multiple definition of '__aeabi_ul2f' on Android NDK (libgcc_real.a)

Question

I'm getting this error about multiple definition of '__aeabi_ul2f' when compiling a big project on Android using CMake. Apparently one of the places where the definition is found is is on

libsmoltcp_cpp_interface_rust.a(compiler_builtins-5ae026d08032e786.compiler_builtins.2kv22hwk-cgu.127.rcgu.o): previous definition here

which is some Rust code mixed with C++ compiled like this in CMake:

add_custom_target(
    lib_smol_tcp_rust
    COMMAND 
    echo "-------------COMPILING LIBSMOLTCP_CPP_INTERFACE RELEASE x86_64"
    && cargo build --release
    && echo "-------------COMPILING LIBSMOLTCP_CPP_INTERFACE RELEASE aarch64-linux-android"
    && cargo build --target aarch64-linux-android --release 
    && echo "-------------COMPILING LIBSMOLTCP_CPP_INTERFACE RELEASE armv7-linux-androideabi"
    && cargo build --target armv7-linux-androideabi --release 
    && echo "-------------COMPILING LIBSMOLTCP_CPP_INTERFACE RELEASE i686"
    && cargo build --target i686-linux-android --release
    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
)

then lsmoltcp_cpp_interface_rust, which is C++ code, links against lib_smol_tcp_rust which is rust.

It looks like the object '__aeabi_ul2f' is already defined on libgcc_real.a from the Android NDK project.

So I shouldn't compile the C++/Rust mixed code against libgcc? How can I do that? I'm confused because I use

The entire error:

  [1369/1369] Linking CXX shared library /home/dev/orwell/orwell_flutter_app/build/app/intermediates/cmake/debug/obj/armeabi-v7a/liborwell_android.so
  FAILED: /home/dev/orwell/orwell_flutter_app/build/app/intermediates/cmake/debug/obj/armeabi-v7a/liborwell_android.so 
  : && /opt/android-sdk-linux/ndk/21.1.6352462/toolchains/llvm/prebuilt/linux-x86_64/bin/clang++ --target=armv7-none-linux-androideabi24 --gcc-toolchain=/opt/android-sdk-linux/ndk/21.1.6352462/toolchains/llvm/prebuilt/linux-x86_64 --sysroot=/opt/android-sdk-linux/ndk/21.1.6352462/toolchains/llvm/prebuilt/linux-x86_64/sysroot -fPIC -g -DANDROID -fdata-sections -ffunction-sections -funwind-tables -fstack-protector-strong -no-canonical-prefixes -D_FORTIFY_SOURCE=2 -march=armv7-a -mthumb -Wformat -Werror=format-security  -std=c++17 -O0 -fno-limit-debug-info  -Wl,--exclude-libs,libgcc.a -Wl,--exclude-libs,libgcc_real.a -Wl,--exclude-libs,libatomic.a -static-libstdc++ -Wl,--build-id -Wl,--fatal-warnings -Wl,--exclude-libs,libunwind.a -Wl,--no-undefined -Qunused-arguments -shared -Wl,-soname,liborwell_android.so -o /home/dev/orwell/orwell_flutter_app/build/app/intermediates/cmake/debug/obj/armeabi-v7a/liborwell_android.so CMakeFiles/orwell_android.dir/orwell_jni.cpp.o CMakeFiles/orwell_android.dir/DecodedFfmpegFrameJNI.cpp.o CMakeFiles/orwell_android.dir/JavaOrwellFlutterRenderer.cpp.o CMakeFiles/orwell_android.dir/MediaCodecDecoder.cpp.o CMakeFiles/orwell_android.dir/JavaSimpleFileWriter.cpp.o CMakeFiles/orwell_android.dir/JavaHashMapJNI.cpp.o CMakeFiles/orwell_android.dir/JavaFlutterEventMessenger.cpp.o  _liborwell/liborwell_static.a -landroid -llog -lmediandk /home/dev/orwell/orwell_flutter_app/android/app/src/main/cpp/../../../../../../deps/ffmpeg/build/android/armeabi-v7a/lib/libavcodec.so /home/dev/orwell/orwell_flutter_app/android/app/src/main/cpp/../../../../../../deps/ffmpeg/build/android/armeabi-v7a/lib/libavutil.so /home/dev/orwell/orwell_flutter_app/android/app/src/main/cpp/../../../../../../deps/ffmpeg/build/android/armeabi-v7a/lib/libswscale.so /home/dev/orwell/orwell_flutter_app/android/app/src/main/cpp/../../../../../../deps/ffmpeg/build/android/armeabi-v7a/lib/libswresample.so /home/dev/orwell/orwell_flutter_app/build/app/intermediates/cmake/debug/obj/armeabi-v7a/libmyRtspClient.so /home/dev/orwell/orwell_flutter_app/build/app/intermediates/cmake/debug/obj/armeabi-v7a/libjrtp.so _liborwell/_myRtspClient/libmyRtspClient-static.a _liborwell/_myRtspClient/_JTRPLIB/src/libjrtp.a _liborwell/_ZLMediaKit/libzlmediakit.a _liborwell/_ZLMediaKit/libzltoolkit.a _liborwell/_ZLMediaKit/libmpeg.a _liborwell/_ZLMediaKit/libmov.a _liborwell/_ZLMediaKit/libflv.a _liborwell/common/openvpn_zl_socket/libopenvpn_zl_socket.a _liborwell/_libopenvpn3/src/libopenvpn/libopenvpn_lib.a _liborwell/_libopenvpn3/openvpn3/libssl.a _liborwell/_libopenvpn3/openvpn3/libcrypto.a _liborwell/_libopenvpn3/openvpn3/liblzo.a _liborwell/_libopenvpn3/openvpn3/liblz4.a _liborwell/_libopenvpn3/libtins/lib/libtins.a _liborwell/_libopenvpn3/smoltcp_cpp_interface/libsmoltcp_cpp_static.a /home/dev/orwell/deps/libopenvpn3/smoltcp_cpp_interface/target/armv7-linux-androideabi/release/libsmoltcp_cpp_interface_rust.a -ldl _liborwell/common/liborwellebml/liborwell_ebml.a _liborwell/_libebml/libebml.a _liborwell/common/liborwellprofile/liborwell_profile.a _liborwell/common/liborwellprofile2/liborwell_profile2.a _liborwell/common/liborwellprofile2/_protobuf/libprotobufd.a /opt/android-sdk-linux/ndk/21.1.6352462/toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/lib/arm-linux-androideabi/libz.a -latomic -lm && :
  /opt/android-sdk-linux/ndk/21.1.6352462/toolchains/llvm/prebuilt/linux-x86_64/lib/gcc/arm-linux-androideabi/4.9.x/../../../../arm-linux-androideabi/bin/ld: error: /opt/android-sdk-linux/ndk/21.1.6352462/toolchains/llvm/prebuilt/linux-x86_64/lib/gcc/arm-linux-androideabi/4.9.x/armv7-a/thumb/libgcc_real.a(_arm_addsubsf3.o): multiple definition of '__aeabi_ul2f'
  /opt/android-sdk-linux/ndk/21.1.6352462/toolchains/llvm/prebuilt/linux-x86_64/lib/gcc/arm-linux-androideabi/4.9.x/../../../../arm-linux-androideabi/bin/ld: /home/dev/orwell/deps/libopenvpn3/smoltcp_cpp_interface/target/armv7-linux-androideabi/release/libsmoltcp_cpp_interface_rust.a(compiler_builtins-5ae026d08032e786.compiler_builtins.2kv22hwk-cgu.127.rcgu.o): previous definition here
  clang++: error: linker command failed with exit code 1 (use -v to see invocation)                                
  ninja: build stopped: subcommand failed.

