After an undo operation, the media position is at the beginning of the media.

Undo is unlimited. However, after a save, any previous editing actions (such as cut, delete, paste) are cleared and cannot be undone. If there are no possible actions to be undone (the file is in the state where the last change was made) then MCIERR_CANNOT_UNDO is returned.

If undo interrupts an in-progress operation, such as play, the command is aborted and an MM_MCINOTIFY message is sent to the application.

Not all devices support this command. Use the MCI_GETDEVCAPS message to determine whether the device supports MCI_UNDO.