This function dispatches messages while a modal dialog window is displayed.

#define INCL_WINDIALOGS /* Or use INCL_WIN, INCL_PM, */
#include <os2.h>

HWND     hwndDlg;  /*  Dialog-window handle. */
ULONG    ulReply;  /*  Reply value. */

ulReply = WinProcessDlg(hwndDlg);