If an MCI message constant does not already exist for your command in mmsystem.h, you must define one. Place this definition in an include file that is available both to your driver and to application developers. MCI message constants must be assigned values between the constants MCI_USER_MESSAGES and MCI_LAST, which are defined in mmsystem.h.
Note that message constants cannot be defined using an expression, such as “MCI_USER_MESSAGES + 1”, because you are using the constant in a command table. Command tables are defined within resource files, using RCDATA type, and this type does not accept expressions. Define each messageconstant as a numeric value, such as:
#define MCI_MYCOMMAND 0xA01