https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87296
mostlyaspambox at protonmail dot com changed:
What |Removed |Added
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CC| |mostlyaspambox at protonmail
dot c
| |om
--- Comment #9 from mostlyaspambox at protonmail dot com ---
I think I've also hit this or a variation of it with the following reproducer:
```
#include <string.h>
#include <algorithm>
#include <assert.h>
#include <array>
struct Base1
{
char arr[5];
};
struct Base2
{
char arr[25];
};
struct A : Base1, Base2
{
};
extern const char* str;
void f(A& a)
{
Base1& base1 = a;
std::fill(std::begin(base1.arr), std::end(base1.arr), ' ');
Base2& base2 = a;
strncpy(reinterpret_cast<char*>(&base2), str, 6);
}
int main()
{
A a{};
f(a);
}
```
If `a` is created inside `f` instead of passed in by reference, there is no
error.
When compiled with `g++ -std=c++11 -O2 -Wall -Wextra -fno-strict-aliasing
-fwrapv -fno-aggressive-loop-optimizations -fsanitize=undefined` on x86-64 with
gcc 8.1 this prints:
test.cpp:27:12: warning: 'char* strncpy(char*, const char*, size_t)' writing 6
bytes into a region of size 5 overflows the destination [-Wstringop-overflow=]
strncpy(reinterpret_cast<char*>(&base2), str, 6);
~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If `str` in the strncpy call is replaced with a string literal, there is the
same warning unless the literal has length 6 including the null terminator.
