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")

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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.

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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().