Segfault on writing to a Shared Memory Queue

Question

I created boost deque on a shared memory using boost managed shared memory. I have one process ( process A ) that pushes data to the back of the queue and another process (process B )that reads from the front of the queue and pops the front.

My process A can push data into the queue without any issues. But when I start my process B, it segfaults the moment it reads the front of the queue. process B can read the queue size correctly, it segfaults only when I read or pop an element from the queue.

Process A creates the shared memory and constructs the queue object. While my process B finds the constructed object.

My SharedMemQueue is,

struct QueueType
{       
   boost::interprocess::interprocess_mutex mutex;
   boost::interprocess::interprocess_condition signal;
   boost::interprocess::deque < int > queue;
};


class ShmQueue
{
public:
    ShmQueue ( std::string name )
    :   _shm_name ( name )
    {
    }
    ~ShmQueue ( )
    {
        Deinit();
    }
    bool Init ( bool value = false )
    {
        bool result ( false );
        try
        {
            if ( value )
            {
                shared_memory_object::remove ( _shm_name.c_str() );
                _shm_mem = managed_shared_memory ( create_only, _shm_name.c_str(), 65356 );
                _queue = _shm_mem.construct < QueueType > ( "Queue" )();
            }
            else
            {
                _shm_mem = managed_shared_memory ( open_only, _shm_name.c_str() );
                _queue = _shm_mem.find < QueueType > ( "Queue" ).first;
            }
        }
        catch ( interprocess_exception &e )
        {
            std::cout << e.what() << std::endl;
            _queue = NULL;
        }
        if ( _queue != NULL )
        {
            result = true;
        }
        return result;
    }

    bool Deinit ( )
    {
        bool result ( false );
        _shm_mem.destroy < QueueType > ( "Queue" );
        shared_memory_object::remove ( _shm_name.c_str() );
        return result;
    }
    void Push ( int value )
    {
        scoped_lock < interprocess_mutex > lock ( _queue->mutex );
        if ( _queue->queue.size ( ) < 20 )
        {
            _queue->queue.push_back ( value );
        }
    }

    int Pop ( )
    {
        int result ( -1 );
        scoped_lock < interprocess_mutex > lock ( _queue->mutex );
        if ( !_queue->queue.empty( ) )
        {
            result = _queue->queue.front();
            _queue->queue.pop_front();
        }
        return result;
    }

private:
    std::string _shm_name;
    QueueType * _queue;
    managed_shared_memory _shm_mem;
};

Any help is much appreciated, Thanks

score 1 · Answer 1 · answered Nov 06 '13 at 20:19

1

If you can read the size of queue but not its ellements, than its probably that queue is just a handler and stores its ellements somewhere else. Are you sure that boost::interprocess::deque < int > queue; uses your shared memory to allocate its ellements?

answered Nov 06 '13 at 20:19

VaSaKed

33
5

thanks for your response, but how do I find out if the queue is inside my allocated memory ? – slash_axl Nov 06 '13 at 21:26
it must stay in docs, btw you can try an fixed array like int queue[20]; instead of boost::deque to see if problem lies there – VaSaKed Nov 06 '13 at 21:55

Segfault on writing to a Shared Memory Queue

1 Answers1