(cffi:defcstruct vec3-t
  (value :float :count 4)) ; place for 4 single floats  

The graphene:vec3-t structure is capable of holding a vector with three dimensions x, y, and z. Use the graphene:vec3-x, graphene:vec3-y, and graphene:vec3-z functions to get the values of the components. There are no functions to set the values directly.