[Valgrind-users] understanding uninitialised value error

[Ross B. <ro...@bi...>]

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)
==14820==    by 0x40FE394: std::ostreambuf_iterator<char, std::char_traits<char> > std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::_M_insert_int<unsigned long>(std::ostreambuf_iterator<char, std::char_tra\
its<char> >, std::ios_base&, char, unsigned long) const (in /usr/lib/libstdc++.so.6.0.10)
==14820==    by 0x40FE60C: std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::do_put(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, unsigned long) const (in /usr/lib/libstdc++\
.so.6.0.10)
==14820==    by 0x410FEFF: std::ostream& std::ostream::_M_insert<unsigned long>(unsigned long) (in /usr/lib/libstdc++.so.6.0.10)
==14820==    by 0x4110073: std::ostream::operator<<(unsigned) (in /usr/lib/libstdc++.so.6.0.10)
==14820==    by 0x806903B: boost::test_tools::tt_detail::print_log_value<unsigned>::operator()(std::ostream&, unsigned const&) (test_tools.hpp:299)
==14820==    by 0x806905E: std::ostream& boost::test_tools::tt_detail::operator<< <unsigned>(std::ostream&, boost::test_tools::tt_detail::print_helper_t<unsigned> const&) (test_tools.hpp:405)
==14820==    by 0x8069085: boost::basic_wrap_stringstream<char>& boost::operator<< <char, boost::test_tools::tt_detail::print_helper_t<unsigned> >(boost::basic_wrap_stringstream<char>&, boost::test_tools::tt_detail::print_helper_t<uns\
igned> const&) (wrap_stringstream.hpp:66)
==14820==    by 0x8069EE8: void boost::test_tools::tt_detail::check_frwd<boost::test_tools::tt_detail::equal_impl_frwd, unsigned, unsigned>(boost::test_tools::tt_detail::equal_impl_frwd, boost::basic_wrap_stringstream<char>&, boost::u\
nit_test::basic_cstring<char const>, unsigned, boost::test_tools::tt_detail::tool_level, boost::test_tools::tt_detail::check_type, unsigned const&, char const*, unsigned const&, char const) (test_tools.hpp:461)
==14820==    by 0x810E4DA: data::test_method() (Data_test.cc:133)
==14820==    by 0x810EE7B: _ZL12data_invokerv (Data_test.cc:40)
==14820==    by 0x807BC1A: boost::unit_test::ut_detail::unused boost::unit_test::ut_detail::invoker<boost::unit_test::ut_detail::unused>::invoke<void (*)()>(void (*)()&) (callback.hpp:56)
==14820==    by 0x807BC44: boost::unit_test::ut_detail::callback0_impl_t<boost::unit_test::ut_detail::unused, void (*)()>::invoke() (callback.hpp:89)
==14820==    by 0x406765C: (within /usr/lib/libboost_unit_test_framework-gcc42-1_34_1.so.1.34.1)
==14820==    by 0x40561B4: boost::execution_monitor::catch_signals(boost::unit_test::callback0<int> const&, bool, int) (in /usr/lib/libboost_unit_test_framework-gcc42-1_34_1.so.1.34.1)
==14820==    by 0x4056545: boost::execution_monitor::execute(boost::unit_test::callback0<int> const&, bool, int) (in /usr/lib/libboost_unit_test_framework-gcc42-1_34_1.so.1.34.1)
==14820==    by 0x40677B8: boost::unit_test::unit_test_monitor_t::execute_and_translate(boost::unit_test::test_case const&) (in /usr/lib/libboost_unit_test_framework-gcc42-1_34_1.so.1.34.1)
==14820==    by 0x405AC3E: boost::unit_test::framework_impl::visit(boost::unit_test::test_case const&) (in /usr/lib/libboost_unit_test_framework-gcc42-1_34_1.so.1.34.1)
==14820==    by 0x406DE66: boost::unit_test::traverse_test_tree(boost::unit_test::test_case const&, boost::unit_test::test_tree_visitor&) (in /usr/lib/libboost_unit_test_framework-gcc42-1_34_1.so.1.34.1)
==14820==    by 0x406E63F: boost::unit_test::traverse_test_tree(unsigned long, boost::unit_test::test_tree_visitor&) (in /usr/lib/libboost_unit_test_framework-gcc42-1_34_1.so.1.34.1)
==14820==
==14820== Conditional jump or move depends on uninitialised value(s)
==14820==    at 0x40F684A: (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?  My understanding is that
the first at (0x40F6841) is the instruction location, not the memory being referenced.

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.

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.

The program is compiled with g++ 4.3.2 with -O0 -g, valgrind 3.3.1 on Debian GNU/Linux Lenny with 32 bit AMD Athlon chip.