Re: [wish] Flexible array members in unions

[Martin Uecker <uecker@tugraz.at>]

Am Donnerstag, dem 18.05.2023 um 20:59 +0000 schrieb Qing Zhao:
> 
> > On May 18, 2023, at 12:25 PM, Martin Uecker via Gcc <gcc@gcc.gnu.org> wrote:
> > 
> > 
> > 
> > > On Thu, May 11, 2023 at 11:14 PM Kees Cook via Gcc <gcc@gcc.gnu.org> wrote:
> > > > 
> > > > On Thu, May 11, 2023 at 08:53:52PM +0000, Joseph Myers wrote:
> > > > > On Thu, 11 May 2023, Kees Cook via Gcc wrote:
> > > > > 
> > > > > > On Thu, May 11, 2023 at 06:29:10PM +0200, Alejandro Colomar wrote:
> > > > > > > On 5/11/23 18:07, Alejandro Colomar wrote:
> > > > > > > [...]
> > > > > > > > Would you allow flexible array members in unions?  Is there any
> > > > > > > > strong reason to disallow them?
> > > > > > 
> > > > > > Yes please!! And alone in a struct, too.
> > > > > > 
> > > > > > AFAICT, there is no mechanical/architectural reason to disallow them
> > > > > > (especially since they _can_ be constructed with some fancy tricks,
> > > > > > and they behave as expected.) My understanding is that it's disallowed
> > > > > > due to an overly strict reading of the very terse language that created
> > > > > > flexible arrays in C99.
> > > > > 
> > > > > Standard C has no such thing as a zero-size object or type, which would
> > > > > lead to problems with a struct or union that only contains a flexible
> > > > > array member there.
> > 
> > (I think it is fundamentally not too problematic to have zero-size
> > objects, although it would take some work to specify the semantics
> > exactly.)
> > 
> > But my preference would be to make structs / unions with FAM an
> > incomplete type which would then restrict their use (for the cases
> > now supported we would need backwards compatible exceptions).
> > We could then allow such a struct / union as the last member
> > of another struct / union which would make this an incomplete
> > type too.
> 
> Yes, I like this approach. 
> And we can make them GCC extensions first,  promote to C standard later.
> 
> My proposed patch sets (originally targeted on GCC13, now might need to target on GCC14) will
> make one part of the above a GCC extension:
>     Allowing the struct with FAM as the last member of another struct. (See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101832)
> 
> https://gcc.gnu.org/pipermail/gcc-patches/2023-March/614794.html
> https://gcc.gnu.org/pipermail/gcc-patches/2023-March/614793.html
> https://gcc.gnu.org/pipermail/gcc-patches/2023-March/614790.html
> 
> I’d like these changes going into GCC first to improve this area.

Seems reasonable to me. Thanks!

Martin


> 
> > 
> > We then would need a special macro (based on a builtin) instead
> > of sizeof to get the size, but this would be safer anyway.
> > 
> > In principle, an even better solution would be to allow dynamic
> > arrays because then it has a dynamic bound where the type with
> > the bound could propagate to some user. Bounds checking would
> > work as expected and more cases.
> > 
> > struct foo {
> >  int len;
> >  char buf[.len];
> > };
> > 
> > But this takes a bit more work to get right.
> > 
> > > > 
> > > > Ah-ha, okay. That root cause makes sense now.
> > > 
> > > Hmm. but then the workaround
> > > 
> > > struct X {
> > >  int n;
> > >  union u {
> > >      char at_least_size_one;
> > >      int iarr[];
> > >      short sarr[];
> > >  };
> > > };
> > > 
> > > doesn't work either.  We could make that a GNU extension without
> > > adverse effects?
> > 
> > I think we could allow this even without the "at_least_size_one"
> > without a problem when allowing the use of such unions only as
> > a last member of some structure. Allowing it elsewhere seems
> > questionable anyway.
> 
> Yes,  Such an union can be treated as an flexible array member 
> (just multiple flexible arrays sharing the same storage).  Therefore it’s reasonable
> To only allow it as the last field of a structure. 
> 
> thanks.
> 
> Qing.
> 
> > 
> > > Richard.
> > > 
> > > > Why are zero-sized objects missing in Standard C? Or, perhaps, the better
> > > > question is: what's needed to support the idea of a zero-sized object?
> > 
> > Probably a lot of convincing that it actually does not cause problems,
> > and is useful. Also a lot of work in making sure the standard is revised
> > everywhere where it is necessary. I think zero sized objects and
> > especially arrays are very useful also to avoid special code for corner
> > cases in numerical algorithms. But I think here some restrictions on
> > the use of the FAM will do.
> > 
> > 
> > Martin
>