Each file and directory on a volume that supports compression for individual files and directories has a compression state.
Whereas the compression attribute of a file or directory indicates simply whether the file or directory is compressed or not compressed, the compression state also specifies the format of any compressed data.
Use the FSCTL_GET_COMPRESSION control code to determine the compression state of a file or directory.
Compression state is encoded as a 16-bit value. A compression state value of COMPRESSION_FORMAT_NONE indicates that a file is not compressed. A value of COMPRESSION_FORMAT_DEFAULT indicates that a file is compressed, using the default compression format. Any other value indicates that a file is compressed, using the compression format specified by the compression state value.
Use the FSCTL_SET_COMPRESSION control code to set the compression state of a file or directory. This operation also sets the compression attribute of the file or directory.
Setting the compression state of a file to a nonzero value compresses the file, using the compression format encoded by the compression state value. Setting a file's compression state to zero decompresses the file. These are synchronous operations. The file is compressed or decompressed immediately when you set its compression state.
Setting a directory's compression state does not cause any immediate compression or decompression. Instead, setting a directory's compression state sets a default compression state that will be given to all newly created files and subdirectories.