apple-clang not exposing __apple_build_version__

[Raphael-Martin-TS3D]

Issue and reason

We figured out apple-clang version 12 and 14 (tested on both) do not expose __apple_build_version__ define. It is an issue because CMake bases its heuristics on this variable to differentiate clang and apple-clang. Which leads some projects to fail due to clang parameters given to apple clang (i.e: --start-group linker parameter).

Steps to reproduce

We built apple-clang with:

# Build Apple Clang
RUN wget -O osxcross.zip https://github.com/tpoechtrager/osxcross/archive/${OSXCROSS_COMMIT_HASH}.zip \
    && unzip osxcross.zip \
    && rm osxcross.zip \
    && mv osxcross-${OSXCROSS_COMMIT_HASH}/ osxcross \
    && cd osxcross \
    && UNATTENDED=1 \
       CLANG_VERSION=${CLANG_VERSION} \
       ENABLE_CLANG_INSTALL=1 \
       ENABLE_FULL_BOOTSTRAP=1 \
       PORTABLE=1 \
       INSTALLPREFIX=${CLANG_INSTALL_PREFIX} \
       ./build_apple_clang.sh

And tested with a simple .cpp file:

#include <iostream>

int main(void)
{
#ifdef __apple_build_version__
    std::cout << "__apple_build_version__ set" << std::endl;
#else
    std::cout << "__apple_build_version__ NOT set" << std::endl;
#endif

#ifdef __clang__
    std::cout << "__clang__ set" << std::endl;
#else
    std::cout << "__clang__ NOT set" << std::endl;
#endif
}

The cpp was pre-processed into our osxcross docker with command: /usr/local/osxcross/bin/x86_64-apple-darwin20.2-clang++ -E testCompiler.cpp

And the output (curated) is quite clear:

# 2 "testCompiler.cpp" 2

int main(void)
{
    std::cout << "__apple_build_version__ NOT set" << std::endl;
    std::cout << "__clang__ set" << std::endl;
}

Did we forget a parameter when building apple-clang?

[tpoechtrager]

Afaik __apple_build_version__ is only defined by Apple's internal build system. There is no option to expose it otherwise: https://github.com/search?q=repo%3Aswiftlang%2Fllvm-project%20__apple_build_version__&type=code

The CMake check should be fixed instead.

[Raphael-Martin-TS3D]

I totally agree the CMake check is not a proper way to do it. Nonetheless, since we are building apple-clang, I'd tend to think it should be done so the executable behaves the same without differences. Thank you for your answer.

[Raphael-Martin-TS3D]

@tpoechtrager if you agree, we can try and patch the build-clang script to, when building apple-clang, expose this variable.

[tpoechtrager]

I am not a fan of defining __apple_build_version__ on our own. I'm not exactly sure what it does, nor do I know the specifics of how Apple builds their compiler.

Not having the compiler recognized as Apple Clang shouldn't be a problem - the relevant project should be fixed instead.

If you still want to experiment with defining __apple_build_version__, you could add the following:

fargs.push_back("-D__apple_build_version__=1");

to wrapper/target.cpp right before

if (SDKOSNum >= OSVersion(14, 0) && clangversion < ClangVersion(17, 0)) {

Then re-build the wrapper with:

./wrapper/build_wrapper.sh

Verify __apple_build_version__ is defined:

$ OCDEBUG=1 xcrun clang
osxcross: debug: --> /home/thomas/dev/osxcross/target/bin/x86_64-apple-darwin24.5-clang
osxcross: debug: <-- /opt/llvm/20.1.5/bin/clang-20 (clang) -target x86_64-apple-darwin24.5  -isysroot /home/thomas/dev/osxcross/target/bin/../SDK/MacOSX15.5.sdk -isystem /home/thomas/dev/osxcross/target/bin/../SDK/MacOSX15.5.sdk/usr/include/c++/v1 -isystem /opt/llvm/20.1.5/bin/../lib/clang/20/include -mmacosx-version-min=10.13.0 -arch x86_64 -fuse-ld=lld -D__apple_build_version__=1 -Wno-liblto
osxcross: debug: === time spent in wrapper: 0.052552 ms
clang: error: no input files

[Raphael-Martin-TS3D]

Thanks again for your answer, I will try this and present a PR. I totally understand the impact is impossible to analyze, I'll add your proposal between a ifdef EXPOSE_APPLE_CLANG_VERSION so osxcross users can opt-in only if they want it.

[tpoechtrager]

That'd be fine 👍

[HinTak]

A runtime define (I.e. if you set an environment like OSXCROSS_PRETEND_TO_BE_APPLE_CLANG=1) instead of a compile time define, might be even more useful: people could switch it on when they think they need it, without recompiling.

[Raphael-Martin-TS3D]

Please see PR : https://github.com/tpoechtrager/osxcross/pull/464