WinCE and Function Pointer Addresses

One of the more strange bits of code that didn’t work out of the box during my Windows CE project is this nugget from Mozilla NPAPI plugin handling code. Basically what it attempts to do is discover if the given window has already been subclassed by comparing the current WNDPROC to a locally defined function. If the equality matches, then we know that the window has been subclassed by us (unless someone else also subclassed this window).

static LRESULT CALLBACK PluginWndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
//…
}

WNDPROC currentWndProc = (WNDPROC)::GetWindowLong(hWnd, GWL_WNDPROC);
if (PluginWndProc == currentWndProc)

Looks fine, doesn’t it?

Well, looking at this in the debugger — if you “mouse over” the two operands — looks fine too. However, when you run the code, the wrong thing happens.

Now if you look at the ASM you see this:

2865E274 ldr r0, [pc, #0x124]
2865E278 ldr r1, [sp, #8]
2865E27C cmp r0, r1

Loading one of the address off of the PC. I think that might work, but I am not totally sure.. hmm.

Now, looking at the registers after the two loads I see something really messed up (the actual addresses I don’t have at the moment):

r0′s 0x00ABCDEF
r1′s 0x83ABCDEF

what is going on here? R0 looks like a NEAR 24bit pointer… This shouldn’t be!

I have tried a few things including, changing the declaration of PluginWndProc, doing the compare in a separate DLL, and various forms of casting. Nothing has worked yet.

Here is the actual code.