This is how I created my Rust toolchain:

&& ${ANDROID_NDK_HOME}/build/tools/make_standalone_toolchain.py --api 26 --arch arm64 --install-dir /opt/RUST_NDK_TOOLCHAIN/arm64 \
    && ${ANDROID_NDK_HOME}/build/tools/make_standalone_toolchain.py --api 26 --arch arm --install-dir /opt/RUST_NDK_TOOLCHAIN/arm \
    && ${ANDROID_NDK_HOME}/build/tools/make_standalone_toolchain.py --api 26 --arch x86 --install-dir /opt/RUST_NDK_TOOLCHAIN/x86

As you can see, the linking uses

--exclude-libs,libgcc_real.a

so it shouldn't use libgcc_real.a at all, but it somehow complains about libgcc_real.a anyways!

user_2604708 · Answer

I've run into this before linking a Rust staticlib inside a C/C++ project.

It is incredibly hard (sometimes impossible) to get gcc to not link libgcc. It is hardcoded as a "default" library and the linker will always pull it (you can try -nodefaultlibs or -nostdlib to prevent that). It can also be pulled in from linker scripts.

The trick for me was that order matters in the linker arguments.

The linker works (roughly) like this:

All arguments are processed left-to-right, only once each.
For each non--l argument, the linker loads the given .a/.o and pulls in ALL the symbols, erroring if there are duplicates. If the pulled-in symbols need other symbols that aren't defined anywhere, the linker keeps track of them as "undefined symbols".
For each -l argument, the linker loads the .a and pulls in the symbols matching seen previously-undefined symbols ONLY. If the .a has symbols that are already defined, they're ignored (instead of erroring!).

Try putting libsmoltcp_cpp_interface_rust.a as early as possible in the linker args. Any -l that goes after it won't cause the duplicate error, since -l ignores duplicates.

multiple definition of '__aeabi_ul2f' on Android NDK (libgcc_real.a)

1 Answers