On Mon, 2023-02-20 at 10:37 +0000, Jonathan Wakely wrote:
> On Mon, 20 Feb 2023 at 10:26, Xi Ruoyao <xry111@xxxxxxxxxxx> wrote:
> >
> > On Sun, 2023-02-19 at 21:33 +0000, Jonny Grant wrote:
> >
> > > I noticed -Wanalyzer-null-dereference reports at build time a
> > > dereference. Also works if a function parameter. I wondered why
> > > std::string isn't detected by this static analyser option.
> >
> > Because the analyzer does not know the C++ standard disallows to use
> > NULL here. It just analyzes the code. The code in libstdc++ reads:
> >
> > basic_string(const _CharT* __s, const _Alloc& __a = _Alloc())
> > : _M_dataplus(_M_local_data(), __a)
> > {
> > // NB: Not required, but considered best practice.
> > if (__s == 0)
> > std::__throw_logic_error(__N("basic_string: "
> > "construction from null is not valid"));
> > const _CharT* __end = __s + traits_type::length(__s);
> > _M_construct(__s, __end, forward_iterator_tag());
> > }
> >
> > As you can see yourself, though the standard implies using NULL here is
> > a UB, libstdc++ does not really code a UB here. So the analyzer will
> > consider the code absolutely valid.
>
> Right, it's defined behaviour in libstdc++, as an extension.
>
> >
> > Note that throwing a C++ exception is not a programming error. It's
> > perfectly legal to catch the exception elsewhere. It's also perfectly
> > legal not to catch it and treat it as an abort() (calling abort is also
> > not a programming error).
> >
> >
> > > It's not pretty, but this wrapper catches NULL passed at compile time:
> > >
> > > std::string make_std_string(const char * const str)
> > > {
> > > // This line ensures: warning: dereference of NULL '0' [CWE-476]
> > > [-Wanalyzer-null-dereference]
> > > char b = *str;
> >
> > You are invoking an undefined behavior here if str is NULL, so it's
> > essentially same as using a nonnull attribute for make_std_string.
>
> And turned defined behaviour back into UB. The warning isn't reliable
> (only if the compiler can see the point is null, which isn't the case
> without optimization, or if the pointer comes from some non-inline
> function), the exception is. You're trading guaranteed exception for a
> not guaranteed warning and unbounded misoptimization due to undefined
> behaviour.
Well, maybe we should have a warning here with -Wpedantic (or something)
as the standard does not allow people to pass NULL and expect a
logic_error. But "deliberately making a UB to raise the warning" is not
good.
--
Xi Ruoyao <xry111@xxxxxxxxxxx>
School of Aerospace Science and Technology, Xidian University
