MCI_UNDO

This message undoes the operation most recently performed by a cut, paste, or delete command.

Syntax

param1
ULONG ulParam1; /* Message flags. */

param2
PMCI_GENERIC_PARMS pParam2; /* Pointer to generic structure. */

Parameters

ulParam1 (ULONG) - input

This parameter can contain any of the following flags:

* MCI_NOTIFY — Posts a notification message to the window specified in the `hwndCallback` field of the structure pointed to by `pParam2` when the action completes or an error occurs.

* MCI_WAIT — Does not return control until the action is completed or an error occurs.

pParam2 (PMCI_GENERIC_PARMS) - input

A pointer to the MCI_GENERIC_PARMS data structure.

Return Value

rc (ULONG)

Return codes indicating success or type of failure:

* MCIERR_SUCCESS — The UNDO was successful.

* MCIERR_INVALID_DEVICE_ID — The device ID is not valid.

* MCIERR_INVALID_FLAG — Flag (`ulParam1`) is invalid.

* MCIERR_INSTANCE_INACTIVE — The device is currently inactive. Issue MCI_ACQUIREDEVICE to make the device context active.

* MCIERR_INVALID_CALLBACK_HANDLE — Given callback handle is invalid.

* MCIERR_CANNOT_UNDO — Undo is not possible in the current state (e.g., no actions in the undo stack).

Remarks

• **Positioning:** After a successful undo operation, the media position is reset to the beginning of the media.
• **Undo Stack:** Undo is unlimited. However, performing a save operation clears the undo history; any previous editing actions (cut, delete, paste) can no longer be undone.
• **Interruption:** If an undo command is issued while another operation (like playback) is in progress, that operation is aborted, and an `MM_MCINOTIFY` message is sent.
• **Support:** Not all devices support this command. Use MCI_GETDEVCAPS to check for `MCI_GETDEVCAPS_CAN_UNDO`.

Related Messages

• MCI_COPY
• MCI_CUT
• MCI_PASTE
• MCI_DELETE
• MCI_REDO

Example Code

The following code illustrates undoing the last editing operation.

    USHORT                usDeviceID;
    MCI_GENERIC_PARMS     mep;

    mep.hwndCallback = hwndMyWindow;

    mciSendCommand( usDeviceID,
                    MCI_UNDO,
                    MCI_NOTIFY,
                    (PVOID) &mep,
                    0 );