FSCTL_SET_COMPRESSION

The FSCTL_SET_COMPRESSION operation sets the compression state of a file or directory on a volume whose file system supports per-file and per-directory compression. You can use this operation to compress or uncompress a file or directory on such a volume. To perform this operation, call the DeviceIoControl function using the following parameters.

BOOL DeviceIoControl(
  (HANDLE) hDevice,        // handle to a file
  FSCTL_SET_COMPRESSION,   // dwIoControlCode, control code of
                           // operation to perform
  (LPVOID) lpInBuffer,     // pointer to buffer with input data
  (DWORD) nInBufferSize,   // size, in bytes, of lpInBuffer
  NULL,                    // lpOutBuffer is not used; must be NULL
  0,                       // nOutBufferSize is not used;
                           // must be zero
  (LPDWORD) lpBytesReturned,   // pointer to variable to receive
                           // output byte count
  (LPOVERLAPPED) lpOverlapped  // pointer to OVERLAPPED structure
                           // for asynchronous operation
);

Parameters

hDevice

Handle to the object on which the operation is to be performed. To obtain a device handle, call the CreateFile function.

dwIoControlCode

Control code for the operation. This value identifies the specific operation to be performed and the type of device on which the operation is to be performed. Use FSCTL_SET_COMPRESSION for this operation.

lpInBuffer

Pointer to a buffer containing a USHORT that specifies a new compression state for the file or directory.

The following values are defined:

Value	Meaning
COMPRESSION_FORMAT_NONE	Uncompress the file or directory.
COMPRESSION_FORMAT_DEFAULT	Compress the file or directory, using the default compression format.
COMPRESSION_FORMAT_LZNT1	Compress the file or directory, using the LZNT1 compression format.
all other values	Reserved for future use.

nInBufferSize

Size, in bytes, of the buffer pointed to by lpInBuffer. The buffer must be large enough to contain one USHORT value.

lpOutBuffer

Pointer to the output buffer. Not used; must be NULL

nOutBufferSize

Size, in bytes, of the output buffer. Not used; must be zero.

lpBytesReturned

Pointer to a DWORD variable that receives the size, in bytes, of output data returned. If the lpOutBuffer buffer is too small, lpBytesReturned receives the required buffer size.

If lpOverlapped is NULL (nonoverlapped I/O), lpBytesReturned cannot be NULL.

If lpOverlapped is not NULL (overlapped I/O), lpBytesReturned can be NULL. If this is an overlapped operation, you can retrieve the number of bytes returned by calling the GetOverlappedResult function. If hDevice is associated with an I/O completion port, you can get the number of bytes returned by calling the GetQueuedCompletionStatus function.

lpOverlapped

Pointer to an OVERLAPPED structure.

If hDevice was opened with the FILE_FLAG_OVERLAPPED flag, this parameter must point to a valid OVERLAPPED structure. In this case, DeviceIoControl is performed as an overlapped (asynchronous) operation. If the device was opened with the FILE_FLAG_OVERLAPPED flag and lpOverlapped is NULL, the function fails in unpredictable ways.

If hDevice was opened without specifying the FILE_FLAG_OVERLAPPED flag, this parameter is ignored and the DeviceIoControl function does not return until the operation has been completed, or until an error occurs.

Return Values

If the operation succeeds, DeviceIoControl returns a nonzero value.

If the operation fails, DeviceIoControl returns zero. To get extended error information, call GetLastError.

Remarks

LZNT1 is the only compression algorithm implemented. As a result, LZNT1 is used as the DEFAULT compression method. Future releases may have additional compression methods which may be used as the DEFAULT.

If the file system of the volume containing the specified file or directory does not support per-file or per-directory compression, the FSCTL_SET_COMPRESSION operation fails.

Windows NT versions 3.51 and later support file compression on volumes formatted with NTFS.

The compression state change of the file or directory occurs synchronously with the call to DeviceIoControl.

You can obtain the compression state of a file or directory by using the FSCTL_GET_COMPRESSION DeviceIoControl operation.

You can obtain the compression attribute of a file or directory by calling the GetFileAttributes function. The compression attribute indicates whether a file or directory is compressed. The compression state indicates whether a file or directory is compressed and, if it is, the format of the compressed data.

Directories are not actually compressed by the FSCTL_SET_COMPRESSION operation. Rather, this operation sets the default state for files created in the directory to be compressed.

QuickInfo

  Windows NT: Requires version 3.51 or later.
  Windows: Unsupported.
  Windows CE: Unsupported.
  Header: Declared in winioctl.h.

See Also

Device Input and Output Overview, Device Input and Output Operations, DeviceIoControl, FSCTL_GET_COMPRESSION, GetFileAttributes