Thread: Debugging Memory Corruption

Good evening all. I am currently having problems with memory corruption in one of my programs this results in the following error:

Code:

*** glibc detected *** malloc(): memory corruption: 0x0804ec90 ***
./buildandrun.sh: line 3: 15773 Aborted                 ./build

It is proving very difficult to fix and I have yet to find a solution. It only happens with a specific set of apparently harmless data (an object which holds "foobar\n") and only after another string before comprising of "!!!!!!!" is created. This seems to only happen when I attempt to insert these values into my Doubly Linked List. What limited I know about gdb does not seem to be assisting myself much. The output is below:

Code:

Program received signal SIGABRT, Aborted.
0xb7f17410 in ?? ()
(gdb) bt
#0  0xb7f17410 in ?? ()
#1  0xbf877d10 in ?? ()
#2  0x00000006 in ?? ()
#3  0x00003dc5 in ?? ()
#4  0xb7dd1811 in raise () from /lib/tls/i686/cmov/libc.so.6
#5  0xb7dd2fb9 in abort () from /lib/tls/i686/cmov/libc.so.6
#6  0xb7e06d3a in __fsetlocking () from /lib/tls/i686/cmov/libc.so.6
#7  0xb7e0f106 in free () from /lib/tls/i686/cmov/libc.so.6
#8  0xb7e1092f in malloc () from /lib/tls/i686/cmov/libc.so.6
#9  0xb7ee21fe in DLLNCreate (previous=0x0, next=0x0, index=0, data=0x804a190) at DoublyLinkedList.c:7
#10 0xb7ee22e9 in DLLAddObject (list=0x804ccd8, object=0x804a190) at DoublyLinkedList.c:38
#11 0x08048afd in main () at main.m:28

Contrary to the above I can find no fault in DLLNCreate (below) or DDLAddObject.

Code:

struct DLLNode {
	struct DLLNode * previous;
	struct DLLNode * next;
	unsigned index;
	void * data;
};

struct DLList {
	struct DLLNode * first;
	struct DLLNode * last;
	unsigned count;
};

....

DLLNode *
DLLNCreate(DLLNode * previous, DLLNode * next, unsigned index, void * data)
{
	DLLNode * node = (DLLNode *)malloc(sizeof(DLLNode));
	node->previous = previous;
	node->next = next;
	node->index = index;
	node->data = data;
	return node;
}

DLList *
DLLAddObject(DLList * list, void * object)
{
	DLLNode * last = list->last;
	DLLNode * newNode = DLLNCreate(last,NULL,last==NULL ? 0 : (last->index+1),object);
	
	if (last != NULL) {
		last->next = newNode;
	}
	if ((list->first) == NULL) {
		list->first = newNode;
	}
	list->last = newNode;
	list->count++;
	return list;
}

My question is this, does anyone know of any tutorials or other guides which I can follow to troubleshoot this? I would like to learn how to fix it on my own.

I can provide any information and any of the source required to resolve this issue.

Thank you.

Could you show the code where you are using DLLAddObject function?

> node->previous = previous;
> node->next = next;
So where do you do
previous->next = node;
next->previous = node;
to complete the insertion of a node into the list?

These of course need to be guarded for NULL in the cases of inserting a node into an empty list, at the head of the list or at the tail of the list.

Personally, I would move all the next/prev updating back into DLLAddObject(), and make

Code:

DLLNode *
DLLNCreate(unsigned index, void * data)
{
	DLLNode * node = malloc(sizeof(*node));
	node->previous = NULL;
	node->next = NULL;
	node->index = index;
	node->data = data;
	return node;
}

and put all the next/prev smarts into whatever function calls it.


Do you correctly initialise your DLList variable to NULL, NULL, 0 ?