traceback a pointer in c++ code gdb

Question

I am getting seg fault while running a c++ application. In gdb it shows somehow my one pointer location getting corrupted. But I have 100 thousands of such object pointers c

Answer 1

I had some mem-leaks problems in the past (caused by compiler bug) in apps with about 256K pointers so i had to check it somehow. After some struggle I created a table of all allocated pointers and their sizes and some functions to keep it updated. the result is this:

File: mmap.h

//---------------------------------------------------------------------------
//--- Memory map system ver: 2.03 -------------------------------------------
//---------------------------------------------------------------------------
#ifndef _mmap_h
#define _mmap_h
//---------------------------------------------------------------------------
#define _mmap_aprox
//---------------------------------------------------------------------------
/*
    new
    #ifdef _mmap_h
    if () mmap_new('Main',,sizeof());
    #endif

    #ifdef _mmap_h
    if () mmap_del('Main',);
    #endif
    delete
*/
//---------------------------------------------------------------------------
struct _mmap_entry
    {
    char ids[4];                            // id string
    DWORD beg,end;                          // mem adr <beg,end)
    _mmap_entry(){ beg=0; end=0; ((DWORD*)(ids))[0]='LLUN'; };
    _mmap_entry(_mmap_entry& a) { *this=a; }
    ~_mmap_entry()  {}
    _mmap_entry* operator = (const _mmap_entry *a) { *this=*a; return this; }
    //_mmap_entry* operator = (const _mmap_entry &a) { ...copy... return this; }
    };
//---------------------------------------------------------------------------
const int _mmap_entries=4*1024;             // max num of allocated memory chunks (pointers)
const int _mmapn_entries=32;                // num of last news to remember
const int _mmapd_entries=32;                // num of last dels to remember
static _mmap_entry mmap [_mmap_entries];    // memory map table active ptrs
static _mmap_entry mmapn[_mmapn_entries];   // memory map table last news
static _mmap_entry mmapd[_mmapd_entries];   // memory map table last dels
static int mmaps=0;                         // num of used entries in memory map table
static int mmapn_ix=0;                      // num of last deletes to remember
static int mmapd_ix=0;                      // num of last deletes to remember
static int mmap_errs=0;                     // error count
static int mmap_news=0;                     // allocations count
static int mmap_dels=0;                     // deallocations count
//---------------------------------------------------------------------------
void mmap_err(const char* msg,DWORD ptr)    // breakpointeable error
    {
    mmap_errs++;
    }
//---------------------------------------------------------------------------
int mmap_new(DWORD ids,void* ptr,DWORD siz) // tracks all allocations return false if error
    {
    mmap_news++;
    int i,j; _mmap_entry e,*p;
    e.beg=DWORD(ptr);
    e.end=e.beg+siz;
    e.ids[0]=((char*)&ids)[3];
    e.ids[1]=((char*)&ids)[2];
    e.ids[2]=((char*)&ids)[1];
    e.ids[3]=((char*)&ids)[0];
    if (e.beg==0)
        {
        mmap_err("Not enough memory.",e.beg);
        return 0;
        }
    // find first i where mmap[i].end >= e.beg
    #ifdef _mmap_aprox
    if (!mmaps) i=0;
    else{
        for (j=1;j<mmaps;j<<=1); j>>=1; if (!j) j=1;
        for (i=0;j;j>>=1)
            {
            i|=j;
            p=mmap+i;
            if ((i>=mmaps)||(e.beg<p->end)) i^=j;
            if ((e.beg<p->end)&&(e.end>p->beg))
                {
                mmap_err("Bad allocation.",e.beg);      // memory already allocated
                return 0;
                }
            }
        if (e.beg>=mmap[i].end) i++;
        }
    #endif
    #ifndef _mmap_aprox
    for (i=mmaps-1,p=mmap+i;i>=0;i--,p--)
     if (e.beg<p->end)
        {
        if (e.end>p->beg)
            {
            mmap_err("Bad allocation.",e.beg);      // memory already allocated
            return 0;
            }
        } else break; i++;
    #endif
    // insert new pointer at i
    if (mmaps>=_mmap_entries)
        {
        mmap_err("Too many pointers.",e.beg);   // _mmap_entries is too low
        return 0;
        }
    for (j=mmaps;j>i;j--) mmap[j]=mmap[j-1];
    mmap[i]=e; mmaps++;
    // remember last new in mmapn table
    mmapn[mmapn_ix]=e; mmapn_ix++;
    if (mmapn_ix>=_mmapn_entries) mmapn_ix=0;
    return 1;
    };
//---------------------------------------------------------------------------
int  mmap_del(DWORD ids,void* ptr)              // tracks all deallocations return false if error
    {
    mmap_dels++;
    int i,j; _mmap_entry *p;
    DWORD adr=DWORD(ptr);
    if (adr==0)
        {
        mmap_err("Can not delete NULL.",adr);
        return 0;
        }
    if (mmap<=0)
        {
        mmap_err("Nothing to delete.",adr);
        return 0;
        }
    // find mmap[i] where beg==ptr and delete it if found
    #ifdef _mmap_aprox
    if (!mmaps) i=0;
    else{
        for (j=1;j<mmaps;j<<=1); j>>=1; if (!j) j=1;
        for (i=0;j;j>>=1)
            {
            i|=j;
            p=mmap+i;
            if ((i>=mmaps)||(adr<p->beg)) i^=j;
            }
        if (adr==mmap[i].beg)
            {
            if (mmaps>1) for (j=i;j<mmaps-1;j++) mmap[j]=mmap[j+1];
            mmaps--;
            // remember last delete in mmapd table
            mmapd[mmapd_ix]=mmap[mmaps]; mmapd_ix++;
            if (mmapd_ix>=_mmapd_entries) mmapd_ix=0;
            // delete ptr from mmap table
            mmap[mmaps].beg=0;
            mmap[mmaps].end=0;
            return 1;
            }
        for (p=mmap,j=0;j<=i;j++,p++)               // test all mmap[j].beg < adr
         if (adr<p->end)                            // if overlap then ...
            {
            mmap_err("Wrong delete pointer.",adr);  // pointer inside already allocated space
            return 0;
            }
        }
    #endif
    #ifndef _mmap_aprox
    for (p=mmap,i=0;i<mmaps;i++,p++)
     if (p->beg==adr)
        {
        if (mmaps>1) for (j=i;j<mmaps-1;j++) mmap[j]=mmap[j+1];
        mmaps--;
        mmap[mmaps].beg=0;
        mmap[mmaps].end=0;
        return 1;
        } else if (p->beg>adr) break;
    for (p=mmap,i=0;i<mmaps;i++,p++)
     if ((adr>p->beg)&&(adr<p->end))
        {
        mmap_err("Wrong delete pointer.",adr);  // pointer inside already allocated space
        return 0;
        }
    #endif
    mmap_err("Delete pointer not found.",adr);
    return 0;
    };
//---------------------------------------------------------------------------
#endif
//---------------------------------------------------------------------------
//---------------------------------------------------------------------------

