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macos stage3: add link support for system libc++ #23264

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macos stage3: add link support for system libc++ #23264

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12 changes: 11 additions & 1 deletion build.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -782,7 +782,17 @@ fn addCmakeCfgOptionsToExe(
mod.linkSystemLibrary("unwind", .{});
},
.ios, .macos, .watchos, .tvos, .visionos => {
mod.link_libcpp = true;
if (static or !std.zig.system.darwin.isSdkInstalled(b.allocator)) {
mod.link_libcpp = true;
Comment on lines +785 to +786
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Per my comment here, it's probably still worth rethinking linking to the vendored libc++ when you are using a third-party LLVM (i.e. one built not using the vendored libc++).

} else {
// Avoid using `mod.linkSystemLibrary()`, which:
// - is semantically equivalent to `-lc++`
// - and enables `mod.link_libcpp`
// Instead, add the full object pathname.
Comment on lines +788 to +791
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I see, thank you. This reveals a contradiction in this implementation:

  • by checking for the string "c++", linkSystemLibrary reveals an assumption that there is only one possible way to link libc++.
  • The entire point of this patch rests on the assumption that there are multiple ways to link libc++.

Instead of working around it, the build system should be modified to recognize that there are two choices when linking libc++, because the logic that makes my code snippet not work for this use case is based on this incorrect assumption.

Then, my simpler code snippet which does not rely on something that requires access outside of a build sandbox (#14286) will work correctly.

const sdk = std.zig.system.darwin.getSdk(b.allocator, b.graph.host.result) orelse return error.SdkDetectFailed;
const libcxx = b.pathJoin(&.{ sdk, "usr/lib/libc++.tbd" });
exe.root_module.addObjectFile(.{ .cwd_relative = libcxx });
}
},
.windows => {
if (target.abi != .msvc) mod.link_libcpp = true;
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