[Bug c/53760] New: attribute packed doesn't pack inner structs

[Bug c/53760] New: attribute packed doesn't pack inner structs

[stsp at users dot sourceforge.net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53760

             Bug #: 53760
           Summary: attribute packed doesn't pack inner structs
    Classification: Unclassified
           Product: gcc
           Version: 4.6.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: stsp@users.sourceforge.net


The following code:

---
#include <stdio.h>

struct A {
    char a:1;
    struct {
        char a:1;
    } __attribute__((packed)) s1;
    struct {
        char a:1;
    } __attribute__((packed)) s2;
    struct {
        char a:1;
    } __attribute__((packed)) s3;
} __attribute__((packed));

int main()
{
    printf("%lu\n", sizeof(struct A));
    return 0;
}
---

prints '4' instead of '1'.
Every bitfield seems to be occupying an entire byte.

[Bug c/53760] attribute packed doesn't pack inner structs

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53760

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-06-24 17:59:08 UTC ---
I think having a size of 4 is correct.  Packed is on a byte boundary and not a
bit boundary.

[Bug c/53760] attribute packed doesn't pack inner structs

[stsp at users dot sourceforge.net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53760

--- Comment #2 from Stas Sergeev <stsp at users dot sourceforge.net> 2012-06-24 18:09:18 UTC ---
Even if "attribute packed" have nothing to do with this,
then why doesn't it pack regardless?
Eg,
struct {
char a:1;
char b:1;
char c:1;
}
would be packed, no matter what.
Why does the same not happen to the struct?
Any way to force such a packing? There is an
obvious (IMHO) need for that.

[Bug c/53760] attribute packed doesn't pack inner structs

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53760

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-06-24 18:12:39 UTC ---
Because each inner struct has to be at a byte boundary.

[Bug c/53760] attribute packed doesn't pack inner structs

[stsp at users dot sourceforge.net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53760

--- Comment #4 from Stas Sergeev <stsp at users dot sourceforge.net> 2012-06-24 18:20:00 UTC ---
Is this mandated in some spec or doc?
Any way to defeat that? :)
Btw, why can't the "attribute packed" be
allowed to pack on a bit boundary too?
Sounds like a useless limitation, no?

[Bug c/53760] attribute packed doesn't pack inner structs

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53760

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-06-25 06:51:12 UTC ---
It is not a useless limitation.  While you can't take address of a bitfield,
you can take address of a struct field, even when it contains only bitfields.

[Bug c/53760] attribute packed doesn't pack inner structs

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53760

Richard Guenther <rguenth at gcc dot gnu.org> changed:

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

--- Comment #6 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-06-25 08:11:17 UTC ---
Indeed.

[Bug c/53760] attribute packed doesn't pack inner structs

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53760

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ebotcazou at gcc dot
                   |                            |gnu.org

--- Comment #7 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-06-25 10:06:23 UTC ---
> Is this mandated in some spec or doc?
> Any way to defeat that? :)

If you're ready to switch language.  The Ada language supports bit-packedness.

[Bug c/53760] attribute packed doesn't pack inner structs

[stsp at users dot sourceforge.net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53760

--- Comment #8 from Stas Sergeev <stsp at users dot sourceforge.net> 2012-06-25 10:29:40 UTC ---
I think gcc C could support it too by
some "pack_harder" attribute (and then not
allow pointers to such structs), but I've got
the point. :)