pcap_setnonblock, pcap_getnonblock - set or get the state of non-blocking mode
  on a capture device
#include <pcap/pcap.h>
char errbuf[PCAP_ERRBUF_SIZE];
int pcap_setnonblock(pcap_t *p, int nonblock, char *errbuf);
int pcap_getnonblock(pcap_t *p, char *errbuf);
pcap_setnonblock() puts a capture handle into ``non-blocking'' mode, or
  takes it out of ``non-blocking'' mode, depending on whether the
  nonblock argument is non-zero or zero. It has no effect on
  ``savefiles''. If there is an error, -1 is returned and errbuf is
  filled in with an appropriate error message; otherwise, 0 is returned. In
  ``non-blocking'' mode, an attempt to read from the capture descriptor with
  pcap_dispatch() will, if no packets are currently available to be read,
  return 0 immediately rather than blocking waiting for packets to arrive.
  pcap_loop() and pcap_next() will not work in ``non-blocking''
  mode.
When first activated with pcap_activate() or opened with
    pcap_open_live() , a capture handle is not in ``non-blocking mode'';
    a call to pcap_setnonblock() is required in order to put it into
    ``non-blocking'' mode.
pcap_getnonblock() returns the current ``non-blocking'' state of the
  capture descriptor; it always returns 0 on ``savefiles''. If there is an
  error, -1 is returned and errbuf is filled in with an appropriate error
  message.
errbuf is assumed to be able to hold at least
    PCAP_ERRBUF_SIZE chars.
pcap(3), pcap_loop(3), pcap_next_ex(3), pcap_geterr(3)