Allocation and deallocation of memory in unmanaged code using platform Invoke (C#)

Question

I want to allocate and deallocate memory in unmanaged code (C++) and we call them functions from managed code (C#). Iam not sure whether the following code is fine without memory leaks or not?

C# code:

[DllImport("SampleDLL.dll")]
public extern void getString([MarshalAs(UnmanagedType.LPStr)] out String strbuilder);

[DllImport("SampleDLL.dll")]
public extern void freeMemory([MarshalAs(UnmanagedType.LPStr)] out String strBuilder);

....

//call to unmanaged code
getString(out str);
Console.WriteLine(str);
freeMemory(out str);

C++ code:

extern void _cdecl getString(char **str)
{
    *str = new char[20];
    std::string temp = "Hello world";
    strncpy(*str,temp.c_str(),temp.length()+1);
}

extern void _cdecl freeMemory(char **str)
{
    if(*str)
        delete []*str;
    *str=NULL;
}

score 4 · Accepted Answer · answered Sep 03 '11 at 16:41

4

No, this cannot work. The pinvoke marshaller is going to try to release the memory for the string with CoTaskMemFree(). It doesn't otherwise know that you have a release function. That's not going to work well, you didn't allocate the string with CoTaskMemAlloc. This is going to be a silent memory leak in XP, a crash in Vista and up.

You have to stop the marshaller from trying to do the right job:

[DllImport("SampleDLL.dll")]
public extern void getString(out IntPtr strptr);

[DllImport("SampleDLL.dll")]
public extern void freeMemory(IntPtr strptr);

Which then requires Marshal.PtrToStringAnsi() in your C# code to marshal the string yourself from the returned pointer.

answered Sep 03 '11 at 16:41

Hans Passant

922,412
146
1,693
2,536

Hi,why didn't u used 'out' keyword for the parameter in freeMemory function ? – user186246 Sep 05 '11 at 01:54
Btw why CoTaskMemFree() is called? what it does actually? – user186246 Sep 05 '11 at 04:30
Because freeMemory doesn't return a pointer. CoTaskMemAlloc() is the memory allocator for COM. If there's no way to guess what heap was used then the marshaller guesses at the COM heap. – Hans Passant Sep 05 '11 at 08:18

score 0 · Answer 2 · answered Sep 03 '11 at 18:54

Personally I think this is most easily done using a BSTR and thus avoiding the need to export a deallocator.

C++

BSTR ANSItoBSTR(const char* input)
{
    BSTR result = NULL;
    int lenA = lstrlenA(input);
    int lenW = ::MultiByteToWideChar(CP_ACP, 0, input, lenA, NULL, 0);
    if (lenW > 0)
    {
        result = ::SysAllocStringLen(0, lenW);
        ::MultiByteToWideChar(CP_ACP, 0, input, lenA, result, lenW);
    } 
    return result;
}

BSTR __stdcall getString()
{
    return ANSItoBSTR("Hello world");
}

Of course, if you are working with Unicode strings its even easier.

BSTR __stdcall getString()
{
    return ::SysAllocString(L"Hello world");
}

C#

[DllImport(@"test.dll")]
[return: MarshalAs(UnmanagedType.BStr)]
private static extern string getString();

And on the C# side that's it. You just call getString() and it returns a .net string and you don't need to marshall anything or call a deallocator.

Allocation and deallocation of memory in unmanaged code using platform Invoke (C#)

2 Answers2

Linked