General manager Reggie McKenzie announced the deals Thursday.
Smith played four years with San Francisco after being a third-round pick in 2008. He had two interceptions and seven passes defensed in 45 games. He was with Carolina in training camp last year but was cut before the season.
After missing the 2011 season with a knee injury, Jennings returned to play 10 games last season and start six. He averaged just 2.8 yards per carry.