glyphless-char-display-control

glyphless-char-display-control is a variable defined in `characters.el'.
Its value is

((format-control . thin-space) (no-font . hex-code))

Documentation:
List of directives to control display of glyphless characters.

Each element has the form (GROUP . METHOD), where GROUP is a
symbol specifying the character group, and METHOD is a symbol
specifying the method of displaying characters belonging to that
group.

GROUP must be one of these symbols:
`c0-control': U+0000..U+001F, but excluding newline and TAB.
`c1-control': U+0080..U+009F.
`format-control': Characters of Unicode General Category `Cf',
such as U+200C (ZWNJ), U+200E (LRM), but
excluding characters that have graphic images,
such as U+00AD (SHY).
`no-font': characters for which no suitable font is found.
For character terminals, characters that cannot
be encoded by `terminal-coding-system'.

METHOD must be one of these symbols:
`zero-width': don't display.
`thin-space': display a thin (1-pixel width) space. On character
terminals, display as 1-character space.
`empty-box': display an empty box.
`acronym': display an acronym of the character in a box. The
acronym is taken from `char-acronym-table', which see.
`hex-code': display the hexadecimal character code in a box.

Do not set its value directly from Lisp; the value takes effect
only via a custom `:set'
function (`update-glyphless-char-display'), which updates
`glyphless-char-display'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.