When i try to start dnsmasq ,I face the following error
dnsmasq: failed to bind DHCP server socket: Adress already in use
Given the information provided I'd say you already have a DHCP server running on that machine.
Perhaps you have started dnsmasq more than once?
Try the following command to see if anything else is acting as a DHCP server on port 67.
sudo netstat -tulpn|grep 67