FSCTL_QUERY_ALLOCATED_RANGES

Using the FSCTL_QUERY_ALLOCATED_RANGES control code scans a file or alternate stream looking for ranges that may contain nonzero data. Only compressed or sparse files can have zeroed ranges known to the operating system. For other files, the output buffer will contain only a single entry that contains the starting point and the length requested.

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

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

Parameters

hDevice

[in] Handle to the file or alternate stream to be queried for allocated ranges. Call the CreateFile function to obtain a handle.

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_QUERY_ALLOCATED_RANGES for this operation.

lpInBuffer

[in] Pointer to a FILE_ALLOCATED_RANGE_BUFFER structure that indicates the portion of the file to search for allocated ranges. The FileOffset member indicates the offset, in bytes, to the first byte of the range to search, and the Length member indicates its size, in bytes.

nInBufferSize

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

lpOutBuffer

[out] Pointer to a buffer that receives an array of FILE_ALLOCATED_RANGE_BUFFER structures. Each structure in the array provides information about an allocated range within the file. The FileOffset member of each structure indicates the beginning of a range, and the Length member indicates its size, in bytes.

Ranges returned are always at least partially within the range specified in the lpInBuffer buffer.

nOutBufferSize

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

lpBytesReturned

[out] Pointer to a variable that receives the size, in bytes, of output data returned.

If the output buffer is too small to return any data, then the call fails, GetLastError returns the error code ERROR_INSUFFICIENT_BUFFER, and the returned byte count is zero.

If the output buffer is too small to hold all of the data but can hold some entries, then the operating system returns as much as fits, the call fails, GetLastError returns the error code ERROR_MORE_DATA, and lpBytesReturned indicates the amount of data returned. Your application should call DeviceIoControl again with the same operation, specifying a new starting point.

If lpOverlapped is NULL, lpBytesReturned cannot be NULL.

If lpOverlapped is not NULL, lpBytesReturned can be NULL. If this is an overlapped operation, you can get 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

[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 FILE_FLAG_OVERLAPPED 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 an error occurs.

Return Values

If the operation succeeds, DeviceIoControl returns a nonzero value, and the output buffer pointed to by lpOutBuffer contains an array of valid FILE_ALLOCATED_RANGE_BUFFER structures.

If the operation fails, DeviceIoControl returns zero. The contents of the output buffer pointed to by lpOutBuffer are meaningless. For extended error information, call GetLastError.

Remarks

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

NTFS rounds the input file offset down to a convenient boundary and the length up to a convenient boundary and then begins to walk through the file.

The operating system does not track every piece of zero or nonzero data. Because zero is often a perfectly legal datum, it would be misleading. Instead, the system tracks ranges where disk space is allocated. Where no disk space is allocated, all data are assumed to be zero. Allocated storage can contain zero or nonzero data. So all this operation does is return information about parts of the file where nonzero data may be located. It is up to the application to scan these parts of the file in accordance with the application's data conventions.

Each entry in the output array contains an offset and a length that indicates a range in the file that may contain nonzero data. The actual nonzero data, if any, is somewhere within this range, and the calling program must scan further within the range to locate it and determine if it really is valid data. Multiple instances of valid data may exist within the range.

Allocated ranges are subject to the rule that a memory mapped remote (network) file and an open handle to the file are not necessarily coherent. If you memory mapped a sparse network file and wrote nonzero data to previously unallocated regions of the file, disk space would be allocated for the new data. However, a call to FSCTL_QUERY_ALLOCATED_RANGES thereafter would not necessarily return a correct list of allocated regions. To ensure coherency between the view memory and the file handle, flush the data to the file with the FlushViewOfFile function.

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_ALLOCATED_RANGE_BUFFER, FSCTL_SET_SPARSE, FSCTL_SET_ZERO_DATA