On Sun, 24 Mar 2019, 13:53 Xi Ruoyao, <xry111@xxxxxxxxxxxxxxxx> wrote:
>
> On 2019-03-24 12:45 +0000, Jonny Grant wrote:
> > Hello!
> >
> > Is this an issue? G++ doesn't identify problem in this test case doesn't
> > when compiling or running.
> >
> > "hello" is not displayed, std::cout never works again after nullptr.
> >
> >
> > // g++-8 -Wall -Wextra -Wconversion-null -Wno-nonnull-compare
> > -Wzero-as-null-pointer-constant
> > -fsanitize=null,returns-nonnull-attribute,signed-integer-
> > overflow,leak,undefined,address
> > -o cout cout.cpp
> > #include <iostream>
> > int main()
> > {
> > const char * p = nullptr;
> > std::cout << p;
> > std::cout << "hello";
> > }
>
> The standard *Requires* p is not a null pointer here (N4618 27.7.3.2.4 p3). So
> if p is a null pointer, the implementation (GCC and libstdc++) can do anything.
>
> Libstdc++ is friendly. It does not abort, raise exception, or run
> /usr/game/starwar, it just sets ios::badbit to cout (ostream:559) so actually
> you can detect this issue by checking the state of cout.
And you can use it for more output after clearing the badbit flag.
> But do NOT rely on
> this behavior. This is really undefined.
>
> Because libstdc++ is so friendly, the sanitizer don't even know this is an
> undefined behavior. And, it can not sanitize undefined behaviors in
> libstdc++.so because it was built w/o -fsanitize= option.
>
> Well, if you want this to be detected at compile time, maybe you can convince
> Jonathan to add an __attribute__((nonnull(2))) to operator<<(basic_ostream&,
> const char *). Then this case can be detected with "-O -Wnonnull".
That seems like a good idea.
