[Bug libc/12831] New: mtrace location not usefull for C++

["yair.lenga at citi dot com" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=12831

           Summary: mtrace location not usefull for C++
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
        AssignedTo: drepper.fsp@gmail.com
        ReportedBy: yair.lenga@citi.com


Summary:

The address reported by mtrace for C++ program is the location of the new
operators, and not the name of the calling program.

Long Description:

I've been working with mtrace output for the last few days to resolve a complex
memory management bug in C++ program. While the mtrace output provides
essential information, the address field is provided with each line is useless,
as it points to the new operators inside the c++ runtime library.

The correct/better behavior is to retrieve the address of the calling program
for the c++ new operators (using backtrace-like function), and using it in the
trace output file. This could be done whenever the address resolve to
"/usr/lib/libstdc++.so.6:(_Znwj+0x27)[0x258bb7]" address.

More general solution will allow specifying allowing user-defined env var to
control list of ".so", which should be ignored.

Even better solution will include addresses from multiple frames of the
backtrace, which will make it easier to capture the calling stack of the code
during the memory allocation.

In theory, all of those can be done using the malloc hooks. However,
implementing the hooks is very hard for most users. Implementing the proposed
solution in glibc will make the mtrace function significantly more valuable for
c++ developers.

Example:

consider the following C++ memory leaking program (complete source below). The
output is:

Memory not freed:
-----------------
   Address     Size     Caller
0x084d9378  0x186a0  at prog.cc:11
0x084f1a20   0x2710  at prog.cc:12
0xb755a008  0xf4240  at 0x258bb7
0xb764f008  0xf4240  at 0x258bb7


The location of the malloc/calloc calls is resolved to line number. However,
all the "new" calls do not show the correct line number.


Complete example:
g++ -g prog.cc
export MALLOC_TRACE=trace.txt
./a.out
cat trace.txt
mtrace a.out trace.txt

Program: prog.cc

#include <mcheck.h>
#include <stdlib.h>

class T1 { char c[10000] ; } ;

int f(void)
{
    mtrace() ;

    static T1 *x = new T1[100] ;
    static T1 *u = new T1[100] ;
    static T1 *y = (T1 *)calloc(10, sizeof(T1)) ;
    static T1 *z = (T1 *)malloc(sizeof(T1)) ;
}

int main(void)
{
    f();
}

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