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From: "qinzhao at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/101832] __builtin_object_size(P->M, 1) where M ends with a
 flex-array behaves like sizeof()
Date: Wed, 25 Jan 2023 21:54:34 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101832

--- Comment #8 from qinzhao at gcc dot gnu.org ---
(In reply to Jakub Jelinek from comment #3)
> This is intentional, if you embed an aggregate with flex array into anoth=
er
> struct and ask not to cross the field boundaries (i.e. bos1), then the si=
ze
> of that field is exactly what is the maximum size.

As we discussed in PR 107952
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107952):

GCC extension accepts the following two cases:

**Case 1:

struct A { char data[1]; };
struct B { int n; struct A a; };

as if the a.data[] array is a flex-array.=20=20

**Case 2:=20

struct C { int n; struct A a; int x; };

as if a.data[] can be up to 4 elements.=20

So, what's you mean by "not to cross the field boundaries" is for the above
Case 2?=20
For Case 1, we should treat A.data as flexible array, and then B.A as a
structure that has flexible array, therefore B.A's size is flexible too.=20

Is my understanding correct?=