Read Memory Memory Mapped File C++ and C#

Question

I am trying to share a structure coming from C++ to C# using memory mapped file. So far I managed to write on the file, but I am unable to read the content in C#.

1. SendData in C++

struct Bus_1553 // this is the structure to send
{
    string name;
    int directions; 
};

struct Bus_1553* p_1553; // set the pointer to it
HANDLE handle; // create the handle


// here we define the data to send
string name = "IFF";
int directions = 3;

bool startShare() // Open the shared memory
{
    try
    {
        handle = CreateFileMappingW(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, 0, sizeof(Bus_1553), L"DataSend");
        p_1553 = (struct Bus_1553*) MapViewOfFile(handle, FILE_MAP_READ | FILE_MAP_WRITE, 0, 0, sizeof(Bus_1553));
        return true;
    }
    catch (...)
    {
        return false;
    }

}


int main()
{

    if (startShare() == true)
    {

        while (true)
        {
            if (p_1553 != 0) // populate the memory
            {  

                p_1553->name = name;
                p_1553->directions = directions;
            }

            else
                puts("create shared memory error");
        }
    }
    if (handle != NULL)
        CloseHandle(handle);
    return 0;
} 

2. Trying to read in C#

namespace sharedMemoryGET
{
    class sharedMemoryGET
    {
        [StructLayout(LayoutKind.Sequential, Pack = 1)]
        public unsafe struct Bus_Data_1553
        {
            public string name;
            public int directions; // which directions used
        }

        public static MemoryMappedFile mmf;
        public static MemoryMappedViewStream mmfvs;

        static public bool MemOpen() // open the mapped file
        {
            try
            {
                mmf = MemoryMappedFile.OpenExisting("DataSend");
                return true;
            }
            catch
            {
                return false;
            }

        }

        public static void readData()
        {
            if (MemOpen())
                {
                    using (var accessor = mmf.CreateViewAccessor())
                    {
                    accessor.Read(0, out Bus_Data_1553 a);
                    Console.WriteLine(a.name);
                    Console.WriteLine(a.directions);
                    }
                }
            
        }
    }
} 

When a string is present in the structure to share, I have the following error: The specified Type must be a struct containing no references.

When I remove the string and share only the int directions, i get a value of 0. Can someone help me figure this out?

Answer 1

Let's start with what's wrong with the C++ version. I'll bold this to make sure nobody ever passes over this, it's very important: NEVER WRITE POINTERS TO DISK

std::string is a wrapper around a pointer (2 pointers actually) that handle allocation and reallocation for you as needed. You absolutely cannot write them to a "file" anywhere, you must write the contents of those pointers instead.

One simplistic way (and prevalent in C) to do this is to simply define a buffer large enough to hold your data and then use as much of it as needed:

struct Bus_1553 // this is the structure to send
{
    char name[128];
    int directions; 
};

To write to name, use strcpy_s or your OS equivalent.

Now once you write this structure in C++ to your shared file, reading it in C# is about letting the system (the marshaller) decode that soup of bytes into useful managed objects. You do this by using attributes on your structure and field definition:

    [StructLayout(LayoutKind.Sequential, Pack = 1, CharSet = CharSet.Ansi)]
    public struct Bus_Data_1553
    {
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
        public string name;
        public int directions; // which directions used
    }

Also you don't need unsafe for this if you use the marshaller properly.