[Bug c/45317] struct union misalignment

[Bug c/45317] struct union misalignment

[sherpya at netfarm dot it]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45317

--- Comment #2 from Gianluigi Tiesi <sherpya at netfarm dot it> 2012-11-15 00:13:28 UTC ---
so mingw < 4.7.0 has same packing behavior of linux gcc, why in _mingw_ >=
4.7.0 should I add -malign-double to be compatible with mingw < 4.7.0 and msvc
abi?

[Bug c/45317] struct union misalignment

[sherpya at netfarm dot it]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45317

--- Comment #3 from Gianluigi Tiesi <sherpya at netfarm dot it> 2012-11-15 00:21:58 UTC ---
I've also tested with -malign-double and nothing changes, I don't need mingw32
abi on linux, I need mingw abi on mingw, somehow changed with 4.7.0, I'm not
sure if it's a bug or not

[Bug c/45317] struct union misalignment

[sherpya at netfarm dot it]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45317

--- Comment #4 from Gianluigi Tiesi <sherpya at netfarm dot it> 2012-11-15 00:36:10 UTC ---
sorry my comment are for another bug, please ignore them

[Bug c/45317] New: struct union misalignment

[sherpya at netfarm dot it]

Hi, bug or feature? I have this sample:

#include <stdio.h>
#include <stdlib.h>
#include <stddef.h>
#include <stdint.h>

int main(void)
{
    union U
    {
        struct
        {
            uint32_t l;
            uint32_t h;
        } d;
        uint64_t q;
    };
    struct S
    {
        uint32_t one;
        union U two;
        uint32_t three;
    };

    printf("sizeof(union) = %u\n", sizeof(union U));
    printf("offsets %u, %u, %u\n", offsetof(struct S, one), offsetof(struct S,
two), offsetof(struct S, three));
    return 0;
}

I have different results:

gcc version 4.4.5 20100816 (prerelease) (Debian 4.4.4-9) (32bit)
and gcc version 4.3.5 (Debian 4.3.5-2)

sizeof(union) = 8
offsets 0, 4, 12
----------------
gcc version 4.2.5 20090330 (prerelease) [Sherpya] (32bit latest changeset 4.2.5
on mingw)

and gcc version 4.4.3 on mingw32 (built from release tarball)

sizeof(union) = 8
offsets 0, 8, 16

---

I've discovered the problem by passing struct between a binary compiled with
msvc and my code, looks like the 'correct way' is to pad the first int32 in the
struct S with 4 byte to make union 'two' aligned to union size (8)

Something changed in gcc behavior that I don't known?

Regards


-- 
           Summary: struct union misalignment
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: sherpya at netfarm dot it
 GCC build triplet: i486-linux-gnu
  GCC host triplet: i486-linux-gnu
GCC target triplet: i486-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45317

[Bug c/45317] struct union misalignment

[schwab at linux-m68k dot org]

------- Comment #1 from schwab at linux-m68k dot org  2010-08-18 08:19 -------
That's how it is defined by the respective ABIs.  On i386-linux a struct field
has at most 32-bit alignment.  Add explicit padding or compile with
-malign-double if you want to be compatible to the mingw32 ABI, but that won't
be the only difference between those ABIs.


-- 

schwab at linux-m68k dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45317