You ought to be able to test that with something like:
struct {long long N} a;
char b;
struct {long long N} c;
int d;
d=&c - &a; /* I can't remember if you need to cast c & a as char to
get byte distance */
printf("a is at %d, c is at %d, they are %d apart\n",&a,&c, d);
-y
On Sun, May 4, 2014 at 6:26 AM, Nicholas Clark <[email protected]> wrote:
> Just for the record:
>
> $ cat sizer.c
> #include <stdio.h>
> int main() {
> printf(" sizeof(long long) = %2u
> __alignof__(long long) = %2u\n", (unsigned) sizeof(long long), (unsigned)
> __alignof__(long long));
> printf(" sizeof(struct {long long N;}) = %2u __alignof__(struct
> {long long N;}) = %2u\n", (unsigned) sizeof(struct {long long N;}),
> (unsigned) __alignof__(struct {long long N;}));
> return 0;
> }
> $ gcc -Wall -o sizer sizer.c
> $ ./sizer
> sizeof(long long) = 8 __alignof__(long long) =
> 8
> sizeof(struct {long long N;}) = 8 __alignof__(struct {long long N;}) =
> 4
>
> Yes, this was a surprise.
>
> I knew that structures can cause padding to be introduced, and arrays can have
> higher alignment constraints than their elements.
>
> But lowering alignment constraints?
>
> Is __alignof__() telling the truth here?
> Or is it giving a truthful less-than-useful answer?
>
> (Because what matters to MoarVM is the alignment of, and therefor padding
> needed for, structure members)
>
> Nicholas Clark
