(cffi:defcstruct font-extents-t
  (ascent :double)
  (descent :double)
  (height :double)
  (max-xadvance :double)
  (max-yadvance :double))  

ascent

The distance that the font extends above the baseline. Note that this is not always exactly equal to the maximum of the extents of all the glyphs in the font, but rather is picked to express the font designer's intent as to how the font should align with elements above it.

descent

The distance that the font extends below the baseline. This value is positive for typical fonts that include portions below the baseline. Note that this is not always exactly equal to the maximum of the extents of all the glyphs in the font, but rather is picked to express the font designer's intent as to how the font should align with elements below it.

height

The recommended vertical distance between baselines when setting consecutive lines of text with the font. This is greater than ascent + descent by a quantity known as the line spacing or external leading. When space is at a premium, most fonts can be set with only a distance of ascent + descent between lines.

max-xadvance

The maximum distance in the x direction that the origin is advanced for any glyph in the font.

max-yadvance

The maximum distance in the y direction that the origin is advanced for any glyph in the font. This will be zero for normal fonts used for horizontal writing. The scripts of East Asia are sometimes written vertically.