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The MCI_RESERVE command allocates contiguous disk space for the workspace of the device driver instance for use with subsequent recording. Digital-video devices recognize this command.

To send this command, call the mciSendCommand function with the following parameters.

MCIERROR mciSendCommand(
  DWORD dwFlags, 



Device identifier of the MCI device that is to receive the command message.


MCI_NOTIFY, MCI_WAIT, or MCI_TEST. For information about these flags, see The Wait, Notify, and Test Flags.


Pointer to an MCI_DGV_RESERVE_PARMS structure.

Return Value

Returns zero if successful or an error otherwise.


If the workspace contains unsaved data, this data is lost. If disk space is not reserved prior to recording, the MCI_RECORD command performs an implied reserve with device-specific default parameters. On some implementations, reserve is not required and might be ignored by the device driver. Explicitly reserving space gives you better control over when the delay for disk allocation occurs, how much space is allocated, and where the disk space is allocated. The amount and location of disk space already reserved for this device instance can be changed by issuing MCI_RESERVE again. Any allocated and still unused disk space is not deallocated until any recorded data is saved or until the device driver instance is closed.

If video is turned off with the MCI_OFF flag of the MCI_SETVIDEO command, the space reserved does not include any video. If audio is turned off with the MCI_OFF flag of the MCI_SETAUDIO command, the space reserved does not include any audio. If both audio and video are turned off or if the requested size is zero, no space is reserved and any existing reserved space is deallocated.

The following additional flags apply to digital-video devices:


The lpstrPath member of the structure identified by lpReserve contains an address of a buffer containing the location of a temporary file. The buffer contains only the drive and directory path of the file used to hold recorded data; the filename is specified by the device driver. This temporary file is deleted when the device instance is closed unless it is explicitly saved. If this flag is omitted, the device driver specifies where disk space is allocated.


The dwSize member of the structure identified by lpReserve specifies the approximate amount of disk space to reserve in the workspace for recording. The value is specified in the current time format. The amount of disk space is estimated from the requested time and from which file format and video and audio algorithm and quality values are in effect. If this flag is omitted, the device driver might use a default value it defines.


Minimum supported client

Windows 2000 Professional [desktop apps only]

Minimum supported server

Windows 2000 Server [desktop apps only]


Mmsystem.h (include Windows.h)

See also

MCI Commands