XSetTransientForHint, XGetTransientForHint - set or read a window's
  WM_TRANSIENT_FOR property
int XSetTransientForHint(Display *display, Window
    w, Window prop_window);
Status XGetTransientForHint(Display *display, Window
    w, Window *prop_window_return);
  - display
- Specifies the connection to the X server.
- w
- Specifies the window.
- prop_window
- Specifies the window that the WM_TRANSIENT_FOR property is to be set
    to.
- prop_window_return
- Returns the WM_TRANSIENT_FOR property of the specified window.
The XSetTransientForHint function sets the WM_TRANSIENT_FOR property of
  the specified window to the specified prop_window.XSetTransientForHint can generate BadAlloc and
    BadWindow errors.
The XGetTransientForHint function returns the
    WM_TRANSIENT_FOR property for the specified window. It returns a nonzero
    status on success; otherwise, it returns a zero status.
XGetTransientForHint can generate a BadWindow
  error.
  - WM_TRANSIENT_FOR
- Set by application programs to indicate to the window manager that a
      transient top-level window, such as a dialog box.
  - BadAlloc
- The server failed to allocate the requested resource or server
    memory.
- BadWindow
- A value for a Window argument does not name a defined Window.
XAllocClassHint(3), XAllocIconSize(3), XAllocSizeHints(3), XAllocWMHints(3),
  XSetCommand(3), XSetTextProperty(3), XSetWMClientMachine(3),
  XSetWMColormapWindows(3), XSetWMIconName(3), XSetWMName(3),
  XSetWMProperties(3), XSetWMProtocols(3), XStringListToTextProperty(3)
Xlib - C Language X Interface