The DRIVER_EXTENSION structure is an adjunct to the well-known (though partially opaque) DRIVER_OBJECT. It was added when introduction of the PnP Manager for Windows 2000 needed that drivers implement another calling point through which to learn of device addition. The address to call might be set into the DRIVER_OBJECT except for compatibility constraints. Pre-existing drivers depended on at least some members, which therefore could not shift. Members could not be appended without risking a future problem for the MajorFunction array were new types of I/O request ever to be defined. The solution was to replace the Count member, which no drivers should have been accessing, by a pointer to an extension. Thereafter, the Count and anything new that is ever wanted for the DRIVER_OBJECT go to the extension.

Documentation Status

Because writers of device drivers that actually do drive a physical device, here meaning one that’s known to the PnP Manager, need to know of the DRIVER_EXTENSION so that they can set the AddDevice member, the DRIVER_EXTENSION has always been documented—but with programmers being told that AddDevice is the only accessible member.

No definition in the WDM.H from any Device Driver Kit (DDK) or Windows Driver Kit (WDK) has yet extended beyond the ServiceKeyName member. This also is the extent of the definition in the NTOSP.H from the WDK for Windows 10.


The DRIVER_EXTENSION has varied from version to version only by appending new members. The following table summarises the growing size:

Version Size (x86) Size (x64)
5.0 0x18  
5.1 to 6.1 0x1C 0x38
6.2 0x24 0x48
6.3 to 10.0 0x28 0x50

These sizes, and the offsets, types and names in the tables that follow, are from Microsoft’s symbol files for the kernel starting with Windows 2000 SP3.

Offset (x86) Offset (x64) Definition Versions
0x00 0x00
DRIVER_OBJECT *DriverObject;
5.0 and higher
0x04 0x08
5.0 and higher
0x08 0x10
ULONG Count;
5.0 and higher
0x0C 0x18
5.0 and higher
0x14 0x28
IO_CLIENT_EXTENSION *ClientDriverExtension;
5.0 and higher
0x18 0x30
5.1 and higher
0x1C 0x38
PVOID KseCallbacks;
6.2 and higher
0x20 0x40
PVOID DvCallbacks;
6.2 and higher
0x24 0x48
PVOID VerifierContext;
6.3 and higher