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, PCAP_ERROR 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(3) will, if no packets are currently
  available to be read, return 0 immediately rather than blocking waiting for
  packets to arrive. pcap_loop(3) and pcap_next(3) will not work
  in ``non-blocking'' mode.
When first activated with pcap_activate(3) or opened with
    pcap_open_live(3) , 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, PCAP_ERROR 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_next_ex(3), pcap_geterr(3)