Using XNA Math in a DLL Class

Question

I'm having a problem in using XNA Math in a DLL I'm creating. I have a class that is in a DLL and is going to be exported. It has a member variable of type XMVECTOR. In the class constructor, I try to initialize the XMVECTOR. I get a Access Violation in reading from reading location 0x0000000000

The code runs something like this:

class DLLClass 
{
public:
    DLLClass(void);
    ~DLLClass(void);
protected:
    XMVECTOR vect;
    XMMATRIX matr;
}

DLLClass::DLLClass(void)
{
    vect = XMLoadFloat3(&XMFLOAT3(0.0f, 0.0f, 0.0f));    //this is the line causing the access violation
}

Note that this class is in a DLL that is going to be exported. I do not know if this will make a difference by just some further info.

Also while I'm at it, I have another question:

I also get the warning: struct '_XMMATRIX' needs to have dll-interface to be used by clients of class 'DLLClass'

Is this fatal? If not, what does it mean and how can I get rid of it? Note this DLLClass is going to be exported and the "clients" of the DLLClass is probably going to use the variable 'matr'.

Any help would be appreciated.

EDIT: just some further info: I've debugged the code line by line and it seems that the error occurs when the return value of XMLoadFloat3 is assigned to the vect.

This compiled and ran with no problems in VS2012 (Visual Studio 2010 (v100) toolset). — David, Jul 18 '13 at 06:04

score 1 · Answer 1 · answered Jun 04 '15 at 01:50

This code is only legal if you are building with x64 native -or- if you use __aligned_malloc to ensure the memory for all instances of DLLClass are 16-byte aligned. x86 (32-bit) malloc and new only provide 8-byte alignment by default. You can 'get lucky' but it's not stable.

class DLLClass 
{
public:
   DLLClass(void);
   ~DLLClass(void);
protected:
   XMVECTOR vect;
   XMMATRIX matr;
}

See DirectXMath Programming Guide, Getting Started

You have three choices:

Ensure DLLClass is always 16-byte aligned
Use XMFLOAT4 and XMFLOAT4X4 instead and do explicit load/stores
Use the SimpleMath wrapper types in DirectX Tool Kit instead which handle the loads/stores for you.

score 0 · Answer 2 · answered Aug 04 '12 at 04:36

0

You shouldn't take the address of an anonymous variable:

vect = XMLoadFloat3(&XMFLOAT3(0.0f, 0.0f, 0.0f));

You need

XMFLOAT3 foo(0.0f, 0.0f, 0.0f);
vect = XMLoadFloat3(&foo);

answered Aug 04 '12 at 04:36

japreiss

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That's not the problem. It still gives the same access violation using your solution. The error occurs on the second line of your solution: vect = XMLoadFloat3(&foo). – l3utterfly Aug 04 '12 at 08:09

Using XNA Math in a DLL Class

2 Answers2