Re: [V5][PATCH 2/2] Update documentation to clarify a GCC extension

[Jakub Jelinek <jakub@redhat.com>]

On Mon, Mar 27, 2023 at 03:57:58PM +0000, Qing Zhao wrote:
> >> +Please use warning option  @option{-Wgnu-variable-sized-type-not-at-end} to
> > This is certainly misnamed.
> 
> The name “-Wgnu-variable-sized-type-not-at-end” was just used the warning name from CLANG. -:)
> 
> Shall we use the same name as CLANG? Or we invent a new name?

The latter IMHO.  Having a warning with completely nonsensical name will
just confuse users.

> >  GNU variable sized type not at the end of a
> > struct is something like
> > void bar (void *);
> > void foo (int n) {
> >  struct S { int a; int b[n]; int c; } s;
> >  s.a = 1;
> >  __builtin_memset (s.b, 0, sizeof (s.b));
> >  s.c = 3;
> >  bar (&s);
> > }
> > Certainly not flexible array members in the middle of structure.
> 
> Right now, with -Wpedantic, we have the following warning for the above small case:
> 
> t2.c:3:24: warning: a member of a structure or union cannot have a variably modified type [-Wpedantic]
>     3 |  struct S { int a; int b[n]; int c; } s;
>       |                        ^

Sure, it is a GNU C extension (not allowed in C++ BTW).
It is documented in https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html
though just very briefly:
As an extension, GCC accepts variable-length arrays as a member of a structure or a union. For example: 
void
foo (int n)
{
  struct S { int x[n]; };
}

> Do we have a definition for “GNU variable sized type” now?

Naturally, variable sized type should have non-constant sizeof, because
otherwise it is constant sized type.  That is not
the case for flexible array members, there is nothing variable sized on
them, especially if they are in the middle of a structure.

	Jakub