Struct alignment is always 4

Question

Why pragma pack does not work in here? sizeof() returning the size aligned to 4. I use compiler which comes with Microsoft Visual Studio 2015

#pragma pack(push,1)

struct Header {
    uint16_t zero = 0;
    uint32_t id{};
};

struct ImageBlock : Header {
    uint32_t currentValidLength{};
};

#pragma pack(pop)
constexpr int sz = sizeof(ImageBlock);  // returns 12 but should be 10

Tried to also set Struct member alignment (/Zp1) compiler option with no luck

Can you please reduce your code to a [mre]? I doubt that a structure of size `1260` with that many different members is actually required to reproduce your issue. — Brian61354270, Aug 23 '23 at 13:04
I couldn't reproduce, I got 1258, but admittedly with different compiler versions. — harold, Aug 23 '23 at 13:06
Still [could not reproduce](https://godbolt.org/z/3a7dc3oY7) — harold, Aug 23 '23 at 13:16

score 3 · Answer 1 · edited Aug 23 '23 at 14:19

3

Looks like it's an IntelliSense bug. At runtime, the size is correct, but Visual Studio shows me an incorrect value of sz while editing. I've also tried static_assert on the size: VS shows that it will fail at compile time, but in fact, everything is ok.

edited Aug 23 '23 at 14:19

Jan Schultke

17,446
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answered Aug 23 '23 at 13:20

Valerii Chistyakov

41
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Struct alignment is always 4

1 Answers1