traceback a pointer in c++ code gdb
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 created during my application. How can I watch one which is causing the crash Can I do any operation in bt command to see the lifetime of that pointer?
Thanks Ruchi
Solution 1:
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
Solution 2:
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:
- delete pointer more than once
- 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; }
};