#ifndef _NTIOAPI_H

/**
 * The FILE_COMPRESSION_INFORMATION structure describes the state of a compressed data buffer.
 * \sa https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/ntifs/ns-ntifs-_file_compression_information
 */
typedef struct _FILE_COMPRESSION_INFORMATION
{
    LARGE_INTEGER CompressedFileSize;
    USHORT CompressionFormat;
    UCHAR CompressionUnitShift;
    UCHAR ChunkShift;
    UCHAR ClusterShift;
    UCHAR Reserved[3];
} FILE_COMPRESSION_INFORMATION, *PFILE_COMPRESSION_INFORMATION;

#endif
View code on GitHub

// ntifs.h

typedef struct _FILE_COMPRESSION_INFORMATION {
  LARGE_INTEGER CompressedFileSize;
  USHORT        CompressionFormat;
  UCHAR         CompressionUnitShift;
  UCHAR         ChunkShift;
  UCHAR         ClusterShift;
  UCHAR         Reserved[3];
} FILE_COMPRESSION_INFORMATION, *PFILE_COMPRESSION_INFORMATION;
View the official Windows Driver Kit DDI reference

NtDoc

This structure is documented in Windows Driver Kit.

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Windows Driver Kit DDI reference (ns-ntifs-_file_compression_information)

_FILE_COMPRESSION_INFORMATION structure

Description

The FILE_COMPRESSION_INFORMATION structure describes the state of a compressed data buffer.

Members

`CompressedFileSize`

The size, in bytes, of the compressed file.

`CompressionFormat`

The compression format. This member must have one of the following values:

Value	Meaning
COMPRESSION_FORMAT_NONE	No compression format.
COMPRESSION_FORMAT_DEFAULT	Default compression format.
COMPRESSION_FORMAT_LZNT1	LZNT1 compression format.

`CompressionUnitShift`

The log, base 2, of the number of clusters per compression

unit. In the NTFS file system, this value is NTFS_CLUSTERS_PER_COMPRESSION.

`ChunkShift`

The log, base 2, of the number of bytes in a chunk. In other words, the size in bytes of the chunk is 2 ** ChunkShift, or (1 << ChunkShift).

`ClusterShift`

The log, base 2, of the minimum number of clusters by which compression must reduce the size of the compression unit. If compression does not reduce the size of the compression unit by at least 2 ** ClusterShift clusters (or 1 << ClusterShift clusters), compression will not occur. Each compression unit must occupy at least one cluster less than the uncompressed data would occupy.

`Reserved`

Reserved

FILE_COMPRESSION_INFORMATION - NtDoc