Result of sizeof() not as expected

[Gronnie]

#pragma pack(1)struct ExampleStruct0 {    union {        struct {            uint32_t xxx    : 23;            uint32_t xxx    : 13;            uint32_t xxx    :  3;            uint32_t xxx    :  3;            uint32_t xxx    :  2;            uint32_t xxx    :  1;            uint32_t xxx    :  2;            uint32_t xxx    :  1;        };        struct {            uint32_t xxxLow     : 32;            uint32_t xxxHigh    :  7;            uint32_t xxx           :  1;        };    };};

#pragma pack(1)struct ExampleStruct1 {    union {        struct {            uint32_t xxx    : 23;            uint32_t xxx    : 13;            uint32_t xxx    :  3;            uint32_t xxx    :  3;            uint32_t xxx    :  2;            uint32_t xxx    :  1;            uint32_t xxx    :  2;            uint32_t xxx    :  1;        };        struct {            uint64_t xxx    : 39;            uint64_t xxx    :  1;        };    };};

What would you expect the result of sizeof(ExampleStruct0) to be?

How about sizeof(ExampleStruct1)?

I am getting something different than expected.

[Gronnie]

Well at work the result is 8, but I just tried it on my home computer and I get what I expected -- 6. The #pragma pack(1) must not be working correctly at work.

It's not actually being done like in the example, it's being set in some weird way with a macro and a file with tons of ifdefs so we can write code to compile with several different compilers.

I've never done much with preprocessors, compilers, etc. I guess now is as good a time as any to learn.

[Ciccioo]

might be worth to give a look at this stackoverflow question: http://stackoverflow.com/questions/3919194/what-is-vc-doing-when-packing-bitfields

it seems that the bits are correctly aligned, but the compiler still reserves enough space for the lowest multiple of sizeof(uint_32t) that will fit your structure declaration

 

#pragma pack has the right to behave differently on every machine and compiler that you use it on, so it shouldn't be too much of a surprise if you get different results on different compilers