Problem with Interop C#/C: AccessViolationException

Question

and thanks in advice for any help.

i have this trivial function in C:

__declspec(dllexport)  Point* createPoint (int x, int y) {
    Point *p;

    p = (Point*) malloc(sizeof(Point)); 
    p->x = x;
    p->y=y;

    return p;       
}

Point is a very simple struct with two int fields, x and y.

I would like calling this function from C#.

I use this code:

[DllImport("simpleC.dll", EntryPoint = "createPoint", CallingConvention = CallingConvention.Cdecl, SetLastError = true, CharSet = CharSet.Auto)]
[return: MarshalAs(UnmanagedType.LPStruct)]
public static extern Point createPoint(int x, int y);

Point p = Wrapper.createPoint(1, 2);

But at runtime I have an AccessViolationException. Watching exception in detail, I found that exception is thrown from Marshal.CoTaskMemFree(IntPtr) method.

It seems that this method is unable to free memory allocated by C malloc.

What am i doing wrong?

Really thanks.

Answer 1

CoTaskMemFree cannot be used to free memory allocated by malloc (because they use different allocators). According to MSDN, "The runtime always uses the CoTaskMemFree method to free memory. If the memory you are working with was not allocated with the CoTaskMemAlloc method, you must use an IntPtr and free the memory manually using the appropriate method."

Additionally, Adam Nathan notes that "UnmanagedType.LPStruct is only supported for one specific case: treating a System.Guid value type as an unmanaged GUID with an extra level of indirection. ... You should probably just stay away from UnmanagedType.LPStruct."

There are two possible solutions:

1. Declare the return type of the method as IntPtr and use Marshal.ReadInt32 to read the fields of the struct, or use Marshal.PtrToStructure to copy the data to a managed struct, or use unsafe code to cast the IntPtr value to a Point *. The C library will need to expose a destroyPoint(Point *) method that frees the memory.
2. Change the C method signature to void getPoint(int x, int y, Point *). This lets C# allocate the struct, and the C method simply fills in the data values. (Most of the Win32 APIs are defined this way).

One final note: Unless your method uses the SetLastError Win32 API, you don't need to specify SetLastError = true on your P/Invoke attribute.