Am Mittwoch, dem 09.08.2023 um 20:10 +0000 schrieb Qing Zhao:
>
> > On Aug 9, 2023, at 12:21 PM, Michael Matz <m...@suse.de> wrote:
...
>
> By definition, the sizeof() of a struct with FAM might not be the same as the
> non-FAM one.
> i.e, for the following two structures, one with FAM, the other with fixed
> array:
>
> struct foo_flex { int a; short b; char t[]; } x = { .t = { 1, 2, 3 } };
> struct foo_fix {int a; short b; char t[3]; }
>
> With current GCC:
> sizeof(foo_flex) == 8
> sizeof(foo_fix) == 12
>
> I think that the current behavior of sizeof for structure with FAM in GCC is
> correct.
Yes, sadly the sizeof has to be like this as required by ISO C.
>
> The major issue is what was pointed out by Martin in the previous email:
>
> Whether using the following formula is correct to compute the allocation?
>
> sizeof(struct foo_flex) + N * sizeof(foo->t);
That formula is safe for allocation, but might allocate more padding
than the minimum amount and it might allocate less storage than a
similar array with fixed array.
> As pointed out in the wikipedia, the value computed by this formula might
> be bigger than the actual size since “sizeof(struct foo_flex)” might include
> paddings that are used as part of the array.
>
> So the more accurate formula should be
>
> offset(struct foo_flex, t[0]) + N * sizeof(foo->t);
>
> With GCC, offset(struct foo_flex,t[0]) == 6, which is also correct.
This formula might be considered incorrect / dangerous because
it might allocate less storage than sizeof(struct foo_flex).
https://godbolt.org/z/8accq75f3
>
...
> > As in: I think sizeof for both structs should return 12, and 12 bytes
> > should be reserved for objects of such types.
> >
> > And then the next question is what __builtin_object_size should do with
> > these: should it return the size with or without padding at end (i.e.
> > could/should it return 9 even if sizeof is 12). I can see arguments for
> > both.
>
> Currently, GCC’s __builtin_object_size use the following formula to compute
> the object size for
> The structure with FAM:
>
> offset(struct foo_flex, t[0]) + N * sizeof(foo->t);
>
> I think it’s correct.
>
> I think that the users might need to use this formula to compute the
> allocation size for a structure with FAM too.
I am not sure for the reason given above. The following
code would not work:
struct foo_flex { int a; short b; char t[]; } x;
x.a = 1;
struct foo_flex *p = malloc(sizeof(x) + x.a);
if (!p) abort();
memcpy(p, &x, sizeof(x)); // initialize struct
Martin
