C++ 11 undefined reference to `__atomic_store_16' [duplicate] - c++

Question

This question already has an answer here:
is_lock_free not defined in std::atomic<T> in gcc 4.7.2?
(1 answer)
Closed 8 years ago.
The following code fails to link:
#include <atomic>
struct A
{
unsigned long a;
unsigned long b;
};
struct B
{
void set(A tmp)
{
_a.store(tmp);
}
std::atomic<A> _a;
};
int main()
{
B b;
b.set(A());
return 0;
}
With the following error:
/tmp/cc8gyaZM.o: In function `std::atomic<A>::store(A, std::memory_order)':
dryn.cpp: (.text._ZNSt6atomicI1AE5storeES0_St12memory_order[_ZNSt6atomicI1AE5storeES0_St12memory_order]+0x3e): undefined reference to `__atomic_store_16'
If I replace the unsigned long-s with anything that is up-to int in size, this compiles just fine. Using g++ 4.7.2 . Do you have any idea why is that?
Compiled with command:
g++ dryn.cpp --std=c++11

Answer 1

As answered by Zeta:
Atomic API isn't complete in GCC 4.7:
When lock free instructions are not available (either through hardware or OS support) atomic operations are left as function calls
to be resolved by a library. Due to time constraints and an API
which is not finalized, there is no libatomic supplied with GCC 4.7.
This is easily determined by encountering unsatisfied external symbols
beginning with __atomic_*.
Since there is no libatomic shipped with GCC 4.7 you need to use
another compiler which actually supports the features you want or
provide the missing features (sample implementation).