Getting wrong output from boost lock free spsc queue

Question

I am trying to implement lock free queue of user defined data type using boost library, but I am getting wrong result.

Please help me out where I am doing wrong.

#include <boost/lockfree/spsc_queue.hpp>
#include <thread>
#include <iostream>
#include <string.h>
#include <time.h>   


class Queue
{
 private:
       unsigned char *m_data;
       int m_len;

 public:
       Queue(unsigned char *data,int len);

       Queue(const Queue &obj);

       ~Queue();  

       Queue & operator =(const Queue &obj);


       unsigned char *getdata()
       {
         return m_data;
       }

       int getint()
       {
           return m_len;
       }
};

Queue::Queue(unsigned char* data, int len)
{
      m_len=len;

      m_data=new unsigned char[m_len];

      memcpy(m_data,data,m_len);
}

Queue::Queue(const Queue& obj)
{
      m_len=  obj.m_len;

      m_data=new unsigned char[m_len];

      memcpy(m_data,(unsigned char *)obj.m_data,m_len);
}

Queue::~Queue()
{
   delete[] m_data;
   m_len=0;
}

Queue & Queue::operator =(const Queue &obj)
{
   if(this != &obj)
   {
      m_len=obj.m_len;

      m_data=new unsigned char[m_len];

      memcpy(m_data,(unsigned char *)obj.m_data,m_len);
   }

   return *this;
}

boost::lockfree::spsc_queue<Queue*> q(10);



void produce()
{
    int i=0;
    unsigned char* data=(unsigned char *)malloc(10);
    memset(data,1,9);

    Queue obj(data,10);

    Queue *pqueue=&obj;

    printf("%d\n",pqueue->getint());


    q.push(pqueue);

}

void consume()
{

    Queue *obj;

    q.pop(&obj);

    printf("%d\n",obj->getint());

}


int main(int argc, char** argv) {


//  std::thread t1{produce};
//  std::thread t2{consume};
//  
//  t1.join();
//  t2.join();
    produce();
    consume();

    return 0;
}

As per boost::lockfree::queue requirements I created following in class.

• Copy Constructor
• Assignment Operator
• Destructor

Please let me know if anything other requires. Thanks.

Answer 1

1. You're using malloc in C++.

   You die.

   You have 2 lives left.

   Seriously, don't do that. Especially since using it with delete[] is clear cut Undefined Behaviour.

   ---

2. Sadly you lose another life here:

   Queue obj(data,10);
   
   Queue *pqueue=&obj;
   q.push(pqueue);
   

   You store a pointer to a local. More Undefined Behaviour

   You have 1 life left.

   ---

3. Last life at

   q.pop(&obj);
   

   You pop using an iterator. It will be treated as an output iterator. You get a return that indicates the number of elements popped, and items will be written to &obj[0], &obj[1], &obj[2], etc.

   Guess what? Undefined Behaviour.

   See also: Boost spsc queue segfault

   You died.

   ---

4. You're already dead. But you forsake your afterlife with

   printf("%d\n",obj->getint());
   

   Since pop might not have popped anything (the queue may have been empty), this in itself is Undefined Behaviour.

   ---

The funny part is, you talk about all these constructor requirements but you store pointers in the lockfree queue...?! Just write it:

typedef std::vector<unsigned char> Data;

class Queue {
    private:
    Data m_data;

    public:
    Queue(Data data) : m_data(std::move(data)) {}
    Queue() : m_data() {}
    unsigned char const *getdata() const { return m_data.data(); } 
    size_t               getint()  const { return m_data.size(); } 
};

boost::lockfree::spsc_queue<Queue> q(10);

Live On Coliru

Notes:

• you need to make the consumer check the return code of pop. The push might not have happened, and lock free queues don't block.

• you don't need that contraption. Just pass vectors all the way:

C++ Code

Live On Coliru

#include <boost/lockfree/spsc_queue.hpp>
#include <thread>
#include <iostream>
#include <vector>

typedef std::vector<unsigned char> Queue;

boost::lockfree::spsc_queue<Queue> q(10);

void produce() {
    Queue obj(10, 1);

    std::cout << __FUNCTION__ << " - " << obj.size() << "\n";

    q.push(std::move(obj));
}

void consume() {
    Queue obj;
    while (!q.pop(obj)) { }

    std::cout << __FUNCTION__ << " - " << obj.size() << "\n";
}

int main() {
    std::thread t1 {produce};
    std::thread t2 {consume};

    t1.join();
    t2.join();
}