shared_ptr in multi-threaded app

[efimd72]

I have a problem with using std and boost shared_ptr in multi-threaded application. It is well illustrated with this test:

#include <memory>
#include <omp.h>
int main () {
    for (int i = 0; ; i++) {
        std::shared_ptr<int> p_source (new int (i));
#pragma omp parallel default(shared) num_threads (2)
        {
            int ithr = omp_get_thread_num ();
            if (ithr == 0) {
                std::shared_ptr <int> p = p_source;
            } else {
                p_source = std::make_shared<int> (0);
                // p_source.reset (new int (0));
            }
        }
    }
    return 0;
}

At some arbitrary iteration the test crashes on access to an already freed sp_counted_block on Linux (RH 6.4). On Windows it works fine. The same test rewritten with boost pointer crashes on both platforms.

Is it supposed to work?

[Cubbi]

it isn't supposed to work: the only special guarantee given by shared_ptr is that different std::shared_ptrs can be accessed using mutable operations even if they share the control block

In this case, the same p_source is modified in one thread and read from in another thread, that's a data race. This is why atomic operations for shared pointers exist: http://en.cppreference.com/w/cpp/memory/shared_ptr/atomic

[efimd72]

Thanks. It helped.