#ifndef _NTIOAPI_H

/**
 * The FILE_ID_INFORMATION structure contains the identification information returned by
 * NtQueryInformationFile with FileIdInformation (59).
 * \remarks This is a query-only information class. The buffer must be at least 24 bytes; the I/O manager
 * probes user-mode buffers with 4-byte alignment. The structure has 8-byte alignment under default packing.
 * VolumeSerialNumber is at offset 0x00 and the 16-byte FileId is at offset 0x08; the total size is 0x18 bytes.
 * Compare both the volume serial number and the complete 128-bit file identifier when comparing file identities.
 * The file system defines the identifier. The FileInternal and low/high-part members are overlapping views
 * of its bytes, not additional returned fields; interpreting FileInternal as the FileInternalInformation result
 * requires knowledge of the target file system.
 * \sa https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/ntifs/ns-ntifs-file_id_information
 */
typedef struct _FILE_ID_INFORMATION
{
    ULONGLONG VolumeSerialNumber; // Serial number of the volume containing the file.
    union
    {
        FILE_ID_128 FileId; // Complete 128-bit identifier generated by the file system.
        struct
        {
            union
            {
                FILE_INTERNAL_INFORMATION FileInternal; // rev: overlays FileId.Identifier[0..7].
                LONGLONG FileIdLowPart; // rev: overlays FileId.Identifier[0..7].
            } DUMMYUNIONNAME;
            LONGLONG FileIdHighPart; // rev: overlays FileId.Identifier[8..15].
        } DUMMYSTRUCTNAME;
    } DUMMYUNIONNAME;
} FILE_ID_INFORMATION, *PFILE_ID_INFORMATION;

#endif
View code on GitHub

// ntifs.h

typedef struct _FILE_ID_INFORMATION {
  ULONGLONG   VolumeSerialNumber;
  FILE_ID_128 FileId;
} FILE_ID_INFORMATION, *PFILE_ID_INFORMATION;
View the official Windows Driver Kit DDI reference