Read access violation for memory mapped vector in debug mode

Question

While attempting to use boost::interprocess for storing a std::vector in a memory mapped file, I am getting the exception Exception thrown: read access violation. when I try to push back on a loaded vector, but only in debug mode.

This minimal example code (written by @sehe) is retrieved from https://stackoverflow.com/a/29602884/2741329, and it crashes on MSVC14 in debug mode and executed more than once:

#include <boost/interprocess/managed_mapped_file.hpp>

namespace bi = boost::interprocess;

int main() {
    std::string vecFile = "vector.dat";
    bi::managed_mapped_file file_vec(bi::open_or_create,vecFile.c_str(), 1000);

    typedef bi::allocator<int, bi::managed_mapped_file::segment_manager> int_alloc;
    typedef std::vector<int, int_alloc>  MyVec;

    MyVec * vecptr = file_vec.find_or_construct<MyVec>("myvector")(file_vec.get_segment_manager());

    vecptr->push_back(rand());
}

EDIT:

This is the Visual Studio error message:

Here the point where the exception happens:

This is the call stack (click on the pic to enlarge it):

Answer 1

As a brainwave, disable MSVC debug iterators.

I'm not sure how (because iterators aren't persisted?) but somehow iterator debugging might add raw pointers inside the memory layout of the std::vector - violating standard library assumptions about allocator use.

Test Results

Creating a VM on azure just for the purpose, tested with the following slightly modified code to better understand the crash reasons:

#include <boost/interprocess/managed_mapped_file.hpp>
#include <iostream>

namespace bi = boost::interprocess;

int main() {
    std::string vecFile = "vector.dat";
    //std::remove(vecFile.c_str());
    std::cout << __LINE__ << "\n";
    {
        bi::managed_mapped_file file_vec(bi::open_or_create, vecFile.c_str(), 100000);

        typedef bi::allocator<int, bi::managed_mapped_file::segment_manager> int_alloc;
        typedef std::vector<int, int_alloc>  MyVec;

        MyVec * vecptr = file_vec.find_or_construct<MyVec>("myvector")(file_vec.get_segment_manager());

        vecptr->push_back(rand());
        std::cout << "size: " << vecptr->size() << "\n";
    }
    std::cout << __LINE__ << "\n";
    {
        bi::managed_mapped_file file_vec(bi::open_or_create, vecFile.c_str(), 100000);

        typedef bi::allocator<int, bi::managed_mapped_file::segment_manager> int_alloc;
        typedef std::vector<int, int_alloc>  MyVec;

        MyVec * vecptr = file_vec.find_or_construct<MyVec>("myvector")(file_vec.get_segment_manager());

        vecptr->push_back(rand());
        std::cout << "size: " << vecptr->size() << "\n";
    }
    std::cout << __LINE__ << "\n";
}

Reproduces the issue. First run:

Subsequent run (with the std::remove line commented as shown):

WORKAROUND DEMO

After putting

#define _ITERATOR_DEBUG_LEVEL 0

at the very top AND REMOVING THE vector.dat file because the change alters binary layout:

Note: in your actual project you may require putting that #define in multiple translation units (especially consider stdafx.cpp). It's probably much better to include it in the project property sheets so it applies to all (future) translation units!