How to use sprintf instead of hardcoded values

I am developing a firewall for Linux as my project. I am able to captu开发者_高级运维re packets and to block them. I am using IPTABLES.

How can I use variables with sprintf instead of hardcoded values?

sprintf(comm, "iptables -A INPUT -s $str -j DROP")
// inplace of:
sprintf(comm, "iptables -A INPUT -s 192.168.0.43 -j DROP")

sprintf(comm, "iptables -A INPUT -s %s -j DROP", "192.168.0.43");
// also:
char ipaddress[] = "192.168.0.43";
sprintf(comm, "iptables -A INPUT -s %s -j DROP", ipaddress);

Read more in man sprintf.

It works just like regular printf(). Same format strings are accepted. However, be careful to avoid overflowing the string buffer. In that sense, snprintf() will be MUCH more acceptable than sprintf().