Now in your code just do this:

// edit the safe big enough number of pointers to use for your application in begin of the mmap.h
_mmap_entries=512*1024;

// before any delete/delete[] of pointer ptr add this:
#ifdef _mmap_h
if (ptr!=NULL) mmap_del('info',ptr);
#endif
if (ptr!=NULL) delete[] ptr;

// after any new of pointer ptr of size siz [byte] add this:
ptr=new BYTE[siz];
#ifdef _mmap_h
if (ptr!=NULL) mmap_new('info',ptr,siz);
#endif

So if you include mmap.h as a first include !!!

• place breakpoint inside function void mmap_err(const char* msg,DWORD ptr)
• run application
• if any allocation error occurs then it will break before exception so you can actually see the info and kind off error and also can step to code where error occurs

I am BDS2006 Turbo C++ user so if I forgot some VCL stuff just convert it to MSVC++ or comment me and i will do it, but I don't see anything what could cause troubles.

PS. I found out that for my compiler is a fatal error to:

1. delete pointer more than once
2. have structs without proper constructor/destructors

in both cases no exception is thrown but memory manager is corrupted afterwards so it allocates wrongly hence exceptions

Proper constructors/destructors for my compiler
for all structs and classes which will be dynamically allocated or any of their component
is like this:

class/struct T
    {
public:
    T()     {}
    T(T& a) { *this=a; }
    ~T()    {}
    T* operator = (const T *a) { *this=*a; return this; }

    // if any dynamic allocation occur then this must be done also else not
    //T* operator = (const T &a) { ... copy a to this ... return this; }
    };

Answer 2

As far as I know you can't get such information with help of gdb, but for these purpose you can try valgrind memcheck with option --track-origins