Packing and Alignment

Packing and Alignment

I've made some effort to understand the differences between the Struct Member Alignment compiler option /Zp, #pragma pack and __declspec(align()) with Microsoft Visual C++ in light of an odd bug that ultimately was caused by inconsistent packing options between compilation units.

/Zp and #pragma pack are equivalent, but /Zp applies to the whole compilation unit (if set per source file) or the whole project (if set as a project option). #pragma pack can apply to a subset of a compilation unit. But they do the same thing: change the alignment of struct members.

__declspec(align()), on the other hand, aligns not the individual data members of a struct, but the struct itself. For example:

__declspec(align(1)) struct s_align 
{ 
    bool b; 
    int i; 
};

The declspec above doesn't actually do anything different from what the default is. If, in contrast, you do this:

__declspec(align(32)) struct s_align32 
{ 
    bool b; 
    int i; 
};

then sizeof(s_align32) is 32, whereas it was 8 in the previous example. In other words, the declspec pads the entire structure to line up on 32 byte boundaries, but doesn't change the member packing.

In short, if you want binary compatibility with, say, an on-disk representation of something, you want to pack, not align.