This can be done for 8 and 16 bit characters.
int XTextWidth(font_struct,str, count)
char *str; /* string */
XFontStruct *font_struct; /* font-id of font to use */
int XTextWidth16(font_struct,str, count)
char *str; /* string */
XFontStruct *font_struct; /* font-id of font to use */
XTextExtents(font_struct,string,nchars,direction,ascent,descent,overall)
XFontStruct *font_struct;
char *str; /* string */
int *direction, *ascent, *descent; /*returned dimensions */
XCharStruct *overall; /* overall retured dimensions */
XTextExtents16(font_struct,string,nchars,direction,ascent,descent,overall)