There were a few other flags I had to override from sysconfig, I'll dig them out.

I also found that we needed to override the include paths set for gcc:

  res = rctx.execute([
      "gcc",
      "-Wp,-v",
      "-xc++",
      "/dev/null",
      "-fsyntax-only",
  ])

  include_paths = [
      paths.normalize(line.replace("(framework directory)", "").strip())
      for line in res.stderr.split("\n")
      if line.startswith(" /usr") or line.startswith(" /Library")
  ]
  include_flags = " ".join(["-I%s" % p for p in include_paths])

And set them on CPPFLAGS.

--sysroot may also need setting so that it applies to libraries too.

My understanding is that all the standard library includes can be found under the sysroot.

From clang docs:

If the headers are installed in some other system root, the -isysroot option can be used provide a different system root from which the headers will be based. For example, -isysroot /Developer/SDKs/MacOSX10.4u.sdk will look for mylib.h in Developer/SDKs/MacOSX10.4u.sdk/usr/include/mylib.h.

Is the difference that when compiling cpp files you need to set CPPFLAGS in addition to CFLAGS? If so does the -isysroot not apply to cpp compiles?

Though the documentation is a bit confusing clang supports --sysroot in some fashion though its undocumented. I believe that the gcc docs provide some answer to why this is:

-isysroot dir
This option is like the --sysroot option, but applies only to header files (except for Darwin targets, where it applies to both header files and libraries). See the --sysroot option for more information.

-isysroot also applies to the library include path on darwin apparently. Since this change only applies to the darwin platform I think that -isysroot should be enough.

@mattem I think I resolved the issues you brought up except for the extra include flags, which I don't think are required based on docs I've read. If there aren't any blockers can we land this and iterate on any edge cases after? Alternatively if you have a repro of an extension that doesn't build with the current PR I could work off of that.

@f0rmiga I resolved your comments. I tried using --sysroot instead of -isysroot but it appears to not include the header files in clang when doing so, so we need to use -isysroot anyway.

@f0rmiga I resolved your comments. I tried using --sysroot instead of -isysroot but it appears to not include the header files in clang when doing so, so we need to use -isysroot anyway.

Oh, interesting!

Thanks for addressing the comments.