Solving ___isOSVersionAtLeast is undefined or CLang intrinsics.

A few weeks ago I wrote about ___isOSVersionAtLeast is undefined problem. At some point I realized I did NOT find a solution.

That time I focused on reproducing the problem with a tiny library of only a few code lines. I did that. A tiny static library. It wrote details in the recent post Minimalistic C library and Kotlin/Native. Now I can check the linking either with Kotlin/Native or with a plain C.

The main.c:

#include "lib.h"
#include <stdio.h>

int main(int arg, char** argv) {
  int z  = foo();
  printf("The result is %d", z);
}

I call the following to compile and link main.c with my lib.a library:

gcc -c  main.c -o main.o
ld -macosx_version_min 10.10  -lc  main.o lib.a

I do the on macOS 10.13.3 with Xcode 9.3.1 and macOS 10.13.5, Xcode 9.4.

The Linkage Error

It was an assumption of mine to check for Clang Language Extensions, and finally, I was able to reproduce the linkage error with only the following code in lib.c targeting macOS 10.10, but not macOS 10.13:

int foo() {
  /// that is a CLang extension
  if(__builtin_available(macOS 10.12, iOS 9.0, tvOS 9.0, watchOS 2.0, *)) {
    return 256;
  }

  return 42;
}

And the linker error:

Undefined symbols for architecture x86_64:
  "___isOSVersionAtLeast", referenced from:
      _foo in lib.a(lib.o)

I ran the ld command with -macosx_version_min 10.10 to target macOS 10.10. The argument specifies the minimal version of macOS the created binary supports.

More experiments with arguments help me to find that CLang is smart to optimize that code if the check makes no sense. For example, CLang optimizes the call __builtin_available(macOS 10.12,*) when I target 10.12 or 10.13. It also means no linkage error. The call __builtin_available(macOS 10.14,*) is never optimized, as long as I cannot target 10.14 yet.

My linkage problem was not in the target version specification at all. Otherwise, the problem was with a missing library. I was missing the compiler-runtime library in my ld call. I found the hacky path (on my machine) to solve the linker error:

/Library/Developer/CommandLineTools/usr/lib/clang/9.1.0/lib/darwin/libclang_rt.osx.a

That one defines the symbol I was missing. The only problem is the path hard-codes too many internals. It is too internal to be used directly from a build. It seems to be a part of Xcode 9.1.0, which is too old.

compiler-rt

LLVM “compiler-rt” that is the place with the documentation of the LLVM and CLang features, that requires a runtime. It includes nice and helpful features inside. There is still no answer with the correct linker options. I have no plans to build my own toolchain.

The right fix

The best fix for ___isOSVersionAtLeast undefined symbol so far is to include the missing runtime library to an ld command. It only needs a path to the static library with no other arguments. Shall a build tool help here? I do not know yet.

The worst here is one needs to generate the path manually. One need to know the Xcode (aka toolchain) version to do that. And the trickiest is to make sure the path is updated once Xcode or something else is updated.

I am looking to find the best way to include the compiler-rt. If you know something, please comment below.

Continue reading the next part of the investigation