pcap_compile(3) pcap_compile(3)
NAME
pcap_compile - compile a filter expression
SYNOPSIS
#include <pcap/pcap.h>
pcap_compile(3) *p, struct bpf_program *fp,
const char *str, int optimize, bpf_u_int32 netmask);
DESCRIPTION
pcap_compile() is used to compile the string str into a filter program.
See pcap-filter(7) for the syntax of that string. fp is a pointer to a
bpf_program structure and is filled in by pcap_compile(). optimize
controls whether optimization on the resulting code is performed.
netmask specifies the IPv4 netmask (in host byte order) 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 the network interface does not have exactly one IPv4 netmask, or
if packets are being captured on the Linux "any" pseudo-interface,
which can capture on more than one network interface, a value of
PCAP_NETMASK_UNKNOWN can be supplied; in this case an attempt to
compile a filter expression that contains the ip broadcast primitive
will fail.
On Linux, if the pcap_t handle corresponds to a live packet capture,
the resulting filter program may use Linux BPF extensions. This works
transparently if the filter program is used to filter packets on the
same pcap_t handle, which should be done when possible. In other use
cases trying to use a filter program with BPF extensions in
pcap_offline_filter(3) or for filtering an input savefile would
reject more packets than expected because the extensions depend on
auxiliary packet data, which would not be available. The workaround is
to compile the filter without the extensions by using a pcap_t handle
from pcap_open_dead(3) or pcap_open_offline(3) rather than a
handle from pcap_create(3) or pcap_open_live(3).
If BPF extensions are disabled as described above or the OS is not
Linux, pcap_compile() may start rejecting some filter expressions for
some link-layer header types, this is the expected behaviour. For
example, the ifindex keyword is valid for any live capture on Linux,
but when reading packets from a savefile, regardless of the OS it is
valid for DLT_LINUX_SLL2 only.
By convention, in this function both a null pointer and an empty string
are valid arguments for the str input parameter and mean the same: to
accept all packets. This should not be confused with an empty
resulting filter program: even a program that accepts or rejects all
packets comprises at least one BPF instruction, so on success
fp.bf_insns is never a null pointer and fp.bf_len is always greater
than zero.
RETURN VALUE
pcap_compile() returns 0 on success and PCAP_ERROR on failure. If
PCAP_ERROR is returned, pcap_geterr(3) or pcap_perror(3) may be
called with p as an argument to fetch or display the error text.
BACKWARD COMPATIBILITY
The PCAP_NETMASK_UNKNOWN constant became available in libpcap release
1.1.0.
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.
SEE ALSO
pcap(3), pcap_setfilter(3), pcap_freecode(3)
11 September 2026 pcap_compile(3)
libpcap 1.11.0 - Generated Sun Sep 13 16:52:46 CDT 2026