#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 PCAP_ERROR on failure. If PCAP_ERROR is returned, pcap_geterr(3PCAP) or pcap_perror(3PCAP) may be called with p as an argument to fetch or display the error text.
The PCAP_NETMASK_UNKNOWN constant became available in libpcap release 1.1.0.
pcap(3PCAP), pcap_setfilter(3PCAP), pcap_freecode(3PCAP),