[Bug c/51568] New: Enum value is not extracted properly via a union

[Bug c/51568] New: Enum value is not extracted properly via a union

[eugene at hutorny dot in.ua]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51568

             Bug #: 51568
           Summary: Enum value is not extracted properly via a union
    Classification: Unclassified
           Product: gcc
           Version: 4.3.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: eugene@hutorny.in.ua
            Target: ARM


Created attachment 26103
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=26103
Test example

A wrong value is received when a value is assigned via union and read its
member.

Consider the following data structures:

enum en { v0, v1, v2, v3, v4, v5, v6, v7 };

struct rec {
  unsigned short w;
  unsigned char  c;
  enum en e;
} __attribute__((__packed__));
union un {
  struct rec r;
  unsigned v;
} __attribute__((__packed__));

void pass(unsigned v) {
  union un u;
  u.v = v;
  printf("r.w=%X r.c=%d r.e=%d\n", u.r.w, u.r.c, u.r.e );
}

int main(int argc, char** argv) {
  union un u;
  u.r.c = '8';
  u.r.e = v2;
  u.r.w = 0xC5FF;
  printf("r.w=%X r.c=%d r.e=%d\n", u.r.w, u.r.c, u.r.e );
  pass(u.v);
  return 0;
}

Sample above compiled with gcc (Debian 4.3.2-1.1) 4.3.2 prints the following:
r.w=C5FF r.c=56 r.e=2
r.w=C5FF r.c=56 r.e=34050

When compiled with arm-linux-gnueabi-gcc (Debian 4.3.2-1.1) 4.3.2 prints:
r.w=C5FF r.c=56 r.e=2
r.w=C5FF r.c=56 r.e=50434

[Bug c/51568] Enum value is not extracted properly via a union

[rearnsha at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51568

Richard Earnshaw <rearnsha at gcc dot gnu.org> changed:

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

--- Comment #1 from Richard Earnshaw <rearnsha at gcc dot gnu.org> 2011-12-15 17:17:34 UTC ---
On linux sizeof (struct rec) is 7, so how do you expect an unsigned (size = 4)
to hold the entire value?

If you want a packed enum, you need to specify the packed on the element
declaration, not just on the overall structure:

struct rec {
  unsigned short w;
  unsigned char  c;
  enum en e __attribute__((packed));
} __attribute__((packed));

[Bug c/51568] Enum value is not extracted properly via a union

[rearnsha at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51568

--- Comment #2 from Richard Earnshaw <rearnsha at gcc dot gnu.org> 2011-12-15 17:27:19 UTC ---
(In reply to comment #1)
> On linux sizeof (struct rec) is 7, so how do you expect an unsigned (size = 4)
> to hold the entire value?
> 
> If you want a packed enum, you need to specify the packed on the element
> declaration, not just on the overall structure:
> 
> struct rec {
>   unsigned short w;
>   unsigned char  c;
>   enum en e __attribute__((packed));
> } __attribute__((packed));

Actually that doesn't work.  The only ways to pack the enum into 8 bits are to
either:

1) Use -fshort-enums (ABI change)
2) Use a bitfield expression in your struct.

struct rec {
  unsigned short w;
  unsigned char  c;
  enum en e:8;
} __attribute__((packed));

Sorry for the confusion.

[Bug c/51568] Enum value is not extracted properly via a union

[eugene at hutorny dot in.ua]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51568

--- Comment #3 from Eugene <eugene at hutorny dot in.ua> 2011-12-15 19:44:34 UTC ---

>On linux sizeof (struct rec) is 7, so how do you expect an unsigned (size = 4)
to hold the entire value?

Agree, that was my mistake.
I misexpected the enum to fit one byte.

[Bug c/51568] Enum value is not extracted properly via a union

[eugene at hutorny dot in.ua]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51568

--- Comment #4 from Eugene <eugene at hutorny dot in.ua> 2011-12-16 10:48:59 UTC ---
Enum can be packed to one byte with __attribute__((__packed__)) :
enum en { v0, v1, v2, v3, v4, v5, v6, v7 } __attribute__((__packed__));