Getting a count from an update

Newbie question here: I want to be able to capture the count of the
number of rows updated from an update command within SQL.  I know that
psql prints out the result, I want to grab it within a function.  The
update is a simple "UPDATE tablename SET col = value WHERE othercol =
otherval;", which will generally update many dozens or hundreds of
rows.  Currently I'm doing a select before doing the update, but since
I'm doing a select count(*) I can't add a 'for update' to the end and
lock the rows.  Is there a better way to do this?

Brian