HOWTO get centroid coordinates

Q: I have a vector of polygons and I want to create a vector of points with the centroids of the polygons, with a connected table showing their coordinates. Which is the simplest way to do that?

A: In Grass 6.4.x:

# Extract centroids from polygon map
v.extract in=poly out=centr type=centroid
# If needed convert centroids to points
v.type in=centr out=centr_pts type=centroid,point
# Add attribute table to points
v.db.addtable centr_pts col="x double,y double,z double"
# Insert coords in table
v.to.db TMP_centr_pts option=coor col=x,y,z

In Grass 7:

v.out.ascii in=poly type=centroid