Directs the disk device to map one or more blocks to its spare-block pool.

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

BOOL DeviceIoControl(
  (HANDLE) hDevice,            // handle to device
  (LPVOID) lpInBuffer,         // input buffer 
  (DWORD) nInBufferSize,       // size of input buffer 
  NULL,                        // lpOutBuffer
  0,                           // nOutBufferSize
  (LPDWORD) lpBytesReturned,   // number of bytes returned
  (LPOVERLAPPED) lpOverlapped  // OVERLAPPED structure



A handle to the object on which blocks are to be reassigned.

To retrieve a device handle, call the CreateFile function.


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


A pointer to a buffer that contains a REASSIGN_BLOCKS data structure specifying the blocks to reassign to its spare block pool.

This structure is really a header followed by an array of block specifiers; thus, it varies in size according to the number of blocks to be reassigned. Each block after the first adds the size of a DWORD to the overall size.


The size of the input buffer, in bytes.


Not used with this operation. Set to NULL.


Not used with this operation. Set to zero (0).


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

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.


A pointer to an OVERLAPPED structure.

If hDevice was opened without specifying FILE_FLAG_OVERLAPPED, lpOverlappedis 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 is complete. Otherwise, the function does not return until the operation is complete 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 (0). To get extended error information, call GetLastError.


The REASSIGN_BLOCKS structure that the IOCTL_DISK_REASSIGN_BLOCKS control code uses only supports drives where the Logical Block Address (LBA) fits into a 4-byte value (typically up to 2 TB). For larger drives the REASSIGN_BLOCKS_EX structure that the IOCTL_DISK_REASSIGN_BLOCKS_EX control code uses supports 8-byte LBAs. For compatibility, the IOCTL_DISK_REASSIGN_BLOCKS control code and REASSIGN_BLOCKS structure should be used where possible.


Minimum supported client

Windows XP [desktop apps only]

Minimum supported server

Windows Server 2003 [desktop apps only]


WinIoCtl.h (include Windows.h)

See also

Disk Management Control Codes