Huge memory leak

I'm getting a large memory leak in my code, its even causing my program to shutdown after a little bit and I'm having trouble finding what is wrong. I ran rational purifyplus on it and it gave me the distinct location but I don't see the problem. Here's some code:

Calling Code:

Code:

std::string str = "";
str += ZPacketData->GetData()->back()->GetMACAddress(TRUE);

GetMACAddress: (where the memory leak supposedly happens)

Code:

std::string ZeroPacket::GetMACAddress(BOOL IsSource)
{
	std::string strReturn = "";
	char cBuff[20];
	char* pChar;

	if( IsSource )
		pChar = (char*)(m_Data);
	else
		pChar = (char*)(m_Data+6);

	for( int i=0; i<6; i++ )
	{
		sprintf(cBuff, ("02x:"), *(BYTE*)pChar++);
		strReturn += cBuff;
	}

	return strReturn;
}

The code is pretty straightforward and I'm assuming it has something to do with std::string. Also, PurifyPlus is stating that the leak is on the heap, but I didn't allocated anything to the heap except for m_Data. Any Ideas?

Code:

sprintf(cBuff, ("02x:"), *(BYTE*)pChar++);

The above line is wrong -- where is the '%'?

Code:

sprintf(cBuff, ("%02x:"), (unsigned)*(BYTE*)pChar++);

Not sure if it will help or not, but you might try typcasting to unsigned int.

yea, I fixed that heh. Still causes a leak tho so I'm switching over to MFC and hopefully CString will return properly

It is highly unlikely that the problem is in the string implementation, since so many people use it and you haven't found anybody else with this problem. It is either a problem with something else, or a problem with how you are using the string.