pcap_compile - compile a filter expression
#include <pcap/pcap.h>
int pcap_compile(pcap_t *p, struct bpf_program *fp,
const char *str, int optimize, bpf_u_int32 netmask);
pcap_compile() is used to compile the string str into a filter
  program. See pcap-filter(7) for the syntax of that string.
  program is a pointer to a bpf_program struct and is filled in by
  pcap_compile(). optimize controls whether optimization on the
  resulting code is performed. netmask specifies the IPv4 netmask of the
  network on which packets are being captured; it is used only when checking for
  IPv4 broadcast addresses in the filter program. If the netmask of the network
  on which packets are being captured isn't known to the program, or if packets
  are being captured on the Linux "any" pseudo-interface that can
  capture on more than one network, a value of PCAP_NETMASK_UNKNOWN can be
  supplied; tests for IPv4 broadcast addresses will fail to compile, but all
  other tests in the filter program will be OK.
NOTE: in libpcap 1.8.0 and later, pcap_compile() can be
    used in multiple threads within a single process. However, in earlier
    versions of libpcap, it is not safe to use pcap_compile() in
    multiple threads in a single process without some form of mutual exclusion
    allowing only one thread to call it at any given time.
pcap_compile() returns 0 on success and -1 on failure. If -1 is returned,
  pcap_geterr() or pcap_perror() may be called with p as an
  argument to fetch or display the error text.
pcap(3), pcap_setfilter(3), pcap_freecode(3), pcap_geterr(3), pcap-filter(7)