DWORD and DWORD_PTR on 64 bit machine

Question

There are few *_PTR types added to the Windows API in order to support Win64's 64bit addressing.

SetItemData(int nIndex,DWORD_PTR dwItemData)

This API works for both 64 and 32 bit machines when I pass second parameter as DWORD.

I want to know, if this particular API will fail on 64 bit machine, if I pass the second parameter as DWORD. How can I test the fail scenario?

Thanks, Nikhil

Answer 1

The function will not fail if you pass a DWORD, because it fits into a DWORD_PTR. A pointer, however, is guaranteed to fit into a DWORD_PTR but not into a DWORD on 64-bit platforms.

Thus, this code is correct:

int *before_ptr = new int;
yourListBox.SetItemData(index, (DWORD_PTR) before_ptr);
int *after_ptr = (int *) yourListBox.GetItemData(index);
ASSERT(before_ptr == after_ptr);  // Succeeds.
delete after_ptr;                 // Works.

But this code is wrong and will silently truncate the pointer to its lower 32 bits:

int *before_ptr = new int;
yourListBox.SetItemData(index, (DWORD) before_ptr);
int *after_ptr = (int *) yourListBox.GetItemData(index);
ASSERT(before_ptr == after_ptr);  // Fails.
delete after_ptr;                 // Undefined behavior, might corrupt the heap.