Bad alloc is thrown

Question

I am getting a 'bad_alloc while trying to work with boost managed shared memory. I've copied the boost example from their quick guide for the impatient and incorporated my own changes. MY code is below, I've commented out the example stuff and wrote my own below it. I've also put in some debugging and testing stuff.

Does anyone have any ideas? Any help is greatly appreciated!

-M

#include <boost/interprocess/managed_shared_memory.hpp>
#include <boost/interprocess/containers/map.hpp>
#include <boost/interprocess/allocators/allocator.hpp>
#include <boost/interprocess/containers/string.hpp>
#include <boost/interprocess/exceptions.hpp>

#include <functional>
#include <utility>
#include <iostream>
#include <string>

#define space_name "MySharedMemorydfgdfhgd"
namespace std{


}

int main ()
{

using namespace boost::interprocess;


//Remove shared memory on construction and destruction

struct shm_remove
{
    shm_remove() { shared_memory_object::remove(space_name); }
    ~shm_remove(){ shared_memory_object::remove(space_name); }
} remover;

typedef int    KeyType;
typedef boost::interprocess::managed_shared_memory::allocator<char>::type char_allocator;
//typedef boost::interprocess::allocator<char, boost::interprocess::managed_shared_memory::segment_manager> char_allocator;
typedef boost::interprocess::basic_string<char, std::char_traits<char>, char_allocator> shm_string;

struct certificateStorage{
    int certificate_id;        
    certificateStorage( int _certificate_id, const char* _certificate, const char* _key, const char_allocator &al) :
    certificate_id(_certificate_id) 
    {}
};

//Note that map<Key, MappedType>'s value_type is std::pair<const Key, MappedType>,
//so the allocator must allocate that pair.

//typedef std::pair<const int, float> ValueType;
typedef std::pair<const int, certificateStorage> certValueType;
//typedef allocator<ValueType, managed_shared_memory::segment_manager> ShmemAllocator;
typedef allocator<certValueType, boost::interprocess::managed_shared_memory::segment_manager> certShmemAllocator;
//typedef map<KeyType, MappedType, std::less<KeyType>, ShmemAllocator> MyMap;
typedef map<KeyType, certificateStorage, std::less<KeyType>, certShmemAllocator> certSHMMap;
// typedef boost::interprocess::map<KeyType, int, std::less<KeyType>, certShmemAllocator> certSHMMap;

std::cout << "\n\n\nStarting the program.\n\n\n";

//Shared memory front-end that is able to construct objects
//associated with a c-string. Erase previous shared memory with the name
//to be used and create the memory segment at the specified address and initialize resources

const int numentries = 20;
const char* elementName = "mymap";
int size = sizeof(certificateStorage) * numentries + 1000;
std::cout << "SHM size is " <<size<< " bytes \n";
int runningsize = 0;


try{
    //this stayed the same
    managed_shared_memory shm_segment
    (create_only
    ,space_name//segment name
    ,size);   

    certSHMMap *mymap;

    //Initialize the shared memory STL-compatible allocator
    //ShmemAllocator alloc_inst (segment.get_segment_manager());
    certShmemAllocator alloc_inst (shm_segment.get_segment_manager());
    char_allocator ca(shm_segment.get_allocator<char>());

    for(int i = 0; i < numentries; i++){

        try{
            //MyMap *mymap =
            // segment.construct<MyMap>("MyMap")      //object name
            //(std::less<int>() //first  ctor parameter
            //  ,alloc_inst);     //second ctor parameter              

            mymap = shm_segment.construct<certSHMMap>(elementName)
            (std::less<int>() 
            ,alloc_inst);     //object name
        }
        catch(boost::interprocess::interprocess_exception &ex){
            std::cout << "Certificates element already exists.";

            try{
                mymap = shm_segment.find<certSHMMap>(elementName).first;   //object name
                 std::cout << " Fetching existing pointer.\n";
            }
            catch(boost::interprocess::interprocess_exception &ex){
                std::cout << "\nCertificates object wont load\n";
                mymap = shm_segment.find<certSHMMap>(elementName).first;   //object name
            }
        }

        certificateStorage thisCert(i, "", "", ca);
         std::cout << "Created object.\n";
        mymap->insert(certValueType(i, thisCert));
         std::cout << "Inserted object. " << i <<" size is " <<sizeof(thisCert)  << " \n";
         runningsize += sizeof(thisCert) ;
         std::cout << "SHM Current size is " << runningsize << " / " << size << "\n";
    }

