Re: [Valgrind-users] understanding uninitialised value error

[Nicholas N. <n.n...@gm...>]

On Fri, Mar 6, 2009 at 5:45 AM, Ross Boylan <ro...@bi...> wrote:
> When I run a test program with leak-check, the first error is
> ==14820== Use of uninitialised value of size 4
> ==14820==    at 0x40F6841: (within /usr/lib/libstdc++.so.6.0.10)
>
> Data_test.cc excerpt
>  BOOST_CHECK_EQUAL(pdi4->next(), true);
>  Data::TIObs p4b = pdi4->begin();
>  //p4b = 2u;
>  Data::TIObs p4bt = 0u;
>  BOOST_CHECK_EQUAL(p4b, p4bt); // line 133 triggering error
>
> First question: why isn't it telling me where the uninitialised value is?  Or is it?

Because when this happens the value is in a register.  It's hard for
Memcheck to know which memory location it came from;  indeed, the
value may be the result of operations combining multiple
originally-in-memory values.

> My understanding is that
> the first at (0x40F6841) is the instruction location, not the memory being referenced.

That's correct.

> It may be relevant that I invoked with
> valgrind --leak-check=yes --num-callers=20 ./test1 --report_level=detailed /home/ross/peter/R/mspath/src/test/inputs
> Does --num-callers disable other information?  I couldn't find a switch that would enable reporting of the offending memory location.

--num-callers doesn't disable anything.

Upgrade to 3.4.1 and use --track-origins=yes if you want more
information, as Tom suggested.

One thing the message doesn't say which it could is that the likely
use of the undefined value is as a pointer in a load or store.  (It
could also be as the shift amount in a shift operation, or one or two
other obscure cases, but they're unlikely.)


> Second question: why is the error reported in the equality test, and not at
>  Data::TIObs p4b = pdi4->begin();
> ?  By uncommenting p4b = 2u I was able to suppress the error, which seems to indicate that p4b is the culprit.
> Data:TIObs is type size_t.

Memcheck doesn't complain about all uses of uninitialised values, only
ones that it thinks are dangerous.  See the manual for more:

  http://www.valgrind.org/docs/manual/mc-manual.html#mc-manual.uninitvals

And this FAQ as a follow-up:

  http://www.valgrind.org/docs/manual/faq.html#faq.undeferrors

And this blog post if you want a more detailed discussion:

  http://blog.mozilla.com/nnethercote/2009/02/27/eliminating-undefined-values-with-valgrind-the-easy-way/

And this paper if you really want all the details:

  http://www.valgrind.org/docs/memcheck2005.pdf

Hope this helps.

Nick