https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96293
--- Comment #8 from lavr at ncbi dot nlm.nih.gov ---
Consider the following code:
struct record {
unsigned char len;
unsigned short data[5];
} __attribute__((packed));
struct attribute {
unsigned char code;
struct record record;
} __attribute__((packed));
If I had "struct attribute* a" in my code then "&a->record.data[0]" would be a
well-aligned access, even though "struct record" is not "properly aligned". If
I had "struct record" defined with the align(2) attribute (to cater to
"short"s), it might have been placed in "struct attribute" at offset 2, leaving
a one byte gap after the "code" member. Newer gcc gives even more warnings
where there should be none (but fortunately, it does not leave a gap: I can't
check the behavior with GCC 9 anymore because I do no longer have it).
$ cat align.c
#include <stdio.h>
#ifdef ALIGN2
# define ALIGN __attribute__((packed, aligned(2)))
#else
# define ALIGN __attribute__((packed))
#endif
void fun(unsigned short* n)
{
*n = 0xCAFE;
}
struct record {
unsigned char len;
unsigned short data[5];
} ALIGN;
struct attribute {
unsigned char code;
struct record record;
} __attribute__((packed));
int main()
{
struct attribute a;
printf("offsetof data = %zu\n",
offsetof(struct attribute, record.data[0]));
fun(&a.record.data[0]);
return 0;
}
$ gcc -Wall -o align align.c
align.c: In function ‘main’:
align.c:34:9: warning: taking address of packed member of ‘struct record’ may
result in an unaligned pointer value [-Waddress-of-packed-member]
34 | fun(&a.record.data[0]);
| ^~~~~~~~~~~~~~~~~
$ ./align
offsetof data = 2
$ gcc -Wall -DALIGN2 -o align align.c
align.c:26:1: warning: alignment 1 of ‘struct attribute’ is less than 2
[-Wpacked-not-aligned]
26 | } __attribute__((packed));
| ^
align.c:25:19: warning: ‘record’ offset 1 in ‘struct attribute’ isn’t aligned
to 2 [-Wpacked-not-aligned]
25 | struct record record;
| ^~~~~~
align.c: In function ‘main’:
align.c:34:9: warning: taking address of packed member of ‘struct record’ may
result in an unaligned pointer value [-Waddress-of-packed-member]
34 | fun(&a.record.data[0]);
| ^~~~~~~~~~~~~~~~~
$ ./align
offsetof data = 2
