Re: Missed warning (-Wuse-after-free)

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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On Fri, 17 Feb 2023, 11:43 Alejandro Colomar, <alx.manpages@gmail.com>
wrote:

> Hi Jonathan,
>
> On 2/17/23 12:24, Jonathan Wakely wrote:
> > Please be aware that in C++ it's implementation-defined, not undefined.
> >
> > That means that an implementation without trap representations for
> pointers
> > can choose to make it behave just like using (uintptr_t)p.
>
> (uintptr_t)p is defined (I believe) for C <= C17.  However, as noted in my
> last email, even that is UB for C2x.  Of course, UB means that the compiler
> might make it defined, but as Martin suggested, that's might have its own
> issues.
>

Yes, I just meant in C++, and C++ has no such rules.



> >
> > https://cplusplus.github.io/CWG/issues/1438.html
> > https://cplusplus.github.io/CWG/issues/623.html
> > https://cplusplus.github.io/CWG/issues/616.html
> > https://cplusplus.github.io/CWG/issues/312.html
> >
> > We could still warn in C++ (because the code isn't portable) but I would
> > strongly suggest we don't influence C++ codegen based on deallocated
> > pointers being undefined. I don't think gcc supports any targets with
> > trapping pointers, and there are quite enough sources of UB already. We
> > don't need to create traps for users where there are no traps for
> pointers
> > :-)
>
> I would warn in C++ too, as some of that code might be in interfaces that
> should be compatible with C.  Maybe not include it in -Wextra in C++ (but
> include it in C's -Wextra)...
>

I agree with warning in C++, just not optimizing based on the assumption
that any use of an invalid pointer implies unreachable code (as in your
original example).