Atomic objects of trivially copyable types in GCC

Question

According to the C++ standard, std::atomic can be combined with any trivially copyable type. However, GCC produces the following error messages:

#include <atomic>
struct TriviallyCopyableType {
  int a, b, c, d;
};
int main() {
  std::atomic<TriviallyCopyableType> a;
  a.store({});      // undefined reference to `__atomic_store_16'
  a.is_lock_free(); // undefined reference to `__atomic_is_lock_free'
}

Clang and Microsoft's compiler do not complain. Am I doing something wrong? Is this a known problem? After all, atomic operations were implemented years ago in GCC 4.4. Are there any workarounds other than using a different compiler? Since Clang implements std::atomic<TriviallyCopyableType> even lock-free, I do not want to use explicit locking.

fine on gcc 6.3. I think you need a newer compiler. Yours is ancient. — Richard Hodges, Dec 25 '16 at 21:20
It seems [this answer](http://stackoverflow.com/a/15204653/1120273) is still relevant. Possibly, you need to includue `-latomic`. — Dietmar Kühl, Dec 25 '16 at 21:34
@DietmarKühl That's the solution. GCC is bundled with the correct library, I only had to specify `-latomic`. I wouldn't have thought that I have to explicitly specify libraries in order to compile code using only standard constructs. Would you like to post your comment as an answer? — user1494080, Dec 26 '16 at 02:38
If you're using that code, you should probably use `-mcx16` on x86-64 to let the compiler use `cmpxchg16b`. That could enable lock-free 16-byte ops. See also http://stackoverflow.com/questions/38984153/implement-aba-counter-with-c11-cas/38991835#38991835. — Peter Cordes, Dec 28 '16 at 00:12

score 1 · Accepted Answer · answered Dec 29 '16 at 00:30

1

This answer is compiled from the comments.

You need to explicitly link the atomic operations library with your program by specifying -latomic on the command line.

-mcx16 may enable lock-free atomic operations on 128-bit data types.

answered Dec 29 '16 at 00:30

user1494080

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Atomic objects of trivially copyable types in GCC

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