Re: Subtleties of __attribute__((packed))

[Phil Endecott]

Jan Blunck wrote:

On 12/6/06, Phil Endecott <[EMAIL PROTECTED]> wrote:

I used to think that this:

struct foo {
   int a  __attribute__((packed));
   char b __attribute__((packed));
   ... more fields, all packed ...
};

was exactly the same as this:

struct foo {
   int a;
   char b;
   ... more fields ...
} __attribute__((packed));

but it is not, in a subtle way.

The same code is generated. [...]

I don't think so.  Example:

struct test {
  int a __attribute__((packed));
  int b __attribute__((packed));
};

char c = 1;
struct test t = { .a=2, .b=3 };

$ arm-linux-gnu-gcc -O2 -S -W -Wall test1.c

        .file   "test2.c"
        .global c
        .data
        .type   c, %object
        .size   c, 1
c:
        .byte   1
        .global t
        .align  2               <<<<<<<<===== t is aligned
        .type   t, %object
        .size   t, 8
t:
        .word   2
        .word   3
        .ident  "GCC: (GNU) 4.1.2 20061028 (prerelease) (Debian 4.1.1-19)"


Compare with:

struct test {
  int a;
  int b;
} __attribute__((packed));

char c = 1;
struct test t = { .a=2, .b=3 };

$ arm-linux-gnu-gcc -O2 -S -W -Wall test2.c

        .file   "test1.c"
        .global c
        .data
        .type   c, %object
        .size   c, 1
c:
        .byte   1
        .global t                    <<<<<<  "align" has gone, t is unaligned
        .type   t, %object
        .size   t, 8
t:
        .4byte  2
        .4byte  3
        .ident  "GCC: (GNU) 4.1.2 20061028 (prerelease) (Debian 4.1.1-19)"



Phil.





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