为什么atomic_is_lock_free在mac上不能正确链接？

Question

我正在尝试在mac上运行this example：

#include <iostream>
#include <utility>
#include <atomic>

struct A { int a[100]; };
struct B { int x, y; };
int main()
{
    std::atomic<A> a;
    std::atomic<B> b;
    std::cout << std::boolalpha
              << "std::atomic<A> is lock free? "
              << std::atomic_is_lock_free(&a) << '\n'
              << "std::atomic<B> is lock free? "
              << std::atomic_is_lock_free(&b) << '\n';
}

但是我得到了这个错误：

Undefined symbols for architecture x86_64:
  "___atomic_is_lock_free", referenced from:
      std::__1::__atomic_base<main::B, false>::is_lock_free() const volatile in lock_free_main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

this example也是如此。

我注意到，如果我将Struct A更改为只有一个或两个ints，它就会链接。当结果不为真时，看起来它不会链接，但为什么它不链接呢？在我看来像是个虫子。

我发现了一些更老的问题here和here，这让我认为bug现在应该已经解决了。

下面是我的编译器的版本：

 $> g++ --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/4.2.1
Apple clang version 11.0.0 (clang-1100.0.33.17)
Target: x86_64-apple-darwin19.5.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

Answer 1

请改用"is_always_lock_free“。

    std::cout << std::boolalpha
              << "std::atomic<A> is lock free? "
              << a.is_always_lock_free << '\n'
              << "std::atomic<B> is lock free? "
              << b.is_always_lock_free << '\n';

你可能需要另外定义"__cpp_lib_atomic_is_always_lock_free“，因为它还没有达到标准。

实际问题是，"atomic_is_lock_free“在编译时运行，至少对于Mac平台是这样。)