Troubles while reading the struct size

Question

If I debug the following code then I see the size value is 12 (as expected).

#include <cstdint>

int main(int argc, char *argv[])
{
    typedef struct __attribute__((__packed__))  { int8_t value; } time;

    typedef struct __attribute__((__packed__))  {
        uint8_t msg[8];
//        time t1;
        uint32_t count;
    } theStruct;

    theStruct s;
    int size = sizeof(s);

    return 0;
}

Interestingly, removing the comment at "time t1;", the value of size goes to 16. I was expecting 13.

I know (more or less) that this is explained by the data structure padding story...

But, is there some way to avoid this issue? What to do in order to read size = 13?

In my test (g++ 4.8.4) I get a size of `13` with the comment removed. Are you sure you tested it correctly? — Evan Teran, Jul 17 '15 at 16:21
Yes, typed correctly. I'm using (Qt enviroment x86) MinGW 4.8.2 32 bit — KcFnMi, Jul 17 '15 at 16:25
I can reproduce @EvanTeran's results. BTW, what's your programming language? C or C++? How do you invoke the compiler? — 5gon12eder, Jul 17 '15 at 16:25
I'm using Qt 5.3.2. To invoke the compiler just push play... May I provide more details? — KcFnMi, Jul 17 '15 at 16:26
deleted my comment, gcc documentation says: "You may also specify any one of these attributes with '_\_' preceding and following its keyword. This allows you to use these attributes in header files without being concerned about a possible macro of the same name. For example, you may use '_\_aligned_\_' instead of 'aligned'." — Sebastian Riese, Jul 17 '15 at 16:30
Also, why is `time` a struct? if it has only one member? just for syntax? — Iharob Al Asimi, Jul 17 '15 at 16:35
Is there something that will make the compiler ignore `__attribute__((__packed__))`? — KcFnMi, Jul 17 '15 at 16:37
@KcFnMi does it happen if you do this `#pragma pack(1) typedef struct { int8_t value; } time; typedef struct { int8_t t1; uint32_t count; uint8_t msg[8]; } theStruct; #pragma pack(1)`? — Iharob Al Asimi, Jul 17 '15 at 16:41
Just check if it is packing, say, a structure containing an array of 3 bytes, for example. — Eugene Sh., Jul 17 '15 at 16:44
@iharob, problem solved. Now I see 13. May I ask for some explanation? — KcFnMi, Jul 17 '15 at 16:45
`#pragma` is a different way of declaring these attributes in other compilers.. I wonder why it is working here for gcc.. — Eugene Sh., Jul 17 '15 at 16:46
@KcFnMi the `__attribute__` thing is `gcc` specific and it might be causing problems on MinGW. — Iharob Al Asimi, Jul 17 '15 at 16:47

score 4 · Accepted Answer · edited May 23 '17 at 12:29

4

There are some issues with MinGW's emulation of MSVC struct packing.

The workaround is to pass -mno-ms-bitfields flag to the compiler; this will cause it to use its own layout algorithm rather than attempt to emulate MSVC.

See also Struct packing and alignment with mingw (ARM but may be the same issue).

edited May 23 '17 at 12:29

Community

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answered Jul 17 '15 at 16:43

ecatmur

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That's what I thought it had to do with the OS because on my Linux system it didn't happen. And that was why [I suggested what's in my comment](http://stackoverflow.com/questions/31480287/troubles-while-reading-the-struct-size?noredirect=1#comment50926597_31480287) which you can append to this answer if you like. – Iharob Al Asimi Jul 17 '15 at 16:48

score 0 · Answer 2 · edited May 23 '17 at 10:26

It is clearly an alignement problem, meaning it has nothing to do with the language itself but all with the underlying platform.

If the platform knows (or thinks) that int32_t need an alignement of 4, it should add 3 bytes of padding after time.

Anyway __attribute__((__packed__)) is non standard C and will only be interpreted by gcc (*). Worse it leads to non portable programs : according to this answer on another question, it would cause a bus error on a Sparc architecture because of a misaligned int32_t.

I do know that x86 (and derivatives) architecture are now the most common, but other architecture may still exist ...

(*) according to Visual C++ equivalent of GCC's __attribute__ ((__packed__)), the MSVC equivalent is #pragma pack(push, 1)

Troubles while reading the struct size

2 Answers2