Technique 1: Defining a "64Bit" Field

The "64Bit" field is defined in the IOCTL or FSCTL control code. This field contains a bit flag that is always set for 64-bit callers, but is always clear for 32-bit. Which bit in the control code is chosen as the "64Bit" field is driver-specific, but it must be a bit that is never set for 32-bit callers. A good choice for most drivers is the most significant bit (MSB) in the Function field.

For example, the IOCTL (FSCTL) control codes used in 32-bit drivers contain four bitfields:

As long as none of the existing driver-defined control codes set the MSB in the Function field, these control codes can continue to be used by 32-bit user-mode applications.

To accommodate 64-bit callers, the driver defines a Function field that is shorter by one bit. This bit is redefined as a "64Bit" field:

The following code example shows how to define a "64Bit" field in a driver header file:

#define REGISTER_FUNCTION 0     // Define the IOCTL function code

#ifdef  _WIN64
#define CLIENT_64BIT   0x800
#define REGISTER_FUNCTION 0
#define IOCTL_REGISTER   CTL_CODE(FILE_DEVICE_UNKNOWN, \
  CLIENT_64BIT|REGISTER_FUNCTION, METHOD_BUFFERED, FILE_ANY_ACCESS)
#else
#define IOCTL_REGISTER   CTL_CODE(FILE_DEVICE_UNKNOWN, \
  REGISTER_FUNCTION, METHOD_BUFFERED, FILE_ANY_ACCESS)
#endif

typedef struct _IOCTL_PARAMETERS {
    PVOID   Addr;
    SIZE_T  Length;
    HANDLE  Handle;
} IOCTL_PARAMETERS, *PIOCTL_PARAMETERS;