FSCTL_DISMOUNT_VOLUME info  Overview  Group

The FSCTL_DISMOUNT_VOLUME DeviceIoControl operation dismounts a volume.

dwIoControlCode = FSCTL_DISMOUNT_VOLUME;   // operation code 

lpInBuffer = NULL; // pointer to input buffer; not used; must be NULL 

nInBufferSize = 0; // size of input buffer; not used; must be zero 

lpOutBuffer ;      // pointer to output buffer; not used; must be NULL 

nOutBufferSize ;   // size of output buffer; not used; must be zero 

lpBytesReturned ;  // pointer to DWORD used by DeviceIoControl function 

 

Parameters

lpInBuffer

Points to an input buffer. Not used with this operation. Set to NULL.

nInBufferSize

Specifies the size, in bytes, of the buffer pointed to by lpInBuffer. Not used with this operation. Set to zero.

lpOutBuffer

Points to an output buffer. Not used with this operation. Set to NULL.

nOutBufferSize

Specifies the size, in bytes, of the buffer pointed to by lpOutBuffer. Not used with this operation. Set to zero.

lpBytesReturned

Pointer to a DWORD. This value cannot be NULL. Although the FSCTL_DISMOUNT_VOLUME operation produces no output data and lpOutBuffer should be NULL, the DeviceIoControl function uses the variable pointed to by lpBytesReturned. After the operation, the value of this variable is without meaning.

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:

• 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. 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.

See Also

CreateFile, DeviceIoControl, ExitThread, GetLogicalDrives

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