Question

I am using a CList in a multithreaded environment and I keep having problem with the GetHead method. I have one thread that add data to the list, and an other thread who read and remove data from the list.
Here is the reading part :

 value_type get_next()  
        {
          T t;
          if(!queue.IsEmpty()) {
             t = queue.GetHead();
          }
          return t;  //If the queue is empty we return an empty element
        }

Here is the inserting part :

 inline void insert(T &_in) 
        {
          queue.AddTail(_in);
        } 

Here is the removing part

  inline void  pop_next()  
        {
          if(!queue.IsEmpty())  {
            queue.RemoveHead(); 
          }
        }

Why do I get a runtime error when I run this. It always fail at

t = queue.GetHead();

With this assertion :

template<class TYPE, class ARG_TYPE>
AFX_INLINE TYPE& CList<TYPE, ARG_TYPE>::GetHead()
    { ENSURE(m_pNodeHead != NULL);
        return m_pNodeHead->data; }

While the m_pNodeHead value is :

• pNext 0x00000000 {pNext=??? pPrev=??? data={…}
  } CList > >,ATL::CStringT > > &>::CNode *
• pPrev 0x00000000 {pNext=??? pPrev=??? data={…}
  } CList > >,ATL::CStringT > > &>::CNode *
• data “” TESTSETSE ATL::CStringT > >

Answer 1

You have a race condition between inserting and retrieving the value. Add a lock that includes the entire body of get_next(), insert(), and pop_next().