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On Ventura 13.5.2 (22G91), Macbook pro 13 2019 (Intel): The following combination of the C code and Assembly code fails to link conftest_c.c: //conftest_c.c: #ifdef __cplusplus extern "C" { #endif void gsym_test_func(void); #ifdef __cplusplus } #endif int main() { gsym_test_func(); return 0; } Obj compiled using the command clang -O3 -DNDEBUG -finline-functions -fno-strict-aliasing -mcx16 -I. conftest_c.c -c produces conftest_c.o That exposes the following symbols: SYMBOL TABLE: 0000000000000000 g F __TEXT,__text _main 0000000000000000 *UND* _gsym_test_func Now we create assembly conftest.s that define _gsym_test_func: .text # _gsym_test_func .globl _gsym_test_func _gsym_test_func: # _gsym_test_func with the objdumb symbols: SYMBOL TABLE: 0000000000000000 g F __TEXT,__text _gsym_test_func Trying to link the two object files into a binary using the following command clang -O3 -DNDEBUG -v -finline-functions -fno-strict-aliasing -mcx16 conftest_c.o conftest.o -o conftest -lz results in the following error: Apple clang version 15.0.0 (clang-1500.0.40.1) Target: x86_64-apple-darwin22.6.0 Thread model: posix InstalledDir: /Library/Developer/CommandLineTools/usr/bin "/Library/Developer/CommandLineTools/usr/bin/ld" -demangle -lto_library /Library/Developer/CommandLineTools/usr/lib/libLTO.dylib -dynamic -arch x86_64 -platform_version macos 13.0.0 14.0 -syslibroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -O3 -o conftest -L/usr/local/lib conftest_c.o conftest.o -lz -lSystem /Library/Developer/CommandLineTools/usr/lib/clang/15.0.0/lib/darwin/libclang_rt.osx.a ld: Undefined symbols: _gsym_test_func, referenced from: _main in conftest_c.o clang: error: linker command failed with exit code 1 (use -v to see invocation) I do not understand why the linker cannot find the symbol _gsym_test_func defined in conftest.o