8

I'm trying to transfer a std::vector<std::string> to a newly forked process via Boost.Interprocess, so that the child process takes ownership of it and destroy it. Retrieving and reading the vector works, however I'm getting an access violation upon destroying it. I gather that the allocator embarks some state with pointers which point into the parent's address space and are meaningless in the child's one.

How am I supposed to create that vector in the parent and destroy in the child correctly ?

Parent process code:

namespace ip = boost::interprocess;

template <class T>
using ip_allocator = ip::allocator<T, ip::managed_shared_memory::segment_manager>;

template <class T>
using ip_vector = std::vector<T, ip_allocator<T>>;

int CALLBACK WinMain(
    _In_ HINSTANCE,
    _In_ HINSTANCE,
    _In_ LPSTR,
    _In_ int
) {
    ip::shared_memory_object::remove("MyTestShm");
    ip::managed_shared_memory mshm{ ip::open_or_create, "MyTestShm", 1024 * 1024 };

    ip_allocator<int> intAlloc{ mshm.get_segment_manager() };

    mshm.construct<ip_vector<int>>("ForkData")(
        ip_vector<int>{ {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}, intAlloc}
    );

    boost::process::spawn(L"Executor.exe", L"ForkData");
}

Child process (Executor.exe) code:

int wmain(int argc, wchar_t **argv) try {
    namespace ip = boost::interprocess;

    // Kludge to attach the debugger and force continuation
    bool volatile coin = true;
    while (coin)
        ::Sleep(1000);

    ip::managed_shared_memory mshm{ ip::open_only, "MyTestShm" };
    auto const * const fd = mshm.find<ip_vector<int>>("ForkData").first;

    if(!fd)
        return 1;

    for (int i : *fd)
        std::cout << i << ' ';
    std::cout << '\n';

    mshm.destroy_ptr(fd); // Crashing line
}
catch (std::exception const &exc) {
    std::cerr
        << "Unhandled " << boost::typeindex::type_id_runtime(exc).pretty_name() << ":\n"
        << exc.what() << '\n';
    return 1;
}

Error and callstack:

