FSCTL_SET_ZERO_DATA

Using the FSCTL_SET_ZERO_DATA control code fills a specified range of a file with zeroes. If the file is sparse or compressed, NTFS may deallocate disk space within the file. This sets the range of bytes to zeroes without extending the file size.

To perform this operation, call the DeviceIoControl function with the following parameters.

BOOL DeviceIoControl(
  (HANDLE) hDevice,            // handle to a file
  FSCTL_SET_ZERO_DATA,         // dwIoControlCode operation
  (LPVOID) lpInBuffer,         // input buffer
  (DWORD) nInBufferSize,       // size of input buffer
  NULL,                        // lpOutBuffer; must be NULL
  0,                           // nOutBufferSize; must be zero
  (LPDWORD) lpBytesReturned,   // number of bytes returned
  (LPOVERLAPPED) lpOverlapped  // OVERLAPPED structure
);

Parameters

hDevice

[in] Handle to the file or alternate stream on which zero data is to be written. The file must be on a Windows 2000 file system or later. Call the CreateFile function to obtain a file handle. The file need not be a sparse file.

dwIoControlCode

[in] Control code for the operation. This value identifies the specific operation to be performed and the type of device on which to perform it. Use FSCTL_SET_ZERO_DATA for this operation.

lpInBuffer

[in] Pointer to a FILE_ZERO_DATA_INFORMATION structure that specifies the range of the file to set to zeroes. The FileOffset member is the byte offset of the first byte to set to zeroes, and the BeyondFinalZero member is the byte offset of the first byte beyond the last zeroed byte.

nInBufferSize

[in] Size, in bytes, of the input buffer.

lpOutBuffer

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

nOutBufferSize

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

lpBytesReturned

[out] Pointer to a variable that receives the actual count of bytes returned by the function in the output buffer.

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

If lpOverlapped is not NULL (overlapped I/O), lpBytesReturned can be NULL.

lpOverlapped

[in] Pointer to an OVERLAPPED structure.

If hDevice was opened with the FILE_FLAG_OVERLAPPED flag, lpOverlapped must point to a valid OVERLAPPED structure. In this case, the operation 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, lpOverlapped is ignored and DeviceIoControl 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. For extended error information, call GetLastError.

Remarks

For the implications of overlapped I/O on this operation, see the Remarks section of DeviceIoControl.

If you use the WriteFile function to write zeroes to a sparse file, the file system allocates disk space for the data that you are writing. If you use the FSCTL_SET_ZERO_DATA control code to write zeroes to a sparse file and the zeroed region is large enough, the file system may not allocate disk space.

If you use the FSCTL_SET_ZERO_DATA control code to write zeros to a nonsparse file, zeros will be written to the file. The system will allocate disk storage for all of the range zeroed out. It is equivalent to using the WriteFile function to write zeros to a file.

Requirements

  Windows NT/2000: Requires Windows 2000.
  Windows 95/98: Unsupported.
  Header: Declared in Winioctl.h.

See Also

Device Input and Output Overview, Device Input and Output Control Codes, DeviceIoControl, FILE_ZERO_DATA_INFORMATION, FSCTL_QUERY_ALLOCATED_RANGES, FSCTL_SET_SPARSE