Re: [Boost-users] shared_ptr question

Subject: Re: [Boost-users] shared_ptr question
From: Soumen Ghosh (soumen78_at_[hidden])
Date: 2010-05-18 09:40:22

I'm not sure but can you test it following way:

Defer this declaration " boost::shared_ptr<Frame> rx" in following way and
see if that helps ...

boost::shared_ptr<Frame> &rx = processing_queue.back( ) ;

Regards,
~ Soumen

On Tue, May 18, 2010 at 9:30 AM, David Michael Bond
<david.bond_at_[hidden]>wrote:

> Hello,
> I am learning shared_ptrs and I am running into a segfault. I'm fairly
> certain it's because of some misunderstanding I have.
>
> Here is some code. I have altered it to just so the relevant data:
>
> ---- START CODE ----
> std::deque<boost::shared_ptr<Frame> > processing_queue ;
> void this_is_called() {
>  boost::shared_ptr<Frame> rx ;
>  interface->open( "eth0" ) ;
>  while( running ) {
>    interface->rx_frame( this->processing_queue ) ;
>    if( !processing_queue.empty( ) ) {
>      rx = processing_queue.back( ) ;
>      if( rx ) {
>        std::cout
>          << "Received Frame of Ethertype "
>          << ntohs( rx->get_ether_header( ).ether_type )
>          << "." << std::endl ;
>        }
>      }
>    }
>  }
> }
>
> // rx frame from interface and add onto processing_queue
> boost::shared_ptr<Frame> last_rx_frame ; // (CLASSXX member)
> void CLASSXX::rx_frame( std::deque<boost::shared_ptr<Frame> >&
> processing_queue ) {
>  // Do some stuff to receive frame.. (int size= # bytes in frame)
>  last_rx_frame.reset(new Frame( )) ;
>  last_rx_frame->bytes.reset( new u_char[size]) ;
>  memcpy( last_rx_frame->bytes.get(), frame_bytes, size) ;
>  last_rx_frame->size = size ;
>  processing_queue.push_back( this->last_rx_frame ) ;
> }
>
> ---- END CODE ----
>
> The program receives one frame and successful and outputs it:
>  Received Frame of Ethertype 0x800.
> But the second time when it gets to 'rx = processing_queue.back( ) ;' I
> have
> a segfault.
>
> My understanding of shared_ptrs is that when this assignment is done rx
> will
> have its ref count decremented (possibly leading to it being deleted). The
> back of the processing_queue will then become rx do to the assignment and
> that shared ptrs reference count will be increased by one. Anyone know what
> my misconception is? Any idea what's causing the segfault?
>
> Thanks!
> -David
>
>
>
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