This function registers a user message and defines its parameters.

#define INCL_WINMESSAGEMGR /* Or use INCL_WIN, INCL_PM, Also in COMMON section */
#include <os2.h>

HAB      hab;        /*  Anchor-block handle. */
ULONG    msgid;      /*  Message identifier. */
LONG     datatype1;  /*  Data type of message parameter 1. */
LONG     dir1;       /*  Direction of message parameter 1. */
LONG     datatype2;  /*  Data type of message parameter 2. */
LONG     dir2;       /*  Direction of message parameter 2. */
LONG     datatyper;  /*  Data type of message reply. */
BOOL     rc;         /*  Success indicator. */

rc = WinRegisterUserMsg(hab, msgid, datatype1,
       dir1, datatype2, dir2, datatyper);