GCC attribute((aligned(4))) not working as expected

Asked Sep 09 '22 at 19:35

Active Sep 09 '22 at 19:35

Viewed 361 times

I have a GCC question about variable alignment. I thought I understood it, but can't get this to work. This code:

uint8 aaa[4];
uint32 *bbb = (uint32 *)&aaa;

generates this warning:

warning: cast increases required alignment of target type [-Wcast-align]
  175 |     uint32 *bbb = (uint32 *)&aaa;

I've tried several combinations of the aligned() attribute none of which work:

__attribute__((aligned(4))) uint8 aaa[4];
uint8 aaa[4] __attribute__((aligned(4)));

This method fixes it, but it's too ugly (actually the real code is nested structures and adding unions would be difficult):

union {
    uint8 aaa[4];
    uint32 bbb;
} aaa2;
uint32 *bbb2 = (uint32 *)&aaa2;

Any ideas?

asked Sep 09 '22 at 19:35

Symmetric

If it's ugly, hide it in a macro. – Barmar Sep 09 '22 at 19:47
*none of which work* - doesn't work how? – Eugene Sh. Sep 09 '22 at 21:15
Is this a global array or automatic/local? This attribute does not work for stack-allocated variables – Eugene Sh. Sep 09 '22 at 21:16
FWIW, the union method is an "official" alternative to the attribute as per the [docs](https://gcc.gnu.org/onlinedocs/gcc-12.2.0/gcc/Common-Variable-Attributes.html#Common-Variable-Attributes) – Eugene Sh. Sep 09 '22 at 21:19
Doesn’t work as in it doesn’t fix the warning. – Symmetric Sep 10 '22 at 02:28
This is a stack variable. Hmm, I thought I read that it didn’t used to work, like 20 years ago, but it does now. I would think if the union is an alternative, and works, that both would. – Symmetric Sep 10 '22 at 02:36

GCC __attribute__((aligned(4))) not working as expected