Fixing the Size of a Map in Shared Memory

[FrostScYthe]

Hi, I'm wondering how to give a map a fixed size like you can give an array a fixed size by just "byte data[SIZE*DATASIZE]".. or if it's possible anybody know..

struct Table {
map<int, int> table; //
};

struct Table *flightChart;

The reason I have to give this a fixed size, is becuase I have to map this in shared memory :\, so that another process may access it, or is it possible to pass this structure to another process without shared memory :\

 

[chroot]

Even if it were fixed size, sharing your map object in shared memory might not accomplish what you want to do anyway. There's no guarantee that the map object store its data within a single contiguous block of memory -- it may store data in blocks all over the heap.

- Warren

 

[ravenprp]

Hello! To fix the size of a map in shared memory, you can use the boost::interprocess::managed_shared_memory library. This allows you to create a shared memory segment with a fixed size and then use a container allocator to create a map with a fixed size within that segment. Here's an example:

#include <boost/interprocess/managed_shared_memory.hpp>
#include <boost/interprocess/containers/map.hpp>

typedef boost::interprocess::managed_shared_memory Segment;
typedef boost::interprocess::allocator<char, Segment::segment_manager> CharAllocator;
typedef boost::interprocess::map<int, int, std::less<int>, CharAllocator> SharedMap;

int main()
{
// Create a shared memory segment with a fixed size of 1 MB
Segment segment(boost::interprocess::create_only, "MySharedMemory", 1024*1024);

// Create a container allocator for the shared memory segment
CharAllocator charAlloc(segment.get_segment_manager());

// Create a map with a fixed size of 100 elements
SharedMap myMap(charAlloc);
myMap.reserve(100);

// Add elements to the map
myMap.insert(std::make_pair(1, 100));
myMap.insert(std::make_pair(2, 200));

// Access the map from another process by opening the same shared memory segment
Segment segment2(boost::interprocess:pen_only, "MySharedMemory");

// Get a pointer to the map in shared memory
SharedMap *myMap2 = segment2.find<SharedMap>("MyMap").first;

// Access the elements of the map
std::cout << "Value at key 1 is: " << (*myMap2)[1] << std::endl;
std::cout << "Value at key 2 is: " << (*myMap2)[2] << std::endl;

return 0;
}

Note that passing the structure to another process without shared memory is not possible, as each process has its own memory space and cannot access the memory of another process. Shared memory allows for inter-process communication by allowing multiple processes to access and modify the same memory segment. I hope this helps!