為什么 std::atom::is_lock_free 不能編譯？

Question

只是一些 C++11 的代碼：

#include<iostream>
#include<atomic>

struct A { int a[4]; };
struct B { int x, y; };
int main()
{
    std::cout << std::boolalpha
            << "std::atomic<A> is lock free? "
            << std::atomic<A>{}.is_lock_free() << '\n'
            << "std::atomic<B> is lock free? "
            << std::atomic<B>{}.is_lock_free() << '\n';
}

用mac+clang編譯，報錯：

Undefined symbols for architecture x86_64:
"___atomic_is_lock_free", referenced from:
    _main in atomics.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

clang + ubuntu docker 提供：

root@b01946bedcf2:/# clang++ --version
clang version 4.0.0-1ubuntu1 (tags/RELEASE_400/rc1)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
root@b01946bedcf2:/# clang++ 1.cpp -std=c++11 -lpthread
/tmp/1-7cc6e9.o: In function `std::atomic<A>::is_lock_free() const':
1.cpp:(.text._ZNKSt6atomicI1AE12is_lock_freeEv[_ZNKSt6atomicI1AE12is_lock_freeEv]+0x1b): undefined reference to `__atomic_is_lock_free'
/tmp/1-7cc6e9.o: In function `std::atomic<B>::is_lock_free() const':
1.cpp:(.text._ZNKSt6atomicI1BE12is_lock_freeEv[_ZNKSt6atomicI1BE12is_lock_freeEv]+0x1b): undefined reference to `__atomic_is_lock_free'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

在 RHEL7+gcc4.8.5 上，它給出：

$g++ 1.cpp -std=c++11 -lpthread && ./a.out
/tmp/ccW9pNc2.o: In function `std::atomic<A>::is_lock_free() const':
1.cpp:(.text._ZNKSt6atomicI1AE12is_lock_freeEv[_ZNKSt6atomicI1AE12is_lock_freeEv]+0x17): undefined reference to `__atomic_is_lock_free'
collect2: error: ld returned 1 exit status

這真的很奇怪。 代碼有什么問題，還是需要超高版本的編譯器？

Answer 1

您需要包含原子庫。

$ g++ 1.cpp -std=c++11 -latomic && ./a.out