Re: std::string clobbers memory when compiling without optimizations

[Marc Glisse <marc dot glisse at inria dot fr>]

On Fri, 3 Oct 2014, Jonathan Wakely wrote:

> On 3 October 2014 11:08, Henrik MannerstrÃm wrote:
> > Hi,
> >
> > On 10/03/2014 12:58 PM, Jonathan Wakely wrote:
> > > Have you tried valgrind? Or compiling with GCC's -fstack-protector
> > > option? I can't reproduce the problem, and I think it's pretty
> > > unlikely std::string has that kind of bug without someone noticing
> > > years ago.
> >
> > I'm not assuming that std::string has a bug (even though the title might
> > suggest that), I'm just saying that the gdb watchpoint is triggered
> > inside that function. I tried valgrind memcheck, but I'm no expert, so
> > suggestions are welcome. Below is the output with stack-protector, what
> > do you make out of it? Could it be my installation?
> >
> > - Henrik
> >
> >
> > $ g++-4.9 -std=gnu++1y mvu.cc  -o mvu
> > $ ./mvu
> > Correct diagonal: 1 1 1
> > Eigen::Matrix<double, 1, 3, 1, 1, 3>
> > Incorrect diagonal: 1.81749e-316            1            1
> > $ g++-4.9 -std=gnu++1y -fstack-protector-all mvu.cc  -o mvu
> > $ ./mvu
> > Correct diagonal: 1 1 1
> > Eigen::Matrix<double, 1, 3, 1, 1, 3>
> > Incorrect diagonal: 1 2 3
>
> I get the same "1 2 3" result using -fstack-protector-all and valgrind.
>
> My guess is that Sigma holds a dangling reference to some temporary
> object that has gone out of scope, and the function call t2s
> overwrites that stack memory, then when you return to main you go
> through the dangling reference.
>
> Are you sure you're using the Eigen types correctly?

He is using "auto" with an expression template library, so I'd say that it 
is unlikely he is using it correctly, indeed...

--
Marc Glisse