RtlCompareDeviceMemory function (wdm.h)

The RtlCompareDeviceMemory function compares two blocks of device memory and returns the number of bytes that match until the first difference. This function is safe for use on device memory because it uses byte-level accesses that are guaranteed to be performed as specified.

Syntax

SIZE_T RtlCompareDeviceMemory(
  [in] const VOID *Source1,
  [in] const VOID *Source2,
  [in] SIZE_T     Length
);

Parameters

[in] Source1

A pointer to the first block of memory to compare.

[in] Source2

A pointer to the second block of memory to compare.

[in] Length

The number of bytes to compare.

Return value

RtlCompareDeviceMemory returns the number of bytes in the two blocks that match. If all bytes match up to the specified Length value, the Length value is returned.

Remarks

• The function isn't recognized as a compiler intrinsic so the compiler will never optimize away the call (either entirely or replace the call with an equivalent sequence of instructions). This differs from RtlCompareMemory which is subject to various compiler optimizations.

• The function uses byte-level accesses to ensure proper handling of device memory that may have side effects or special access requirements.

• The routine starts by comparing the first byte in the first block to the first byte in the second block, and continues to compare successive bytes in the two blocks while the bytes match. The routine stops comparing bytes when it encounters the first pair of bytes that are not equal, or when the number of matching bytes equals the Length parameter value, whichever occurs first.

• The function might perform unaligned memory accesses if the platform allows for it.

This function works on all versions of Windows, not just the latest. You need to consume the latest WDK to get the function declaration from the wdm.h header. You also need the library (volatileaccessk.lib) from the latest WDK. However, the resulting driver will run fine on older versions of Windows.

Example

UCHAR DeviceBuffer1[256];
UCHAR DeviceBuffer2[256];

// Read data from device memory into buffers
ReadFromDevice(DeviceBuffer1, sizeof(DeviceBuffer1));
ReadFromDevice(DeviceBuffer2, sizeof(DeviceBuffer2));

// Compare the device memory buffers
size_t matchingBytes = RtlCompareDeviceMemory(DeviceBuffer1, DeviceBuffer2, sizeof(DeviceBuffer1));

if (matchingBytes == sizeof(DeviceBuffer1)) {
    // All bytes matched
    DbgPrint("Device buffers are identical\n");
} else {
    // Difference found at byte offset 'matchingBytes'
    DbgPrint("Device buffers differ starting at byte %zu\n", matchingBytes);
}

Requirements

See also

RtlCompareMemory

RtlFillVolatileMemory