When Windows header files need to adjust structure packing, they use the <pshpackN.h> and <poppack.h> header files. But why bother? Why not just issue the #pragma pack(push, N) and #pragma pack(pop) directly?

The #pragma pack directive is a Microsoft compiler extension. It is not part of the C or C++ standard. The <pshpackN.h> and <poppack.h> headers are used so that compiler vendors can provide substitute headers which generate directives that work for their compilers.

For example, here is one example of a replacement header file that records the structure packing stack in preprocessor symbols.

Related reading: What are anonymous structs, and more importantly, how do I tell windows.h to stop using them?