Object pointer randomly points to 0x00000 - c++

Question

I'm running a Visual C++ MFC application in release mode. I'm compiling everything using Visual Studio 2010.
My app runs a mini CNC mill through USB VCP communication.
I have a XML file that stores the app's settings.
My problem is this: ocassionaly (and this is repeatable) the pointer to the tinyxml2::XMLDocument I'm using gets set to 0x000.
Info:
Occasionally, the XML file get written to while the mill is running.
Before the error happens, the mill I'm running siezes for almost 30 seconds.
I'm using mutex locks to make sure the xmldoc doesn't get written to file twice at once.
The mutex locks are working, and the mutex error never occurs. I know the mutex code isn't perfect, but that isn't the issue. Honest.
I never write to the xmldoc pointer except when the parent class is booting up.
And then, all of a sudden, the xmlDoc pointer gets set to zero.
Any thoughts anyone?
Here is my saving code, although the problem may lie elsewhere:
void XMLSettings::SaveToXML()
{
HANDLE g_Mutex = CreateMutex( NULL, TRUE, "XMLSavingMutex");
DWORD wait_success = WaitForSingleObject( g_Mutex, 30000L);
if(wait_success == WAIT_OBJECT_0){
CIsoProApp* pApp = (CIsoProApp*)AfxGetApp();
if(PathFileExists(pApp->DrivePath + "IsoPro\\temp.xml"))
{
DeleteFile(pApp->DrivePath + "IsoPro\\temp.xml");
}
if(0==&xmlDoc)
{
OutputDebugString("xmlDoc == NULL");
}
int errorcode = xmlDoc->SaveFile(pApp->DrivePath + "IsoPro\\temp.xml");
if(errorcode != 0)
{
OutputDebugString("xmlDoc == errorcode");
}
if(0==&xmlDoc)
{
OutputDebugString("xmlDoc == NULL2");
}
if(0==xmlDoc)
{
OutputDebugString("xmlDoc == NULL");
}
if(PathFileExists(pApp->DrivePath + "IsoPro\\Settings.xml"))
{
DeleteFile(pApp->DrivePath + "IsoPro\\Settings.xml");
}
MoveFile(pApp->DrivePath + "IsoPro\\temp.xml",pApp->DrivePath + "IsoPro\\Settings.xml");
ReleaseMutex(g_Mutex);
}
else
{
int errorInt = GetLastError();
CString error;
error.Format("%d",errorInt);
if(errorInt != ERROR_ALREADY_EXISTS)
{
AfxMessageBox("XMLSavingMutex Error. WaitSuccess = " + wait_success);
AfxMessageBox("XMLSavingMutex Error. GetLastError = " + error);
}
}
CloseHandle(g_Mutex);
}

Answer 1

Since it seems that you are creating a Mutex each time SaveToXML is called, you should change your call to
HANDLE g_Mutex = CreateMutex( NULL, FALSE, "XMLSavingMutex");
Doing this will create a named mutex that allows the implementation to dictate who the owner is; other threads will receive the same mutex.
From the doc:
Two or more processes can call CreateMutex to create the same named mutex. The first process actually creates the mutex, and subsequent processes with sufficient access rights simply open a handle to the existing mutex. This enables multiple processes to get handles of the same mutex, while relieving the user of the responsibility of ensuring that the creating process is started first. When using this technique, you should set the bInitialOwner flag to FALSE; otherwise, it can be difficult to be certain which process has initial ownership.
(Credit to WhozCraig for pointing out named mutexes)

Answer 2

It appears that I was accessing the xml getter while writing the xml to a file. I put a single mutex lock in place for all xml actions and things seem to be functioning properly. Thanks to everyone for their help. I'll be in touch with more info if it becomes available.