font-xlfd-name

font-xlfd-name is a built-in function in `C source code'.

(font-xlfd-name FONT &optional FOLD-WILDCARDS)

Return XLFD name of FONT.
FONT is a font-spec, font-entity, or font-object.
If the name is too long for XLFD (maximum 255 chars), return nil.
If the 2nd optional arg FOLD-WILDCARDS is non-nil,
the consecutive wildcards are folded into one.