Question

I was trying to run some drivers coded for 32-bit vista (x86) on 64-bit win7 (amd64) and it was not running. After a lot of debugging, and hit-and-trial, I made it to work on the latter, but I don’t know the reason why it’s working. This is what I did:

At many places, buffer pointers pointed to an array of structures(different at different places), and to increment them, at some places this type of statement was used:

ptr = (PVOID)((PCHAR)ptr + offset);

And at some places:

ptr = (PVOID)((ULONG)ptr + offset);

The 2nd one was returning garbage, so I changed them all to 1st one. But I found many sample drivers on the net following the second one. My questions:

1. Where are these macros
   defined(google didn’t help much)?
2. I understand all the P_ macros are
   pointers, why was a pointer casted
   to ULONG? How does this work on
   32-bit?
3. PCHAR obviously changes the
   width according to the environment. Do you know any place to find documentation for this?

Answer 1

1. they should be defined in WinNT.h (they are in the SDK; don’t have the DDK at hand)
2. ULONG is unsigned long; on a 32-bit system, this is the size of a pointer. So a pointer
   can be converted back and forth to ULONG without loss – but not so on a 64-bit system
   (where casting the value will truncate it). People cast to ULONG to get byte-base pointer
   arithmetic (even though this has undefined behavior, as you found out)
3. Pointer arithmetic always works in units of the underlying type, i.e. in CHARs for PCHAR; this equates to bytes arithmetic
4. Any C book should elaborate on the precise semantics of pointer arithmetic.