Flutter C Interop with omp.h on iOS - c

Question

I'm trying to make a C interop in Flutter in iOS.
The code uses omp.h and when compilling it in Xcode says omp.h not found.
What I've tried:
Install OpenMP library.
Change compiler to GCC in Xcode but didnt find any ways to do that.
Included omp.h manually in code folder.
When included omp.h manually it goes, but now I'm getting this error:
Undefined symbols for architecture x86_64:
"_omp_init_nest_lock", referenced from:
_count_3_initialize in count_3_initialize.o
"_omp_destroy_nest_lock", referenced from:
_count_3_terminate in count_3_terminate.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
I've tried using a dylib but this error appears:
Unhandled Exception: Invalid argument(s): Failed to lookup symbol (dlsym(RTLD_DEFAULT, increment): symbol not found)
Anyone have experiencied a problem like that before?

Answer 1

Need more information to know what happened exactly. But what I guess is:
check your library via file command, does it contains x86_64 arch or not.
use __attribute__((used)) to decorate your C function, like:
__attribute__((used)) void increment(){
//...
}
in your case.

Answer 2

Your installed library could be linked against macOS (e.g. installed by brew install libomp), you cannot use the macOS library for iOS or iPhone Simulator to link.
So we need to compile it ourselves:
wget https://github.com/llvm/llvm-project/releases/download/llvmorg-14.0.6/openmp-14.0.6.src.tar.xz
tar -xf openmp-14.0.6.src.tar.xz
cd openmp-14.0.6.src
mkdir -p build
cmake -B build \
-DCMAKE_TOOLCHAIN_FILE=ios.toolchain.cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=./build/install \
-DIOS_PLATFORM=OS -DENABLE_BITCODE=1 -DENABLE_ARC=0 -DENABLE_VISIBILITY=0 -DIOS_ARCH="arm64;arm64e" \
-DPERL_EXECUTABLE=$(which perl) \
-DLIBOMP_ENABLE_SHARED=OFF -DLIBOMP_OMPT_SUPPORT=OFF -DLIBOMP_USE_HWLOC=OFF
cmake --build build -j 3
cmake --build build --target install
The above would create a static library libomp.a for iOS, if you want a library for Simulator also, you need to set -DIOS_PLATFORM=SIMULATOR and -DIOS_ARCH="x86_64;arm64"
Here's my implementation on creating a OpenMP.xcframework which can support both iOS and iPhone Simulator: https://github.com/flyinghead/flycast/pull/805/