Question

In the code below, why is the size of the packed structure different on Linux and Windows when compiled with gcc?

#include <inttypes.h>
#include <cstdio>

// id3 header from an mp3 file
struct header
{    
        uint8_t version[ 2 ];
        uint8_t flags;
        uint32_t size;
} __attribute__((packed));

int main( int argc, char **argv )
{
        printf( "%u\n", (unsigned int)sizeof( header ) );
        return 0;
}

gcc versions used:

$ g++ --version
g++ (Ubuntu/Linaro 4.5.2-8ubuntu4) 4.5.2
$ x86_64-w64-mingw32-g++ --version
x86_64-w64-mingw32-g++ (GCC) 4.7.0 20110831 (experimental)

Compile and test:

$ g++ -Wall packed.cpp -o packed && ./packed
7
$ x86_64-w64-mingw32-g++ -Wall packed.cpp -o packed.exe
--> prints '8' when run on Windows.

The Linux binary prints the expected size of 7 bytes, the Windows binary 8 bytes. Why the difference?

Answer 1

Section 6.37.3 of the gcc attributes explains it as a difference in ABI specs, see here: http://gcc.gnu.org/onlinedocs/gcc/Type-Attributes.html