[Boost-users] Valgrind false positive in make_shared

16 Nov 2010

      Hi Folks

I've been tracking down some almost-certainly-false-positives from
Valgrind.  Consider the following code:

#include <iostream>
#include <boost/shared_ptr.hpp>
#include <boost/make_shared.hpp>

using namespace std;
using boost::make_shared;

struct StateSpace {
public:

    StateSpace(int spaceType) {
        if (spaceType == 4)
            cout <<  "Some Output" << endl;
    }
};

class State {
public:

    State() : m_rep(make_shared<StateSpace>(0) ) {}

    boost::shared_ptr<StateSpace> m_rep;
};

int main() {
    State state;
}

Valgrind 3.6 reports this:

==8198== Memcheck, a memory error detector
==8198== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et al.
==8198== Using Valgrind-3.6.0.SVN-Debian and LibVEX; rerun with -h for 
copyright info
==8198== Command: testState
==8198== 
==8198== Conditional jump or move depends on uninitialised value(s)
==8198==    at 0x401659: StateSpace::StateSpace(int) (testState.cpp:14)
==8198==    by 0x401796: boost::shared_ptr<StateSpace> 
boost::make_shared<StateSpace, int, >(int&&, ) (make_shared.hpp:150)
==8198==    by 0x4016BD: State::State() (testState.cpp:22)
==8198==    by 0x401317: main (testState.cpp:29)
==8198== 
==8198== 
==8198== HEAP SUMMARY:
==8198==     in use at exit: 464 bytes in 5 blocks
==8198==   total heap usage: 240 allocs, 236 frees, 45,162 bytes allocated
==8198== 
==8198== LEAK SUMMARY:
==8198==    definitely lost: 0 bytes in 0 blocks
==8198==    indirectly lost: 0 bytes in 0 blocks
==8198==      possibly lost: 0 bytes in 0 blocks
==8198==    still reachable: 464 bytes in 5 blocks
==8198==         suppressed: 0 bytes in 0 blocks
==8198== Reachable blocks (those to which a pointer was found) are not shown.
==8198== To see them, rerun with: --leak-check=full --show-reachable=yes
==8198== 
==8198== For counts of detected and suppressed errors, rerun with: -v
==8198== Use --track-origins=yes to see where uninitialised values come from
==8198== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 5 from 5)

Interestingly, if I change 

 State() : m_rep(make_shared<StateSpace>(0) ) {}

to

 State() : m_rep(new StateSpace(0)) {}

the Valgrind error goes away.

This is, of course, a greatly boiled-down version of our actual code,
which we're very confident works correctly.  Any ideas about why
Valgrind doesn't like the call to make_shared?  Any ideas about how to
silence this warning (other than adding it to Valgrind's suppressions
file)?

Thanks very much.

-- 
Dave Steffen, Ph.D. - Software Engineer
Numerica Corporation <http://www.numerica.us>
4850 Hahns Peak Drive, Suite 200
Loveland, Colorado 80538
main (970) 461-2000 x 227   direct (970) 612-2327
Email: dave.steffen@numerica.us
fax (970) 461-2004