[Boost-users] [shmem] problem retrieving objects from map

1 Aug 2006

      Hello,

I have a problem retrieving objects from a map placed in shared
memory. I have two processes:

proc#1: creates the shared memory, puts a map into it and populates it
with objects
proc#2: connects to the shared memory and tries to read out the
objects from the map

Whenever I try to read out the objects from the map, I get zeroes for
the internal members (a, b).

The class definition for the objects:
--------------------------------------------------
class ShmNode
{
private:
	int a;
	int b;
public:
	ShmNode();
	ShmNode(int newa, int newb);
	~ShmNode();
	void set(int newa, int newb);
	int getA();
	int getB();
};
--------------------------------------------------

I'm posting the source codes for the two programs. I removed the
synchronization stuff with semaphores for the sake of clarity:

Proc#1:
--------------------------------------------------
#include <boost/shmem/named_shared_object.hpp>
#include <boost/shmem/allocators/allocator.hpp>
#include <boost/shmem/containers/vector.hpp>
#include <boost/shmem/containers/map.hpp>
#include <boost/shmem/sync/named_semaphore.hpp>

#include <iostream>
#include "../shmem/ShmNode.hpp"

typedef std::pair<
			const int,
			ShmNode*>
		MyPair;

typedef boost::shmem::allocator <
			MyPair,
			boost::shmem::named_shared_object::segment_manager>
		MyAlloc;

typedef boost::shmem::map<
			const int,
			ShmNode*,
			std::less<const int>,
			MyAlloc>
		MyMap;

int main()
{
	boost::shmem::named_shared_object segment;

	segment.create("/myshmem", 20000);

	MyAlloc nodesAlloc(segment.get_segment_manager());
	std::less<const int> comparator;

	MyMap *table = segment.construct<MyMap>("table")(comparator, nodesAlloc);

	MyPair myPair1(1, new ShmNode(1,2));
	MyPair myPair2(2, new ShmNode(3,4));
	MyPair myPair3(3, new ShmNode(5,6));

	table->insert(myPair3);
	table->insert(myPair1);
	table->insert(myPair2);	

	return 0;
}
--------------------------------------------------

Proc#2
--------------------------------------------------
/* same includes */

/* same typedefs */

int main()
{
	boost::shmem::named_shared_object segment;
	segment.open("/myshmem");

	MyMap *table = segment.find<MyMap>("table").first;

	std::cout << "table size: " << table->size() << std::endl;

	MyMap::iterator iter;	
	for (iter = table->begin(); ; iter++)
	{		
		if (iter == table->end())
			break;
		std::cout << iter->second->getA() << "  " << iter->second->getB() <<
std::endl;
	}	
	return 0;
}
--------------------------------------------------

The problem once again: in proc#2,  iter->second->getA() and
iter->second->getB() return 0. If I put the same loop into proc#1, the
correct values are returned. As a further hint, the size of the map
object in proc#2 is returned correctly (=3).

What am I doing wrong?

Regards,
Szilard

[Boost-users] [shmem] problem retrieving objects from map

Pataki Szilard