https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80665
Bug ID: 80665
Summary: dynamic cast on nullptr leads to segfault
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: abenkhadra at protonmail dot com
Target Milestone: ---
Dynamic casts on nullptr should return nullptr as per C++ Standard ยง5.2.7/4.
However, g++ v6.2 segfaults while executing a dynamic_cast wrapped in a
non-virtual method belonging to a parent class. The assumed bug is produced
only at optimization levels higher than -O0.
That is, the code snippet with the following flags doesn't produce the bug
g++ -std=c++11 -Wall -Wextra -Wpedantic -O0 main.cpp -o test
However, compiling it with the following flags does produce a segfault
g++ -std=c++11 -Wall -Wextra -Wpedantic -O2 main.cpp -o test
Note that the following code snippet works on clang v3.8 and g++ v5.4 without
problems.
Code snippet:
-------------
#include <iostream>
class Base{
public:
int foo;
template<typename T>
T* as()
{
return dynamic_cast<T*>(this);
}
virtual ~Base() = default;
};
class Derived: public Base {
public:
int bar;
};
int main(void) {
Derived* fst_ptr = nullptr;
std::cout << "g++ 6.2 segfaults executing next statement" << std::endl;
Derived* snd_ptr = fst_ptr->as<Derived>();
std::cout << "Other compilers segfault on last statement as expected" <<
std::endl;
return (*snd_ptr).foo;
}
Platform:
---------
Used the prepackaged binary of g++ v6.2 for Ubuntu 16.04.2
Compiler details:
--------------------
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
6.2.0-3ubuntu11~16.04' --with-bugurl=file:///usr/share/doc/gcc-6/README.Bugs
--enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-6 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib
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--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib
--with-tune=generic --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 6.2.0 20160901 (Ubuntu 6.2.0-3ubuntu11~16.04)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++11' '-Wall' '-Wextra'
'-Wpedantic' '-O0' '-o' 'test' '-shared-libgcc' '-mtune=generic'
'-march=x86-64'
/usr/lib/gcc/x86_64-linux-gnu/6/cc1plus -E -quiet -v -imultiarch
x86_64-linux-gnu -D_GNU_SOURCE main.cpp -mtune=generic -march=x86-64 -std=c++11
-Wall -Wextra -Wpedantic -O0 -fpch-preprocess -fstack-protector-strong
-Wformat-security -o main.ii
ignoring duplicate directory "/usr/include/x86_64-linux-gnu/c++/6"
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-linux-gnu/6/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/include/c++/6
/usr/include/x86_64-linux-gnu/c++/6
/usr/include/c++/6/backward
/usr/lib/gcc/x86_64-linux-gnu/6/include
/usr/local/include
/usr/lib/gcc/x86_64-linux-gnu/6/include-fixed
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++11' '-Wall' '-Wextra'
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/usr/lib/gcc/x86_64-linux-gnu/6/cc1plus -fpreprocessed main.ii -quiet
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-Wpedantic -std=c++11 -version -fstack-protector-strong -Wformat-security -o
main.s
GNU C++11 (Ubuntu 6.2.0-3ubuntu11~16.04) version 6.2.0 20160901
(x86_64-linux-gnu)
compiled by GNU C version 6.2.0 20160901, GMP version 6.1.0, MPFR
version 3.1.4, MPC version 1.0.3, isl version 0.15
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C++11 (Ubuntu 6.2.0-3ubuntu11~16.04) version 6.2.0 20160901
(x86_64-linux-gnu)
compiled by GNU C version 6.2.0 20160901, GMP version 6.1.0, MPFR
version 3.1.4, MPC version 1.0.3, isl version 0.15
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 23988a38771f71e4676d56931fe884f7
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++11' '-Wall' '-Wextra'
'-Wpedantic' '-O0' '-o' 'test' '-shared-libgcc' '-mtune=generic'
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as -v --64 -o main.o main.s
GNU assembler version 2.26.1 (x86_64-linux-gnu) using BFD version (GNU Binutils
for Ubuntu) 2.26.1
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/6/:/usr/lib/gcc/x86_64-linux-gnu/6/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/6/:/usr/lib/gcc/x86_64-linux-gnu/
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