pcap − Packet Capture library


#include <pcap.h>

pcap_t *pcap_open_live(char *device, int snaplen,
        int promisc, int to_ms, char *ebuf)
pcap_t *pcap_open_offline(char *fname, char *ebuf)
pcap_dumper_t *pcap_dump_open(pcap_t *p, char *fname)

char errbuf[PCAP_ERRBUF_SIZE];
char *pcap_lookupdev(char *errbuf)
int pcap_lookupnet(char *device, bpf_u_int32 *netp,
        bpf_u_int32 *maskp, char *errbuf)

int pcap_dispatch(pcap_t *p, int cnt,
        pcap_handler callback, u_char *user)
int pcap_loop(pcap_t *p, int cnt,
        pcap_handler callback, u_char *user)
void pcap_dump(u_char *user, struct pcap_pkthdr *h,
        u_char *sp)

int pcap_compile(pcap_t *p, struct bpf_program *fp,
        char *str, int optimize, bpf_u_int32 netmask)
int pcap_setfilter(pcap_t *p, struct bpf_program *fp)

u_char *pcap_next(pcap_t *p, struct pcap_pkthdr *h)

int pcap_datalink(pcap_t *p)
int pcap_snapshot(pcap_t *p)
int pcap_is_swapped(pcap_t *p)
int pcap_major_version(pcap_t *p)
int pcap_minor_version(pcap_t *p)
int pcap_stats(pcap_t *p, struct pcap_stat *ps)
FILE *pcap_file(pcap_t *p)
int pcap_fileno(pcap_t *p)
void pcap_perror(pcap_t *p, char *prefix)
char *pcap_geterr(pcap_t *p)
char *pcap_strerror(int error)

void pcap_close(pcap_t *p)
void pcap_dump_close(pcap_dumper_t *p)


The Packet Capture library provides a high level interface to packet capture systems. All packets on the network, even those destined for other hosts, are accessible through this mechanism.


pcap_open_live() is used to obtain a packet capture descriptor to look at packets on the network. device is a string that specifies the network device to open. snaplen specifies the maximum number of bytes to capture. promisc specifies if the interface is to be put into promiscuous mode. (Note that even if this parameter is false, the interface could well be in promiscuous mode for some other reason.) to_ms specifies the read timeout in milliseconds. ebuf is used to return error text and is only set when pcap_open_live() fails and returns NULL.

pcap_open_offline() is called to open a ‘‘savefile’’ for reading. fname specifies the name of the file to open. The file has the same format as those used by tcpdump(1) and tcpslice(1). The name "-" in a synonym for stdin. ebuf is used to return error text and is only set when pcap_open_offline() fails and returns NULL.

pcap_dump_open() is called to open a ‘‘savefile’’ for writing. The name "-" in a synonym for stdout. NULL is returned on failure. p is a pcap struct as returned by pcap_open_offline() or pcap_open_live(). fname specifies the name of the file to open. If NULL is returned, pcap_geterr() can be used to get the error text.

pcap_lookupdev() returns a pointer to a network device suitable for use with pcap_open_live() and pcap_lookupnet(). If there is an error, NULL is returned and errbuf is filled in with with an appropriate error message.

pcap_lookupnet() is used to determine the network number and mask associated with the network device device. Both netp and maskp are bpf_u_int32 pointers. A return of -1 indicates an error in which case errbuf is filled in with with an appropriate error message.

pcap_dispatch() is used to collect and process packets. cnt specifies the maximum number of packets to process before returning. A cnt of -1 processes all the packets received in one buffer. A cnt of 0 processes all packets until an error occurs, EOF is reached, or the read times out (when doing live reads and a non-zero read timeout is specified). callback specifies a routine to be called with three arguments: a u_char pointer which is passed in from pcap_dispatch(), a pointer to the pcap_pkthdr struct (which precede the actual network headers and data), and a u_char pointer to the packet data. The number of packets read is returned. Zero is returned when EOF is reached in a ‘‘savefile.’’ A return of -1 indicates an error in which case pcap_perror() or pcap_geterr() may be used to display the error text.

pcap_dump() outputs a packet to the ‘‘savefile’’ opened with pcap_dump_open(). Note that its calling arguments are suitable for use with pcap_dispatch().

pcap_compile() is used to compile the string str into a filter program. 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 netmask of the local net.

pcap_setfilter() is used to specify a filter program. fp is a pointer to an array of bpf_program struct, usually the result of a call to pcap_compile(). −1 is returned on failure; 0 is returned on success.

pcap_loop() is similar to pcap_dispatch() except it keeps reading packets until cnt packets are processed or an error occurs. It does not return when live read timeouts occur. Rather, specifying a non-zero read timeout to pcap_open_live() and then calling pcap_dispatch() allows the reception and processing of any packets that arrive when the timeout occurs. A negative cnt causes pcap_loop() to loop forever (or at least until an error occurs).

pcap_next() returns a u_char pointer to the next packet.

pcap_datalink() returns the link layer type, e.g. DLT_EN10MB.

pcap_snapshot() returns the snapshot length specified when pcap_open_live was called.

pcap_is_swapped() returns true if the current ‘‘savefile’’ uses a different byte order than the current system.

pcap_major_version() returns the major number of the version of the pcap used to write the savefile.

pcap_minor_version() returns the minor number of the version of the pcap used to write the savefile.

pcap_file() returns the name of the ‘‘savefile.’’

int pcap_stats() returns 0 and fills in a pcap_stat struct. The values represent packet statistics from the start of the run to the time of the call. If there is an error or the under lying packet capture doesn’t support packet statistics, -1 is returned and the error text can be obtained with pcap_perror() or pcap_geterr().

pcap_fileno() returns the file descriptor number of the ‘‘savefile.’’

pcap_perror() prints the text of the last pcap library error on stderr, prefixed by prefix.

pcap_geterr() returns the error text pertaining to the last pcap library error.

pcap_strerror() is provided in case strerror(1) isn’t available.

pcap_close() closes the files associated with p and deallocates resources.

pcap_dump_close() closes the ‘‘savefile.’’


tcpdump(1), tcpslice(1)


Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.

The current version is available via anonymous ftp:



Please send bug reports to libpcap@ee.lbl.gov.