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FSCTL_GET_COMPRESSION control code

Retrieves 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 with the following parameters.


BOOL 
WINAPI 
DeviceIoControl( (HANDLE) hDevice,              // handle to file
                 FSCTL_GET_COMPRESSION,         // dwIoControlCode
                 NULL,                          // lpInBuffer 
                 0,                             // nInBufferSize
                 (LPVOID) lpOutBuffer,          // output buffer
                 (DWORD) nOutBufferSize,        // size of output buffer
                 (LPDWORD) lpBytesReturned,     // number of bytes returned
                 (LPOVERLAPPED) lpOverlapped ); // OVERLAPPED structure

Parameters

hDevice

A handle to the file, alternate stream, or directory whose state of compression is to be retrieved. To retrieve a handle, call the CreateFile function.

dwIoControlCode

The control code for the operation. Use FSCTL_GET_COMPRESSION for this operation.

lpInBuffer

Not used with this operation; set to NULL.

nInBufferSize

Not used with this operation; set to zero.

lpOutBuffer

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

This parameter can be one of the following values.

ValueMeaning
COMPRESSION_FORMAT_NONE

The file or directory is not compressed.

COMPRESSION_FORMAT_LZNT1

The file or directory is compressed, using the LZNT1 compression algorithm.

all other values

Reserved for future use.

 

nOutBufferSize

The size of the output buffer, in bytes. The buffer must be large enough to contain one USHORT value.

lpBytesReturned

A pointer to a variable that receives the size of the data stored in the output buffer, in bytes.

If the output buffer is too small, the call fails, GetLastError returns ERROR_INSUFFICIENT_BUFFER, and lpBytesReturned is zero.

If lpOverlapped is NULL, lpBytesReturned cannot be NULL. Even when an operation returns no output data and lpOutBuffer is NULL, DeviceIoControl makes use of lpBytesReturned. After such an operation, the value of lpBytesReturned is meaningless.

If lpOverlapped is not NULL, lpBytesReturned can be NULL. If this parameter is not NULL and the operation returns data, lpBytesReturned is meaningless until the overlapped operation has completed. To retrieve the number of bytes returned, call GetOverlappedResult. If hDevice is associated with an I/O completion port, you can retrieve the number of bytes returned by calling GetQueuedCompletionStatus.

lpOverlapped

A pointer to an OVERLAPPED structure.

If hDevice was opened without specifying FILE_FLAG_OVERLAPPED, lpOverlapped is ignored.

If hDevice was opened with the FILE_FLAG_OVERLAPPED flag, the operation is performed as an overlapped (asynchronous) operation. In this case, lpOverlapped must point to a valid OVERLAPPED structure that contains a handle to an event object. Otherwise, the function fails in unpredictable ways.

For overlapped operations, DeviceIoControl returns immediately, and the event object is signaled when the operation has been completed. Otherwise, the function does not return until the operation has been completed or an error occurs.

Return value

If the operation completes successfully, DeviceIoControl returns a nonzero value.

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

Remarks

The LZNT1 compression algorithm is the only compression algorithm implemented.

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 control code.

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

You can set the compression state of a file or directory by using the FSCTL_SET_COMPRESSION control code. You can also compress or uncompress a file using this control code.

You can retrieve 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.

In Windows 8 and Windows Server 2012, this code is supported by the following technologies.

TechnologySupported

Server Message Block (SMB) 3.0 protocol

Yes

SMB 3.0 Transparent Failover (TFO)

No

SMB 3.0 with Scale-out File Shares (SO)

No

Cluster Shared Volume File System (CsvFS)

Yes

Resilient File System (ReFS)

No

 

SMB 3.0 Transparent Failover and Scale-Out do not support NTFS compressed files. The FSCTL call is not blocked, but unsupported.

Requirements

Minimum supported client

Windows XP [desktop apps only]

Minimum supported server

Windows Server 2003 [desktop apps only]

Header

WinIoCtl.h (include Windows.h)

See also

DeviceIoControl
File Compression and Decompression
File Management Control Codes
FSCTL_SET_COMPRESSION
GetFileAttributes

 

 

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