cairo:scaled-font-text-extents (font utf8)

font -- a cairo:scaled-font-t instance
utf8 -- a string of text, encoded in UTF-8

The double floats of the cairo:text-extents-t instance with the extents of utf8.

Gets the extents for a string of text. The extents describe a user-space rectangle that encloses the "inked" portion of the text drawn at the origin (0,0), as it would be drawn by the cairo:show-text function if the Cairo graphics state were set to the same cairo:font-face, cairo:set-font-matrix, ctm, and cairo:font-options as font. Additionally, the xadvance and yadvance values indicate the amount by which the current point would be advanced by the cairo:show-text function.

Note that whitespace characters do not directly contribute to the size of the rectangle width and height values. They do contribute indirectly by changing the position of non-whitespace characters. In particular, trailing whitespace characters are likely to not affect the size of the rectangle, though they will affect the xadvance and yadvance values.