SIP conntrack helper

Hello all, I have read that helpers in conntrack would give us the opportunity to pars the packets up to application layer. I want to pars the SIP packet, so I decided to use the SIP conntrack helper;which is nf_conntrack_sip. Unlike the functionality the helper itself gives, I want to find the SIP messages of type MESSAGE and read the request line of the packet. SO I add the following code into the line 256. char sub[mlen+1]; memcpy(sub,&limit[0], mlen)) sub[mlen]='\0'; printk(KERN_ALERT "method is:[%s]", sub); But the problem is, nothing is being printed. My question is, Can I use nf_conntrack_sip for my purpose? and if yes, what is wrong with my code? Thanks in advance.