XTextWidth(), XTextWidth16() - compute text width
int XTextWidth (XFontStruct *font_struct, char *string, int count)
int XTextWidth16 (XFontStruct *font_struct, XChar2b *string,
int count)
The XTextWidth(3) and XTextWidth16(3) functions return the width of the specified 8-bit or 2-byte character strings.
Xlib