Windows provides the following functions to send command messages to devices and to query devices for error information:
| Function | Description |
| mciSendCommand | Sends a command message to an MCI device. |
| mciGetErrorString | Returns the error string corresponding to an error number. |
MCI returns a device ID when you open a device using the MCI_OPEN command. Use this ID to identify the opened device when sending subsequent commands.
MMSYSTEM.H defines a special constant, MCI_ALL_DEVICE_ID, to indicate that a command be sent to all of the MCI devices that an application has opened. You can use MCI_ALL_DEVICE_ID with any command that does not return information. For example, the following code fragment closes all of the MCI devices that are opened by an application:
UINT wDeviceID;
DWORD dwReturn;
/* Closes all MCI devices opened by this application.
* Waits until devices are closed before returning.
*/
if (dwReturn = mciSendCommand(MCI_ALL_DEVICE_ID, MCI_CLOSE,
MCI_WAIT, NULL))
/* Error, unable to close all devices
*/
...
Note:
While using the MCI_ALL_DEVICE_ID identifier is a convenient way to broadcast a command to all your devices, don't rely on it to synchronize devices; the timing between messages can vary.