error running a simple libpcap example

Question

I am trying to compile a simple libpcap example,

#include<stdio.h>
#include<pcap.h>

int main(int argc, char *argv[])
{
  char *dev;
  char errbuf[PCAP_ERRBUF_SIZE];
  struct bpf_program fp;

  char filter_exp[] = "port 23";
  bpf_u_int32 mask;
  bpf_u_int32 net;
  dev = pcap_lookupdev(errbuf);
  if (dev == NULL )
  {
    fprintf(stderr, "couldn't find default device: %s\n", errbuf);
    return (2);
  }
  printf("Device: %s\n", dev);

  //LOOKUP NETMASK and IP
  if (pcap_lookupnet(dev, &net, &mask, errbuf) == -1)
  {
    fprintf(stderr, "can't get netmask for device %s\n", dev);
    net = 0;
    mask = 0;
  }

  printf("lookup\n");

  pcap_t *handle;
  printf("handle defined\n");
  handle = pcap_open_live(dev, BUFSIZ, 1, 1000, errbuf);
  printf("opened\n");
  if (handle = NULL )
  {
    fprintf(stderr, "Couldn't open device %s: %s\n", dev, errbuf);
    return (2);
  }

  printf("pcap_open\n");

  if ((pcap_compile(handle, &fp, filter_exp, 1, net)) == -1)
  {
    printf("compile error block entered\n");
    fprintf(stderr, "Couldn't parse filter %s: %s\n", filter_exp,
        pcap_geterr(handle));
    return (2);
  }

  printf("compiled\n");
  if (pcap_setfilter(handle, &fp) == -1)
  {
    fprintf(stderr, "couldn't install filter %s: %s\n", filter_exp,
        pcap_geterr(handle));
    return (2);
  }

  printf("after filter\n");
  return (0);
}

It compiles without error, but when I try to run it, I'm getting segmentation fault message or if I try to run it with root permission, I don't get the message but the program stops after printing

Device: eth0
lookup
handle defined
opened
pcap_open

Can you please help me with this, I'm confused as to why this error happens. Thanks in advance.

Answer 1

if (handle = NULL) This is the culprit.

You're assigning handle to NULL and handle is used in some other functions after this assignment. So, dereferencing this is causing the seg-fault.

change it to if(handle == NULL) and check.