Function XPLMMeasureString 

pub unsafe extern "C" fn XPLMMeasureString(
    inFontID: XPLMFontID,
    inChar: *const c_char,
    inNumChars: c_int,
) -> f32

XPLMMeasureString

This routine returns the width in pixels of a string using a given font. The string is passed as a pointer plus length (and does not need to be null terminated); this is used to allow for measuring substrings. The return value is floating point; it is possible that future font drawing may allow for fractional pixels.