On Thu, 2013-03-14 at 14:18 -0700, Patrick J. LoPresti wrote: > On Thu, Mar 14, 2013 at 1:58 PM, Philippe Waroquiers > <philippe.waroqui...@skynet.be> wrote: > > On Thu, 2013-03-14 at 19:21 +0000, Phil Longstaff wrote: > >> How hard would it be for memcheck to not report a block as being > >> potentially lost if the internal pointer could be a pointer to a base > >> class? Is there sufficient info in the debug information? > > No, I do not think so. > > Probably correct in general. But... > > For polymorphic C++ classes -- presumably a common case when you have > a pointer to an internal base class -- dynamic_cast<Derived>() has to > work somehow. So I would imagine Valgrind could use the same RTTI > mechanism. In theory.
The problem is that Valgrind only has a piece of memory. It does not know if this piece of memory is a dynamically allocated C++ object or a dynamically allocated string or a dynamically allocated array of integers Assuming this piece of memory is a C++ object, and starting RTTI on that implies to heuristically guess if the memory piece looks like a C++ object. Valgrind cannot be sure of that. In other words, Valgrind will do RTTI by doing an "unchecked cast" of any piece of memory to which it finds an interior pointer. IIUC, DrMemory leak checker uses an heuristic by assuming the V-table pointer is located at the beginning of the piece of memory, and "confirming" this is a V-table pointer by looking if this V-table pointer points to an array of words which are themselves pointing into the text segment of of the application. I do not have a good knowledge of C++ and multiple inheritance v-tables and similar, so I am not sure I properly understand all the above. Such an heuristic might create false negative. There are however already false negative (as e.g. any integer might look like a "start" pointer). E.g. on a 32 bit application which allocates a lot of memory, filled in with a lot of different integers, there is a significant probability to have false negative. Philippe ------------------------------------------------------------------------------ Everyone hates slow websites. So do we. Make your web apps faster with AppDynamics Download AppDynamics Lite for free today: http://p.sf.net/sfu/appdyn_d2d_mar _______________________________________________ Valgrind-users mailing list Valgrind-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/valgrind-users
