From: johan.nilsson@esrange.ssc.se
Sent: Wednesday, January 03, 2001 4:48 AM
To: NT Developers Interest List
Subject: [ntdev] Modify drivers dispatch table

Hi,

would it be legal (and possible) to modify the dispatch table of a running
driver? To be more specific, what I'd like to do is to have one function
called the very first time IRP_MJ_CREATE is dispatched, and another for all
subsequent ones. This is to have specific initialization the first time any
app opens the device, and avoid having a synchronization mechanism combined
with a test for initialization for each following create irp.

The driver will have one device object only, which is created during driver
load, so I'm considering using an executive spinlock inside the device
extension for synchronization, something along the following lines:

*****************

struct MY_DEVICE_EXTENSION
{
	...
	KSPIN_LOCK CreateDispatchSpinLock;	// Initialized during
creation of device
	BOOLEAN CreateInitiallyCalled;		// Initialized during
creation of device to FALSE
	...
};

NTSTATUS
DriverEntry(...)
{
	...
	pDriverObject->MajorFunction[IRP_MJ_CREATE] = InitialCreate;
	...
};

NTSTATUS
InitialCreate(...)
{
	... get device extension ...
	... get spinlock from extension ...
	... acquire spinlock ...
	
	if (FALSE == pDeviceExtension->CreateInitiallyCalled) 
	{
		... do one time initialization ...
		pDeviceExtension->CreateInitiallyCalled = TRUE;
		... get driver object ...
		pDriverObject->MajorFunction[IRP_MJ_CREATE] = NormalCreate;
	}

	... release spinlock
	... complete IRP

	return NT_SUCCESS;
}

NTSTATUS
NormalCreate(...)
{
	... complete IRP

	return NT_SUCCESS;
}


**************************************


// Johan Nilsson

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