Re: [V2][PATCH] gcc-14/changes.html: Deprecate a GCC C extension on flexible array members.

[Richard Biener <rguenther@suse.de>]

On Sat, 4 May 2024, Sebastian Huber wrote:

> On 07.08.23 16:22, Qing Zhao via Gcc-patches wrote:
> > Hi,
> > 
> > This is the 2nd version of the patch.
> > Comparing to the 1st version, the only change is to address Richard's
> > comment on refering a warning option for diagnosing deprecated behavior.
> > 
> > 
> > Okay for committing?
> > 
> > thanks.
> > 
> > Qing
> > 
> > ======
> > 
> > *htdocs/gcc-14/changes.html (Caveats): Add notice about deprecating a C
> > extension about flexible array members.
> > ---
> >   htdocs/gcc-14/changes.html | 13 ++++++++++++-
> >   1 file changed, 12 insertions(+), 1 deletion(-)
> > 
> > diff --git a/htdocs/gcc-14/changes.html b/htdocs/gcc-14/changes.html
> > index dad1ba53..eae25f1a 100644
> > --- a/htdocs/gcc-14/changes.html
> > +++ b/htdocs/gcc-14/changes.html
> > @@ -30,7 +30,18 @@ a work-in-progress.</p>
> >   <!-- ..................................................................
> >   -->
> >   <h2>Caveats</h2>
> >   <ul>
> > -  <li>...</li>
> > +  <li><strong>C:</strong>
> > +      Support for the GCC extension, a structure containing a C99 flexible
> > array
> > +      member, or a union containing such a structure, is not the last field
> > of
> > +      another structure, is deprecated. Refer to
> > +      <a href="https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html">
> > +      Zero Length Arrays</a>.
> > +      Any code relying on this extension should be modifed to ensure that
> > +      C99 flexible array members only end up at the ends of structures.
> > +      Please use the warning option
> > +      <a
> > href="https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wflex-array-member-not-at-end"><code>-Wflex-array-member-not-at-end</code></a>
> > to
> > +      identify all such cases in the source code and modify them.
> > +  </li>
> >   </ul>
> 
> I have a question with respect to the static initialization of flexible array
> members. According to the documentation this is supported by GCC:
> 
> https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> 
> "GCC allows static initialization of flexible array members. This is
> equivalent to defining a new structure containing the original structure
> followed by an array of sufficient size to contain the data. E.g. in the
> following, f1 is constructed as if it were declared like f2.
> 
> struct f1 {
>   int x; int y[];
> } f1 = { 1, { 2, 3, 4 } };
> 
> struct f2 {
>   struct f1 f1; int data[3];
> } f2 = { { 1 }, { 2, 3, 4 } };
> "
> 
> However, when I compile this code, I get a warning like this:
> 
> flex-array.c:6:13: warning: structure containing a flexible array member is
> not at the end of another structure [-Wflex-array-member-not-at-end]
>     6 |   struct f1 f1; int data[3];
>       |
> 
> In general, I agree that flexible array members should be at the end, however
> the support for static initialization is quite important from my point of view
> especially for applications for embedded systems. Here, dynamic allocations
> may not be allowed or feasible.

I do not get a diagnostic for this on trunk?  And I agree there shouldn't
be any.

Richard.