A radio button looks very much like a check box except that it is shaped like a circle rather than a box. A heavy dot within the circle indicates that the radio button has been checked. The radio button has the window style BS_RADIOBUTTON or BS_AUTORADIOBUTTON, but the latter is used only in dialog boxes.
In dialog boxes, groups of radio buttons are conventionally used to indicate mutually exclusive options. (For instance, look at the dialog box in the Windows Terminal program that appears when you select Communications from the Settings menu.) Unlike check boxes, radio buttons do not work as toggles—that is, when you click a radio button a second time, its state remains unchanged.
When you receive a WM_COMMAND message from a radio button, you should display its check by sending it a BM_SETCHECK message with wParam equal to 1:
SendMessage (hwndButton, BM_SETCHECK, 1, 0L) ;
For all other radio buttons in the same group, you can turn off the checks by sending them BM_SETCHECK messages with wParam equal to 0:
SendMessage (hwndButton, BM_SETCHECK, 0, 0L) ;