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. 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".
--
Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx>
School of Aerospace Science and Technology, Xidian University
