Command Strings

To send a string command to an MCI device, use the mciSendString function, which includes parameters for the string command and a buffer for any returned information.

The mciSendString 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 Strings

MCI command strings use a consistent verb-object-modifier syntax. Each command string includes a command, a device identifier, and command arguments. Arguments are optional for some commands and required for others.

A command string has the following form:

command device_id arguments

These components contain the following information:

  • The command specifies an MCI command, such as open, close, or play.
  • The device_id identifies an instance of an MCI driver. The device_id is created when the device is opened.
  • The arguments specify the flags and variables used by the command. Flags are keywords recognized with the MCI command. Variables are numbers or strings that apply to the MCI command or flag.

    For example, the play command uses the arguments "from position " and "to position " to indicate the positions at which to start and end play. You can list the flags used with a command in any order. When you use a flag that has a variable associated with it, you must supply a value for the variable.

    Unspecified (and optional) command arguments assume a default value.

The following example function sends the play command with the "from" and "to" flags.

BOOL PlayFromTo(LPTSTR lpstrAlias, DWORD dwFrom, DWORD dwTo) 
    TCHAR achCommandBuff[128]; 
    int result;
    MCIERROR err;

    // Form the command string.
    result = _stprintf_s(
        TEXT("play %s from %u to %u"), 
        lpstrAlias, dwFrom, dwTo); 

    if (result == -1)
        return FALSE;

    // Send the command string.
    err = mciSendString(achCommandBuff, NULL, 0, NULL); 
    if (err != 0)
        return FALSE;

    return TRUE;

Data Types for Command Variables

You can use the following data types for the variables in a command string.

Data typeDescription
StringsString data types are delimited by leading and trailing white spaces and quotation marks. MCI removes single quotation marks from a string. To put a quotation mark in a string, use a set of two quotation marks where you want to embed your quotation mark. To use an empty string, use two quotation marks delimited by leading and trailing white spaces.
Signed long integersSigned long integer data types are delimited by leading and trailing white spaces. Unless otherwise specified, integers can be positive or negative. If you use negative integers, you should not separate the minus sign and the first digit with a space.
Rectangles Rectangle data types are an ordered list of four signed short values. White space delimits this data type and separates each integer in the list.





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