    std::cout << "\n\nDone Inserting\nStarting output\n.";
    /*    
        //Insert data in the map
        for(int i = 0; i < 100; ++i){
        mymap->insert(std::pair<const int, float>(i, (float)(i*i)));
        }

        for(int i = 0; i < 100; ++i){
        std::cout << "Key: " << i << " Value: " << mymap->at(i) << "\n";

        mymap->insert(std::pair<const int, float>(i, (float)(i*2)));
    } */


    for(int i = 0; i < numentries; i++){

        try{
            mymap = shm_segment.construct<certSHMMap>(elementName)(std::less<int>() ,alloc_inst);     //object name
        }
        catch(boost::interprocess::interprocess_exception &ex){
            std::cout << "Certificates element already exists.\n";

            try{
                mymap = shm_segment.find<certSHMMap>(elementName).first;   //object name
            }
            catch(boost::interprocess::interprocess_exception &ex){
                std::cout << "Certificates object wont load\n";
                mymap = shm_segment.find<certSHMMap>(elementName).first;   //object name
            }
        }
        certificateStorage tmp = mymap->at(i);
        std::cout << "The key is: " << i << " And the value is: " << tmp.certificate_id;
    }
}
catch(boost::interprocess::interprocess_exception &ex){
    std::cout << "\n shm space wont load wont load\n";
    std::cout << "\n Why: " << ex.what() << "\n";


}
shared_memory_object::remove(space_name);
return 0;

}

And here is my program output...

 Starting the program.


SHM size is 1080 bytes
Created object.
Inserted object. 0 size is 4
SHM Current size is 4 / 1080
Certificates element already exists. Fetching existing pointer.
Created object.
Inserted object. 1 size is 4
SHM Current size is 8 / 1080
Certificates element already exists. Fetching existing pointer.
Created object.
Inserted object. 2 size is 4
SHM Current size is 12 / 1080
Certificates element already exists. Fetching existing pointer.
Created object.
Inserted object. 3 size is 4
SHM Current size is 16 / 1080
Certificates element already exists. Fetching existing pointer.
Created object.
Inserted object. 4 size is 4
SHM Current size is 20 / 1080
Certificates element already exists. Fetching existing pointer.
Created object.
Inserted object. 5 size is 4
SHM Current size is 24 / 1080
Certificates element already exists. Fetching existing pointer.
Created object.
Inserted object. 6 size is 4
SHM Current size is 28 / 1080
Certificates element already exists. Fetching existing pointer.
Created object.
Inserted object. 7 size is 4
SHM Current size is 32 / 1080
Certificates element already exists. Fetching existing pointer.
Created object.
Inserted object. 8 size is 4
SHM Current size is 36 / 1080
Certificates element already exists. Fetching existing pointer.
Created object.
Inserted object. 9 size is 4
SHM Current size is 40 / 1080
Certificates element already exists. Fetching existing pointer.
Created object.
Inserted object. 10 size is 4
SHM Current size is 44 / 1080
Certificates element already exists. Fetching existing pointer.
Created object.
Inserted object. 11 size is 4
SHM Current size is 48 / 1080
Certificates element already exists. Fetching existing pointer.
Created object.
Inserted object. 12 size is 4
SHM Current size is 52 / 1080
Certificates element already exists. Fetching existing pointer.
Created object.
Inserted object. 13 size is 4
SHM Current size is 56 / 1080
Certificates element already exists. Fetching existing pointer.
Created object.
Inserted object. 14 size is 4
SHM Current size is 60 / 1080
Certificates element already exists. Fetching existing pointer.
Created object.
Inserted object. 15 size is 4
SHM Current size is 64 / 1080
Certificates element already exists. Fetching existing pointer.
Created object.

 shm space wont load wont load

 Why: boost::interprocess::bad_alloc

Answer 1

It seems you're simply running out of memory. You might reason that you shouldn't since the individual allocations don't occupy the amount of space.

But memory fragmentation can do this: if there is sufficient 'padding' or 'overhead' with the shared memory objects, you can run out of contiguously allocatable space.

Either, store your data in a pre-allocated vector (e.g.), or use one of the smarter interprocess allocation algorithms:

• http://www.boost.org/doc/libs/1_55_0/doc/html/interprocess/allocators_containers.html

The simplest way to resolve it in this instance would seem to be just making the shared memory area twice as big (minimal size is a 4K memory page on most systems, anyway).

