[boost] shared_ptr and threads

From: Timothy Ritchey (tritchey_at_[hidden])
Date: 2005-08-09 18:56:43

We are using shared_ptr extensively in our application, and seem to  
be having some threading issues. In particular, we have an Envelope  
class that is used to hold messages received over the network. We use  
Spirit to parse the message string and construct the Envelope object,  
then we pass is on to the EnvelopeController, which holds a thread  
pool to handle all of the Envelopes. These Envelopes are passed  
around as shared_ptr<Envelope> between these different components.

In essense, we have one thread that is receiving the message from the  
network stack, parsing it, and passing it to the thread_pool, which  
is then using one (or more) additional threads to actually deal with  
the contents of the envelope.

On a Uni-processor box, everything works great. All of our envelopes  
are  being destroyed properly. On a multi-processor machine, none of  
the envelopes are getting destroyed.

We are in a situation where two shared_ptrs to the same underlying  
object may go out of scope and need to decrement the ref count at the  
same time. Is the in the realm of "undefined behavior?" Am I using  
the shared_ptr incorrectly? Thanks for any advice.

Thread 1 ---------
{
     shared_ptr<Envelope> envelope(new Envelope(...));

     ...
     [pass envelope to Thread 2]
     ...
     [envelope goes out of scope]
}

Thread 2 --------

void process(shared_ptr<Envelope> envelope) {
     [do work on envelope]
     [envelope goes out of scope]
}