gtk-entry-invisible-char (object)

object -- a gtk-entry widget
char -- a Unicode character

Accessor of the invisible-char slot of the gtk-entry class.

The gtk-entry-invisible-char slot access function retrieves the character displayed in place of the real characters for entries with visibility set to false. The (setf gtk-entry-invisible-char) slot access function sets the character to use in place of the actual text when the gtk-entry-visibility function has been called to set text visibility to false.

I.e. this is the character used in "password mode" to show the user how many characters have been typed. By default, GTK picks the best invisible char available in the current font. If you set the invisible char to 0, then the user will get no feedback at all. There will be no text on the screen as they type.