http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52991
--- Comment #5 from Mikael Pettersson <mikpe at it dot uu.se> 2012-08-22
08:34:46 UTC ---
The following variation of the test case was compiled with both gcc-4.7.1 and
MS Visual Studio 2008:
#include <stdio.h>
#if defined(__GNUC__)
struct A1 {
short s;
struct { int i; } x;
} __attribute__((__packed__));
#endif
#pragma pack(1)
struct A2 {
short s;
struct { int i; } x;
};
#pragma pack()
int main(void)
{
#if defined(__GNUC__)
printf("sizeof(struct A1) == %u\n", (unsigned)sizeof(struct A1));
#endif
printf("sizeof(struct A2) == %u\n", (unsigned)sizeof(struct A2));
return 0;
}
Note that structs A1 and A2 are identical, except A1 is packed (when compiled
by gcc) using the attribute syntax and A2 is packed using the #pragma syntax.
Compiling this with gcc-4.7.1 (target x86_64-w64-mingw32) with -mms-bitfields
(which is the default) gives:
sizeof(struct A1) == 8
sizeof(struct A2) == 6
proving that attribute packed doesn't work. With -mno-ms-bitfields both
structs are reported as being 6 bytes.
Compiling with Visual Studio 2008 gives:
sizeof(struct A2) == 6
as expected.
