Re: [Valgrind-users] Show where a reference was lost in addition to where it was allocated

[John R. <jr...@bi...>]

> Hi all, I'm wondering if it's possible for memcheck to show the last place that some memory was accessible before being leaked.

The current implementation of memcheck cannot do this.
Reporting the location of the leak for each leak
would take a major re-think of memcheck.  It would be
easier to report the location of only the *last* leak.
(If there were 500 leaks then you could find them all
by running the tool 500 times, fixing them one-by-one
as you go.  And if you couldn't fix a particular last
leak at some point, then you would be stuck; the tool
could not pinpoint any more leaks for you.)

If allocation and freeing were "stable" with respect
to re-running the program, then a two-pass procedure
would be almost easy.  (It would take *lots* of disk
space, but it would be easy.)  Unfortunately "stable"
is not as common as one might hope.

In your example below the difficulty lies in the code that has been elided
by the ellipses "...".  If there are any assignments of x to other pointers,
or if x is passed as an actual argument of a function call, then it is hard
to determine whether a leak occurs.

> 
> int* g() {
>   int* x = new int[256]; <-- allocated here
>   ...
>   return x;
> }
> 
> int f() {
>   int* x = g();
>   ...
>   return 0; <-- leaked here
> }
> 

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