pcap_geterr, pcap_perror - get or print libpcap error message text
#include <pcap/pcap.h>
char *pcap_geterr(pcap_t *p);
void pcap_perror(pcap_t *p, const char *prefix);
pcap_geterr() returns the error text pertaining to the last pcap library
  error. NOTE: the pointer it returns will no longer point to a valid
  error message string after the pcap_t passed to it is closed; you must
  use or copy the string before closing the pcap_t.
pcap_perror() prints the text of the last pcap library
    error on stderr, prefixed by prefix.