$ cat common.c
int common[100];
int other = 42;

$ cat main.c
extern int common[100];
int main(int argc, char* argv[]) {
  return common[0];
}

$ cc -c common.c
$ cc -c main.c
$ ar rcs libcommon.a common.o

$ cc main.o libcommon.a -Wl,-ld_classic
ld: warning: -ld_classic is deprecated and will be removed in a future release
Undefined symbols for architecture arm64:
  "_common", referenced from:
      _main in main.o
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)