This function invokes a window procedure.

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

HAB        hab;        /*  Anchor-block handle. */
PQMSG      pqmsgMsg;   /*  Message structure. */
MRESULT    mresReply;  /*  Message-return data. */

mresReply = WinDispatchMsg(hab, pqmsgMsg);