A window procedure is a function that takes 4 arguments and returns a 32-bit pointer. The arguments of a window procedure consist of a window handle, a ULONG message identifier, and two arguments, called message parameters, that are declared with the MPARAM data type. The system defines an MPARAM as a 32-bit pointer to a VOID data type (a generic pointer). The message parameters actually might contain any of the standard data types. The message parameters are interpreted differently,depending on the value of the message identifier. The operating system includes several macros that enable the application to cast the information from the MPARAM values into the actual data type. SHORT1FROMMP, for example, extracts a 16-bit value from a 32-bit MPARAM. The window-procedure arguments are described in the following table:
旼컴컴컴컴컴컴컴쩡컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 쿌rgument 쿏escription � 쳐컴컴컴컴컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 쿴wnd 쿓andle of the window receiving the message. � 쳐컴컴컴컴컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 쿺sg 쿘essage identifier. The message will � � 쿬orrespond to one of the predefined constants� � �(for example, WM_CREATE) defined in the � � 퀂ystem include files or be an � � 쿪pplication-defined message identifier. The � � 퀆alue of an application-defined message � � 쿶dentifier must be greater than the value of � � 쿥M_USER, and less than or equal to 0xffff. � 쳐컴컴컴컴컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� �mp1,mp2 쿘essage parameters. Their interpretation � � 쿭epends on the particular message. � 읕컴컴컴컴컴컴컴좔컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�
The return value of a window procedure is defined as an MRESULT data type. The interpretation of the return value depends on the particular message. Consult the description of each message to determine the appropriate return value.