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Command Messages

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[The feature associated with this page, MCI, is a legacy feature. It has been superseded by MediaPlayer. MediaPlayer has been optimized for Windows 10 and Windows 11. Microsoft strongly recommends that new code use MediaPlayer instead of MCI, when possible. Microsoft suggests that existing code that uses the legacy APIs be rewritten to use the new APIs if possible.]

The command-message interface is designed to be used by applications requiring a C-language interface to control multimedia devices. It uses a message-passing paradigm to communicate with MCI devices. You can send a command by using the mciSendCommand function.

The mciSendCommand function returns zero if successful. If the function fails, the low-order word of the return value contains an error code. You can pass this error code to the mciGetErrorString function to get a text description of the error.

Syntax of Command Messages

MCI command messages consist of the following elements:

  • A constant message value
  • A structure containing parameters for the command
  • A set of flags specifying options for the command and validating fields in the parameter block

The following example uses the mciSendCommand function to send the MCI_ PLAY command to the device identified by a device identifier.

mciSendCommand(wDeviceID,            // device identifier 
    MCI_PLAY,                        // command message 
    0,                               // flags 
    (DWORD)(LPVOID) &mciPlayParms);  // parameter block

The device identifier given in the first parameter is retrieved when the device is opened using the MCI_ OPEN command. The last parameter is the address of an MCI_ PLAY_ PARMS structure, which might contain information about where to begin and end playback. Many MCI command messages use a structure to contain parameters of this kind. The first member of each of these structures identifies the window that receives an MM_ MCINOTIFY message when the operation finishes.