17, gnu17, 23, gnu23]

Having emcc (emscripten compiler) natively modelled in conan allow us to inyect specific compiler and linker flags.
This also avoids the error you described in your issue.

Additionally, we will soon launch a new repository called conan-toolchains (https://github.com/conan-io/conan-toolchains), where we will be adding and transitioning existing toolchains from CCI. One of these toolchains is emsdk (see conan-io/conan-toolchains#4), which is a refactor of the existing CCI recipe that we plan to deprecate.

We have also implemented proper support for the older asmjs architecture and tested all wasm architectures across different Conan generators, fixing some old bugs.

Please note that in the new conan-toolchains repository, we have added a conan_config folder that can be easily installed using the conan config install command. In this folder, we provide pre-built profiles for the respective toolchains.

Currently, we have the following profiles available:

• emsdk/wasm32
• emsdk/wasm64

These profiles can be used alongside emsdk/native, which allows users to utilize their native emsdk installation instead of the Conan-packaged version.

I hope this meets your needs.
Thank you!