Exception thrown: read access violation. _Pnext was 0x21FE21200B0. occurred

Executor.exe!std::_Container_base12::_Orphan_all() Line 221
Executor.exe!std::_Vector_alloc<std::_Vec_base_types<int,boost::interprocess::allocator<int,boost::interprocess::segment_manager<char,boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family,boost::interprocess::offset_ptr<void,__int64,unsigned __int64,0>,0>,boost::interprocess::iset_index> > > >::_Orphan_all() Line 536
Executor.exe!std::vector<int,boost::interprocess::allocator<int,boost::interprocess::segment_manager<char,boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family,boost::interprocess::offset_ptr<void,__int64,unsigned __int64,0>,0>,boost::interprocess::iset_index> > >::_Tidy() Line 1913
Executor.exe!std::vector<int,boost::interprocess::allocator<int,boost::interprocess::segment_manager<char,boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family,boost::interprocess::offset_ptr<void,__int64,unsigned __int64,0>,0>,boost::interprocess::iset_index> > >::~vector<int,boost::interprocess::allocator<int,boost::interprocess::segment_manager<char,boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family,boost::interprocess::offset_ptr<void,__int64,unsigned __int64,0>,0>,boost::interprocess::iset_index> > >() Line 894
Executor.exe!std::vector<int,boost::interprocess::allocator<int,boost::interprocess::segment_manager<char,boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family,boost::interprocess::offset_ptr<void,__int64,unsigned __int64,0>,0>,boost::interprocess::iset_index> > >::`scalar deleting destructor'(unsigned int)   Executor.exe!boost::interprocess::ipcdetail::placement_destroy<std::vector<int,boost::interprocess::allocator<int,boost::interprocess::segment_manager<char,boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family,boost::interprocess::offset_ptr<void,__int64,unsigned __int64,0>,0>,boost::interprocess::iset_index> > > >::destroy_n(void * mem, unsigned __int64 num, unsigned __int64 & destroyed) Line 61
Executor.exe!boost::interprocess::segment_manager<char,boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family,boost::interprocess::offset_ptr<void,__int64,unsigned __int64,0>,0>,boost::interprocess::iset_index>::priv_generic_named_destroy<char>(const char * name, boost::interprocess::iset_index<boost::interprocess::ipcdetail::index_config<char,boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family,boost::interprocess::offset_ptr<void,__int64,unsigned __int64,0>,0> > > & index, boost::interprocess::ipcdetail::in_place_interface & table, boost::interprocess::ipcdetail::bool_<1> is_intrusive_index) Line 976
Executor.exe!boost::interprocess::segment_manager<char,boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family,boost::interprocess::offset_ptr<void,__int64,unsigned __int64,0>,0>,boost::interprocess::iset_index>::priv_generic_named_destroy<char>(boost::interprocess::ipcdetail::block_header<unsigned __int64> * block_header, boost::interprocess::iset_index<boost::interprocess::ipcdetail::index_config<char,boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family,boost::interprocess::offset_ptr<void,__int64,unsigned __int64,0>,0> > > & index, boost::interprocess::ipcdetail::in_place_interface & table, boost::interprocess::ipcdetail::bool_<0> is_node_index) Line 929
Executor.exe!boost::interprocess::segment_manager<char,boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family,boost::interprocess::offset_ptr<void,__int64,unsigned __int64,0>,0>,boost::interprocess::iset_index>::priv_destroy_ptr(const void * ptr, boost::interprocess::ipcdetail::in_place_interface & dtor) Line 777
Executor.exe!boost::interprocess::segment_manager<char,boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family,boost::interprocess::offset_ptr<void,__int64,unsigned __int64,0>,0>,boost::interprocess::iset_index>::destroy_ptr<std::vector<int,boost::interprocess::allocator<int,boost::interprocess::segment_manager<char,boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family,boost::interprocess::offset_ptr<void,__int64,unsigned __int64,0>,0>,boost::interprocess::iset_index> > > >(const std::vector<int,boost::interprocess::allocator<int,boost::interprocess::segment_manager<char,boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family,boost::interprocess::offset_ptr<void,__int64,unsigned __int64,0>,0>,boost::interprocess::iset_index> > > * p) Line 550
Executor.exe!boost::interprocess::ipcdetail::basic_managed_memory_impl<char,boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family,boost::interprocess::offset_ptr<void,__int64,unsigned __int64,0>,0>,boost::interprocess::iset_index,8>::destroy_ptr<std::vector<int,boost::interprocess::allocator<int,boost::interprocess::segment_manager<char,boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family,boost::interprocess::offset_ptr<void,__int64,unsigned __int64,0>,0>,boost::interprocess::iset_index> > > >(const std::vector<int,boost::interprocess::allocator<int,boost::interprocess::segment_manager<char,boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family,boost::interprocess::offset_ptr<void,__int64,unsigned __int64,0>,0>,boost::interprocess::iset_index> > > * ptr) Line 608
Executor.exe!wmain(int argc, wchar_t * * argv) Line 31
Executor.exe!invoke_main() Line 91
Executor.exe!__scrt_common_main_seh() Line 288
Executor.exe!__scrt_common_main() Line 331
Executor.exe!wmainCRTStartup() Line 17
kernel32.dll!BaseThreadInitThunk()
ntdll.dll!RtlUserThreadStart()
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Quentin
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  • Is it an option to use some callback like "You gave me this vector? Nice. Now delete it for me!" – TobiMcNamobi Nov 08 '18 at 16:10
  • @TobiMcNamobi No, the parent process might terminate before the child even gets to the shared memory. My actual use-case is a shell extension, so I don't have control over the parent process' lifetime. – Quentin Nov 08 '18 at 16:14
  • Weird, I'd assume this is the exact use case for the so called [fancy pointers](https://en.cppreference.com/w/cpp/named_req/Allocator#Fancy_pointers). – Passer By Nov 08 '18 at 18:31
  • @PasserBy by following the trail I ended up on [this example](https://www.boost.org/doc/libs/1_55_0/doc/html/interprocess/quick_guide.html#interprocess.quick_guide.qg_interprocess_container), which is pretty much exactly my code (including the `segment.destroy` which I had before `destroy_ptr` and crashed as well). Guess I'll have to try compiling that example tomorrow to see what's going on... – Quentin Nov 08 '18 at 19:44

1 Answers1

0

Thanks to Passer By who prompted me to go through the Boost.Interprocess documentation again, I stumbled upon this example, which luckily is exactly what I'm trying to do. After confirming that it works on my end (it does), I started gradually morphing it into a copy of my own program, and found the issue.

The difference was the choice of either std::vector or boost::interprocess::vector (also known as boost::container::vector). std::vector is supposed to work fine with a custom allocator, but the issue lies in Microsoft's implementation: _Orphan_all, the function where the access violation happens, is part of the Debug Iterators functionality. That system keeps track of which iterator belongs to which container, but does so by storing raw pointers, which become stale as the vector changes address spaces and end up causing the crash when the vector tries to update the bookkeeping data upon destruction.

Indeed, compiling in Release mode (where _ITERATOR_DEBUG_LEVEL == 0 and debug iterators are deactivated) or using Boost's vector implementation both work flawlessly.

Quentin
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