Tk_SetGrid(tkwin, reqWidth, reqHeight, widthInc, heightInc)
Tk_SetGrid turns on gridded geometry management for tkwin's toplevel window and specifies the geometry of the grid. Tk_SetGrid is typically invoked by a widget when its setGrid option is true. It restricts interactive resizing of tkwin's toplevel window so that the space allocated to the toplevel is equal to its requested size plus or minus even multiples of widthInc and heightInc. Furthermore, the reqWidth and reqHeight values are passed to the window manager so that it can report the window's size in grid units during interactive resizes. If tkwin's configuration changes (e.g., the size of a grid unit changes) then the widget should invoke Tk_SetGrid again with the new information.
Tk_UnsetGrid cancels gridded geometry management for tkwin's toplevel window.
For each toplevel window there can be at most one internal window with gridding enabled. If Tk_SetGrid or Tk_UnsetGrid is invoked when some other window is already controlling gridding for tkwin's toplevel, the calls for the new window have no effect.
See the wm manual entry for additional information on gridded geometry management.