[This is preliminary documentation and subject to change.]
The FSCTL_SET_SPARSE operation marks the indicated file as sparse. In a sparse file, large ranges of zeroes may not require disk allocation. Space for nonzero data will be allocated as needed as the file is written. To perform this operation, call the DeviceIoControl function with the following parameters.
BOOL DeviceIoControl(
(HANDLE) hDevice, // handle to a file
FSCTL_SET_SPARSE, // dwIoControlCode, control code of
// operation to perform
NULL, // lpInBuffer is not used; must be NULL
0, // nInBufferSize is not used; must be zero
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
);
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.
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.
If the operation succeeds, DeviceIoControl returns a nonzero value.
If the operation fails, DeviceIoControl returns zero. To get extended error information, call GetLastError.
See the Remarks section of the DeviceIoControl topic for the implications of overlapped I/O on this operation.
The FSCTL_SET_SPARSE operation sets the FILE_ATTRIBUTE_SPARSE_FILE attribute of the specified file. The only way to clear this attribute is to overwrite the file, for example, by calling the CreateFile function with the CREATE_ALWAYS flag..
Windows NT: Requires version 5.0 or later.
Windows: Unsupported.
Windows CE: Unsupported.
Header: Declared in winioctl.h.
Device Input and Output Overview, Device Input and Output Operations, DeviceIoControl, FSCTL_QUERY_ALLOCATED_RANGES, FSCTL_SET_ZERO_DATA