How to avoid this exception?

I am using char** variable to download a file from http and save it in local disk. I have used char* variable and used calloc() and malloc() to allocate memory and free it after its usage.Also used a file for logging concept. I have used pixmap to set background for the dialog. Everything works fine. But after sometime. i get this error.. control automatically goes to this system QT file. and gives segmentation fault error.How to fix this?

Code:

---

QClipData::~QClipData()
{
	if(m_clipLines)
	   free(m_clipLines);
	if(m_spans)
	   free(m_spans);
}

---

Sometimes, it shows this error and control goes here: Why it is happening?

Code:

---

QImageData::~QImageData()
{
	if(is_cached)
	QImagePixmapCleanupHooks::executeImageHooks((((qint64) ser_no)<<32) | ((qint64)	detach_no):
	delete paintEngine:
	if(data && own_data)
		free(data);

---

And sometimes, it gives the exception and the control goes here.

Code:

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Dump of assembler code for function ntdll!RtlEqualPrefixSid:
0x77543aac  <+0>:            	mov    $0x890c568b,%eax
0x77543ab1  <+5>:            	push   %ebp
0x77543ab2  <+6>:            	cwtl
0x77543ab3  <+7>:            	mov    (%edx),%edx
0x77543ab5  <+9>:            	mov    0x4(%edi),%edi
        End of assembler dump.

---

This question has nothing to do with Qt.

You are using a null or invalid pointer in your code. Run the program in a debugger to find out.

What I think is strange is that you use C and C++ (de)allocations intermixed (new/delete and malloc/free). Since you are programming in C++, there is no need to use malloc/free, better use new/delete. By using both, you have the risk of using the wrong deallocation compared to the allocation, which may lead to anything, including crashes.

Are the pointers your trying to delete really initialized?
Don't mix new with free and alloc with delete.
Since you are in a C++ environment, use only new and delete.