Convert vector types

Q: I want to change closed lines into areas. I tried to convert lines into boundaries with v.type, but now, how to add centroids to turn them into areas? The lines look as follows. In v.digit:

GRASS 6.0:

• Run v.category:

   v.category in=boundaries out=areas option=add

GRASS 6.2 and more recent:

• Run v.centroids to add centroid(s)