std::atomic not supported by clang?

Question

I am trying to use std::atomic with clang. However, whenever I try to include the header file atomic (#include <atomic>), I get the message "atomic not found". Note that I'm including -std=c++11 -stdlib=libc++ while compiling. What am I missing?

The version of clang I'm using is 3.2.

Answer 1

The version of clang I'm using is 3.2.

Clang added atomic support across two different versions according to LLVM CXX Status. The first was Clang 3.1, and the second was Clang 3.2.

I think you can check for it using:

#if defined(__clang__)
#  if __has_feature(cxx_atomic)
#    define CLANG_CXX11_ATOMICS 1
#  endif
#endif

Then, in your code:

#if CLANG_CXX11_ATOMICS
# include <atomic>
#endif

...

#if defined(CLANG_CXX11_ATOMICS)
# define MEMORY_BARRIER() std::atomic_thread_fence(std::memory_order_acq_rel)
#elif defined(__GNUC__) || defined(__clang__)
# define MEMORY_BARRIER() __asm__ __volatile__ ("" ::: "memory")
...
#endif

I can only say "I think" because cxx_atomic is not documented at Clang Language Extensions. However, it shows up on a search of the LLVM site: "cxx_atomic" site:llvm.org.

There's also an open question of the CFE Users mailing list: How to check for std::atomic availability?

---

Note that I'm including -std=c++11 -stdlib=libc++ while compiling. What am I missing?

For this, you might be using one of those Clang/LLVM C++ runtimes that's really just C++03, but pretends to be C++11. It caused me a lot of problems in the past because we support a number of compilers and platforms.

Below is a test Jonathan Wakely helped us craft to see if it really was a C++11 library, or one of Apple's fake C++11 libraries:

// Visual Studio began at VS2010, http://msdn.microsoft.com/en-us/library/hh567368%28v=vs.110%29.aspx.
// Intel and C++11 language features, http://software.intel.com/en-us/articles/c0x-features-supported-by-intel-c-compiler
// GCC and C++11 language features, http://gcc.gnu.org/projects/cxx0x.html
// Clang and C++11 language features, http://clang.llvm.org/cxx_status.html
#if (_MSC_VER >= 1600) || (__cplusplus >= 201103L)
# define CXX11_AVAILABLE 1
#endif

// Hack ahead. Apple's standard library does not have C++'s unique_ptr in C++11. We can't
//   test for unique_ptr directly because some of the non-Apple Clangs on OS X fail the same
//   way. However, modern standard libraries have <forward_list>, so we test for it instead.
//   Thanks to Jonathan Wakely for devising the clever test for modern/ancient versions.
// TODO: test under Xcode 3, where g++ is really g++.
#if defined(__APPLE__) && defined(__clang__)
#  if !(defined(__has_include) && __has_include(<forward_list>))
#    undef CXX11_AVAILABLE
#  endif
#endif

Answer 2

Did you specify -I /path/to/your/c++ (or, almost equivalently, -cxx-isystem /path/to/your/c++) so that clang++ can find its location?

If you think you should not need them, please try clang++ -print-search-dirs to confirm.