Read access violation when accessing shared memory - c++

Question

I implemented, based on the boost examples, a threadsafe queue-class in the shared memory. It works as expected for a simple producer/consumer model.
As a next step, I defined the interface IConcurrentIPCQueue which is implemented by the ConcurrentIPCQueue class. I need the interface so I can implement an adapter to the queue for another issue.
The only difference between my first version and the current one below is the following:
First version:
template <class T> class ConcurrentIPCQueue
now adding the information, that I want to implement the interface like this:
Current version:
`template <class T> class ConcurrentIPCQueue :public IConcurrentIPCQueue<T>`
results in a read access violation on the consumer side. On the producer side, I can easily push_back and pop_front data correctly on its own. But strangely on the consumer side, I cannot access the shared memory (although the pair from segment.find returns correctly an address and 1).
So the question is, why the version with implementing the interface makes a difference on the consumer side and results in this strange error. And how I can solve it?
To keep the example short, I present here a minimalistic model of the queue:
#include <boost/interprocess/allocators/allocator.hpp>
#include <boost/interprocess/containers/deque.hpp>
#include <boost/interprocess/managed_shared_memory.hpp>
#include <boost/interprocess/sync/interprocess_condition.hpp>
#include <boost/interprocess/sync/interprocess_mutex.hpp>
#include <boost/thread/lock_guard.hpp>
#include <sstream>
namespace boost_ipc = boost::interprocess;
static char const *SHMEMNAME= "SHMEM";
static char const *SHQUEUENAME= "MYQUEUE";
template <class T> class IConcurrentIPCQueue
{
public:
virtual void push_back(T const & data) = 0;
virtual bool pop_front(T & data) = 0;
virtual unsigned int size() = 0;
};
template <class T> class ConcurrentIPCQueue :public IConcurrentIPCQueue<T>
{
public:
// allocator for allocating memory from the shared memory
typedef boost_ipc::allocator<T, boost_ipc::managed_shared_memory::segment_manager> ShmemAlloc;
typedef boost_ipc::interprocess_mutex IPC_Mutex;
typedef boost_ipc::interprocess_condition IPC_Cond;
typedef boost::lock_guard<IPC_Mutex> LockGuard;
ConcurrentIPCQueue(ShmemAlloc salloc) : mQueue_(salloc) { }
void push_back(T const & data)
{
{
LockGuard lock(mMutex_);
mQueue_.push_back(data);
}
mWait_.notify_one();
}
bool pop_front(T & data)
{
LockGuard lock(mMutex_);
if (mQueue_.empty())
return false;
data = mQueue_.front(); // return reference to first element
mQueue_.pop_front(); // remove the first element
return true;
}
unsigned int size()
{
LockGuard lock(mMutex_);
return mQueue_.size();
}
private:
boost_ipc::deque<T, ShmemAlloc> mQueue_;
IPC_Mutex mMutex_;
IPC_Cond mWait_;
};
typedef ConcurrentIPCQueue<char> myqueue;
void consumer()
{
boost_ipc::managed_shared_memory openedSegment(boost_ipc::open_only, SHMEMNAME);
myqueue*openedQueue = openedSegment.find<myqueue>(SHQUEUENAME).first;
char tmp;
while (openedQueue->pop_front(tmp)) {
std::cout << "Received " << tmp << "\n";
}
}
void producer() {
boost_ipc::shared_memory_object::remove(SHMEMNAME);
boost_ipc::managed_shared_memory mysegment(boost_ipc::create_only, SHMEMNAME, 131072);
myqueue::ShmemAlloc alloc(mysegment.get_segment_manager());
myqueue*myQueue = mysegment.construct<myqueue>(SHQUEUENAME)(alloc);
char mychar='A';
for (int i = 0; i < 10; ++i)
myQueue->push_back(mychar);
while (myQueue->size() > 0)
continue;
}
int main()
{
//producer(); // delete comment for creating producer process
consumer();
return 0;
}

Answer 1

UPDATE:
I could reproduce it with MSVC15.3 and Boost 1.64.
Turns out that the vtable pointers are the issue: they are different in each process, which leads to Undefined Behaviour as soon as you have runtime polymorphic types (std::is_polymorphic<T>).
It turns out the documentation forbids it clearly: Is it possible to store polymorphic class in shared memory?