Segmentation fault after growing a managed_shared_memory segment

Question

I'm playing around with the shared memory portion of the boost library in preparation for a bigger project. I need a shared memory segment, the size of which I won't necessarily know at initialization, so my plan is to grow this segment.

My initial implementation has a boost::interprocess::vector stored in shared memory. As I add values to the vector, if the size of the vector outgrows the segment (a bad_alloc exception is thrown), I grow the segment. This usually works a few times, until I get a segmentation fault.

Not sure if this helps at all, before this implementation, my code would hang upon invoking the managed_shared_memory::grow() method. After some googling, I figured out it was likely because the segment was still mapped to the main process. The current error occurred after I moved the remover struct out of the constructor. Now, the code doesn't hang, but it does segfault.

Here is the code:

#include <iostream>
#include <exception>

#include <boost/interprocess/containers/vector.hpp>
#include <boost/interprocess/allocators/allocator.hpp>
#include <boost/interprocess/managed_shared_memory.hpp>

using namespace boost::interprocess;

static const std::string SHMEM_NAME = "MySharedMemorySegment";

typedef allocator<int, managed_shared_memory::segment_manager> IntAllocator;
typedef vector<int, IntAllocator> ShmemVector;

class ShmemTest {
private:
    managed_shared_memory *segment;
    const IntAllocator *allocator_instance;
    ShmemVector *test_vector;
    size_t shmem_block = 10000;  // Originally, I could allocate 65536 without problems
    struct shm_remove {
        shm_remove()  { shared_memory_object::remove(SHMEM_NAME.data()); }
        ~shm_remove() { shared_memory_object::remove(SHMEM_NAME.data()); }
    } remover;

public:
    ShmemTest() {
        segment = new managed_shared_memory(create_only, SHMEM_NAME.data(), shmem_block);
        allocator_instance = new IntAllocator(segment->get_segment_manager());
    }

    void create_vector() {
        test_vector = segment->construct<ShmemVector>("ShmemVector")(*allocator_instance);
    }

    void append(int value) {
        try {
            test_vector->push_back(value);
        } catch (std::exception & e) {  // Grow shared memory if out of memory
            std::cout << e.what() << std::endl;
            std::cout << "Growing the shared memory block" << std::endl;
            managed_shared_memory::grow(SHMEM_NAME.data(), 1000);
            std::cout << "Done growing the shared memory block, current size: " << size();
            std::cout << " max size: " << max_size() << std::endl;
            test_vector->push_back(value);
            std::cout << "Added value after growing shared memory block" << std::endl;
        }
    }

    void print() {
        std::cout << "ShmemVector values: ";
        for (int value : *test_vector) {
            std::cout << " " << value << " ";
        }
        std::cout << std::endl;
    }

    size_t max_size() {
        return test_vector->max_size();
    }

    size_t size() {
        return test_vector->size();
    }

    void destroy() {
        segment->destroy<ShmemVector>("ShmemVector");
    }
};

int main() {
    ShmemTest shmem_test;
    shmem_test.create_vector();
    std::cout << "Max size: " << shmem_test.max_size() << std::endl;
    for (int i = 0; i < 16380; ++i) {
        shmem_test.append(i);
        if (i > 16200) {
            std::cout << "Current size: " << shmem_test.size() << " ";
        }
    }
    std::cout << std::endl;
    shmem_test.print();
    shmem_test.destroy();
}

This is the typical output I get:

Max size: 2496
boost::interprocess::bad_alloc
Growing the shared memory block
Done growing the shared memory block, current size: 2414 max size: 2746
Added value after growing shared memory block
boost::interprocess::bad_alloc
Growing the shared memory block
Done growing the shared memory block, current size: 2662 max size: 2996
Added value after growing shared memory block
boost::interprocess::bad_alloc
Growing the shared memory block
Done growing the shared memory block, current size: 2914 max size: 3246
Segmentation fault (core dumped)

Solution

As pointed out by oxuf, re-mapping the segment and then using find worked. Here is the updated function, in case anyone stumbles upon the same problem:

void append(int value) {
    try {
        test_vector->push_back(value);
    } catch (std::exception & e) {  // Grow shared memory if out of memory
        managed_shared_memory::grow(SHMEM_NAME.data(), 1000);
        // Re-map the shared memory segment
        segment = new managed_shared_memory(open_only, SHMEM_NAME.data());
        // Find the vector object in the newly mapped shared memory segment
        std::pair<ShmemVector*, size_t> return_value = segment->find<ShmemVector>("ShmemVector");
        test_vector = return_value.first;
        // Append the value
        test_vector->push_back(value);
    }
}

Answer 1

Every time a segment is grown, the OS is free to reallocate the whole memory chunk, and then, all pointers to that memory become invalid. Then, test_vector could point to invalid memory after growing. Try mapping the segment again, this time with the open_only option and use managed_shared_memory::find to reach to your vector.