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vbapointersoperating-system32bit-64bitconditional-compilation

Does the bitness of the OS ever matter, or is it just the application I need to worry about?

Some assumptions: (correct me if wrong)

Ignoring 16-bit stuff, VBA can be run on 32 or 64-bit Office hosts. 64-bit Office can only be run on a 64-bit OS, whereas you can run 32-Bit office on a 32 or 64-bit version of Windows/macOS/other Operating System.

As of VBA7, we have the LongPtr type, which becomes a Long on 32-bit Office (#Win64 = False), LongLong on 64-bit Office (#Win64 = True), regardless of the OS bitness

My question: In APIs that deal with pointers (addresses in memory), does the OS bitness ever matter, or is it only the application running the code that we care about (32/64-bit Office host/VBA)?

Current understanding:

On the one hand I can see why it might not matter:

  • VBA running in 32-bit Excel will have a 32-bit address space (regardless of OS)
  • Therefore any pointers to its own memory should be 32-bits (i.e. Longs - LongPtr gives us this)
  • Similarly 64-bit VBA (necessarily in 64-bit OS, but really could be anywhere) has a 64 bit address space (each pointer is itself 64-bits long and refers to a 64-bit wide block of memory)
  • Therefore any pointers to its own memory should be 64-bits (i.e. LongLongs - LongPtr gives us this)
  • So LongPtr accurately represents a pointer to my code's own memory, regardless of OS bitness

However I can imagine times where the OS bitness could matter

  • Running 32-bit VBA on 64-bit OS, but wanting to refer to areas in memory of a different 64-bit application, the LongPtr type would evaluate to Long and would be too short to represent the maximum pointer size available in that OS.
  • Similarly a 64-bit VBA accessing memory of a 32-bit application would find its pointer type too large to hold the address
  • Now the much trusted LongPtr type is actually the wrong length to represent a pointer to an address in memory outside VBA's own address space!

Problems can now arise depending on OS bitness, not Office/VBA bitness. If we're running VBA7 on 32-bit OS, then LongPtr will be the right length for any chunk of memory you could want to throw at it; it is a suitable pointer datatype in 99.9% of cases since everything the OS does is 32-bit (ignoring 16 bit).

However the same 32-bit VBA7 code instead running on a 64-bit OS would run into difficulties when trying to use LongPtr to hold 32-bit addresses. I feel it's quite common to mix 32-bit and 64-bit applications on a 64-bit OS so this could be a real issue.

32-bit VBA6 and earlier applications running on 32 vs 64 bit Operating Systems could face similar issues, only without the help of LongPtr

Now I appreciate that's a pretty contrived situation, who would ever want to access another application's memory, right? In fact it's so contrived that I'm not sure I could ever come up with a situation where this is important and worth worrying about. Could an application ever obtain an address to the memory of another application with different bitness? Maybe there are some read-write protections preventing this.

Perhaps accessing another application's window handle would be one such occasion; that's public memory and perhaps the bitness of the window handle reflects the bitness of the application or the Operating system, in which case it would be feasible for my 32-bit VBA to want to hold a reference to a 64-bit Hwnd, I'm not sure...

PS I'm aware there are situations other than pointer length where OS bitness may be important. I know of one; the SetWindowLong function required different declarations on 64-bit vs 32-bit Windows - although IIUC that's now been solved with the SetWindowLongPtr function which is identical for both. But any other similar quirks would be useful to know about, I'm only focussing on pointer length here because I have a problem that requires that specific info.

PPS come to think of it, can you even get OS bitness at compile time; I think you can infer it from MAC_OFFICE_VERSION, and ofc Win64 = True means 64-bit Office and de-facto 64-bit OS. But I'm not sure if there's a way to tell whether 32-bit VBA is running on 64-bit Windows...

Solution

  • A LongPtr is always correct for your process. You do not need to worry about its size. You do not need the WIN64 constant to use it. In fact, the only constant you normally need is VBA7 that tells you whether the LongPtr is even available. If it is, use it, if it's not, you are definitely x86 so use Long.

    Additionally, Windows x64 has an entire compatibility layer called WoW64. As a 32-bit app running on 64-bit Windows, you do not notice anything and you run as if the OS was 32-bit. Your pointer size is four bytes, your pointer-sized data types such as HWNDs are four bytes, so again, you do not need to worry about any of that if you only consult VBA7 and correctly place LongPtr in all places where a pointer-sized argument must appear.

    So, for routine things inside your process, and for interacting with the OS and its objects, you don't need to worry about either your own or the OS'es bitness, and you don't need the WIN64 constant either.

    Now, you specifically mention obtaining and using pointers that are valid for processes with different bitness than your own. Yes, that might be a problem, but this problem is not specific to VBA.

    If, as a VBA app, you find yourself in a need to read memory of an arbitrary process of arbitrary bitness, you do need to compare your own bitness to its bitness. At this point you could make use of the WIN64 constant, but it's much more convenient in this case to check the Len(long_ptr_variable) at runtime than to have separate code branches.

    Having made the test,

    • If your bitness is higher, you can read the other process'es memory without restrictions.
    • If your bitness is smaller, you can reach as far up the big guy's virtual memory as your 4-byte pointer allows.
      • And if you need to reach beyond that, you would have to fail (or not).

    But note that even in this case, you don't care about the OS bitness or the WIN64 const either! You only care about your process bitness vs the other process bitness.