Instead of using /Zp, you should use #pragma pack along with #pragma
push/pop to control the packing of specific structs. Here's why.

You should not fool around with the default packing. While the CRT headers
and most of the Windows headers are diligent about setting the packing they
expect and restoring the original packing, this is far from universal.
You're begging for trouble if you use /Zp on code you didn't write
yourself, such as libraries you may be using, and even some of the Windows
headers, e.g. <wincon.h>, which last I checked (Jan 2004), has a problem
with its KEY_EVENT_RECORD struct under non-default packing. You can easily
cause your code to observe different class layouts than the libraries
you're using, which will really mess you up. On a non-Intel CPU, straying
from the default would almost certainly result in a horrendously slow
application or outright crashing due to misaligned data access. That's true
for cases in which changing the default packing *works*, in the sense that
everyone agrees on class layouts; on Intel CPUs, you would merely observe
slightly slower data access.

--
Doug Harrison
Microsoft MVP - Visual C++