FSCTL_DISMOUNT_VOLUME

Using the FSCTL_DISMOUNT_VOLUME control code dismounts a volume.

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

BOOL DeviceIoControl(
  (HANDLE) hDevice,            // handle to volume
  FSCTL_DISMOUNT_VOLUME,       // dwIoControlCode operation
  NULL,                        // lpInBuffer; must be NULL
  0,                           // nInBufferSize; must be zero
  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 volume to be dismounted. To obtain a handle, call the CreateFile function.

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_DISMOUNT_VOLUME for this operation.

lpInBuffer

[in] Pointer to an input buffer. Not used, set to NULL.

nInBufferSize

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

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. 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 by another process, the operation fails. To prevent another process from locking the volume, lock it as soon as you open it.

A dismounted volume has the following properties:

• There are no open files.
• The operating system does not "know" about the volume.

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. Dismount the volume
5. Unlock 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.

Requirements

  Windows NT/2000: Requires Windows NT 3.5 or later.
  Windows 95/98: Unsupported.
  Header: Declared in Winioctl.h.

See Also

Device Input and Output Overview, Device Input and Output Control Codes, CreateFile, DeviceIoControl, ExitThread, GetLogicalDrives