#ifndef _PHNT_NTDEF_H
#ifndef _NTDEF_
#if defined(__INTELLISENSE__) || defined(DOXYGEN)
#ifndef BYTE_OFFSET

/**
 * The BYTE_OFFSET macro returns the byte offset of an address within its page.
 *
 * \param Address The address/pointer value.
 * \return The offset in bytes within the containing page.
 * \remarks This masks the address with PAGE_MASK.
 */
#define BYTE_OFFSET(Address) ((SIZE_T)((ULONG_PTR)(Address) & PAGE_MASK))

#endif
#endif
#endif
#endif
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#ifndef _PHNT_NTDEF_H
#ifndef _NTDEF_
#if defined(__INTELLISENSE__) || defined(DOXYGEN)
// ...
#else
#ifndef BYTE_OFFSET

#define BYTE_OFFSET(Address) ((SIZE_T)((ULONG_PTR)(Address) & PAGE_MASK))

#endif
#endif
#endif
#endif
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// wdm.h

void BYTE_OFFSET(
  [in] Va
);
View the official Windows Driver Kit DDI reference