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supporting-32-bit-i-o-in-your-64-bit-driver.md

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Supporting 32-Bit I/O in your 64-Bit Driver
Supporting 32-Bit I/O in Your 64-Bit Driver
32-bit I/O support WDK 64-bit
64-bit WDK kernel , 32-bit I/O support
thunking WDK
WOW64 thunking layer WDK
converting parameters to fixed-precision types
32-bit I/O support WDK 64-bit , about 32-bit I/O support in 64-bit
control codes WDK 64-bit
I/O control codes WDK kernel , 32-bit I/O in 64-bit drivers
IOCTLs WDK kernel , 32-bit I/O in 64-bit drivers
file system control codes WDK 64-bit
FSCTL WDK 64-bit
buffer pointers WDK 64-bit
06/16/2017

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 32-bit pointer types to 64-bit pointer 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.