c++ - Why does g++ still require -latomic

Question

In 29.5 Atomic types of the C++ Standard November 2014 working draft it states:

1. There is a generic class template atomic. The type of the template argument T shall be trivially copyable (3.9). [ Note: Type arguments that are not also statically initializable may be difficult to use. —end note ]

So - as far as I can tell - this:

#include <atomic>

struct Message {
    unsigned long int a;
    unsigned long int b;
};

std::atomic<Message> sharedState;

int main() {    
    Message tmp{1,2};       
    sharedState.store(tmp);         
    Message tmp2=sharedState.load();
}

should be perfectly valid standard c++14 (and also c++11) code. However, if I don't link libatomic manually, the command

g++ -std=c++14 <filename>

gives - at least on Fedora 22 (gcc 5.1) - the following linking error:

/tmp/ccdiWWQi.o: In function `std::atomic<Message>::store(Message, std::memory_order)':
main.cpp:(.text._ZNSt6atomicI7MessageE5storeES0_St12memory_order[_ZNSt6atomicI7MessageE5storeES0_St12memory_order]+0x3f): undefined reference to `__atomic_store_16'
/tmp/ccdiWWQi.o: In function `std::atomic<Message>::load(std::memory_order) const':
main.cpp:(.text._ZNKSt6atomicI7MessageE4loadESt12memory_order[_ZNKSt6atomicI7MessageE4loadESt12memory_order]+0x1c): undefined reference to `__atomic_load_16'
collect2: error: ld returned 1 exit status

If I write

g++ -std=c++14 -latomic <filename>

everything is fine. I know that the standard doesn't say anything about compiler flags or libraries that have to be included, but so far I thought that any standard conformant, single file code can be compiled via the first command.

So why doesn't that apply to my example code? Is there a rational why -latomic is still necessary, or is it just something that hasn't been addressed by the compiler maintainers, yet?

Answer 1

Relevant reading on the GCC homepage on how and why GCC makes library calls in certain cases regarding <atomic> in the first place.

GCC and libstdc++ are only losely coupled. libatomic is the domain of the library, not the compiler -- and you can use GCC with a different library (which might provide the necessary definitions for <atomic> in its main proper, or under a different name), so GCC cannot just assume -latomic.

Also:

GCC 4.7 does not include a library implementation as the API has not been firmly established.

The same page claims that GCC 4.8 shall provide such a library implementation, but plans are the first victims of war. I'd guess the reason for -latomic still being necessary can be found in that vicinity.

Besides...

...so far I thought that any standard conformant, single file code can be compiled via the first command.

...-lm has been around for quite some time if you're using math functions.