A field in a packed struct will often not have the normal alignment for its
type. So taking the address of such a field may yield a misaligned pointer,
but gcc does not warn about that. This is similar to casting a pointer from a
less aligned type to a more aligned type, which does give a warning (with
-Wcast-align).
Example:
> cat aopf.c
#include <stdio.h>
void __attribute__((noinline)) f(int *p)
{
printf("%p\n", p);
}
struct s {
int x;
char c;
} __attribute__((__packed__));
struct s A[10];
int main(void)
{
unsigned int i;
for (i = 0; i < sizeof(A)/sizeof(A[0]); ++i) {
f(&A[i].x);
f((int*)(char*)&A[i].x);
}
return 0;
}
> gcc -Wall -Wextra -Wcast-align -O aopf.c
aopf.c: In function 'main':
aopf.c:21:4: warning: cast increases required alignment of target type
Ideally both calls to f() should trigger alignment warnings.
--
Summary: escaping address of packed field should trigger warning
Product: gcc
Version: 4.5.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: other
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: mikpe at it dot uu dot se
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41809
