Error in libnet_pblock_coalesce()

Hi,
i've written a routine to send TCP SYN packets.
When this routine is executed,the first packet is sent
and the it terminates with the following error
message:

libnet_pblock_coalesce(): packet assembly cannot find
an IPv4 header.

The routine's code is the following:

void send_tcp_pkt(u_int32_t daddr) {
  seq = rand();
  tcp =
libnet_build_tcp(lport,dport,seq,ack,TH_SYN,32767,0,0,LIBNET_TCP_H,NULL,0,l,tcp);
  if (tcp == -1){
    fprintf(stderr,"Can't build tcp header:
%s\n",libnet_geterror(l));
    exit(1);
  }
  ip =
libnet_build_ipv4(LIBNET_IPV4_H+LIBNET_TCP_H,0,id,0,64,IPPROTO_TCP,0,laddr,daddr,NULL,0,l,ip);
  if (ip == -1) {
    fprintf(stderr,"Can't build ip header:
%s\n",libnet_geterror(l));
    exit(1);
  }
  if (libnet_write(l) == -1){
      fprintf(stderr,"libnet_write():
%s\n",libnet_geterror(l));
      exit(1);
  }
}

What's wrong with this code?

Thanks.


                
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