Supporting 32-Bit I/O in Your 64-Bit Driver

Windows on Windows (WOW64) enables Microsoft Win32 user-mode applications to run on 64-bit Windows. It does this by intercepting Win32 function calls and converting parameters from pointer-precision types to fixed-precision types as appropriate before making the transition to the 64-bit kernel. This conversion, which is called thunking, is done automatically for all Win32 functions, with one important exception: the data buffers passed to DeviceIoControl. The contents of these buffers, which are pointed to by the InputBuffer and OutputBuffer parameters, are not thunked, because their structure is driver-specific.

Note Although the buffer contents are not thunked, the buffer pointers are converted into 64-bit pointers.

User-mode applications call DeviceIoControl to send an I/O request directly to a specified kernel-mode driver. This request contains an I/O control code (IOCTL) or file system control code (FSCTL) and pointers to input and output data buffers. The format of these data buffers is specific to the IOCTL or FSCTL, which in turn is defined by the kernel-mode driver. Because the buffer format is arbitrary, and because it is known to the driver and not WOW64, the task of thunking the data is left to the driver.

Your 64-bit driver must support 32-bit I/O if all of the following are true:

  • The driver exposes an IOCTL (or FSCTL) to user-mode applications.

  • At least one of the I/O buffers used by the IOCTL contains pointer-precision data types.

  • Your IOCTL code cannot easily be rewritten to eliminate the use of pointer-precision buffer data types.


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