MSVC++ 2013 Debug vs. Release Compiling Issue

Question

So I am experimenting with how to call C++ programs from ASM. It works on debug mode, but not so much on release mode.

I have assembly code that looks like this :

sub rsp, 30h
mov rcx, 1
mov rdx, 2
mov r8, 3
mov r9, 4
mov qword ptr [rsp + 20h], 5
mov qword ptr [rsp + 28h], 6
call MyProc
add rsp, 30h
ret

And the MyProc function looks like this :

extern "C"
{
    *other irrelevant methods here*

    void MyProc(int a, int b, int c, int d, int e, int f)
    {
        cout << "First & Last Param : " << a << " and " << f << endl;
    }
}

In debug mode, it runs properly. On release mode, it fails with : Error 0xC0000005: Access violation reading location 0xFFFFFFFFFFFFFFFF

Whats going on, and how do I fix this?

Answer 1

After some experimenting, all I need to do is to sub the stack pointer by any number greater or equal than the (number of parameters of the function * 8) bytes, but the number has to end with an 8 to keep the stack aligned (credit to Hans), so numbers like 28h for (< 5 param), 38h(6 - 7 param), 48h (8 - 9 param) etc. would work.

Not sure if I'm wrong about this, but the first 4 bytes of the stack are reserved, which is why I need to sub the stack by more than 32 bytes...