FSCTL_DISMOUNT_VOLUME

The FSCTL_DISMOUNT_VOLUME operation dismounts a volume. To perform this operation, call the DeviceIoControl function using the following parameters.

BOOL DeviceIoControl(
  (HANDLE) hDevice,        // handle to a file
  FSCTL_DISMOUNT_VOLUME,   // 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
);

Parameters

hDevice
Handle to the object on which the operation is to be performed. To obtain a device handle, call the CreateFile function.
dwIoControlCode
Control code for the operation. This value identifies the specific operation to be performed and the type of device on which the operation is to be performed. Use FSCTL_DISMOUNT_VOLUME for this operation.
lpInBuffer
Pointer to an input buffer. Not used, set to NULL.
nInBufferSize
Size, in bytes, of the input buffer. Not used; must be zero.
lpOutBuffer
Pointer to the output buffer. Not used; must be NULL
nOutBufferSize
Size, in bytes, of the output buffer. Not used; must be zero.
lpBytesReturned
Pointer to a DWORD variable.

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
Pointer to an OVERLAPPED structure.

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.

Return Values

If the operation succeeds, DeviceIoControl returns a nonzero value.

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

Remarks

The hDevice handle passed to DeviceIoControl must be a handle to a volume, opened for direct access. An application can obtain such a handle by calling CreateFile with lpFileName set to a string that looks like this:

\\.\X:

where X is a hard-drive partition letter, floppy disk drive, or CD-ROM drive. The application must also specify the FILE_SHARE_READ and FILE_SHARE_WRITE flags in the dwShareMode parameter of CreateFile.

If the specified volume is locked, the operation fails.

A dismounted volume has the following properties:

The operating system tries to mount an unmounted volume as soon as any attempt is made to access it. For example, a call to GetLogicalDrives triggers the operating system to mount any unmounted volumes.

Dismounting a volume is useful whenever a volume needs to disappear for a while. For example, an application that changes a volume's file system from FAT to NTFS might follow these steps:

  1. Open the volume
  2. Lock the volume
  3. Format the volume
  4. Unlock the volume
  5. Dismount the volume
  6. Close the volume handle

A dismounting operation removes the volume from the FAT file system's "awareness." When the operating system mounts the volume, it appears as an NTFS volume.

QuickInfo

  Windows NT: Requires version 3.5 or later.
  Windows: Unsupported.
  Windows CE: Unsupported.
  Header: Declared in winioctl.h.

See Also

Device Input and Output Overview, Device Input and Output Operations, CreateFile, DeviceIoControl, ExitThread, GetLogicalDrives