I just used 2*size and the tests ran to completion.

Update/fixes

I've just verified that indeed doing things "the vector way" is much more efficient: replacing std::map by boost's flat_map gets you vector storage.

The big difference is that each node in a map is dynamically allocated, incurring a fixed overhead, linearly consuming available memory.

Observations

• There's considerable initial overhead, consuming 320 bytes before anything happened.
• with the flat_map, you also reserve the vector capacity up front, you see that you can win just a little extra storage efficiency.

The above graph was created from the output of the following program. Look for the calls to get_free_memory(). To switch map implementation, just change #if 0 into #if 1. (Note how I cleaned up some of the code that was needless repetitious and using exceptions for flow control).

#include <boost/interprocess/managed_shared_memory.hpp>
#include <boost/interprocess/containers/map.hpp>
#include <boost/interprocess/allocators/allocator.hpp>
#include <boost/interprocess/containers/string.hpp>
#include <boost/interprocess/containers/flat_map.hpp>
#include <boost/interprocess/exceptions.hpp>

#include <functional>
#include <utility>
#include <iostream>
#include <string>

#define space_name "MySharedMemory"

int main ()
{
    using namespace boost::interprocess;
    //Remove shared memory on construction and destruction

    struct shm_remove
    {
        shm_remove() { shared_memory_object::remove(space_name); }
        ~shm_remove(){ shared_memory_object::remove(space_name); }
    } remover;

    typedef int KeyType;
    typedef boost::interprocess::managed_shared_memory::allocator<char>::type char_allocator;
    //typedef boost::interprocess::allocator<char, boost::interprocess::managed_shared_memory::segment_manager> char_allocator;
    //typedef boost::interprocess::basic_string<char, std::char_traits<char>, char_allocator> shm_string;

    struct certificateStorage{
        int certificate_id;        
        certificateStorage( int _certificate_id, const char* _certificate, const char* _key, const char_allocator &al) :
            certificate_id(_certificate_id) 
        {}
    };

#if 0 // STL
    typedef std::pair<const int, certificateStorage> certValueType;
    typedef allocator<certValueType, boost::interprocess::managed_shared_memory::segment_manager> certShmemAllocator;
    typedef map<KeyType, certificateStorage, std::less<KeyType>, certShmemAllocator> certSHMMap;
#else // FLAT_MAP
    typedef std::pair<int, certificateStorage> certValueType; // not const key for flat_map
    typedef allocator<certValueType, boost::interprocess::managed_shared_memory::segment_manager> certShmemAllocator;
    typedef boost::container::flat_map<KeyType, certificateStorage, std::less<KeyType>, certShmemAllocator> certSHMMap;
#endif 

    std::cout << "\n\n\nStarting the program.\n\n\n";

    const int numentries    = 20;
    const char* elementName = "mymap";
    int size                = sizeof(certificateStorage) * numentries + 1000;
    int runningsize         = 0;

    std::cout << "SHM size is " <<size<< " bytes \n";

    try{
        managed_shared_memory shm_segment(create_only, space_name/*segment name*/, size);   

        certShmemAllocator alloc_inst (shm_segment.get_segment_manager());
        char_allocator ca(shm_segment.get_allocator<char>());

        certSHMMap *mymap = shm_segment.find_or_construct<certSHMMap>(elementName)
            (std::less<int>(), alloc_inst);

        mymap->reserve(numentries);

        for(int i = 0; i < numentries; i++){
            std::cout << "Free memory: " << shm_segment.get_free_memory() << "\n";

            certificateStorage thisCert(i, "", "", ca);
            std::cout << "Created object.\n";
            mymap->insert(certValueType(i, thisCert));
            std::cout << "Inserted object. " << i <<" size is " <<sizeof(thisCert)  << " \n";
            runningsize += sizeof(thisCert) ;
            std::cout << "SHM Current size is " << runningsize << " / " << size << "\n";
        }

        std::cout << "\n\nDone Inserting\nStarting output\n";

        for(int i = 0; i < numentries; i++){
            certificateStorage tmp = mymap->at(i);
            std::cout << "The key is: " << i << " And the value is: " << tmp.certificate_id << "\n";
        }
    }
    catch(boost::interprocess::interprocess_exception &ex){
        std::cout << "\n shm space wont load wont load\n";
        std::cout << "\n Why: " << ex.what() << "\n";
    }
}