#ifndef _NTIOAPI_H

// Reparse structure for FSCTL_SET_REPARSE_POINT_EX

typedef struct _REPARSE_DATA_BUFFER_EX
{
    ULONG Flags;

    //
    //  This is the existing reparse tag on the file if any,  if the
    //  caller wants to replace the reparse tag too.
    //
    //    - To set the reparse data  along with the reparse tag that
    //      could be different,  pass the current reparse tag of the
    //      file.
    //
    //    - To update the reparse data while having the same reparse
    //      tag,  the caller should give the existing reparse tag in
    //      this ExistingReparseTag field.
    //
    //    - To set the reparse tag along with reparse data on a file
    //      that doesn't have a reparse tag yet, set this to zero.
    //
    //  If the ExistingReparseTag  does not match the reparse tag on
    //  the file,  the FSCTL_SET_REPARSE_POINT_EX  would  fail  with
    //  STATUS_IO_REPARSE_TAG_MISMATCH. NOTE: If a file doesn't have
    //  a reparse tag, ExistingReparseTag should be 0.
    //

    ULONG ExistingReparseTag;

    //  For non-Microsoft reparse tags, this is the existing reparse
    //  guid on the file if any,  if the caller wants to replace the
    //  reparse tag and / or guid along with the data.
    //
    //  If ExistingReparseTag is 0, the file is not expected to have
    //  any reparse tags, so ExistingReparseGuid is ignored. And for
    //  non-Microsoft tags ExistingReparseGuid should match the guid
    //  in the file if ExistingReparseTag is non zero.

    GUID ExistingReparseGuid;

    //
    //  Reserved
    //
    ULONGLONG Reserved;

    //
    //  Reparse data to set
    //
    union
    {
        REPARSE_DATA_BUFFER ReparseDataBuffer;
        REPARSE_GUID_DATA_BUFFER ReparseGuidDataBuffer;
    };
} REPARSE_DATA_BUFFER_EX, *PREPARSE_DATA_BUFFER_EX;

#endif
View code on GitHub

// ntifs.h

typedef struct _REPARSE_DATA_BUFFER_EX {
  ULONG     Flags;
  ULONG     ExistingReparseTag;
  GUID      ExistingReparseGuid;
  ULONGLONG Reserved;
  union {
    REPARSE_DATA_BUFFER      ReparseDataBuffer;
    REPARSE_GUID_DATA_BUFFER ReparseGuidDataBuffer;
  } DUMMYUNIONNAME;
} REPARSE_DATA_BUFFER_EX, *PREPARSE_DATA_BUFFER_EX;
View the official Windows Driver Kit DDI reference