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

2 Aug 2006


      Hi!

Thanks for answering! Figured it out in the end...
-------------------------------------------
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;
(...)
ShmNode shmNode1(1, 2);
ShmNode shmNode2(3, 4);
ShmNode shmNode3(5, 6);

MyPair myPair1(1, shmNode1);
MyPair myPair2(2, shmNode2);
MyPair myPair3(3, shmNode3);

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


Still, can anybody explain what's the purpose of "comparator" and
"nodesAlloc" in the following piece of code? I found no documentation
whatsoever regarding this, and I managed to compile it by pure
accident. Why is there a need for a second comparator when one was
already defined for MyMap. Same story with the allocator...
-------------------------------------------
MyAlloc nodesAlloc(segment.get_segment_manager());
std::less<const int> comparator;
	
MyMap *table = segment.construct<MyMap>("table")(comparator, nodesAlloc);
-------------------------------------------

Regards,
Szilard


On 8/1/06, Adam Lewis  wrote:
...
The problem is you are storing pointers in the map,
which are not in the address space of
proc#2. Check
http://ice.prohosting.com/newfunk/boost/libs/shmem/doc/html/shmem/limitation...
On 8/1/06, Pataki Szilard  wrote:
...
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 
#include 
#include 
#include 
#include 
#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 mailing list
Boost-users@lists.boost.org
http://lists.boost.org/mailman/listinfo.cgi/boost-users
_______________________________________________
Boost-users mailing list
Boost-users@lists.boost.org
http://lists.boost.org/mailman/listinfo.cgi/boost-users