Re: Preventing ISO C errors when using macros for builtin types

[Segher Boessenkool <segher at kernel dot crashing dot org>]

On Wed, Jun 05, 2019 at 08:49:39PM +0100, Jozef Lawrynowicz wrote:
> On Wed, 5 Jun 2019 11:49:21 -0500
> Segher Boessenkool <segher@kernel.crashing.org> wrote:
> > The documentation says
> > 
> >   '-pedantic' and other options cause warnings for many GNU C extensions.
> >   You can prevent such warnings within one expression by writing
> >   '__extension__' before the expression.  '__extension__' has no effect
> >   aside from this.
> > 
> > It's not clear to me why you cannot simply put __extension__ earlier in
> > your case?
> 
> If I am modifying tests on an individual basis, then indeed I can put
> __extension__ earlier in the declaration and it fixes the issue.

Or you could fix it like this in your header file?

> But for a fix within the compiler to prevent having to modify individual tests,
> I could add __extension__ to the beginning of the macro - but the macro may
> not end up at the beginning of a declaration in the source code.
> 
> For example, say __SIZE_TYPE__ now expands to "__extension__ __int20 unsigned",
> then the following usage of __SIZE_TYPE__ would be ok, as __extension__ is at
> the beginning of the declaration:
> 
> > __SIZE_TYPE__ size;

Don't use macros for types?  You could use something like

  __extension__ typedef unsigned __int20 __SIZE_TYPE__;

(stddef.h already uses typedefs like that, btw, for __SIZE_TYPE__ in fact).

> I'm mainly just wondering if a change to the compiler to allow the usage of
> __extension__ within a declaration would be allowed.

Well, how would that work?  We'd need to modify the grammar to allow
__extension__ pretty much anywhere, and then change all compiler code
to not pedwarn until it has parsed a full expression (or statement, or
file, or however this will work).  Or make it not warn for things after
the __extension__ only, or make it only *guaranteed* not to warn for
things *after* the __extension__.

None of those options are very appetising, IMO.


Segher