FSCTL_GET_COMPRESSION

The FSCTL_GET_COMPRESSION operation obtains the current compression state of a file or directory on a volume whose file system supports per-stream compression. To perform this operation, call the DeviceIoControl function using the following parameters.

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

Parameters

hDevice

Handle to the file or directory 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_GET_COMPRESSION for this operation.

lpInBuffer

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

nInBufferSize

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

lpOutBuffer

Pointer to a buffer that receives a USHORT indicating the current compression state of the file or directory.

The following values are defined:

Value	Meaning
COMPRESSION_FORMAT_NONE	The file or directory is not compressed.
COMPRESSION_FORMAT_LZNT1	The file or directory is compressed, using the LZNT1 compression format.
all other values	Reserved for future use.

nOutBufferSize

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

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

On this release, LZNT1 is the only compression algorithm implemented. Future releases may have additional compression methods.

COMPRESSION_FORMAT_DEFAULT is not a compression state so it is not included in the table under the lpOutBuffer parameter. This value is only used with the FSCTL_SET_COMPRESSION operation

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

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

You can set the compression state of a file or directory by using the FSCTL_SET_COMPRESSION DeviceIoControl operation. You can also compress or uncompress a file with that 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.

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_SET_COMPRESSION, GetFileAttributes